{"id":330,"date":"2021-12-17T21:41:19","date_gmt":"2021-12-17T21:41:19","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes\/"},"modified":"2025-01-21T01:03:28","modified_gmt":"2025-01-21T01:03:28","slug":"4-igneous-processes-and-volcanoes","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes\/","title":{"raw":"4 Igneous Processes and Volcanoes","rendered":"4 Igneous Processes and Volcanoes"},"content":{"raw":"[caption id=\"attachment_315\" align=\"aligncenter\" width=\"2031\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/IT-PompeiiVesuvius.jpg\"><img class=\"wp-image-235 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius.jpg\" alt=\"The town in Italy is a ruin.\" width=\"2031\" height=\"1355\"><\/a> Mount Vesuvius towers over the ruins of Pompeii, a city destroyed by the eruption in 79 CE.[\/caption]\n<h1>4 Igneous Processes and Volcanoes<\/h1>\n<b>KEY CONCEPTS<\/b>\n\n<strong>By the end of this chapter, students should be able to:<\/strong>\n<ul>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain the origin of [pb_glossary id=\"2672\"]magma[\/pb_glossary] it relates to [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary]<\/span><\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe how the [pb_glossary id=\"1174\"]Bowen\u2019s Reaction Series[\/pb_glossary]\u00a0relates [pb_glossary id=\"2687\"]mineral[\/pb_glossary] [pb_glossary id=\"2674\"]crystallization[\/pb_glossary] and melting temperatures<\/span><\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain how cooling of [pb_glossary id=\"2672\"]magma[\/pb_glossary] leads to rock compositions and textures, and how these are used to classify [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks<\/span><\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Analyze the features of common [pb_glossary id=\"2675\"]igneous[\/pb_glossary] landforms and how they relate to their origin<\/span><\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain [pb_glossary id=\"1177\"]partial melting[\/pb_glossary] and [pb_glossary id=\"1180\"]fractionation[\/pb_glossary], and how they change [pb_glossary id=\"2672\"]magma[\/pb_glossary] compositions<\/span><\/li>\n \t<li>Describe how silica content affects [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] and eruptive style of [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]<\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe [pb_glossary id=\"1181\"]volcano[\/pb_glossary] types, eruptive styles, [pb_glossary id=\"2831\"]composition[\/pb_glossary], and their [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] settings<\/span><\/li>\n \t<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] hazards<\/span><\/li>\n<\/ul>\n<span style=\"font-weight: 400\"><strong>[pb_glossary id=\"2675\"]Igneous[\/pb_glossary]\u00a0rock<\/strong> is\u00a0formed when liquid rock freezes into a solid rock. This molten material is called <strong>[pb_glossary id=\"2672\"]magma[\/pb_glossary]<\/strong>\u00a0when it is in the ground and\u00a0<strong>[pb_glossary id=\"2673\"]lava[\/pb_glossary]<\/strong>\u00a0when it is on the surface. <\/span><span style=\"font-weight: 400\">Only the Earth's [pb_glossary id=\"2595\"]outer core[\/pb_glossary] is liquid; the Earth's [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and [pb_glossary id=\"2580\"]crust[\/pb_glossary] is naturally solid. However, there are<\/span><span style=\"font-weight: 400\">\u00a0a few minor pockets of [pb_glossary id=\"2672\"]magma[\/pb_glossary] that form near the surface where geologic processes cause melting. It is this [pb_glossary id=\"2672\"]magma[\/pb_glossary] that becomes the source for [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] and [pb_glossary id=\"2675\"]igneous rocks[\/pb_glossary]. This chapter will describe the classification of [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks, the unique processes that form [pb_glossary id=\"2672\"]magmas[\/pb_glossary], types of [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] and [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] processes, [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] hazards, and [pb_glossary id=\"2675\"]igneous[\/pb_glossary]\u00a0landforms.\u00a0<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii.jpg\"><img class=\"size-medium wp-image-236\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-300x200.jpg\" alt=\"Pahoehoe lava flow in Hawaii\" width=\"300\" height=\"200\"><\/a> Lava flow in Hawaii[\/caption]\n\n&nbsp;\n\n<span style=\"font-weight: 400\">[pb_glossary id=\"2673\"]Lava[\/pb_glossary] cools quickly on the surface of the earth and forms tiny microscopic crystals. These are known as fine-grained\u00a0<b>[pb_glossary id=\"1938\"]extrusive[\/pb_glossary]<\/b>, or [pb_glossary id=\"1181\"]volcanic[\/pb_glossary], [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks. [pb_glossary id=\"1938\"]Extrusive[\/pb_glossary] rocks are often <strong>[pb_glossary id=\"1945\"]vesicular[\/pb_glossary]<\/strong>, filled with holes from escaping gas bubbles. <strong>[pb_glossary id=\"1181\"]Volcanism[\/pb_glossary]<\/strong> is the process in which [pb_glossary id=\"2673\"]lava[\/pb_glossary] is erupted. Depending on the properties of the [pb_glossary id=\"2673\"]lava[\/pb_glossary] that is erupted, the [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] can be drastically different, from smooth and gentle to dangerous and explosive. This leads to different types of [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] and different [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] hazards.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Half-Dome-a-granitic-batholith-in-Yosemite.jpg\"><img class=\"size-medium wp-image-237\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-300x257.jpg\" alt=\"An intrusive igneous mass now exposed at the surface by erosion\" width=\"300\" height=\"257\"><\/a> Half Dome, an intrusive igneous batholith in Yosemite National Park[\/caption]\n\n<span style=\"font-weight: 400\">In contrast, [pb_glossary id=\"2672\"]magma[\/pb_glossary] that cools slowly below the earth\u2019s surface forms larger crystals which can be seen with the naked eye. These are known as coarse-grained\u00a0<b>[pb_glossary id=\"1939\"]intrusive[\/pb_glossary]<\/b>, or [pb_glossary id=\"1939\"]plutonic[\/pb_glossary], [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks. This relationship between cooling rates and grain sizes of the solidified [pb_glossary id=\"2687\"]minerals[\/pb_glossary] in [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks is important for interpreting the rock\u2019s geologic history.<\/span>\n<h2>\u00a0<span style=\"font-weight: 400\">4.1 Classification of Igneous Rocks<\/span><\/h2>\n<span style=\"font-weight: 400\">[pb_glossary id=\"2675\"]Igneous[\/pb_glossary] rocks are classified based on [pb_glossary id=\"2919\"]texture[\/pb_glossary] and [pb_glossary id=\"2831\"]composition[\/pb_glossary]. [pb_glossary id=\"2919\"]Texture[\/pb_glossary] describes the physical characteristics of the [pb_glossary id=\"2687\"]minerals[\/pb_glossary], such as grain size. This relates to the cooling history of the molten [pb_glossary id=\"2672\"]magma[\/pb_glossary] from which it came. [pb_glossary id=\"2831\"]Composition[\/pb_glossary] refers to the rock's specific mineralogy and chemical [pb_glossary id=\"2831\"]composition[\/pb_glossary]. Cooling history is also related to changes that can occur to the [pb_glossary id=\"2831\"]composition[\/pb_glossary] of [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks.<\/span>\n<h3><b>4.1.1 Texture<\/b><\/h3>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Fig-6.3-Granite-vs-Gneiss-e1495050932921.jpg\"><img class=\"size-medium wp-image-238\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-300x271.jpg\" alt=\"Image showing three or four distinct colors of clearly visible minerals.\" width=\"300\" height=\"271\"><\/a> Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. The different colors are unique minerals. The black colors are likely two or three different minerals.[\/caption]\n\nIf [pb_glossary id=\"2672\"]magma[\/pb_glossary] cools slowly, deep within the [pb_glossary id=\"2580\"]crust[\/pb_glossary], the resulting rock is called [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] or [pb_glossary id=\"1939\"]plutonic[\/pb_glossary]. The slow cooling process allows crystals to grow large, giving [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] a coarse-grained or <strong>[pb_glossary id=\"1940\"]phaneritic[\/pb_glossary]<\/strong> [pb_glossary id=\"2919\"]texture[\/pb_glossary]. The individual crystals in [pb_glossary id=\"1940\"]phaneritic[\/pb_glossary] [pb_glossary id=\"2919\"]texture[\/pb_glossary] are readily visible to the unaided eye.\n\n&nbsp;\n\n&nbsp;\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"284\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/IMG_2623-e1495051966958-scaled.jpg\"><img class=\"size-medium wp-image-2840\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/IMG_2623-e1495051966958-scaled-1.jpg\" alt=\"Show dark rock with no visible minerals except for a few tiny green minerals that are olivine.\" width=\"284\" height=\"300\"><\/a> Basalt is a classic fine-grained extrusive igneous rock. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine.[\/caption]\n\nWhen [pb_glossary id=\"2673\"]lava[\/pb_glossary] is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] is called [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] or [pb_glossary id=\"1181\"]volcanic[\/pb_glossary]. [pb_glossary id=\"1938\"]Extrusive[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks have a fine-grained or <strong>[pb_glossary id=\"1941\"]aphanitic[\/pb_glossary]<\/strong> [pb_glossary id=\"2919\"]texture[\/pb_glossary], in which the grains are too small to see with the unaided eye. The fine-grained [pb_glossary id=\"2919\"]texture[\/pb_glossary] indicates the quickly cooling [pb_glossary id=\"2673\"]lava[\/pb_glossary] did not have time to grow large crystals. These tiny crystals can be viewed under a petrographic microscope. In some cases, [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] cools so rapidly it does not develop crystals at all. This non-crystalline material is not classified as [pb_glossary id=\"2687\"]minerals[\/pb_glossary], but as [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] glass<span style=\"font-weight: 400\">. This is a common component of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1949\"]ash[\/pb_glossary] and rocks like [pb_glossary id=\"1947\"]obsidian[\/pb_glossary].<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.5_Porphyritic_texture.jpg\"><img class=\"size-medium wp-image-240\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-300x223.jpg\" alt=\"Porphyritic teture with large crystals in a finer grained groundmass\" width=\"300\" height=\"223\"><\/a> Porphyritic texture[\/caption]\n\nSome [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks have a mix of coarse-grained [pb_glossary id=\"2687\"]minerals[\/pb_glossary] surrounded by a matrix of fine-grained material in a [pb_glossary id=\"2919\"]texture[\/pb_glossary] called <strong>[pb_glossary id=\"1942\"]porphyritic[\/pb_glossary]<\/strong>. The large crystals are called <strong>[pb_glossary id=\"1943\"]phenocrysts[\/pb_glossary]<\/strong> and the fine-grained matrix is called the <strong>[pb_glossary id=\"1959\"]groundmass[\/pb_glossary]<\/strong> or <strong>matrix<\/strong>. [pb_glossary id=\"1942\"]Porphyritic[\/pb_glossary] [pb_glossary id=\"2919\"]texture[\/pb_glossary] indicates the [pb_glossary id=\"2672\"]magma[\/pb_glossary] body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.6_We-pegmatite.jpg\"><img class=\"size-medium wp-image-241\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-300x224.jpg\" alt=\"Pegmatic texture with large grains of minerals, mostly of felsic composition\" width=\"300\" height=\"224\"><\/a> Pegmatitic texture[\/caption]\n\nResidual molten material expelled from [pb_glossary id=\"2675\"]igneous[\/pb_glossary] intrusions may form veins or masses containing very large crystals of [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like [pb_glossary id=\"1916\"]feldspar[\/pb_glossary], [pb_glossary id=\"1915\"]quartz[\/pb_glossary], beryl, tourmaline, and [pb_glossary id=\"1914\"]mica[\/pb_glossary]. This [pb_glossary id=\"2919\"]texture[\/pb_glossary], which indicates a very slow [pb_glossary id=\"2674\"]crystallization[\/pb_glossary], is called <strong>[pb_glossary id=\"1944\"]pegmatitic[\/pb_glossary]<\/strong>. A rock that chiefly consists of [pb_glossary id=\"1944\"]pegmatitic[\/pb_glossary] [pb_glossary id=\"2919\"]texture[\/pb_glossary] is known as a <strong>[pb_glossary id=\"1944\"]pegmatite[\/pb_glossary]<\/strong>. To give an example of how large these crystals can get, transparent cleavage sheets of [pb_glossary id=\"1944\"]pegmatitic[\/pb_glossary] [pb_glossary id=\"1914\"]muscovite[\/pb_glossary] [pb_glossary id=\"1914\"]mica[\/pb_glossary] were used as windows during the Middle Ages.\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.7_Scoria_Macro_Digon3-e1495227072616.jpg\"><img class=\"size-medium wp-image-242\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-300x286.jpg\" alt=\"A lava rock full of bubbles called scoria\" width=\"300\" height=\"286\"><\/a> Scoria[\/caption]\n\nAll [pb_glossary id=\"2672\"]magmas[\/pb_glossary] contain gases [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] in solution called <strong>[pb_glossary id=\"2606\"]volatiles[\/pb_glossary]<\/strong>. As the [pb_glossary id=\"2672\"]magma[\/pb_glossary] rises to the surface, the drop in pressure causes the [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] to come bubbling out of [pb_glossary id=\"2705\"]solution[\/pb_glossary], like the fizz in an opened bottle of soda. The gas bubbles become trapped in the solidifying [pb_glossary id=\"2673\"]lava[\/pb_glossary] to create a <strong>[pb_glossary id=\"1945\"]vesicular[\/pb_glossary]<\/strong> [pb_glossary id=\"2919\"]texture[\/pb_glossary], with the holes specifically called vesicles. The type of [pb_glossary id=\"1938\"]volcanic rock[\/pb_glossary] with common vesicles is called <strong>scoria<\/strong>.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.9_Pumice_stone-e1495052465796.jpg\"><img class=\"wp-image-243\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-298x300.jpg\" alt=\"A pumice stone, a hardened froth of volcanic glass\" width=\"250\" height=\"251\"><\/a> Pumice[\/caption]\n\nAn extreme version of scoria occurs when volatile-rich [pb_glossary id=\"2673\"]lava[\/pb_glossary]\u00a0is very quickly quenched and becomes a meringue-like froth of glass called <b>[pb_glossary id=\"1946\"]pumice[\/pb_glossary]<\/b>. Some [pb_glossary id=\"1946\"]pumice[\/pb_glossary] is so full of vesicles that the density of the rock drops low enough that it will float.\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"200\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.8_Lipari-Obsidienne_5.jpg\"><img class=\"wp-image-244\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-300x225.jpg\" alt=\"Photo of obsidian, a volcanic glass\" width=\"200\" height=\"150\"><\/a> Obsidian (volcanic glass). Note conchoidal fracture.[\/caption]\n\n&nbsp;\n\n<span style=\"font-weight: 400\">[pb_glossary id=\"2673\"]Lava[\/pb_glossary] that cools extremely quickly may not form crystals at all, even microscopic ones. The resulting rock is called\u00a0<strong>[pb_glossary id=\"1181\"]volcanic[\/pb_glossary] glass<\/strong>. <strong>O<\/strong><\/span><b>bsidian<\/b> is a rock consisting of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] glass.\u00a0<span style=\"font-weight: 400\">[pb_glossary id=\"1947\"]Obsidian[\/pb_glossary] as a glassy rock shows an excellent example of [pb_glossary id=\"1935\"]conchoidal[\/pb_glossary] [pb_glossary id=\"1934\"]fracture[\/pb_glossary] similar to the [pb_glossary id=\"2687\"]mineral[\/pb_glossary] [pb_glossary id=\"1915\"]quartz[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/3-minerals\/\">Chapter 3<\/a>).<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.11_HoleInTheWallTuff.jpg\"><img class=\"size-medium wp-image-245\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-300x225.jpg\" alt=\"Tuff showing various size fragments of minerals and ash blown out of a volcano\" width=\"300\" height=\"225\"><\/a> Welded tuff[\/caption]\n\nWhen [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] erupt explosively, vast amounts of [pb_glossary id=\"2673\"]lava[\/pb_glossary], rock, [pb_glossary id=\"1949\"]ash[\/pb_glossary], and gases are thrown into the [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary]. The solid parts, called [pb_glossary id=\"1948\"]tephra[\/pb_glossary], settle back to earth and cool into rocks with <strong>[pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary]<\/strong> textures. <em>Pyro,<\/em> meaning fire, refers to the [pb_glossary id=\"2675\"]igneous[\/pb_glossary] source of the [pb_glossary id=\"1948\"]tephra[\/pb_glossary] and <em>[pb_glossary id=\"2825\"]clastic[\/pb_glossary] <\/em>refers to the rock fragments. [pb_glossary id=\"1948\"]Tephra[\/pb_glossary] fragments are named based on size\u2014<strong>[pb_glossary id=\"1949\"]ash[\/pb_glossary]<\/strong> (&lt;2 mm), <strong>[pb_glossary id=\"1950\"]lapilli[\/pb_glossary]<\/strong> (2-64 mm), and <strong>[pb_glossary id=\"1951\"]bombs[\/pb_glossary] or blocks<\/strong> (&gt;64 mm). [pb_glossary id=\"1952\"]Pyroclastic[\/pb_glossary] [pb_glossary id=\"2919\"]texture[\/pb_glossary] is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. Rock formed from large deposits of [pb_glossary id=\"1948\"]tephra[\/pb_glossary] fragments is called <strong>[pb_glossary id=\"1953\"]tuff[\/pb_glossary]<\/strong>. If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded [pb_glossary id=\"1953\"]tuff[\/pb_glossary].\n<h3><b>4.1.2 Composition<\/b><\/h3>\n[pb_glossary id=\"2831\"]Composition[\/pb_glossary] refers to a rock\u2019s chemical and [pb_glossary id=\"2687\"]mineral[\/pb_glossary] make-up . For [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary], [pb_glossary id=\"2831\"]composition[\/pb_glossary] is divided into four groups: <strong>[pb_glossary id=\"1954\"]felsic[\/pb_glossary]<\/strong>, <strong>[pb_glossary id=\"1955\"]intermediate[\/pb_glossary]<\/strong>, <strong>[pb_glossary id=\"1956\"]mafic[\/pb_glossary]<\/strong>, and <strong>[pb_glossary id=\"1957\"]ultramafic[\/pb_glossary]<\/strong><em>.<\/em> These groups refer to differing amounts of silica, iron, and magnesium found in the [pb_glossary id=\"2687\"]minerals[\/pb_glossary] that make up the rocks. It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of [pb_glossary id=\"2687\"]minerals[\/pb_glossary].\u00a0<span style=\"font-weight: 400\">As an example, [pb_glossary id=\"1962\"]granite[\/pb_glossary] is a commonly-used term, but has a very specific definition which includes exact quantities of [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like [pb_glossary id=\"1916\"]feldspar[\/pb_glossary] and [pb_glossary id=\"1915\"]quartz[\/pb_glossary]. Rocks labeled as '[pb_glossary id=\"1962\"]granite[\/pb_glossary]' in laymen applications can be several other rocks, including\u00a0 syenite, tonalite, and monzonite.\u00a0<\/span>To avoid these complications, the following figure presents a simplified version of [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] nomenclature focusing on the four main groups, which is adequate for an introductory student.\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"1672\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-rock-composition-as-volume-percentages-of-minerals.jpg\"><img class=\"size-full wp-image-246\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals.jpg\" alt=\"Diagram showing the mineral composition of the four classes of igneous rocks, ultramafic, mafic, intermediate, and felsic.\" width=\"1672\" height=\"1182\"><\/a> Mineral composition of common igneous rocks. Percentage of minerals is shown on the vertical axis. Percentage of silica is shown on the horizontal axis. Rock names at the top include a continuous spectrum of compositions grading from one into another.[\/caption]\n\n<b>Fel<\/b><span style=\"font-weight: 400\">sic refers to a predominance of the light-colored ([pb_glossary id=\"1954\"]felsic[\/pb_glossary]) [pb_glossary id=\"2687\"]minerals[\/pb_glossary]\u00a0<\/span><span style=\"text-decoration: underline\"><b>fel<\/b><\/span><span style=\"font-weight: 400\">dspar and <span style=\"text-decoration: underline\"><strong>si<\/strong><\/span>lica in the form of [pb_glossary id=\"1915\"]quartz[\/pb_glossary]. These light-colored [pb_glossary id=\"2687\"]minerals[\/pb_glossary] have more silica as a proportion of their overall chemical formula. Minor amounts of dark-colored ([pb_glossary id=\"1956\"]mafic[\/pb_glossary]) [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like [pb_glossary id=\"2713\"]amphibole[\/pb_glossary] and [pb_glossary id=\"4444\"]biotite[\/pb_glossary] [pb_glossary id=\"1914\"]mica[\/pb_glossary] may be present as well. [pb_glossary id=\"1954\"]Felsic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks are rich in silica (in the 65-75% range, meaning the rock would be 65-75% weight percent SiO<sup>2<\/sup>) and poor in iron and magnesium.<\/span>\n\n<b>[pb_glossary id=\"1955\"]Intermediate[\/pb_glossary]<\/b> is a [pb_glossary id=\"2831\"]composition[\/pb_glossary] between [pb_glossary id=\"1954\"]felsic[\/pb_glossary] and [pb_glossary id=\"1956\"]mafic[\/pb_glossary].\u00a0 It usually contains roughly-equal amounts of light and dark [pb_glossary id=\"2687\"]minerals[\/pb_glossary], including light grains of [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] [pb_glossary id=\"1916\"]feldspar[\/pb_glossary] and dark [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like amphibole.\u00a0 It is [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] in silica in the 55-60% range.\n\n<b>Maf<\/b><span style=\"font-weight: 400\">ic refers to a abundance of ferromagnesian [pb_glossary id=\"2687\"]minerals[\/pb_glossary] (with magnesium and iron, chemical symbols <\/span><b>M<\/b>g<span style=\"font-weight: 400\">\u00a0and <\/span><b>F<\/b><span style=\"font-weight: 400\">e) plus [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] [pb_glossary id=\"1916\"]feldspar[\/pb_glossary]. It is mostly made of dark [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like [pb_glossary id=\"2712\"]pyroxene[\/pb_glossary] and [pb_glossary id=\"2711\"]olivine[\/pb_glossary], which are rich in iron and magnesium and relatively poor in silica. [pb_glossary id=\"1956\"]Mafic[\/pb_glossary] rocks are low in silica, in the 45-50% range.<\/span>\n\n<span style=\"font-weight: 400\"><strong>[pb_glossary id=\"1957\"]Ultramafic[\/pb_glossary]<\/strong> refers to the extremely [pb_glossary id=\"1956\"]mafic[\/pb_glossary] rocks [pb_glossary id=\"2831\"]composed[\/pb_glossary] of mostly [pb_glossary id=\"2711\"]olivine[\/pb_glossary] and some [pb_glossary id=\"2712\"]pyroxene[\/pb_glossary] which have even more magnesium and iron and even less silica. T<\/span><span style=\"font-weight: 400\">hese rocks are rare on the surface, but make up [pb_glossary id=\"2588\"]peridotite[\/pb_glossary], the rock of the upper [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. It is poor in silica, in the 40% or less range.<\/span>\n\n<span style=\"font-weight: 400\">On the figure above, the top row has both [pb_glossary id=\"1939\"]plutonic[\/pb_glossary] and [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks arranged in a continuous spectrum from [pb_glossary id=\"1954\"]felsic[\/pb_glossary] on the left to [pb_glossary id=\"1955\"]intermediate[\/pb_glossary], [pb_glossary id=\"1956\"]mafic[\/pb_glossary], and [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] toward the right.\u00a0<\/span><b>[pb_glossary id=\"1958\"]Rhyolite[\/pb_glossary]<\/b><span style=\"font-weight: 400\">\u00a0thus refers to the [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] and [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks, and <\/span><b>[pb_glossary id=\"1962\"]granite[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> thus refer to [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] and [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks. <\/span><b>[pb_glossary id=\"1960\"]Andesite[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> and <\/span><b>[pb_glossary id=\"1963\"]diorite[\/pb_glossary]<\/b><span style=\"font-weight: 400\">\u00a0likewise refer to [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] and [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] rocks (with <\/span>dacite<span style=\"font-weight: 400\"> and <\/span>granodiorite<span style=\"font-weight: 400\"> applying to those rocks with [pb_glossary id=\"2831\"]composition[\/pb_glossary] between [pb_glossary id=\"1954\"]felsic[\/pb_glossary] and [pb_glossary id=\"1955\"]intermediate[\/pb_glossary]).\u00a0<\/span><b>[pb_glossary id=\"1961\"]Basalt[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> and <\/span><b>[pb_glossary id=\"1964\"]gabbro[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> are the [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] and [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] names for [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks, and <\/span><b>[pb_glossary id=\"2588\"]peridotite[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> is [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary], with\u00a0<b>komatiite<\/b> as the fine-grained [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] equivalent. Komatiite is a rare rock because [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] material that comes direct from the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] is not common, although some examples can be found in ancient [pb_glossary id=\"2205\"]Archean[\/pb_glossary] rocks<\/span><span style=\"font-weight: 400\">. Nature rarely has sharp boundaries and the classification and naming of rocks often imposes what appear to be sharp boundary names onto a continuous spectrum.<\/span>\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"2048\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-Classification-cropped-scaled.jpg\"><img class=\"size-full wp-image-2848\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-Classification-cropped-scaled-1.jpg\" alt=\"Classification table of igneous rock.\" width=\"2048\" height=\"2560\"><\/a> Igneous rock classification table with composition as vertical columns and texture as horizontal rows.[\/caption]\n<h4><strong>Aphanitic\/Phaneritic Rock Types with images<\/strong><\/h4>\n<table style=\"height: 1296px\">\n<tbody>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;vertical-align: middle;height: 15px;width: 810.062px\" colspan=\"2\">\n<div class=\"mceTemp\"><strong>[pb_glossary id=\"1954\"]Felsic[\/pb_glossary] [pb_glossary id=\"2831\"]Composition[\/pb_glossary]<\/strong><\/div><\/td>\n<\/tr>\n<tr style=\"height: 279px\">\n<td style=\"text-align: center;vertical-align: middle;height: 279px;width: 397.672px\">\n\n[caption id=\"attachment_2849\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/550px-Quartz_monzonite_36mw1037.jpg\"><img class=\"size-medium wp-image-248\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-300x262.jpg\" alt=\"Photograph of cut granite. showing a variety of visible minerals, including quartz and k-feldspar.\" width=\"300\" height=\"262\"><\/a> Granite from Cape Cod, Massachusetts.[\/caption]<\/td>\n<td style=\"text-align: center;vertical-align: middle;height: 279px;width: 399.453px\">\n\n[caption id=\"attachment_2850\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/lossy-page1-640px-PinkRhyolite.tif_.jpg\"><img class=\"size-medium wp-image-249\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-300x214.jpg\" alt=\"Fine Crystalline rock with larger glassy fragments floating in the groundmass.\" width=\"300\" height=\"214\"><\/a> Rhyolite (source: Michael C. Rygel via Wikimedia Commons)[\/caption]<\/td>\n<\/tr>\n<tr style=\"height: 221px\">\n<td style=\"height: 221px;width: 397.672px\">[pb_glossary id=\"1962\"]Granite[\/pb_glossary] is a course-crystalline [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] rock. \u00a0The presence of [pb_glossary id=\"1915\"]quartz[\/pb_glossary] is a good indicator of [pb_glossary id=\"1962\"]granite[\/pb_glossary]. \u00a0[pb_glossary id=\"1962\"]Granite[\/pb_glossary] commonly has large amounts of salmon pink potassium [pb_glossary id=\"1916\"]feldspar[\/pb_glossary] and white [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] crystals that have visible\u00a0cleavage planes. [pb_glossary id=\"1962\"]Granite[\/pb_glossary] is a good approximation for the [pb_glossary id=\"2575\"]continental crust[\/pb_glossary], both in density and [pb_glossary id=\"2831\"]composition[\/pb_glossary].<\/td>\n<td style=\"height: 221px;width: 399.453px\">[pb_glossary id=\"1958\"]Rhyolite[\/pb_glossary]\u00a0is a fine-crystalline [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] rock. \u00a0[pb_glossary id=\"1958\"]Rhyolite[\/pb_glossary] is commonly pink and will often have glassy [pb_glossary id=\"1915\"]quartz[\/pb_glossary] [pb_glossary id=\"1943\"]phenocrysts[\/pb_glossary].\u00a0 Because [pb_glossary id=\"1954\"]felsic[\/pb_glossary] lavas are less mobile, it is less common than [pb_glossary id=\"1962\"]granite[\/pb_glossary]. Examples of [pb_glossary id=\"1958\"]rhyolite[\/pb_glossary] include several [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows in Yellowstone National Park and the altered [pb_glossary id=\"1958\"]rhyolite[\/pb_glossary] that makes up the Grand Canyon of the Yellowstone.<\/td>\n<\/tr>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;height: 15px;width: 810.062px\" colspan=\"2\">\u00a0\u00a0<strong>[pb_glossary id=\"1955\"]Intermediate[\/pb_glossary] [pb_glossary id=\"2831\"]Composition[\/pb_glossary]<\/strong><\/td>\n<\/tr>\n<tr style=\"height: 257px\">\n<td style=\"height: 257px;width: 397.672px\">\n\n[caption id=\"attachment_2851\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Diorite_MA-e1496027879779.jpg\"><img class=\"size-medium wp-image-250\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-300x225.jpg\" alt=\"Rock with visible black and white crystals.\" width=\"300\" height=\"225\"><\/a> Diorite[\/caption]<\/td>\n<td style=\"height: 257px;width: 399.453px\">\n\n[caption id=\"attachment_2852\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Andesite2.tif_.jpg\"><img class=\"size-medium wp-image-251\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-300x240.jpg\" alt=\"Grey rock with fine crystals and black phenocrysts.\" width=\"300\" height=\"240\"><\/a> Andesite[\/caption]<\/td>\n<\/tr>\n<tr style=\"height: 158px\">\n<td style=\"height: 158px;width: 397.672px\">\u00a0[pb_glossary id=\"1963\"]Diorite[\/pb_glossary] is a coarse-crystalline [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary]. Diorite is identifiable by it's Dalmatian-like appearance of black hornblende and [pb_glossary id=\"1914\"]biotite[\/pb_glossary]\u00a0and white [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] [pb_glossary id=\"1916\"]feldspar[\/pb_glossary]. It is found in its namesake, the Andes Mountains as well as the Henry and Abajo mountains of Utah.<\/td>\n<td style=\"height: 158px;width: 399.453px\">\u00a0[pb_glossary id=\"1960\"]Andesite[\/pb_glossary] is a fine crystalline [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] rock. \u00a0It is commonly grey and [pb_glossary id=\"1942\"]porphyritic[\/pb_glossary]. \u00a0It can be found in the Andes Mountains and in some island arcs (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\">Chapter 2<\/a>). It is the fine grained compositional equivalent of [pb_glossary id=\"1963\"]diorite[\/pb_glossary].<\/td>\n<\/tr>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;height: 15px;width: 810.062px\" colspan=\"2\"><strong>[pb_glossary id=\"1956\"]Mafic[\/pb_glossary] [pb_glossary id=\"2831\"]Composition[\/pb_glossary]<\/strong><\/td>\n<\/tr>\n<tr style=\"height: 242px\">\n<td style=\"height: 242px;width: 397.672px\">\n\n[caption id=\"attachment_2853\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/GabbroRockCreek1.jpg\"><img class=\"size-medium wp-image-252\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-300x215.jpg\" alt=\"Dark rock with visible crystals.\" width=\"300\" height=\"215\"><\/a> Gabbro[\/caption]<\/td>\n<td style=\"height: 242px;width: 399.453px\">\n\n[caption id=\"attachment_2854\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/VessicularBasalt1.jpg\"><img class=\"size-medium wp-image-253\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-300x225.jpg\" alt=\"Dark grey rock with many visible holes and no visible crystals.\" width=\"300\" height=\"225\"><\/a> Vesicular Basalt[\/caption]<\/td>\n<\/tr>\n<tr style=\"height: 94px\">\n<td style=\"height: 94px;width: 397.672px\">\n<div class=\"mceTemp\">[pb_glossary id=\"1964\"]Gabbro[\/pb_glossary] is a coarse-grained [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary], made with mainly [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like [pb_glossary id=\"2712\"]pyroxene[\/pb_glossary] and only minor [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary]. Because [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] is more mobile, it is less common than [pb_glossary id=\"1961\"]basalt[\/pb_glossary]. [pb_glossary id=\"1964\"]Gabbro[\/pb_glossary] is a major component of the lower [pb_glossary id=\"2581\"]oceanic crust[\/pb_glossary].<\/div><\/td>\n<td style=\"height: 94px;width: 399.453px\">[pb_glossary id=\"1961\"]Basalt[\/pb_glossary] is a fine-grained [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary]. It is commonly [pb_glossary id=\"1945\"]vesicular[\/pb_glossary] and [pb_glossary id=\"1941\"]aphanitic[\/pb_glossary]. When [pb_glossary id=\"1942\"]porphyritic[\/pb_glossary], it often has either [pb_glossary id=\"2711\"]olivine[\/pb_glossary] or [pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] [pb_glossary id=\"1943\"]phenocrysts[\/pb_glossary]. [pb_glossary id=\"1961\"]Basalt[\/pb_glossary] is the main rock which is formed at [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], and is therefore the most common rock on the Earth's surface, making up the entirety of the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] (except where covered by [pb_glossary id=\"2678\"]sediment[\/pb_glossary]).<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3><b>4.1.3 Igneous Rock Bodies<\/b><\/h3>\n<span style=\"font-weight: 400\">[pb_glossary id=\"2675\"]Igneous[\/pb_glossary] rocks are common in the geologic record, but surprisingly, it is the [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] rocks that are more common. [pb_glossary id=\"1938\"]Extrusive[\/pb_glossary] rocks, because of their small crystals and glass, are less durable. Plus, they are, by definition, exposed to the [pb_glossary id=\"2700\"]elements[\/pb_glossary] of [pb_glossary id=\"2677\"]erosion[\/pb_glossary] immediately. [pb_glossary id=\"1939\"]Intrusive[\/pb_glossary] rocks, forming underground with larger, stronger crystals, are more likely to last. Therefore, most landforms and rock groups that owe their origin to [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks are [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] bodies. A significant exception to this is active [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary], which are discussed in a <a href=\"#4-5Volcanism\">later section on volcanism<\/a>. This section will [pb_glossary id=\"3080\"]focus[\/pb_glossary] on the common [pb_glossary id=\"2675\"]igneous[\/pb_glossary] bodies which are found in many places within the [pb_glossary id=\"1971\"]bedrock[\/pb_glossary] of Earth.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic.jpg\"><img class=\"size-medium wp-image-254\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-300x201.jpg\" alt=\"Igneous dike cuts across Baffin Island in the Canadian Arctic.\" width=\"300\" height=\"201\"><\/a> Dike of olivine gabbro cuts across Baffin Island in the Canadian Arctic[\/caption]\n\nWhen [pb_glossary id=\"2672\"]magma[\/pb_glossary] intrudes into a weakness like a crack or fissure and solidifies, the resulting cross-cutting feature is called a <strong>[pb_glossary id=\"1969\"]dike[\/pb_glossary]<\/strong>\u00a0(sometimes spelled [pb_glossary id=\"1969\"]dyke[\/pb_glossary])<span style=\"font-weight: 400\">. Because of this, [pb_glossary id=\"1969\"]dikes[\/pb_glossary] are often vertical or at an angle relative to the pre-existing rock layers that they intersect. [pb_glossary id=\"1969\"]Dikes[\/pb_glossary] are therefore discordant intrusions, not following any layering that was present. [pb_glossary id=\"1969\"]Dikes[\/pb_glossary] are important to geologists, not only for the study of [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks themselves but also for dating rock sequences and interpreting the geologic history of an area. The [pb_glossary id=\"1969\"]dike[\/pb_glossary] is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time (<a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/7-geologic-time\/\">Chapter 7<\/a>), may be used to assign actual numeric ages to sedimentary sequences, which are notoriously difficult to age date.\u00a0<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.14_Horton_Bluff_mid-Carboniferous_sill.jpg\"><img class=\"size-medium wp-image-255\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-300x225.jpg\" alt=\"Igneous sill intruding in between Paleozoic strata in Nova Scotia\" width=\"300\" height=\"225\"><\/a> Igneous sill intruding between Paleozoic strata in Nova Scotia[\/caption]\n\n<strong>[pb_glossary id=\"1970\"]Sills[\/pb_glossary]<\/strong> are another type of [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] structure. A [pb_glossary id=\"1970\"]sill[\/pb_glossary] is a concordant intrusion that runs parallel to the sedimentary layers in the [pb_glossary id=\"1971\"]country rock[\/pb_glossary]. They are formed when [pb_glossary id=\"2672\"]magma[\/pb_glossary] exploits a weakness between these layers, shouldering them apart and squeezing between them. As with [pb_glossary id=\"1969\"]dikes[\/pb_glossary], [pb_glossary id=\"1970\"]sills[\/pb_glossary] are younger than the surrounding layers and may be radioactively dated to study the age of sedimentary [pb_glossary id=\"2857\"]strata[\/pb_glossary].\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah.jpg\"><img class=\"size-medium wp-image-256\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-300x245.jpg\" alt=\"Exposure of Cottonwood Stock in Little Cottonwood Canyon, Utah\" width=\"300\" height=\"245\"><\/a> Cottonwood Stock, a quartz monzonite pluton exposed at the mouth of Little Cottonwood Canyon, Utah[\/caption]\n\n<span style=\"font-weight: 400\">A [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary] is a large underground [pb_glossary id=\"3341\"]reservoir[\/pb_glossary] of molten rock. The path of rising [pb_glossary id=\"2672\"]magma[\/pb_glossary] is called a <strong>[pb_glossary id=\"1966\"]diapir[\/pb_glossary]<\/strong>. The processes by which a [pb_glossary id=\"1966\"]diapir[\/pb_glossary] intrudes into the surrounding [pb_glossary id=\"1924\"]native[\/pb_glossary] or [pb_glossary id=\"1971\"]country rock[\/pb_glossary] are not well understood and are the subject of ongoing geological inquiry. For example, it is not known what happens to the pre-existing [pb_glossary id=\"1971\"]country rock[\/pb_glossary] as the [pb_glossary id=\"1966\"]diapir[\/pb_glossary] intrudes. One [pb_glossary id=\"2655\"]theory[\/pb_glossary] is the overriding rock gets shouldered aside, displaced by the increased volume of [pb_glossary id=\"2672\"]magma[\/pb_glossary]. Another is the [pb_glossary id=\"1924\"]native[\/pb_glossary] rock is melted and consumed into the rising [pb_glossary id=\"2672\"]magma[\/pb_glossary] or broken into pieces that settle into the [pb_glossary id=\"2672\"]magma[\/pb_glossary], a process known as <strong>[pb_glossary id=\"1967\"]stoping[\/pb_glossary]<\/strong>. It has also been proposed that diapirs are not a real phenomenon, but just a series of [pb_glossary id=\"1969\"]dikes[\/pb_glossary] that blend into each other. The [pb_glossary id=\"1969\"]dikes[\/pb_glossary] may be intruding over millions of years, but since they may be made of similar material, they would be appearing to be formed at the same time. Regardless, when a [pb_glossary id=\"1966\"]diapir[\/pb_glossary] cools, it forms an mass of [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] rock called a <strong>[pb_glossary id=\"1965\"]pluton[\/pb_glossary]<\/strong>. [pb_glossary id=\"1965\"]Plutons[\/pb_glossary] can have irregular shapes, but can often be somewhat round.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.2_Yosemite_Half-Dome.jpg\"><img class=\"size-medium wp-image-257\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-300x225.jpg\" alt=\"View showing an expansive area of a mountain range with exposed white granite in many places.\" width=\"300\" height=\"225\"><\/a> Half-Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite.[\/caption]\n\n&nbsp;\n\nWhen many [pb_glossary id=\"1965\"]plutons[\/pb_glossary] merge together in an extensive single feature, it is called a <strong>[pb_glossary id=\"1968\"]batholith[\/pb_glossary]<\/strong>. [pb_glossary id=\"1968\"]Batholiths[\/pb_glossary] are found in the cores of many mountain ranges, including the [pb_glossary id=\"1962\"]granite[\/pb_glossary] [pb_glossary id=\"2960\"]formations[\/pb_glossary] of Yosemite National Park in the Sierra Nevada of California. They are typically more than 100 km<sup>2<\/sup> in area, associated with [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, and mostly [pb_glossary id=\"1954\"]felsic[\/pb_glossary] in [pb_glossary id=\"2831\"]composition[\/pb_glossary]. A <strong>stock<\/strong> is a type of [pb_glossary id=\"1965\"]pluton[\/pb_glossary] with less surface exposure than a [pb_glossary id=\"1968\"]batholith[\/pb_glossary], and may represent a narrower neck of material emerging from the top of a [pb_glossary id=\"1968\"]batholith[\/pb_glossary]. [pb_glossary id=\"1968\"]Batholiths[\/pb_glossary] and stocks are discordant intrusions that cut across and through surrounding [pb_glossary id=\"1971\"]country rock[\/pb_glossary].\n\n&nbsp;\n\n&nbsp;\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Henry-Mountains-a-laccolith.jpg\"><img class=\"wp-image-258\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-300x198.jpg\" alt=\"Henry Mountains, Utah, interpreted to be a laccolith.\" width=\"250\" height=\"165\"><\/a> The Henry Mountains in Utah are interpreted to be a laccolith, exposed by erosion of the overlying layers.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.15_diagram_of_formation_of_laccolith.jpg\"><img class=\"wp-image-259\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-300x225.jpg\" alt=\"Laccolith forms as a blister in between sedimentary layers\" width=\"250\" height=\"188\"><\/a> Laccolith forms as a blister in between sedimentary strata.[\/caption]\n\n<strong>[pb_glossary id=\"1172\"]Laccoliths[\/pb_glossary]<\/strong> are blister-like, concordant intrusions of [pb_glossary id=\"2672\"]magma[\/pb_glossary] that form between sedimentary layers. The Henry Mountains of Utah are a famous topographic landform formed by this process. [pb_glossary id=\"1172\"]Laccoliths[\/pb_glossary] bulge upwards; a similar downward-bulging intrusion is called a <strong>[pb_glossary id=\"1172\"]lopolith[\/pb_glossary]<\/strong>.\n\n&nbsp;\n\n[h5p id=\"23\"]\n\n<em>Click on the plus signs the illustration for descriptions of several [pb_glossary id=\"2675\"]<em>igneous<\/em>[\/pb_glossary] features.<\/em>\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code.png\"><img class=\"size-thumbnail wp-image-260\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this interactive activity via this QR Code.[\/caption]\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"24\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.1-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-261\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the quiz for section 4.1 via this QR Code.[\/caption]\n<h2><strong><a id=\"4-2BowensReaction\" href=\"\"><\/a>4.2 Bowen's Reaction Series<\/strong><\/h2>\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"696\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.4_Bowens_Reaction_Series.png\"><img class=\"size-full wp-image-262\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series.png\" alt=\"Diagram of Bowen's Reaction Series, Y-shpaed with 8 minerals and a temperature scale\" width=\"696\" height=\"369\"><\/a> Bowen's Reaction Series. Higher temperature minerals shown at top (olivine) and lower temperature minerals shown at bottom (quartz). (Source Colivine, modified from Bowen, 1922)[\/caption]\n\n&nbsp;\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Peridot2.jpg\"><img class=\"wp-image-209\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-272x300.jpg\" alt=\"The crystal is light green.\" width=\"150\" height=\"165\"><\/a> Olivine, the first mineral to crystallize in a melt.[\/caption]\n\n&nbsp;\n\n<strong>[pb_glossary id=\"1174\"]Bowen\u2019s Reaction Series[\/pb_glossary]<\/strong> describes the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] at which [pb_glossary id=\"2687\"]minerals[\/pb_glossary] crystallize when cooled, or melt when heated. The low end of the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] scale where all [pb_glossary id=\"2687\"]minerals[\/pb_glossary] crystallize into solid rock, is approximately 700\u00b0C (1292\u00b0F). The upper end of the range where all [pb_glossary id=\"2687\"]minerals[\/pb_glossary] exist in a molten state, is approximately 1,250\u00b0C (2,282\u00b0F). These numbers reference [pb_glossary id=\"2687\"]minerals[\/pb_glossary] that crystallize at standard sea-level pressure, 1 bar. The values will be different for [pb_glossary id=\"2687\"]minerals[\/pb_glossary] located deep below the Earth\u2019s surface due to the increased pressure, which affects [pb_glossary id=\"2674\"]crystallization[\/pb_glossary] and melting temperatures (see <a href=\"#4-4PartialMelting\">Chapter 4.4<\/a>). However, the order and relationships are maintained.\n\nIn the figure, the righthand column lists the four groups of [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] from top to bottom: [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary], [pb_glossary id=\"1956\"]mafic[\/pb_glossary], [pb_glossary id=\"1955\"]intermediate[\/pb_glossary], and [pb_glossary id=\"1954\"]felsic[\/pb_glossary]. The down-pointing arrow on the far right shows increasing amounts of silica, sodium, aluminum, and potassium as the [pb_glossary id=\"2687\"]mineral[\/pb_glossary] [pb_glossary id=\"2831\"]composition[\/pb_glossary] goes from [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] to [pb_glossary id=\"1954\"]felsic[\/pb_glossary]. The up-pointing arrow shows increasing ferromagnesian components, specifically iron, magnesium, and calcium.\u00a0\u00a0 To the far left of the diagram is a [pb_glossary id=\"2689\"]temperature[\/pb_glossary] scale. [pb_glossary id=\"2687\"]Minerals[\/pb_glossary] near the top of diagram, such as olivine and anorthite (a type of plagioclase), crystallize at higher temperatures. Minerals near the bottom, such as [pb_glossary id=\"1915\"]quartz[\/pb_glossary] and [pb_glossary id=\"1914\"]muscovite[\/pb_glossary], crystalize at lower temperatures.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"200\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/NormanLBowen_1909.jpg\"><img class=\"size-medium wp-image-263\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-200x300.jpg\" alt=\"Photo of Normal L. Bowen in 1909.\" width=\"200\" height=\"300\"><\/a> Normal L. Bowen[\/caption]\n\nThe most important aspect of [pb_glossary id=\"1174\"]Bowen's Reaction Series[\/pb_glossary] is to notice the relationships between [pb_glossary id=\"2687\"]minerals[\/pb_glossary] and [pb_glossary id=\"2689\"]temperature[\/pb_glossary]. Norman L. Bowen (1887-1956) was an early 20th Century geologist who studied igneous rocks. He noticed that in [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks, certain [pb_glossary id=\"2687\"]minerals[\/pb_glossary] always occur together and these [pb_glossary id=\"2687\"]mineral[\/pb_glossary] assemblages exclude other [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. Curious as to why, and with the\u00a0[pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] in mind that it had to do with the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] at which the rocks cooled, he set about conducting experiments on [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks in the early 1900s.\u00a0He conducted experiments on [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary]\u2014grinding combinations of rocks into powder, sealing the powders into metal capsules, heating them to various temperatures, and then cooling them.\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"286\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/nlbowenexperimentingsm.jpg\"><img class=\"size-medium wp-image-264\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-286x300.jpg\" alt=\"Photo of Bowen working over his pertrographic microscope\" width=\"286\" height=\"300\"><\/a> Norman L. Bowen working with his petrographic microscope[\/caption]\n\nWhen he opened the quenched capsules, he found a glass surrounding [pb_glossary id=\"2687\"]mineral[\/pb_glossary] crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a [pb_glossary id=\"2192\"]period[\/pb_glossary] of several years, showed that the common [pb_glossary id=\"2675\"]igneous[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary] crystallize from [pb_glossary id=\"2672\"]magma[\/pb_glossary] at different temperatures. He also saw that [pb_glossary id=\"2687\"]minerals[\/pb_glossary] occur together in rocks with others that crystallize within similar [pb_glossary id=\"2689\"]temperature[\/pb_glossary] ranges, and never crystallize with other [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. This relationship can explain the main difference between [pb_glossary id=\"1956\"]mafic[\/pb_glossary] and [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks. [pb_glossary id=\"1956\"]Mafic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks contain more [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary], and therefore, crystallize at higher temperatures than [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks. This is even seen in [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows, with [pb_glossary id=\"1954\"]felsic[\/pb_glossary] lavas erupting hundreds of degrees cooler than their [pb_glossary id=\"1956\"]mafic[\/pb_glossary] counterparts. Bowen\u2019s work laid the foundation for understanding [pb_glossary id=\"2675\"]igneous[\/pb_glossary]<b> [pb_glossary id=\"1173\"]petrology[\/pb_glossary]<\/b> (the study of rocks) and resulted in his book, <i>The Evolution of the [pb_glossary id=\"2675\"]Igneous[\/pb_glossary] Rocks<\/i> in 1928.\n<h3><\/h3>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"25\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.2-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-265\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the quiz for section 4.2 via this QR Code.[\/caption]\n<h2><a id=\"4-3MagmaGeneration\" href=\"\"><\/a>4.3 Magma Generation<\/h2>\n[pb_glossary id=\"2672\"]Magma[\/pb_glossary] and [pb_glossary id=\"2673\"]lava[\/pb_glossary] contain three components: melt, solids, and [pb_glossary id=\"2606\"]volatiles[\/pb_glossary]. The melt is made of ions from [pb_glossary id=\"2687\"]minerals[\/pb_glossary] that have liquefied. The solids are made of crystallized [pb_glossary id=\"2687\"]minerals[\/pb_glossary] floating in the liquid melt. These may be [pb_glossary id=\"2687\"]minerals[\/pb_glossary] that have already cooled\u00a0<strong>[pb_glossary id=\"2606\"]Volatiles[\/pb_glossary]<\/strong> are gaseous components\u2014such as water vapor, carbon dioxide, sulfur, and chlorine\u2014[pb_glossary id=\"2815\"]dissolved[\/pb_glossary] in the [pb_glossary id=\"2672\"]magma[\/pb_glossary]. The presence and amount of these three components affect the physical behavior of the [pb_glossary id=\"2672\"]magma[\/pb_glossary] and will be discussed more below.\n<h3>4.3.1 Geothermal Gradient<\/h3>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.2_Temperature_schematic_of_inner_Earth.jpg\"><img class=\"size-medium wp-image-266\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-300x300.jpg\" alt=\"Diagram showing temperature increase with depth in the Earth\" width=\"300\" height=\"300\"><\/a> Geothermal gradient[\/caption]\n\nBelow the surface, the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the Earth rises. This heat is caused by residual heat left from the [pb_glossary id=\"2960\"]formation[\/pb_glossary] of Earth and ongoing [pb_glossary id=\"2966\"]radioactive[\/pb_glossary] decay. The rate at which [pb_glossary id=\"2689\"]temperature[\/pb_glossary] increases with depth is called the <strong>[pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary]<\/strong>. The average [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] in the upper 100 km (62 mi) of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] is about 25\u00b0C per kilometer of depth. So for every kilometer of depth, the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] increases by about 25\u00b0C.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"283\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Simple-P-T-Diagram-modified-from-Woudloper.jpg\"><img class=\"size-full wp-image-267\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Simple-P-T-Diagram-modified-from-Woudloper.jpg\" alt=\"Diagram showing pressures and temperatures of the geothermal gradient increasing deeper in the earth. The solidus line shows that temperatures need to be much higher or pressure needs to be lower in order for rocks to start to melt.\" width=\"283\" height=\"256\"><\/a> Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and the green solidus line represents the temperature and pressure regime at which melting begins. Rocks at pressures and temperatures left of the green line are solid. If pressure\/temperature conditions change so that rocks pass to the right of the green line, then they will start to melt. (Source: Woudloper)[\/caption]\n\nThe depth-[pb_glossary id=\"2689\"]temperature[\/pb_glossary] graph (see figure) illustrates the relationship between the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] (geotherm, red line) and the start of rock melting (solidus, green line). The [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] changes with depth (which has a direct relationship to pressure) through the [pb_glossary id=\"2580\"]crust[\/pb_glossary] into upper [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing [pb_glossary id=\"2689\"]temperature[\/pb_glossary] with depth makes the depth of about 125 kilometers (78 miles) where the natural [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] is closest to the solidus.\n\nThe [pb_glossary id=\"2689\"]temperature[\/pb_glossary] at 100 km (62 mi) deep is about 1,200\u00b0C (2,192\u00b0F). At bottom of the [pb_glossary id=\"2580\"]crust[\/pb_glossary], 35 km (22 mi) deep, the pressure is about 10,000 bars. A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the [pb_glossary id=\"2580\"]crust[\/pb_glossary] and [pb_glossary id=\"2586\"]mantle[\/pb_glossary] are solid. To a depth of 150 km (93 mi), the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] line stays to the left of the solidus line. This relationship continues through the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] to the [pb_glossary id=\"2589\"]core[\/pb_glossary]-[pb_glossary id=\"2586\"]mantle[\/pb_glossary] boundary, at 2,880 km (1,790 mi).\n\nThe solidus line slopes to the right because the melting [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of any substance depends on pressure. The higher pressure created at greater depth increases the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100\u00b0C. But if the pressure is lowered, as shown on the video below, water boils at a much lower [pb_glossary id=\"2689\"]temperature[\/pb_glossary].\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Boiling-Water-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-268\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[embed]https:\/\/www.youtube.com\/embed\/Ks4VuXTTKmo[\/embed]\n\n&nbsp;\n\nThere are three principal ways rock behavior crosses to the right of the green solidus line to create molten [pb_glossary id=\"2672\"]magma[\/pb_glossary]: 1) [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary] caused by lowering the pressure, 2) [pb_glossary id=\"2608\"]flux melting[\/pb_glossary] caused by adding [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] (see more below), and 3) heat-induced melting caused by increasing the [pb_glossary id=\"2689\"]temperature[\/pb_glossary]. The [pb_glossary id=\"1174\"]Bowen\u2019s Reaction Series[\/pb_glossary] shows that [pb_glossary id=\"2687\"]minerals[\/pb_glossary] melt at different temperatures. Since [pb_glossary id=\"2672\"]magma[\/pb_glossary] is a mixture of different [pb_glossary id=\"2687\"]minerals[\/pb_glossary], the solidus boundary is more of a fuzzy zone rather than a well-defined line; some [pb_glossary id=\"2687\"]minerals[\/pb_glossary] are melted and some remain solid. This type of rock behavior is called <strong>[pb_glossary id=\"1177\"]partial melting[\/pb_glossary]<\/strong> and represents real-world [pb_glossary id=\"2672\"]magmas[\/pb_glossary], which typically contain solid, liquid, and volatile components.\n\n<span style=\"font-weight: 400\">The figure below uses P-T diagrams to show how melting can occur at three different [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] settings.\u00a0 The green line is called the <strong>solidus<\/strong>, the melting point [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the rock at that pressure. Setting\u00a0<\/span>A is a situation (called \"normal\") in the middle of a stable [pb_glossary id=\"2591\"]plate[\/pb_glossary] in which no [pb_glossary id=\"2672\"]magma[\/pb_glossary] is generated. In the other three situations, rock at a lettered location with a [pb_glossary id=\"2689\"]temperature[\/pb_glossary] at the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] is moved to a new P-T situation on the diagram. This shift is indicated by the arrow and its [pb_glossary id=\"2689\"]temperature[\/pb_glossary] relative to the solidus is shown by the red line. Partial melting occurs where the red line [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the rock crosses the green solidus on the diagram. Setting B is at a [pb_glossary id=\"2630\"]mid-ocean ridge[\/pb_glossary] <em>([pb_glossary id=\"1176\"]<em>decompression melting<\/em>[\/pb_glossary])<\/em> where reduction of pressure carries the rock at its [pb_glossary id=\"2689\"]temperature[\/pb_glossary] across the solidus. Setting C is a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] where [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary] plus <em>addition of heat<\/em> carries the rock across the solidus, and setting D is a [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone where a process called <em>[pb_glossary id=\"2608\"]<em>flux melting<\/em>[\/pb_glossary]<\/em> takes place where the solidus (melting point) is actually shifted to below the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the rock.\n\nGraphs A-D below, along with the side view of the Earth\u2019s layers in various [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] settings (see figure), show how melting occurs in different situations. Graph A illustrates a normal situation, located in the middle of a stable [pb_glossary id=\"2591\"]plate[\/pb_glossary], where no melted rock can be found. The remaining three graphs illustrate rock behavior relative to shifts in the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] or solidus lines. [pb_glossary id=\"1177\"]Partial melting[\/pb_glossary] occurs when the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] line crosses the solidus line. Graph B illustrates behavior of rock located at a [pb_glossary id=\"2630\"]mid-ocean ridge[\/pb_glossary], labeled X in the graph and side view. Reduced pressure shifts the geotherm to the right of the solidus, causing [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary]. Graph C and label Y illustrate a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] situation. [pb_glossary id=\"1176\"]Decompression melting[\/pb_glossary], plus an addition of heat, shifts the geotherm across the solidus. Graph D and label Z show a [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone, where an addition of [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] lowers the melting point, shifting the solidus to the left of the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary]. B, C, and D all show different ways the Earth produces intersections of the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] and the solidus, which results in melting each time.\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"1024\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_P-T-diagrams-in-mantle.jpg\"><img class=\"size-large wp-image-269\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-1024x585.jpg\" alt=\"Pressure-Temperature diagrams showing temperture in the mantle plotted against pressure (depth)\" width=\"1024\" height=\"585\"><\/a> Four P-T diagrams show temperature in degrees Celsius on x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and green solidus line represents at temperature and pressure regime at which melting begins. Each of the four P-T diagrams are associated a tectonic setting as shown by a side-view (cross-section) of the lithosphere and mantle.[\/caption]\n<h3>4.3.2 Decompression Melting<\/h3>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"212\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ocean-birth.svg_.png\"><img class=\"size-medium wp-image-2867\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-1.png\" alt=\"The ocean starts as a valley and then gets wider and wider.\" width=\"212\" height=\"300\"><\/a> Progression from rift to mid-ocean ridge, the divergent boundary types. Note the rising material in the center.[\/caption]\n\n[pb_glossary id=\"2672\"]Magma[\/pb_glossary] is created at [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] via <strong>[pb_glossary id=\"1176\"]decompression melting[\/pb_glossary]<\/strong>. Strong [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents cause the solid [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] to slowly flow beneath the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary]. The upper part of the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] ([pb_glossary id=\"2580\"]crust[\/pb_glossary]) is a poor heat conductor, so the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] remains about the same throughout the underlying [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material. Where the [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents cause [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material to rise, the pressure decreases, which causes the melting point to drop. In this situation, the rock at the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] is rising toward the surface, thus hotter rock is now shallower, at a lower pressure, and the rock, still at the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] at its old location, shifts past the its melting point (shown as the red line crossing over the solidus or green line in example B in previous figure) and [pb_glossary id=\"1177\"]partial melting[\/pb_glossary] starts. As this magma continues to rise, it cools and crystallizes to form new lithospheric [pb_glossary id=\"2580\"]crust[\/pb_glossary].\n<h3>4.3.3 Flux Melting<\/h3>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"500\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Subduction-en.svg_.png\"><img class=\"wp-image-94\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-300x147.png\" alt=\"Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental\" width=\"500\" height=\"244\"><\/a> Diagram of ocean-continent subduction. Note water vapor driven out of hydrated minerals in the descending oceanic slab.[\/caption]\n\n<strong>[pb_glossary id=\"2608\"]Flux melting[\/pb_glossary]<\/strong> or <strong>[pb_glossary id=\"2608\"]fluid-induced melting[\/pb_glossary]<\/strong> occurs in island arcs and [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones when volatile gases are added to [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material (see figure: graph D, label Z). Flux-melted [pb_glossary id=\"2672\"]magma[\/pb_glossary] produces many of the [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] in the circum-Pacific [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, also known as the Ring of Fire. The [pb_glossary id=\"2602\"]subducting[\/pb_glossary] [pb_glossary id=\"2605\"]slab[\/pb_glossary] contains [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] and hydrated [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. As covered in <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\">Chapter 2<\/a>, these hydrated forms are created when water ions [pb_glossary id=\"2703\"]bond[\/pb_glossary] with the crystal structure of [pb_glossary id=\"2709\"]silicate[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. As the [pb_glossary id=\"2605\"]slab[\/pb_glossary] descends into the hot [pb_glossary id=\"2586\"]mantle[\/pb_glossary], the increased [pb_glossary id=\"2689\"]temperature[\/pb_glossary] causes the hydrated [pb_glossary id=\"2687\"]minerals[\/pb_glossary] to emit water vapor and other volatile gases, which are expelled from the [pb_glossary id=\"2605\"]slab[\/pb_glossary] like water being squeezed out of a sponge. The [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] [pb_glossary id=\"2815\"]dissolve[\/pb_glossary] into the overlying asthenospheric [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and decrease its melting point. In this situation the applied pressure and [pb_glossary id=\"2689\"]temperature[\/pb_glossary] have not changed, the [pb_glossary id=\"2586\"]mantle[\/pb_glossary]'s melting point has been lowered by the addition of volatile substances. The previous figure (graph D) shows the green solidus line shifting to the left of and below the red [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] line, and melting begins. This is analogous to adding salt to an icy roadway. The salt lowers the freezing [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the solid ice so it turns into liquid water.\n<h3>4.3.4 Heat-Induced Melting<\/h3>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/06.2-15-Mt-Blaca-Migmatite-1.jpg\"><img class=\"size-medium wp-image-48\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-300x225.jpg\" alt=\"Swirling bands of light and dark minerals.\" width=\"300\" height=\"225\"><\/a> Migmatite, a rock which was partially molten. (Source: Peter Davis)[\/caption]\n\nHeat-induced melting, transforming solid [pb_glossary id=\"2586\"]mantle[\/pb_glossary] into liquid [pb_glossary id=\"2672\"]magma[\/pb_glossary] by simply applying heat, is the least common process for generating [pb_glossary id=\"2672\"]magma[\/pb_glossary] (see figure: graph C, label Y). Heat-induced melting occurs at a [pb_glossary id=\"2586\"]mantle[\/pb_glossary] plumes or [pb_glossary id=\"2638\"]hotspots[\/pb_glossary]. The rock surrounding the plume is exposed to higher temperatures, the [pb_glossary id=\"1175\"]geothermal gradient[\/pb_glossary] crosses to the right of the green solidus line, and the rock begins to melt. The [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary] includes rising [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material, meaning some [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary] is occurring as well. A small amount of [pb_glossary id=\"2672\"]magma[\/pb_glossary] is also generated by intense [pb_glossary id=\"2946\"]regional metamorphism[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/6-metamorphic-rocks\/\">Chapter 6<\/a>). This [pb_glossary id=\"2672\"]magma[\/pb_glossary] becomes a hybrid [pb_glossary id=\"2914\"]metamorphic[\/pb_glossary]-[pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] called [pb_glossary id=\"2933\"]migmatite[\/pb_glossary].\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"26\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.3-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-271\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the quiz for section 4.3 via this QR Code.[\/caption]\n<h2><b><a id=\"4-4PartialMelting\" href=\"\"><\/a><\/b><strong>4.4<\/strong>\u00a0<b>Partial Melting and Crystallization<\/b><\/h2>\nEven though all [pb_glossary id=\"2672\"]magmas[\/pb_glossary] originate from similar [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rocks, and start out as similar [pb_glossary id=\"2672\"]magma[\/pb_glossary], other things, like\u00a0[pb_glossary id=\"1177\"]partial melting[\/pb_glossary] and [pb_glossary id=\"2674\"]crystallization[\/pb_glossary] processes like [pb_glossary id=\"1179\"]magmatic differentiation[\/pb_glossary], can change the chemistry of the [pb_glossary id=\"2672\"]magma[\/pb_glossary]. This explains the wide variety of resulting [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks that are found all over Earth.\n<h3>4.4.1 Partial Melting<\/h3>\nBecause the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] is [pb_glossary id=\"2831\"]composed[\/pb_glossary] of many different minerals, it does not melt uniformly. As [pb_glossary id=\"2687\"]minerals[\/pb_glossary] with lower melting points turn into liquid [pb_glossary id=\"2672\"]magma[\/pb_glossary], those with higher melting points remain as solid crystals. This is known as [pb_glossary id=\"1177\"]partial melting[\/pb_glossary]. As [pb_glossary id=\"2672\"]magma[\/pb_glossary] slowly rises and cools into solid rock, it undergoes physical and chemical changes in a process called [pb_glossary id=\"1179\"]magmatic differentiation[\/pb_glossary].\n\nAccording to [pb_glossary id=\"1174\"]Bowen\u2019s Reaction Series[\/pb_glossary] (<a href=\"#4-2BowensReaction\">Section 4.2<\/a>), each [pb_glossary id=\"2687\"]mineral[\/pb_glossary] has a unique melting and [pb_glossary id=\"2674\"]crystallization[\/pb_glossary] [pb_glossary id=\"2689\"]temperature[\/pb_glossary]. Since most rocks are made of many different [pb_glossary id=\"2687\"]minerals[\/pb_glossary], when they start to melt, some [pb_glossary id=\"2687\"]minerals[\/pb_glossary] begin melting sooner than others. This is known as [pb_glossary id=\"1177\"]partial melting[\/pb_glossary], and creates [pb_glossary id=\"2672\"]magma[\/pb_glossary] with a different [pb_glossary id=\"2831\"]composition[\/pb_glossary] than the original [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material.\n\nThe most important example occurs as [pb_glossary id=\"2672\"]magma[\/pb_glossary] is generated from [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rocks (as discussed in <a href=\"#4-3MagmaGeneration\">Section 4.3<\/a>). The chemistry of [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rock ([pb_glossary id=\"2588\"]peridotite[\/pb_glossary]) is [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary], low in [pb_glossary id=\"2709\"]silicates[\/pb_glossary] and high in iron and magnesium. When [pb_glossary id=\"2588\"]peridotite[\/pb_glossary] begins to melt, the silica-rich portions melt first due to their lower melting point. If this continues, the [pb_glossary id=\"2672\"]magma[\/pb_glossary] becomes increasingly silica-rich, turning [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] [pb_glossary id=\"2586\"]mantle[\/pb_glossary] into [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary], and [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2586\"]mantle[\/pb_glossary] into [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary]. The [pb_glossary id=\"2672\"]magma[\/pb_glossary] rises to the surface because it is more buoyant than the [pb_glossary id=\"2586\"]mantle[\/pb_glossary].\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"500\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/CratonGeolProv.jpg\"><img class=\"wp-image-92\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-300x159.jpg\" alt=\"The legend shows shields, platforms, orogens, basins, large igneous provinces, and extended crust.\" width=\"500\" height=\"265\"><\/a> Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents.[\/caption]\n\n[pb_glossary id=\"1177\"]Partial melting[\/pb_glossary] also occurs as existing crustal rocks melt in the presence of heat from [pb_glossary id=\"2672\"]magmas[\/pb_glossary]. In this process, existing rocks melt, allowing the [pb_glossary id=\"2672\"]magma[\/pb_glossary] formed to be more [pb_glossary id=\"1954\"]felsic[\/pb_glossary] and less [pb_glossary id=\"1956\"]mafic[\/pb_glossary] than the pre-existing rock. Early in the Earth\u2019s history when the continents were forming, silica-rich [pb_glossary id=\"2672\"]magmas[\/pb_glossary] formed and rose to the surface and solidified into granitic continents. In the figure, the old granitic cores of the continents, called <strong>[pb_glossary id=\"2207\"]shields[\/pb_glossary]<\/strong>, are shown in orange.\n<h3>4.4.2 Crystallization and Magmatic Differentiation<\/h3>\nLiquid [pb_glossary id=\"2672\"]magma[\/pb_glossary] is less dense than the surrounding solid rock, so it rises through the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and [pb_glossary id=\"2580\"]crust[\/pb_glossary]. As [pb_glossary id=\"2672\"]magma[\/pb_glossary] begins to cool and crystallize, a process known as <strong>[pb_glossary id=\"1179\"]magmatic differentiation[\/pb_glossary]<\/strong> changes the chemistry of the resultant rock towards a more [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2831\"]composition[\/pb_glossary]. This happens via two main methods: [pb_glossary id=\"1178\"]assimilation[\/pb_glossary] and [pb_glossary id=\"1180\"]fractionation[\/pb_glossary].\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.16_Xenoliths_Little_Cottonwood_Canyon.jpg\"><img class=\"size-medium wp-image-272\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-300x211.jpg\" alt=\"Xenoliths are bits of surrounding counjtry rock incorporated in intrusive magma and solidified within it.\" width=\"300\" height=\"211\"><\/a> Xenoliths in Little Cottonwood Stock, Utah[\/caption]\n\nDuring <strong>[pb_glossary id=\"1178\"]assimilation[\/pb_glossary]<\/strong>, pieces of [pb_glossary id=\"1971\"]country rock[\/pb_glossary] with a different, often more [pb_glossary id=\"1954\"]felsic[\/pb_glossary], [pb_glossary id=\"2831\"]composition[\/pb_glossary] are added to the [pb_glossary id=\"2672\"]magma[\/pb_glossary]. These solid pieces may melt, which changes the composition of the original [pb_glossary id=\"2672\"]magma[\/pb_glossary]. At times, the solid fragments may remain intact within the cooling [pb_glossary id=\"2672\"]magma[\/pb_glossary] and only partially melt. The unmelted country rocks within an [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] mass are called <strong>[pb_glossary id=\"2587\"]xenoliths[\/pb_glossary]<\/strong>.\n\n[pb_glossary id=\"2587\"]Xenoliths[\/pb_glossary] are also common in the processes of [pb_glossary id=\"2672\"]magma[\/pb_glossary] mixing and rejuvenation, two other processes that can contribute to [pb_glossary id=\"1179\"]magmatic differentiation[\/pb_glossary]. [pb_glossary id=\"2672\"]Magma[\/pb_glossary] mixing occurs when two different [pb_glossary id=\"2672\"]magmas[\/pb_glossary] come into contact and mix, though at times, the [pb_glossary id=\"2672\"]magmas[\/pb_glossary] can remain heterogeneous and create [pb_glossary id=\"2587\"]xenoliths[\/pb_glossary], [pb_glossary id=\"1969\"]dikes[\/pb_glossary], and other features. Magmatic rejuvenation happens when a cooled and crystallized body of rock is remelted and pieces of the original rock may remain as [pb_glossary id=\"2587\"]xenoliths[\/pb_glossary].\n\nMuch of the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is [pb_glossary id=\"1954\"]felsic[\/pb_glossary] (i.e. granitic), and normally more buoyant than the underlying [pb_glossary id=\"1956\"]mafic[\/pb_glossary]\/[pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. When [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] rises through thick [pb_glossary id=\"2575\"]continental crust[\/pb_glossary], it does so slowly, more slowly than when [pb_glossary id=\"2672\"]magma[\/pb_glossary] rises through [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary]. This gives the [pb_glossary id=\"2672\"]magma[\/pb_glossary] lots of time to react with the surrounding [pb_glossary id=\"1971\"]country rock[\/pb_glossary]. The [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] tends to [pb_glossary id=\"1178\"]assimilate[\/pb_glossary] [pb_glossary id=\"1954\"]felsic[\/pb_glossary] rock, becoming more silica-rich as it migrates through the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] and changing into [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] or [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] by the time it reaches the surface. This is why [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2672\"]magmas[\/pb_glossary] are much more common within continents.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Magmatism_and_volcanism_EN.png\"><img class=\"size-medium wp-image-273\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-300x235.png\" alt=\"Shows large pools of magma rising from the source in the mantle, up into the crust under a volcano.\" width=\"300\" height=\"235\"><\/a> Rising magma diapirs in mantle and crust. Fractional crystallization assimilation occurs to the diapirs in the crust.[\/caption]\n\n<strong>[pb_glossary id=\"1180\"]Fractionation[\/pb_glossary]<\/strong> or <strong>fractional [pb_glossary id=\"2674\"]crystallization[\/pb_glossary]<\/strong> is another process that increase [pb_glossary id=\"2672\"]magma[\/pb_glossary] silica content, making it more [pb_glossary id=\"1954\"]felsic[\/pb_glossary]. As the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] drops within a [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"1966\"]diapir[\/pb_glossary] rising through the [pb_glossary id=\"2580\"]crust[\/pb_glossary], some [pb_glossary id=\"2687\"]minerals[\/pb_glossary] will crystallize and settle to the bottom of the [pb_glossary id=\"1185\"]magma\u00a0chamber[\/pb_glossary], leaving the remaining melt depleted of those ions. [pb_glossary id=\"2711\"]Olivine[\/pb_glossary] is a [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2687\"]mineral[\/pb_glossary] at the top of the [pb_glossary id=\"1174\"]Bowen\u2019s Reaction series[\/pb_glossary] with a high melting point and a smaller percentage of silica verses other common [pb_glossary id=\"2675\"]igneous[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. When [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] cools, the [pb_glossary id=\"2711\"]olivine[\/pb_glossary] crystallizes first and settles to the bottom of the [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary] (see figure). This means the remaining melt becomes more silica-rich and [pb_glossary id=\"1954\"]felsic[\/pb_glossary]. As the [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] further cools, the next [pb_glossary id=\"2687\"]minerals[\/pb_glossary] on [pb_glossary id=\"1174\"]Bowen's Reaction Series[\/pb_glossary] ([pb_glossary id=\"1916\"]plagioclase[\/pb_glossary] and [pb_glossary id=\"2712\"]pyroxene[\/pb_glossary]) crystallize next, removing even more low-silica components from the [pb_glossary id=\"2672\"]magma[\/pb_glossary], making it even more [pb_glossary id=\"1954\"]felsic[\/pb_glossary]. This crystal [pb_glossary id=\"1180\"]fractionation[\/pb_glossary] can occur in [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], but the [pb_glossary id=\"2960\"]formation[\/pb_glossary] of more differentiated, highly evolved [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2672\"]magmas[\/pb_glossary] is largely confined to [pb_glossary id=\"2575\"]continental[\/pb_glossary] regions where the longer time to the surface allows more [pb_glossary id=\"1180\"]fractionation[\/pb_glossary] to occur.\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"750\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Fractional_crystallization.png\"><img class=\"size-full wp-image-274\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization.png\" alt=\"Complicated diagram showing minerals settling out in the magma chamber and thus making the remaining liquid magma (the melt) more silica-rich in composition.\" width=\"750\" height=\"320\"><\/a> Schematic diagram illustrating fractional crystallization. If magma at composition A is ultramafic, as the magma cools it changes composition as different minerals crystallize from the melt and settle to the bottom of the magma chamber. In section 1, olivine crystallizes; section 2: olivine and pyroxene crystallize; section 3: pyroxene and plagioclase crystallize; and section 4: plagioclase crystallizes. The crystals are separated from the melt and the remaining magma (composition B) is more silica-rich. (Source: Woudloper)[\/caption]\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"27\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.4-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-275\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the quiz for section 4.4 via this QR Code.[\/caption]\n<h2><span style=\"font-weight: 400\"><a id=\"4-5Volcanism\" href=\"\"><\/a>4.5 Volcanism<\/span><\/h2>\nWhen [pb_glossary id=\"2672\"]magma[\/pb_glossary] emerges onto the Earth\u2019s surface, the molten rock is called [pb_glossary id=\"2673\"]lava[\/pb_glossary]. A <strong>[pb_glossary id=\"1181\"]volcano[\/pb_glossary]<\/strong> is a type of land [pb_glossary id=\"2960\"]formation[\/pb_glossary] created when [pb_glossary id=\"2673\"]lava[\/pb_glossary] solidifies into rock. [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary] have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] has made them less mysterious.\u00a0<span style=\"font-weight: 400\">This section\u00a0describes [pb_glossary id=\"1181\"]volcano[\/pb_glossary] location, type, hazards, and monitoring.<\/span>\n<h3><b>4.5.1. Distribution and Tectonics<\/b><\/h3>\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"775\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Main-types-of-plate-boundaries.gif\"><img class=\"size-full wp-image-276\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Main-types-of-plate-boundaries.gif\" alt=\"Diagram showing how volcanoes are associated with plate boundaries\" width=\"775\" height=\"429\"><\/a> Association of volcanoes with plate boundaries. (Source: USGS)[\/caption]\n\nMost [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are [pb_glossary id=\"1182\"]interplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]. [pb_glossary id=\"1182\"]Interplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are located at active [pb_glossary id=\"2591\"]plate[\/pb_glossary] boundaries created by [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] at [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifts[\/pb_glossary]. The prefix \"<em>inter-\"<\/em> means between. Some volcanoes are [pb_glossary id=\"1183\"]intraplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]. The prefix \"<em>intra-\"<\/em>\u00a0means within, and intraplate volcanoes are located within [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary], far removed from plate boundaries. Many [pb_glossary id=\"1183\"]intraplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are formed by [pb_glossary id=\"2638\"]hotspots[\/pb_glossary].\n<h4><span style=\"font-weight: 400\">Volcanoes at Mid-Ocean Ridges<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Spreading_ridges_volcanoes_map-en.svg_.png\"><img class=\"size-medium wp-image-277\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-300x151.png\" alt=\"Map showing spreading ridges throughout the world. These ridges are all over the world.\" width=\"300\" height=\"151\"><\/a> Map of spreading ridges throughout the world.[\/caption]\n\nMost [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] on Earth occurs on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] along [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], a type of [pb_glossary id=\"2599\"]divergent[\/pb_glossary] [pb_glossary id=\"2592\"]plate boundary[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\" target=\"_blank\" rel=\"noopener\">Chapter 2<\/a>). These [pb_glossary id=\"1182\"]interplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are also the least observed and famous, since most of them are located under 3,000-4,500 m (10,000-15,000 ft) of ocean and the eruptions are slow, gentle, and oozing. One exception is the [pb_glossary id=\"1182\"]interplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] of Iceland. The diverging and thinning [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2576\"]plates[\/pb_glossary] allow hot [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rock to rise, releasing pressure and causing [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary]. [pb_glossary id=\"1957\"]Ultramafic[\/pb_glossary] [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rock, consisting largely of [pb_glossary id=\"2588\"]peridotite[\/pb_glossary], partially melts and generates [pb_glossary id=\"2672\"]magma[\/pb_glossary] that is basaltic. Because of this, almost all [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] are basaltic. In fact, most [pb_glossary id=\"2885\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is basaltic near the surface, with [pb_glossary id=\"1940\"]phaneritic[\/pb_glossary] [pb_glossary id=\"1964\"]gabbro[\/pb_glossary] and [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] [pb_glossary id=\"2588\"]peridotite[\/pb_glossary] underneath.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pillow-basalt.jpg\"><img class=\"size-medium wp-image-278\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-300x180.jpg\" alt=\"Pillow basalt on sea floor near Hawaii.\" width=\"300\" height=\"180\"><\/a> Pillow basalt on sea floor near Hawaii.[\/caption]\n\nWhen basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] erupts underwater it emerges in small explosions and\/or forms pillow-shaped structures called pillow basalts. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary]. This ecosystem exists around tall vents emitting black, hot [pb_glossary id=\"2687\"]mineral[\/pb_glossary]-rich water called deep-sea [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] vents, also known as [pb_glossary id=\"2922\"]black smokers[\/pb_glossary].\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"133\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/BlackSmoker.jpg\"><img class=\"wp-image-119\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-233x300.jpg\" alt=\"There is a large build up of minerals around the vent\" width=\"133\" height=\"171\"><\/a> Black smoker hydrothermal vent with a colony of giant (6'+) tube worms.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"500\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.16_Distribution_of_hydrothermal_vent_fields.jpg\"><img class=\"wp-image-279\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-300x178.jpg\" alt=\"Map showing worldwide distgrbution of hydrothermal vent fields;\" width=\"500\" height=\"297\"><\/a> Distribution of hydrothermal vent fields[\/caption]\n\nWithout sunlight to support photosynthesis, these organisms instead utilize a process called <strong>[pb_glossary id=\"1184\"]chemosynthesis[\/pb_glossary]<\/strong>. Certain bacteria are able to turn hydrogen [pb_glossary id=\"1921\"]sulfide[\/pb_glossary] (H<sub>2<\/sub>S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies. The three videos show some of the ecosystems found around deep-sea [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] vents.\n\n[embed]https:\/\/youtu.be\/a5aQ4W9GbpU[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-1-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-280\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[embed]https:\/\/youtu.be\/dXOQFnU-49k[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-2-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-281\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[embed]https:\/\/youtu.be\/eUzz_ilsFa0[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-3-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-282\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n<h4><span style=\"font-weight: 400\">Volcanoes at Subduction Zones<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"500\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Map_plate_tectonics_world.gif\"><img class=\"wp-image-283\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Map_plate_tectonics_world.gif\" alt=\"Map showing volcanoes follow the edges of tectonic plates.\" width=\"500\" height=\"314\"><\/a> Distribution of volcanoes on the planet. Click here for an <a href=\"https:\/\/maps.ngdc.noaa.gov\/viewers\/hazards\/?layers=3\" target=\"_blank\" rel=\"noopener\">interactive map<\/a>\u00a0of volcano distributions.[\/caption]\n\nThe second most commonly found location for [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] is adjacent to [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, a type of [pb_glossary id=\"2600\"]convergent[\/pb_glossary] [pb_glossary id=\"2592\"]plate boundary[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\" target=\"_blank\" rel=\"noopener\">Chapter 2<\/a>). The process of [pb_glossary id=\"2602\"]subduction[\/pb_glossary] expels water from hydrated [pb_glossary id=\"2687\"]minerals[\/pb_glossary] in the descending [pb_glossary id=\"2605\"]slab[\/pb_glossary], which causes [pb_glossary id=\"2608\"]flux melting[\/pb_glossary] in the overlying [pb_glossary id=\"2586\"]mantle[\/pb_glossary] rock. Because [pb_glossary id=\"2602\"]subduction[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] occurs in a [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary], the thickened [pb_glossary id=\"2580\"]crust[\/pb_glossary] promotes [pb_glossary id=\"1177\"]partial melting[\/pb_glossary] and [pb_glossary id=\"2672\"]magma[\/pb_glossary] differentiation. These evolve the [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] from the mantle into more silica-rich [pb_glossary id=\"2672\"]magma[\/pb_glossary]. The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by [pb_glossary id=\"2602\"]subduction[\/pb_glossary]-generated eruptions of mostly silica-rich [pb_glossary id=\"2673\"]lava[\/pb_glossary]; the [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] and [pb_glossary id=\"1965\"]plutons[\/pb_glossary] consist largely of [pb_glossary id=\"1955\"]intermediate[\/pb_glossary]-to-[pb_glossary id=\"1954\"]felsic[\/pb_glossary] rock such as [pb_glossary id=\"1960\"]andesite[\/pb_glossary], [pb_glossary id=\"1958\"]rhyolite[\/pb_glossary], [pb_glossary id=\"1946\"]pumice[\/pb_glossary], and [pb_glossary id=\"1953\"]tuff[\/pb_glossary].\n<h4><span style=\"font-weight: 400\">Volcanoes at Continental Rifts<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Black_Rock_Desert_volcanic_field.jpg\"><img class=\"size-medium wp-image-284\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-300x199.jpg\" alt=\"A barren landscape of lava flows in central Utah.\" width=\"300\" height=\"199\"><\/a> Basaltic cinder cones of the Black Rock Desert near Beaver, Utah.[\/caption]\n\nSome [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are created at [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifts[\/pb_glossary], where crustal thinning is caused by diverging lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary], such as the East African [pb_glossary id=\"2624\"]Rift[\/pb_glossary] [pb_glossary id=\"1461\"]Basin[\/pb_glossary] in Africa. [pb_glossary id=\"1181\"]Volcanism[\/pb_glossary] caused by crustal thinning without [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary] is found in the [pb_glossary id=\"2462\"]Basin and Range[\/pb_glossary] Province in North America. In this location, [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activity is produced by rising [pb_glossary id=\"2672\"]magma[\/pb_glossary] that stretches the overlying [pb_glossary id=\"2580\"]crust[\/pb_glossary] (see figure). Lower [pb_glossary id=\"2580\"]crust[\/pb_glossary] or upper [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material rises through the thinned [pb_glossary id=\"2580\"]crust[\/pb_glossary], releases pressure, and undergoes decompression-induced [pb_glossary id=\"1177\"]partial melting[\/pb_glossary]. This [pb_glossary id=\"2672\"]magma[\/pb_glossary] is less dense than the surrounding rock and continues to rise through the [pb_glossary id=\"2580\"]crust[\/pb_glossary] to the surface, erupting as basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary]. These eruptions usually result in [pb_glossary id=\"1197\"]flood basalts[\/pb_glossary], [pb_glossary id=\"1195\"]cinder[\/pb_glossary] cones, and basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows (see video). Relatively young [pb_glossary id=\"1195\"]cinder[\/pb_glossary] cones of basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] can be found in south-central Utah, in the Black Rock Desert [pb_glossary id=\"1181\"]Volcanic[\/pb_glossary] Field, which is part of the zone of [pb_glossary id=\"2462\"]Basin and Range[\/pb_glossary] crustal [pb_glossary id=\"1445\"]extension[\/pb_glossary]. These Utah [pb_glossary id=\"1195\"]cinder[\/pb_glossary] cones and [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago.\n\n[embed]https:\/\/youtu.be\/4VgMe-JXOAM[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcanic-Processes-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-285\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n<h4><span style=\"font-weight: 400\">Hotspots<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"193\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hotspotgeology-1.svg_.png\"><img class=\"size-medium wp-image-129\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-193x300.png\" alt=\"The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.\" width=\"193\" height=\"300\"><\/a> Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.[\/caption]\n\n[pb_glossary id=\"2638\"]Hotspots[\/pb_glossary] are the main source of [pb_glossary id=\"1183\"]intraplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. <strong>[pb_glossary id=\"2638\"]Hotspots[\/pb_glossary]<\/strong> occur when lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary] glide over a hot [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary], an ascending column of solid heated rock originating from deep within the [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. The [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary] generates melts as material rises, with the [pb_glossary id=\"2672\"]magma[\/pb_glossary] rising even more. When the ascending [pb_glossary id=\"2672\"]magma[\/pb_glossary] reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/YellowstoneHotspot.jpg\"><img class=\"size-medium wp-image-133\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-300x206.jpg\" alt=\"The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.\" width=\"300\" height=\"206\"><\/a> The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.[\/caption]\n\nSince most [pb_glossary id=\"2586\"]mantle[\/pb_glossary] plumes are located beneath the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], the early stages of [pb_glossary id=\"1183\"]intraplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] typically take place underwater. Over time, basaltic [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] may build up from the sea floor into islands, such as the Hawaiian Islands. Where a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is found under a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary], contact with the hot [pb_glossary id=\"1956\"]mafic[\/pb_glossary] magma may cause the overlying [pb_glossary id=\"1954\"]felsic[\/pb_glossary] rock to melt and mix with the mafic material below, forming [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary]. Or the [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] may continue to rise, and cool into a granitic [pb_glossary id=\"1968\"]batholith[\/pb_glossary] or erupt as a [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"1181\"]volcano[\/pb_glossary]. The Yellowstone [pb_glossary id=\"1188\"]caldera[\/pb_glossary] is an example of [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] that resulted in an explosive eruption.\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"296\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hawaii-Emperor_engl.png\"><img class=\"size-medium wp-image-131\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-296x300.png\" alt=\"There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.\" width=\"296\" height=\"300\"><\/a> The Hawaii-Emperor seamount and island chain.[\/caption]\n\nA zone of actively erupting [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] connected to a chain of [pb_glossary id=\"1708\"]extinct[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] indicates [pb_glossary id=\"1183\"]intraplate[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] located over a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary]. These [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] chains are created by the overriding [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] slowly moving over a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary]. These chains are seen on the seafloor and continents and include [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] that have been inactive for millions of years. The Hawaiian Islands on the Pacific [pb_glossary id=\"2581\"]Oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] are the active end of a long [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] chain that extends from the northwest Pacific Ocean to the Emperor [pb_glossary id=\"2899\"]Seamounts[\/pb_glossary], all the way to the to the [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone beneath the Kamchatka Peninsula. The overriding North American [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] moved across a [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary] [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] for several million years, creating a chain of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] calderas that extends from Southwestern Idaho to the presently active Yellowstone [pb_glossary id=\"1188\"]caldera[\/pb_glossary] in Wyoming.\n\n<span style=\"font-weight: 400\">Two three<\/span><span style=\"font-weight: 400\">-minute videos (below)<\/span><span style=\"font-weight: 400\"> illustrates [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary].<\/span>\n\n[embed]https:\/\/youtu.be\/AhSaE0omw9o[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-286\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[embed]https:\/\/youtu.be\/t5go-78gCJU[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-287\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n<h3><b>4.5.2 Volcano Features and Types<\/b><\/h3>\n<span style=\"font-weight: 400\">There are several different types of [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] based on their shape, eruption style, magmatic [pb_glossary id=\"2831\"]composition[\/pb_glossary], and other aspects. <\/span>\n\n<span style=\"font-weight: 400\">[h5p id=\"28\"] <\/span>\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code.png\"><img class=\"size-thumbnail wp-image-288\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this interactive activity via this QR Code.[\/caption]\n\n<span style=\"font-weight: 400\">The figure shows the main features of a typical [pb_glossary id=\"1193\"]stratovolcano[\/pb_glossary]:<\/span><span style=\"font-weight: 400\">\u00a01) <strong>[pb_glossary id=\"1185\"]magma chamber[\/pb_glossary]<\/strong>, 2) upper layers of [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], 3) the<\/span><span style=\"font-weight: 400\">\u00a0<\/span><b>[pb_glossary id=\"1186\"]conduit[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> or narrow pipe through which the<\/span><span style=\"font-weight: 400\">\u00a0[pb_glossary id=\"2673\"]lava[\/pb_glossary] erupts, 4) the base or edge of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary], 5) a <strong>[pb_glossary id=\"1970\"]sill[\/pb_glossary]<\/strong> of [pb_glossary id=\"2672\"]magma[\/pb_glossary] between layers of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary], 6) a <strong>[pb_glossary id=\"1966\"]diapir[\/pb_glossary]<\/strong> or feeder tube to the [pb_glossary id=\"1970\"]sill[\/pb_glossary], 7) layers of <strong>[pb_glossary id=\"1948\"]tephra[\/pb_glossary]<\/strong> ([pb_glossary id=\"1949\"]ash[\/pb_glossary]) from previous eruptions, 8 &amp; 9) layers of [pb_glossary id=\"2673\"]lava[\/pb_glossary] erupting from the [pb_glossary id=\"1187\"]vent[\/pb_glossary] and flowing down the sides of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary], 10) the <strong>crater<\/strong> at the top of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary], 11) layers of [pb_glossary id=\"2673\"]lava[\/pb_glossary] and [pb_glossary id=\"1948\"]tephra[\/pb_glossary] on (12), a [pb_glossary id=\"1189\"]parasitic cone[\/pb_glossary]. A\u00a0<b>[pb_glossary id=\"1189\"]parasitic cone[\/pb_glossary]<\/b> is a small [pb_glossary id=\"1181\"]volcano[\/pb_glossary] located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a [pb_glossary id=\"1189\"]parasitic cone[\/pb_glossary] because it has its own separate [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary],\u00a0 13) the <strong>vents<\/strong> of the parasite and the main [pb_glossary id=\"1181\"]volcano[\/pb_glossary], 14) the rim of the crater, 15) clouds of [pb_glossary id=\"1949\"]ash[\/pb_glossary] blown into the sky by the eruption; this settles back onto the [pb_glossary id=\"1181\"]volcano[\/pb_glossary] and surrounding land.\n<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mt.-Shasta-and-Shastina-in-Washington.jpg\"><img class=\"size-medium wp-image-289\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-300x253.jpg\" alt=\"A smaller parasitic cone called Shastina on the flanks of Mt. Shasta in Washington\" width=\"300\" height=\"253\"><\/a> Mt. Shasta in Washington state with Shastina, its parasitic cone[\/caption]\n\n<span style=\"font-weight: 400\">The largest craters are called\u00a0<\/span><b>calderas<\/b><span style=\"font-weight: 400\">, such as the<\/span><span style=\"font-weight: 400\">\u00a0Crater Lake [pb_glossary id=\"1188\"]Caldera[\/pb_glossary]<\/span><span style=\"font-weight: 400\">\u00a0in Oregon. <\/span><span style=\"font-weight: 400\">Many [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] features are produced by\u00a0<strong>[pb_glossary id=\"3368\"]viscosity[\/pb_glossary]<\/strong>, a basic property of a [pb_glossary id=\"2673\"]lava[\/pb_glossary]. [pb_glossary id=\"3368\"]Viscosity[\/pb_glossary] is the resistance to flowing by a fluid. Low [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] flows easily more like syrup, the basaltic [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] that occurs in Hawaii on [pb_glossary id=\"2207\"]shield[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary].\u00a0High [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] means a thick and sticky [pb_glossary id=\"2672\"]magma[\/pb_glossary], typically [pb_glossary id=\"1954\"]felsic[\/pb_glossary] or [pb_glossary id=\"1955\"]intermediate[\/pb_glossary], that flows slowly, similar to toothpaste.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Crater_lake_oregon-1.jpg\"><img class=\"size-medium wp-image-45\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-300x200.jpg\" alt=\"The mountain has a large hole in the center that is filled with the lake.\" width=\"300\" height=\"200\"><\/a> Oregon's Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.[\/caption]\n\n&nbsp;\n\n&nbsp;\n\n&nbsp;\n\n&nbsp;\n\n&nbsp;\n<h4><span style=\"font-weight: 400\">Shield Volcano<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Kilauea_Shield_Volcano_Hawaii_20071209A.jpg\"><img class=\"size-medium wp-image-290\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-300x200.jpg\" alt=\"The mountain has low-angle flanks\" width=\"300\" height=\"200\"><\/a> Kilauea in Hawai'i.[\/caption]\n\nThe largest [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are <strong>[pb_glossary id=\"2207\"]shield[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]<\/strong>. They are characterized by broad low-angle flanks, small vents at the top, and [pb_glossary id=\"1956\"]mafic[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] chambers. The name comes from the side view, which resembles a medieval warrior\u2019s [pb_glossary id=\"2207\"]shield[\/pb_glossary]. They are typically associated with [pb_glossary id=\"2638\"]hotspots[\/pb_glossary], [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], or [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifts[\/pb_glossary] with rising upper [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material. The low-angle flanks are built up slowly from numerous low-[pb_glossary id=\"3368\"]viscosity[\/pb_glossary] basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows that spread out over long distances. The basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] erupts effusively, meaning the eruptions are small, localized, and predictable.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Kiluea-eruption-2018.jpg\"><img class=\"size-medium wp-image-291\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-300x223.jpg\" alt=\"Lava from Kiluea destroying road in Hawaii.\" width=\"300\" height=\"223\"><\/a> Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. This eruption caused much property damage[\/caption]\n\nTypically, [pb_glossary id=\"1190\"]shield volcano[\/pb_glossary] eruptions are not much of a hazard to human life\u2014although non-explosive eruptions of Kilauea (Hawaii) in 2018 produced uncharacteristically large lavas that damaged roads and structures.\u00a0<span style=\"font-weight: 400\">Mauna Loa (see <a href=\"http:\/\/hvo.wr.usgs.gov\/maunaloa\/\">USGS page<\/a>) and Kilauea (see <a href=\"http:\/\/hvo.wr.usgs.gov\/kilauea\/\">USGS page<\/a>) in Hawaii are examples of [pb_glossary id=\"2207\"]shield[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]. [pb_glossary id=\"2207\"]Shield[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African [pb_glossary id=\"2624\"]Rift[\/pb_glossary]<\/span><span style=\"font-weight: 400\">. <\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"215\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Olympus-Mons-on-Mars.jpg\"><img class=\"wp-image-292\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-300x278.jpg\" alt=\"\" width=\"215\" height=\"200\"><\/a> Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system, standing about two and a half times higher than Everest is above sea level.[\/caption]\n\nThe largest [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] edifice in the [pb_glossary id=\"2201\"]Solar System[\/pb_glossary] is Olympus Mons on Mars. This (possibly [pb_glossary id=\"1708\"]extinct[\/pb_glossary]) [pb_glossary id=\"1190\"]shield volcano[\/pb_glossary] covers an area the size of the state of Arizona. This may indicate the [pb_glossary id=\"1181\"]volcano[\/pb_glossary] erupted over a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] for millions of years, which means Mars had little, if any, [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] activity<span style=\"font-weight: 400\">.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2022\/01\/ReU_PtFournaise_Lavastrome.jpg\"><img class=\"size-medium wp-image-293\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-300x225.jpg\" alt=\"The lava is ropey\" width=\"300\" height=\"225\"><\/a> Ropey pahoehoe lava[\/caption]\n<p style=\"text-align: left\"><span style=\"font-weight: 400\">Basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] forms special landforms based on [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"2689\"]temperature[\/pb_glossary], [pb_glossary id=\"2831\"]composition[\/pb_glossary], and content of [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] gases and water vapor. The two main types of basaltic [pb_glossary id=\"1938\"]volcanic rock[\/pb_glossary] have Hawaiian names\u2014<em>[pb_glossary id=\"1192\"]<em>pahoehoe<\/em>[\/pb_glossary]<\/em> and <em>[pb_glossary id=\"1191\"]<em>aa<\/em>[\/pb_glossary]<\/em>. <strong>[pb_glossary id=\"1192\"]Pahoehoe[\/pb_glossary]<\/strong> might come from low-[pb_glossary id=\"3368\"]viscosity[\/pb_glossary] lava that flows easily into ropey strands.<\/span><\/p>\n\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Etna_02aa.jpg\"><img class=\"size-medium wp-image-294\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-300x225.jpg\" alt=\"The lava is sharp and jagged\" width=\"300\" height=\"225\"><\/a> Blocky a'a lava[\/caption]\n<p style=\"text-align: left\"><b>[pb_glossary id=\"1191\"]Aa[\/pb_glossary]<\/b><span style=\"font-weight: 400\"> (sometimes spelled a\u2019a or <\/span><span style=\"font-weight: 400\">[pb_glossary id=\"1191\"]\u02bba\u02bb\u0101 [\/pb_glossary]and pronounced \"ah-ah\"<\/span><span style=\"font-weight: 400\">) is more [pb_glossary id=\"3368\"]viscous[\/pb_glossary] and has a crumbly blocky appearance. The exact details of what forms the two types of flows are still up for debate. <\/span><span style=\"font-weight: 400\">[pb_glossary id=\"1954\"]Felsic[\/pb_glossary] lavas have lower temperatures and more silica, and thus are higher [pb_glossary id=\"3368\"]viscosity[\/pb_glossary]. These also form [pb_glossary id=\"1191\"]aa[\/pb_glossary]-style flows.<\/span><\/p>\n\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano_fissure_tube.jpg\"><img class=\"size-medium wp-image-295\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-300x199.jpg\" alt=\"The magma is sputtering outward\" width=\"300\" height=\"199\"><\/a> Volcanic fissure and flow, which could eventually form a lava tube.[\/caption]\n\nLow-[pb_glossary id=\"3368\"]viscosity[\/pb_glossary], fast-flowing basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] tends to harden on the outside into a tube and continue to flow internally. Once [pb_glossary id=\"2673\"]lava[\/pb_glossary] flow subsides, the empty outer shell may be left as a [pb_glossary id=\"2673\"]lava[\/pb_glossary] tube. [pb_glossary id=\"2673\"]Lava[\/pb_glossary] tubes, with or without collapsed roofs, make famous caves in Hawaii, Northern California, the Columbia [pb_glossary id=\"3134\"]River[\/pb_glossary] [pb_glossary id=\"1961\"]Basalt[\/pb_glossary] Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho.\n\n<strong>Fissures<\/strong> are cracks that commonly originate from [pb_glossary id=\"2207\"]shield[\/pb_glossary]-style eruptions. [pb_glossary id=\"2673\"]Lava[\/pb_glossary] emerging from fissures is typically [pb_glossary id=\"1956\"]mafic[\/pb_glossary] and very fluid. The 2018 Kiluaea eruption included fissures associated with the [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows. Some fissures are caused by the [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"3087\"]seismic[\/pb_glossary] activity rather than [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows. Some fissures are influenced by [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary], such as the common fissures located parallel to the [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundary in Iceland.\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"200\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/DT2-scaled.jpg\"><img class=\"wp-image-2892\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/DT2-scaled-1.jpg\" alt=\"The rock is full of columns\" width=\"200\" height=\"150\"><\/a> Devils Tower in Wyoming has columnar jointing.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"192\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Columnar-jointing-on-Giants-Causeway-in-Ireland.jpg\"><img class=\"wp-image-297\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-288x300.jpg\" alt=\"Columnar jointing on Giant's Causeway in Ireland.\" width=\"192\" height=\"200\"><\/a> Columnar jointing on Giant's Causeway in Ireland.[\/caption]\n\nCooling [pb_glossary id=\"2673\"]lava[\/pb_glossary] can contract into columns with semi-hexagonal cross sections called <strong>columnar jointing<\/strong>. This feature forms the famous Devils Tower in Wyoming, possibly an ancient [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1187\"]vent[\/pb_glossary] from which the surrounding layers of [pb_glossary id=\"2673\"]lava[\/pb_glossary] and [pb_glossary id=\"1949\"]ash[\/pb_glossary] have been removed by [pb_glossary id=\"2677\"]erosion[\/pb_glossary]. Another well-known exposed example of columnar jointing is the Giant\u2019s Causeway in Ireland.\n<h4><span style=\"font-weight: 400\">Stratovolcano<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"225\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_Rainier_over_Tacoma.jpg\"><img class=\"wp-image-298\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-300x200.jpg\" alt=\"The mountain is very tall, and looms over the city\" width=\"225\" height=\"150\"><\/a> Mount Rainier towers over Tacoma, Washington.[\/caption]\n\nA <strong>[pb_glossary id=\"1193\"]stratovolcano[\/pb_glossary]<\/strong>, also called a [pb_glossary id=\"1193\"]composite cone[\/pb_glossary] [pb_glossary id=\"1181\"]volcano[\/pb_glossary], has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] fragments like [pb_glossary id=\"1949\"]ash[\/pb_glossary] and [pb_glossary id=\"1951\"]bombs[\/pb_glossary], and solidified [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows of varying [pb_glossary id=\"2831\"]composition[\/pb_glossary]. Examples include Mount Rainier in Washington state and Mount Fuji in Japan.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mt.-Fuji-300x190-1.jpg\"><img class=\"wp-image-299\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Fuji-300x190-1.jpg\" alt=\"\" width=\"250\" height=\"158\"><\/a> Mt. Fuji in Japan, a typical stratovolcano, symmetrical, increasing slope, visible crater at the top.[\/caption]\n\nStratovolcanoes usually have [pb_glossary id=\"1954\"]felsic[\/pb_glossary] to [pb_glossary id=\"1955\"]intermediate[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] chambers, but can even produce [pb_glossary id=\"1956\"]mafic[\/pb_glossary] lavas. Stratovolcanoes have [pb_glossary id=\"3368\"]viscous[\/pb_glossary]\u00a0[pb_glossary id=\"2673\"]lava[\/pb_glossary] flows and [pb_glossary id=\"1460\"]domes[\/pb_glossary], punctuated by explosive eruptions. This produces [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] with steep flanks.\n<h4><span style=\"font-weight: 400\">Lava Domes<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSH06_aerial_crater_from_north_high_angle_09-12-06.jpg\"><img class=\"size-medium wp-image-300\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-300x200.jpg\" alt=\"The mountain has a hole, but the hole has filled in somewhat\" width=\"300\" height=\"200\"><\/a> Lava domes have started the rebuilding process at Mount St. Helens, Washington.[\/caption]\n\n<strong>[pb_glossary id=\"2673\"]Lava[\/pb_glossary] [pb_glossary id=\"1460\"]domes[\/pb_glossary]<\/strong> are accumulations of silica-rich [pb_glossary id=\"1938\"]volcanic rock[\/pb_glossary], such as [pb_glossary id=\"1958\"]rhyolite[\/pb_glossary] and [pb_glossary id=\"1947\"]obsidian[\/pb_glossary]. Too [pb_glossary id=\"3368\"]viscous[\/pb_glossary] to flow easily, the [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] tends to pile up near the [pb_glossary id=\"1187\"]vent[\/pb_glossary] in blocky masses. [pb_glossary id=\"2673\"]Lava[\/pb_glossary] [pb_glossary id=\"1460\"]domes[\/pb_glossary] often form in a [pb_glossary id=\"1187\"]vent[\/pb_glossary] within the collapsed crater of a [pb_glossary id=\"1193\"]stratovolcano[\/pb_glossary], and grow by internal expansion. As the [pb_glossary id=\"1460\"]dome[\/pb_glossary] expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. Mount Saint Helens has a good example of a [pb_glossary id=\"2673\"]lava[\/pb_glossary] [pb_glossary id=\"1460\"]dome[\/pb_glossary] inside of a collapsed [pb_glossary id=\"1193\"]stratovolcano[\/pb_glossary] crater. Examples of stand-alone [pb_glossary id=\"2673\"]lava[\/pb_glossary] [pb_glossary id=\"1460\"]domes[\/pb_glossary] are Chaiten in Chile and Mammoth Mountain in California.\n<h4><span style=\"font-weight: 400\">Caldera<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"149\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_Mazama_eruption_timeline.png\"><img class=\"wp-image-301\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline-89x300.png\" alt=\"It shows the eruption forming a caldera.\" width=\"149\" height=\"500\"><\/a> Timeline of events at Mount Mazama.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/CL1-scaled.jpg\"><img class=\"wp-image-2898\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/CL1-scaled-1.jpg\" alt=\"The island is forested, as are the flanks\" width=\"250\" height=\"188\"><\/a> Wizard Island sits in the caldera at Crater Lake.[\/caption]\n\n<strong>Calderas<\/strong> are steep-walled, [pb_glossary id=\"1461\"]basin[\/pb_glossary]-shaped depressions formed by the collapse of a [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] edifice into an empty [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary]. Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term [pb_glossary id=\"1188\"]caldera[\/pb_glossary] specifically refers to a [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1187\"]vent[\/pb_glossary]; however, it is frequently used to describe a [pb_glossary id=\"1181\"]volcano[\/pb_glossary] type. [pb_glossary id=\"1188\"]Caldera[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] are\u00a0typically formed by eruptions of high-[pb_glossary id=\"3368\"]viscosity[\/pb_glossary] [pb_glossary id=\"1954\"]felsic[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] having high [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] content.\n\nCrater Lake, Yellowstone, and the Long Valley [pb_glossary id=\"1188\"]Caldera[\/pb_glossary] are good examples of this type of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. The [pb_glossary id=\"1188\"]caldera[\/pb_glossary] at Crater Lake National Park in Oregon was created about 6,800 years ago when Mount Mazama, a [pb_glossary id=\"1193\"]composite volcano[\/pb_glossary], erupted in a huge explosive blast. The [pb_glossary id=\"1181\"]volcano[\/pb_glossary] ejected large amounts of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1949\"]ash[\/pb_glossary] and rapidly drained the [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary], causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent [pb_glossary id=\"1194\"]lava dome[\/pb_glossary] that formed within the [pb_glossary id=\"1188\"]caldera[\/pb_glossary] [pb_glossary id=\"1461\"]basin[\/pb_glossary]<span style=\"font-weight: 400\">.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Yellowstone_Caldera_map2.jpg\"><img class=\"size-medium wp-image-303\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-300x242.jpg\" alt=\"The map shows locations of calderas and rocks within Yellowstone\" width=\"300\" height=\"242\"><\/a> Map of calderas and related rocks around Yellowstone.[\/caption]\n\nThe Yellowstone [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"2664\"]system[\/pb_glossary] erupted three times in the recent geologic past\u20142.1, 1.3, and 0.64 million years ago\u2014leaving behind three [pb_glossary id=\"1188\"]caldera[\/pb_glossary] basins. Each eruption created large [pb_glossary id=\"1958\"]rhyolite[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows as well as [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows that solidified into [pb_glossary id=\"1953\"]tuff[\/pb_glossary] [pb_glossary id=\"2960\"]formations[\/pb_glossary]. These extra-large eruptions rapidly emptied the [pb_glossary id=\"1185\"]magma chamber[\/pb_glossary], causing the roof to collapse and form a [pb_glossary id=\"1188\"]caldera[\/pb_glossary]. The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent [pb_glossary id=\"2673\"]lava[\/pb_glossary] [pb_glossary id=\"1460\"]domes[\/pb_glossary]. The calderas are difficult to see today due to the amount of time since their eruptions and subsequent [pb_glossary id=\"2677\"]erosion[\/pb_glossary] and [pb_glossary id=\"1700\"]glaciation[\/pb_glossary].\n\nYellowstone [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] started about 17-million years ago as a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] under the North American lithospheric [pb_glossary id=\"2591\"]plate[\/pb_glossary] near the Oregon\/Nevada border. As the [pb_glossary id=\"2591\"]plate[\/pb_glossary] moved to the southwest over the stationary [pb_glossary id=\"2638\"]hotspot[\/pb_glossary], it left behind a track of past [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activities. Idaho\u2019s Snake [pb_glossary id=\"3134\"]River[\/pb_glossary] Plain was created from [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] that produced a series of calderas and [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows. The [pb_glossary id=\"2591\"]plate[\/pb_glossary] eventually arrived at its current location in northwestern Wyoming, where [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] formed the Yellowstone calderas<span style=\"font-weight: 400\">.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Yellowstone_volcano_-_ash_beds.jpg\"><img class=\"size-medium wp-image-134\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-300x195.jpg\" alt=\"The eruptions trend eastward due to prevailing winds.\" width=\"300\" height=\"195\"><\/a> Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).[\/caption]\n\nThe Long Valley [pb_glossary id=\"1188\"]Caldera[\/pb_glossary] near Mammoth, California, is the result of a large [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of [pb_glossary id=\"1949\"]ash[\/pb_glossary] across the United States, in a manner similar to the Yellowstone eruptions. The Bishop [pb_glossary id=\"1953\"]Tuff[\/pb_glossary] deposit near Bishop, California, is made of [pb_glossary id=\"1949\"]ash[\/pb_glossary] from this eruption. The current [pb_glossary id=\"1188\"]caldera[\/pb_glossary] [pb_glossary id=\"1461\"]basin[\/pb_glossary] is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent [pb_glossary id=\"1460\"]dome[\/pb_glossary], and several hot springs<span style=\"font-weight: 400\">.<\/span>\n<h4><span style=\"font-weight: 400\">Cinder Cone<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Sunset_Crater10.jpg\"><img class=\"size-medium wp-image-304\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-300x202.jpg\" alt=\"The cone is relatively small and red\" width=\"300\" height=\"202\"><\/a> Sunset Crater, Arizona is a cinder cone.[\/caption]\n\n<strong>[pb_glossary id=\"1195\"]Cinder[\/pb_glossary] cones<\/strong> are small [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] with steep sides, and made of [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] fragments that have been ejected from a pronounced central [pb_glossary id=\"1187\"]vent[\/pb_glossary]. The small fragments are called <strong>[pb_glossary id=\"1195\"]cinders[\/pb_glossary]<\/strong> and the largest are <strong>[pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1951\"]bombs[\/pb_glossary]<\/strong>. The eruptions are usually short-lived events, typically consisting of [pb_glossary id=\"1956\"]mafic[\/pb_glossary] lavas with a high content of [pb_glossary id=\"2606\"]volatiles[\/pb_glossary]. Hot [pb_glossary id=\"2673\"]lava[\/pb_glossary] is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary]. [pb_glossary id=\"1195\"]Cinder[\/pb_glossary] cones are found throughout western North America<span style=\"font-weight: 400\">.<\/span>\n\n&nbsp;\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"200\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Paricutin_30_612.jpg\"><img class=\"size-medium wp-image-305\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-200x300.jpg\" alt=\"A person looks at the eruption of ash\" width=\"200\" height=\"300\"><\/a> Soon after the birth of Par\u00edcutin in 1943.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Paracutin-lava-in-San-Juan-300x224-1.jpg\"><img class=\"size-full wp-image-306\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Paracutin-lava-in-San-Juan-300x224-1.jpg\" alt=\"\" width=\"300\" height=\"224\"><\/a> Lava from Par\u00edcutin covered the local church and destroyed the town of San Juan, Mexico[\/caption]\n\n<span style=\"font-weight: 400\">A recent and striking example of a [pb_glossary id=\"1196\"]cinder cone[\/pb_glossary] is the eruption near the village of Par\u00edcutin, Mexico that started in 1943. The [pb_glossary id=\"1196\"]cinder cone[\/pb_glossary] started explosively shooting [pb_glossary id=\"1195\"]cinders[\/pb_glossary] out of the [pb_glossary id=\"1187\"]vent[\/pb_glossary] in the middle of a farmer\u2019s field. The [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] quickly built up the cone to a height of over 90 m (300 ft) within a week, and 365 m (1,200 ft) within the first 8 months. After the initial explosive eruption of gases and [pb_glossary id=\"1195\"]cinders[\/pb_glossary], basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] poured out from the base of the cone. This is a common order of events for [pb_glossary id=\"1195\"]cinder[\/pb_glossary] cones: violent eruption, cone and crater [pb_glossary id=\"2960\"]formation[\/pb_glossary], low-[pb_glossary id=\"3368\"]viscosity[\/pb_glossary] [pb_glossary id=\"2673\"]lava[\/pb_glossary] flow from the base. The [pb_glossary id=\"1196\"]cinder cone[\/pb_glossary] is not strong enough to support a column of [pb_glossary id=\"2673\"]lava[\/pb_glossary] rising to the top of the crater, so the [pb_glossary id=\"2673\"]lava[\/pb_glossary] breaks through and emerges near the bottom of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary]. During nine years of eruption activity, the ashfall covered about 260 km<sup>2<\/sup> (100 mi<sup>2<\/sup>) and destroyed the nearby town of San Juan<\/span><span style=\"font-weight: 400\">.<\/span>\n<h4><span style=\"font-weight: 400\">Flood Basalts<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"500\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/World-map-or-flood-basalts.jpg\"><img class=\"wp-image-307\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-300x168.jpg\" alt=\"World map of flood basalts. Note the largest is the Siberian Traps\" width=\"500\" height=\"280\"><\/a> World map of flood basalts. Note the largest is the Siberian Traps[\/caption]\n\nA rare [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] eruption type,\u00a0unobserved in modern times, is the\u00a0<strong>[pb_glossary id=\"1197\"]flood basalt[\/pb_glossary]<\/strong>. [pb_glossary id=\"1197\"]Flood basalts[\/pb_glossary] are some of the largest and lowest [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia [pb_glossary id=\"3134\"]River[\/pb_glossary] [pb_glossary id=\"1197\"]Flood Basalts[\/pb_glossary] in Washington, Oregon, and Idaho, the Deccan [pb_glossary id=\"3342\"]Traps[\/pb_glossary], which cover about 1\/3 of the country of India, and the Siberian [pb_glossary id=\"3342\"]Traps[\/pb_glossary], which may have been involved in the Earth's largest [pb_glossary id=\"2223\"]mass extinction[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/8-earth-history\/\" target=\"_blank\" rel=\"noopener\">chapter 8<\/a>).\n<h4><span style=\"font-weight: 400\">Carbonatites<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Crater_of_Ol_Doinyo_Lengai_Jan_2011-1.jpg\"><img class=\"size-medium wp-image-3187\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_of_Ol_Doinyo_Lengai_Jan_2011-1-300x169-1.jpg\" alt=\"The crater has white rocks in the walls\" width=\"300\" height=\"169\"><\/a> Crater of Ol Doinyo Lengai in 2011. Note the white carbonatite in the walls of the crater.[\/caption]\n\nArguably the most unusual [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activity are <strong>[pb_glossary id=\"3365\"]carbonatite[\/pb_glossary]<\/strong> eruptions. Only one actively erupting [pb_glossary id=\"3365\"]carbonatite[\/pb_glossary] [pb_glossary id=\"1181\"]volcano[\/pb_glossary] exists on Earth today, Ol Doinyo Lengai, in the East African [pb_glossary id=\"2624\"]Rift[\/pb_glossary] Zone of Tanzania. While all other [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] on Earth originates from [pb_glossary id=\"2709\"]silicate[\/pb_glossary]-based [pb_glossary id=\"2672\"]magma[\/pb_glossary], [pb_glossary id=\"3365\"]carbonatites[\/pb_glossary] are a product of [pb_glossary id=\"1917\"]carbonate[\/pb_glossary]-based [pb_glossary id=\"2672\"]magma[\/pb_glossary] and produce [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] rocks containing greater than 50% [pb_glossary id=\"1917\"]carbonate[\/pb_glossary] [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. [pb_glossary id=\"3365\"]Carbonatite[\/pb_glossary] lavas are very low [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] and relatively cold for [pb_glossary id=\"2673\"]lava[\/pb_glossary]. The erupting [pb_glossary id=\"2673\"]lava[\/pb_glossary] is black, and solidifies to brown\/grey rock that eventually turns white. These rocks are occasionally found in the geologic record and require special study to distinguish them from [pb_glossary id=\"2914\"]metamorphic[\/pb_glossary] marbles (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/6-metamorphic-rocks\/\">Chapter 6<\/a>). They are mostly associated with [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary].\n\n<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-e1546649689814.png\"><img class=\"size-full wp-image-309\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814.png\" alt=\"Table of igneous rocks and related volcano types. Horizontal axis is arranged from low to high silica content (i.e. from ultramafic to felsic). First row shows the extrusive (surface) igneous rocks basalt, andesite, and rhyolite. Second row shows volcano types: mid-ocean ridge, shield, cinder cone, and strato (composite). Third row shows examples of each volcano: mid-atlantic ridge, Mauna Kea (Hawaii), Paricutin, and Mt. St. Helens. Forth row shows intrusive rocks from mafic to felsic: Dunite, gabbro, diorige, granite. Fifth row shows common plate-tectonic settings: divergent oceanic hot spot, and convergent boundaries. Sixth row is typical composition: ultramafic, mafic, intermediate, and felsic.\" width=\"759\" height=\"820\"><\/a>\n<blockquote>\n<p style=\"text-align: left\"><em>[pb_glossary id=\"2675\"]<em>Igneous rock<\/em>[\/pb_glossary] types and related [pb_glossary id=\"1181\"]<em>volcano<\/em>[\/pb_glossary] types. [pb_glossary id=\"2630\"]<em>Mid-ocean ridges<\/em>[\/pb_glossary] and [pb_glossary id=\"2207\"]<em>shield<\/em>[\/pb_glossary] [pb_glossary id=\"1181\"]<em>volcanoes<\/em>[\/pb_glossary] represent more [pb_glossary id=\"1956\"]<em>mafic<\/em>[\/pb_glossary] compositions, and strato (composite) [pb_glossary id=\"1181\"]<em>volcanoes<\/em>[\/pb_glossary] generally represent a more [pb_glossary id=\"1955\"]<em>intermediate<\/em>[\/pb_glossary] or [pb_glossary id=\"1954\"]<em>felsic<\/em>[\/pb_glossary] [pb_glossary id=\"2831\"]<em>composition<\/em>[\/pb_glossary] and a [pb_glossary id=\"2600\"]<em>convergent<\/em>[\/pb_glossary] [pb_glossary id=\"2591\"]<em>plate<\/em>[\/pb_glossary] [pb_glossary id=\"2576\"]<em>tectonic<\/em>[\/pb_glossary] boundary. Note that there are exceptions to this generalized layout of volcano types and [pb_glossary id=\"2675\"]<em>igneous rock<\/em>[\/pb_glossary] [pb_glossary id=\"2831\"]<em>composition<\/em>[\/pb_glossary].<\/em><\/p>\n<\/blockquote>\n<h3><b>4.5.3 Volcanic Hazards and Monitoring<\/b><\/h3>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"258\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Types_of_volcano_hazards_usgs.gif\"><img class=\"wp-image-310 size-medium\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Types_of_volcano_hazards_usgs-258x300.gif\" alt=\"It shows many hazards\" width=\"258\" height=\"300\"><\/a> General diagram of volcanic hazards.[\/caption]\n\nWhile the most obvious [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] hazard is [pb_glossary id=\"2673\"]lava[\/pb_glossary], the dangers posed by [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] go far beyond [pb_glossary id=\"2673\"]lava[\/pb_glossary] flows. For example, on May 18, 1980, Mount Saint Helens (Washington, United States) erupted with an explosion and [pb_glossary id=\"1199\"]landslide[\/pb_glossary] that removed the upper 400 m (1,300 ft) of the mountain. The initial explosion was immediately followed by a lateral blast, which produced a [pb_glossary id=\"1198\"]pyroclastic\u00a0flow[\/pb_glossary] that covered nearly 600 km<sup>2<\/sup> (230 mi<sup>2<\/sup>) of forest with hot [pb_glossary id=\"1949\"]ash[\/pb_glossary] and debris. The pyroclastic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and ejecting clouds of ash into the air. The USGS video provides an account of this explosive eruption that killed 57 people.\n\n[embed]https:\/\/youtu.be\/Ec30uU0G56U[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount-St.-helens-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-311\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/DSC01727.jpg\"><img class=\"wp-image-312\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-300x225.jpg\" alt=\"The body is outlined with a cast, and the bones are seen.\" width=\"250\" height=\"188\"><\/a> Human remains from the 79 CE eruption of Vesuvius.[\/caption]\n\nIn 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a [pb_glossary id=\"1198\"]pyroclastic flow[\/pb_glossary] over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains ([pb_glossary id=\"2179\"]casts[\/pb_glossary]) of people suffocated by ash and covered by 10 feet (3 m) of [pb_glossary id=\"1949\"]ash[\/pb_glossary], [pb_glossary id=\"1946\"]pumice[\/pb_glossary] [pb_glossary id=\"1950\"]lapilli[\/pb_glossary], and collapsed roofs.\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_St._Helens_one_day_before_the_devastating_eruption.jpg\"><img class=\"size-medium wp-image-313\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-300x203.jpg\" alt=\"The volcano is conical and forested.\" width=\"300\" height=\"203\"><\/a> Mount St. Helens, the day before the May 18th, 1980 eruption.[\/caption]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSH80_st_helens_from_johnston_ridge_09-10-80.jpg\"><img class=\"size-medium wp-image-314\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-300x201.jpg\" alt=\"The top of the mountain is gone\" width=\"300\" height=\"201\"><\/a> Picture 4 months after the major eruption of Mount St. Helens.[\/caption]\n\n[caption id=\"attachment_315\" align=\"alignnone\" width=\"1500\"]<img class=\"wp-image-315 size-full\" title=\"By Associated Press, via The Atlantic, https:\/\/www.theatlantic.com\/photo\/2015\/05\/the-eruption-of-mount-st-helens-in-1980\/393557\/\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt_Saint_Helens_Eruption_main_1500.gif\" alt=\"Series of images showing half of the mountain releasing as a giant landslide and ash billowing out from underneath.\" width=\"1500\" height=\"943\"> Series of still images of the May 18, 1980, eruption of Mt. Saint Helens, Washington showing largest recorded landslide in history and subsequent eruption and pyroclastic flow (By The Associated Press via The Atlantic)[\/caption]\n<h4><span style=\"font-weight: 400\">Pyroclastic flows<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pyroclastic_flows_at_Mayon_Volcano.jpg\"><img class=\"size-medium wp-image-316\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-300x188.jpg\" alt=\"Most of the material is heading up, but small portions of the eruption column head downward.\" width=\"300\" height=\"188\"><\/a> The material coming down from the eruption column is a pyroclastic flow.[\/caption]\n\n<span style=\"font-weight: 400\">The most dangerous [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] hazard are <strong>[pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows<\/strong> (<\/span><a href=\"https:\/\/volcanoes.usgs.gov\/vsc\/movies\/movie_101\/PF_Animation.mp4\"><span style=\"font-weight: 400\">video<\/span><\/a><span style=\"font-weight: 400\">). These flows are a mix of [pb_glossary id=\"2673\"]lava[\/pb_glossary] blocks, [pb_glossary id=\"1946\"]pumice[\/pb_glossary], [pb_glossary id=\"1949\"]ash[\/pb_glossary], and hot gases between 200\u00b0C-700\u00b0C (400\u00b0F-1,300\u00b0F). The turbulent cloud of [pb_glossary id=\"1949\"]ash[\/pb_glossary] and gas races down the steep flanks at high speeds up to 193 kph (120 mph) into the valleys around composite [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]<\/span><span style=\"font-weight: 400\">. Most explosive, silica-rich, high [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] such as composite cones usually have [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows. The rock <\/span>[pb_glossary id=\"1953\"]tuff[\/pb_glossary] <span style=\"font-weight: 400\">and <\/span>welded [pb_glossary id=\"1953\"]tuff[\/pb_glossary]<span style=\"font-weight: 400\"> is often formed from these [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"256\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pelee_1902_3.jpg\"><img class=\"size-medium wp-image-317\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3-256x300.jpg\" alt=\"A man is seen overlooking the destroyed city\" width=\"256\" height=\"300\"><\/a> The remains of St. Pierre.[\/caption]\n\n<span style=\"font-weight: 400\">There are numerous examples of deadly [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows. In 2014, the Mount Ontake [pb_glossary id=\"1198\"]pyroclastic flow[\/pb_glossary] in Japan killed 47 people. The flow was caused by [pb_glossary id=\"2672\"]magma[\/pb_glossary] heating [pb_glossary id=\"3129\"]groundwater[\/pb_glossary] into steam, which then rapidly ejected with [pb_glossary id=\"1949\"]ash[\/pb_glossary] and [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"1951\"]bombs[\/pb_glossary]. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs<\/span><span style=\"font-weight: 400\">. <\/span><span style=\"font-weight: 400\">Two short videos below document<\/span><span style=\"font-weight: 400\">\u00a0eye-witness video of [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows. In the early 1990s, Mount Unzen erupted several times with [pb_glossary id=\"1952\"]pyroclastic[\/pb_glossary] flows. The [pb_glossary id=\"1198\"]pyroclastic flow[\/pb_glossary]\u00a0shown in this famous <\/span><span style=\"font-weight: 400\">short video<\/span><span style=\"font-weight: 400\"> killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee <\/span><span style=\"font-weight: 400\">erupted with a violent [pb_glossary id=\"1198\"]pyroclastic flow[\/pb_glossary] that destroyed the entire town of St. Pierre and killing 28,000 people in moments<\/span><span style=\"font-weight: 400\">.<\/span>\n\n[embed]https:\/\/youtu.be\/3ObsOj9Q2Do[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Japans-Mount-Ontake-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-318\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n[embed]https:\/\/youtu.be\/Cvjwt9nnwXY[\/embed]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes.png\"><img class=\"size-thumbnail wp-image-319\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n<h4><span style=\"font-weight: 400\">Landslides and Landslide-Generated Tsunamis<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Msh_may18_sequence.gif\"><img class=\"size-medium wp-image-320\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-300x197.gif\" alt=\"The landslide opened an area for the eruption\" width=\"300\" height=\"197\"><\/a> Sequence of events for Mount St. Helens, May 18th, 1980. Note that an earthquake caused a landslide, which caused the \"uncorking\" of the mountain and started the eruption.[\/caption]\n\nThe steep and unstable flanks of a [pb_glossary id=\"1181\"]volcano[\/pb_glossary] can lead to slope failure and dangerous [pb_glossary id=\"1199\"]landslides[\/pb_glossary]. These [pb_glossary id=\"1199\"]landslides[\/pb_glossary] can be triggered by [pb_glossary id=\"2672\"]magma[\/pb_glossary] movement, explosive eruptions, large earthquakes, and\/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the [pb_glossary id=\"1181\"]volcano[\/pb_glossary] collapsed and released a huge [pb_glossary id=\"1199\"]landslide[\/pb_glossary] that moved at speeds of 160-290 kph (100-180 mph).\n\nIf enough [pb_glossary id=\"1199\"]landslide[\/pb_glossary] material reaches the ocean, it may cause a [pb_glossary id=\"3194\"]tsunami[\/pb_glossary]. In 1792, a [pb_glossary id=\"1199\"]landslide[\/pb_glossary] caused by the Mount Unzen eruption reached the Ariaka Sea, generating a [pb_glossary id=\"3194\"]tsunami[\/pb_glossary] that killed 15,000 people (see <a href=\"http:\/\/volcano.oregonstate.edu\/describe-1883-eruption-krakatau\" target=\"_blank\" rel=\"noopener\">USGS page<\/a>). When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The [pb_glossary id=\"3194\"]tsunami[\/pb_glossary] killed 36,000 people and destroyed 165 villages.\n<h4><span style=\"font-weight: 400\">Tephra<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01.jpg\"><img class=\"size-medium wp-image-321\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-300x204.jpg\" alt=\"The man is wearing a mask to prevent pneumonoultramicroscopicsilicovolvanoconiosis.\" width=\"300\" height=\"204\"><\/a> Aman sweeps ash from an eruption of Kelud, Indonesia.[\/caption]\n\n<span style=\"font-weight: 400\">[pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary], especially composite [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary], eject large amounts of <\/span><b>[pb_glossary id=\"1948\"]tephra[\/pb_glossary] <\/b><span style=\"font-weight: 400\">(ejected rock materials), most notably\u00a0<\/span><b>[pb_glossary id=\"1949\"]ash[\/pb_glossary] <\/b><span style=\"font-weight: 400\">([pb_glossary id=\"1948\"]tephra[\/pb_glossary] fragments less than 0.08 inches [2 mm]). Larger [pb_glossary id=\"1948\"]tephra[\/pb_glossary] is heavier and [pb_glossary id=\"3119\"]falls[\/pb_glossary] closer to the [pb_glossary id=\"1187\"]vent[\/pb_glossary]. Larger blocks and [pb_glossary id=\"1951\"]bombs[\/pb_glossary] pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier.<\/span>\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"250\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Micrograph-of-volcanic-ash-particle.jpg\"><img class=\"size-full wp-image-322\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Micrograph-of-volcanic-ash-particle.jpg\" alt=\"Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.\" width=\"250\" height=\"183\"><\/a> Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.[\/caption]\n\nHot [pb_glossary id=\"1949\"]ash[\/pb_glossary] poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. [pb_glossary id=\"1949\"]Ash[\/pb_glossary] is fine grained (&lt; 2mm) and can travel airborne long distances away from the eruption site. Heavy accumulations of [pb_glossary id=\"1949\"]ash[\/pb_glossary] can cause buildings to collapse. In people, it may cause respiratory issues like silicosis. [pb_glossary id=\"1949\"]Ash[\/pb_glossary] is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services. In 2010, the Eyjafjallaj\u00f6kull [pb_glossary id=\"1181\"]volcano[\/pb_glossary] in Iceland emitted a large [pb_glossary id=\"1949\"]ash[\/pb_glossary] cloud into the upper [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary], causing the largest air-travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars.\n<h4><span style=\"font-weight: 400\">Volcanic Gases<\/span><\/h4>\nAs [pb_glossary id=\"2672\"]magma[\/pb_glossary] rises to the surface the [pb_glossary id=\"2917\"]confining[\/pb_glossary] pressure decreases, and allows [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] gases to escape into the [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary]. Even [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO<sub>2<\/sub>), sulfur dioxide (SO<sub>2<\/sub>), hydrogen [pb_glossary id=\"1921\"]sulfide[\/pb_glossary] (H<sub>2<\/sub>S), and hydrogen [pb_glossary id=\"1920\"]halides[\/pb_glossary] (HF, HCl, or HBr).\n\nCarbon dioxide tends to sink and accumulate in depressions and basins. In [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] areas known to emit carbon dioxide, low-lying areas may [pb_glossary id=\"3342\"]trap[\/pb_glossary] hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley [pb_glossary id=\"1188\"]Caldera[\/pb_glossary], is one such area of carbon dioxide-producing [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole\u00a0<span style=\"font-weight: 400\">(<\/span><a href=\"http:\/\/volcanoes.usgs.gov\/Imgs\/Jpg\/Unzen\/MayuyamaSlide_caption.html\" target=\"_blank\" rel=\"noopener\"><span style=\"font-weight: 400\">info<\/span><\/a><span style=\"font-weight: 400\">)<\/span>.\n\nIn rare cases, [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] may create a sudden emission of gases without warning. Limnic eruptions (<em>limne<\/em> is Greek for lake), occur in crater lakes associated with active [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. The water in these lakes is supercharged with high concentrations of [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] gases. If the water is physically jolted by a [pb_glossary id=\"1199\"]landslide[\/pb_glossary] or earthquake, it may [pb_glossary id=\"3117\"]trigger[\/pb_glossary] an immediate and [pb_glossary id=\"1933\"]massive[\/pb_glossary] release of gases out of [pb_glossary id=\"2705\"]solution[\/pb_glossary]. An analogous example would be what happens to vigorously shaken bottle of carbonated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2,000 people were killed by a [pb_glossary id=\"1933\"]massive[\/pb_glossary] release of carbon dioxide.\n<h4><span style=\"font-weight: 400\">Lahars<\/span><\/h4>\n[caption id=\"attachment_315\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSHlahar.jpg\"><img class=\"size-medium wp-image-323\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-300x200.jpg\" alt=\"The mud line is far up on the trees\" width=\"300\" height=\"200\"><\/a> Mud line shows the extent of lahars around Mount St. Helens.[\/caption]\n\n<strong>[pb_glossary id=\"1201\"]Lahar[\/pb_glossary]<\/strong> is an Indonesian word and is used to describe a [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] mudflow that forms from rapidly melting snow or [pb_glossary id=\"2464\"]glaciers[\/pb_glossary]. [pb_glossary id=\"1201\"]Lahars[\/pb_glossary] are slurries resembling wet concrete, and consist of water, [pb_glossary id=\"1949\"]ash[\/pb_glossary], rock fragments, and other debris. These mudflows flow down the flanks of [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] or mountains covered with freshly-erupted [pb_glossary id=\"1949\"]ash[\/pb_glossary] and on steep slopes can reach speeds of up to 80 kph (50 mph).\n\n[caption id=\"attachment_315\" align=\"alignright\" width=\"235\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/LaharsRaineer.jpg\"><img class=\"size-medium wp-image-324\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-235x300.jpg\" alt=\"The cities are on top of old lahar deposits\" width=\"235\" height=\"300\"><\/a> Old lahars around Tacoma, Washington.[\/caption]\n\n<span style=\"font-weight: 400\">Several major cities, including Tacoma, are located on prehistoric [pb_glossary id=\"1201\"]lahar[\/pb_glossary] flows that extend for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). Mount Baker in Washington has a similar hazard for [pb_glossary id=\"1201\"]lahar[\/pb_glossary] flows. A tragic scenario played out recently, in 1985, when a [pb_glossary id=\"1201\"]lahar[\/pb_glossary] from the Nevado del Ruiz [pb_glossary id=\"1181\"]volcano[\/pb_glossary] in Colombia buried the town of Armero and killed an estimated 23,000 people.<\/span>\n<h4><span style=\"font-weight: 400\">Monitoring<\/span><\/h4>\nGeologists use various instruments to detect changes or indications that an eruption is imminent. The three videos show different types of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] monitoring used to predict eruptions 1) earthquake activity; 2) increases in gas emission; and 3) changes in land surface orientation and elevation.\n\nOne video shows how monitoring earthquake frequency, especially special vibrational earthquakes called harmonic tremors, can detect [pb_glossary id=\"2672\"]magma[\/pb_glossary] movement and possible eruption. Another video shows how gas monitoring may be used to predict an eruption. A rapid increase of gas emission may indicate [pb_glossary id=\"2672\"]magma[\/pb_glossary] that is actively rising to surface and releasing [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] gases out of [pb_glossary id=\"2705\"]solution[\/pb_glossary], and that an eruption is imminent. The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the [pb_glossary id=\"2672\"]magma[\/pb_glossary] is moving underneath it.\n\nhttps:\/\/youtu.be\/nlo-2JoNHrw\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-325\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\nhttps:\/\/youtu.be\/owk4fWbw4qM\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-326\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n&nbsp;\n\nhttps:\/\/youtu.be\/sNYQkxxd_0Q\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-327\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this YouTube video via this QR Code.[\/caption]\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"29\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.5-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-328\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the quiz for section 4.5 via this QR Code.[\/caption]\n<h2>Summary<\/h2>\n[pb_glossary id=\"2675\"]Igneous rock[\/pb_glossary] is divided into two major groups: [pb_glossary id=\"1939\"]intrusive[\/pb_glossary] rock that solidifies from underground [pb_glossary id=\"2672\"]magma[\/pb_glossary], and [pb_glossary id=\"1938\"]extrusive[\/pb_glossary] rock formed from [pb_glossary id=\"2673\"]lava[\/pb_glossary] that erupts and cools on the surface. [pb_glossary id=\"2672\"]Magma[\/pb_glossary] is generated from [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material at several [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonics[\/pb_glossary] situations by three types of melting: [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary], [pb_glossary id=\"2608\"]flux melting[\/pb_glossary], or heat-induced melting. [pb_glossary id=\"2672\"]Magma[\/pb_glossary] [pb_glossary id=\"2831\"]composition[\/pb_glossary] is determined by differences in the melting temperatures of the [pb_glossary id=\"2687\"]mineral[\/pb_glossary] components ([pb_glossary id=\"1174\"]Bowen\u2019s Reaction Series[\/pb_glossary]). The processes affecting [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"2831\"]composition[\/pb_glossary] include [pb_glossary id=\"1177\"]partial melting[\/pb_glossary], [pb_glossary id=\"1179\"]magmatic differentiation[\/pb_glossary], [pb_glossary id=\"1178\"]assimilation[\/pb_glossary], and [pb_glossary id=\"2620\"]collision[\/pb_glossary]. [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary] come in a wide variety of shapes and sizes, and are classified by a multiple factors, including [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"2831\"]composition[\/pb_glossary], and [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] activity. Because [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] presents serious hazards to human civilization, geologists carefully monitor [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activity to mitigate or avoid the dangers it presents.\n<h3>Take this quiz to check your comprehension of this Chapter.<\/h3>\n[h5p id=\"30\"]\n\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ch.4-Review-QR-Code.png\"><img class=\"size-thumbnail wp-image-329\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access the review quiz for Chapter 4 via this QR Code.[\/caption]\n<h2><strong>References<\/strong><\/h2>\n<div class=\"csl-bib-body\">\n<ol>\n \t<li class=\"csl-entry\">Arndt, N.T., 1994, Chapter 1 [pb_glossary id=\"2205\"]Archean[\/pb_glossary] Komatiites, <i>in<\/i> K.C. Condie, editor, Developments in [pb_glossary id=\"2218\"]Precambrian[\/pb_glossary] Geology: Elsevier, p. 11\u201344.<\/li>\n \t<li class=\"csl-entry\">Bateman, P.C., and Chappell, B.W., 1979, [pb_glossary id=\"2674\"]Crystallization[\/pb_glossary], [pb_glossary id=\"1180\"]fractionation[\/pb_glossary], and solidification of the Tuolumne [pb_glossary id=\"1939\"]Intrusive[\/pb_glossary] Series, Yosemite National Park, California: Geological Society of America Bulletin, v. 90, no. 5, p. 465\u2013482., doi: &lt;a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1979)902.0.CO;2\"&gt;10.1130\/0016-7606(1979)90&lt;465:CFASOT&gt;2.0.CO;2.<\/li>\n \t<li class=\"csl-entry\">Bell, K., and Keller, J., 2012, [pb_glossary id=\"3365\"]Carbonatite[\/pb_glossary] [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]: Oldoinyo Lengai and the petrogenesis of natrocarbonatites: Springer Science &amp; Business Media.<\/li>\n \t<li class=\"csl-entry\">Boehler, R., 1996, Melting temperatures of the Earth\u2019s [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and [pb_glossary id=\"2589\"]core[\/pb_glossary]: Earth\u2019s thermal structure: Annual Review of Earth and Planetary Sciences, v. 24, no. 1, p. 15\u201340., doi: <a href=\"https:\/\/doi.org\/10.1146\/annurev.earth.24.1.15\">10.1146\/annurev.earth.24.1.15<\/a>.<\/li>\n \t<li class=\"csl-entry\">Bowen, N.L., 1922, The Reaction Principle in Petrogenesis: J. Geol., v. 30, no. 3, p. 177\u2013198.<\/li>\n \t<li class=\"csl-entry\">Bowen, N.L., 1928, The evolution of the [pb_glossary id=\"2675\"]igneous[\/pb_glossary] rocks: Dover Publications, 334 p.<\/li>\n \t<li class=\"csl-entry\">Carr, M.H., 1975, Geologic map of the Tharsis Quadrangle of Mars: IMAP.<\/li>\n \t<li class=\"csl-entry\">Earle, S., 2015, Physical geology OER textbook: BC Campus OpenEd.<\/li>\n \t<li class=\"csl-entry\">EarthScope, 2014, Mount Ontake [pb_glossary id=\"1181\"]Volcanic[\/pb_glossary] Eruption: Online, <a href=\"http:\/\/www.earthscope.org\/science\/geo-events\/mount-ontake-volcanic-eruption\">http:\/\/www.earthscope.org\/science\/geo-events\/mount-ontake-volcanic-eruption<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">Frankel, C., 2005, Worlds on Fire: [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary] on the Earth, the Moon, Mars, Venus and Io: Cambridge University Press, 396 p.<\/li>\n \t<li class=\"csl-entry\">Glazner, A.F., Bartley, J.M., Coleman, D.S., Gray, W., and Taylor, R.Z., 2004, Are [pb_glossary id=\"1965\"]plutons[\/pb_glossary] assembled over millions of years by amalgamation from small [pb_glossary id=\"2672\"]magma[\/pb_glossary] chambers? GSA Today, v. 14, no. 4, p. 4., doi: &lt;a href=\"https:\/\/doi.org\/10.1130\/1052-5173(2004)0142.0.CO;2\"&gt;10.1130\/1052-5173(2004)014&lt;0004:APAOMO&gt;2.0.CO;2.<\/li>\n \t<li class=\"csl-entry\">Luongo, G., Perrotta, A., Scarpati, C., De Carolis, E., Patricelli, G., and Ciarallo, A., 2003, Impact of the AD 79 explosive eruption on Pompeii, II. Causes of death of the inhabitants inferred by [pb_glossary id=\"2859\"]stratigraphic[\/pb_glossary] analysis and areal distribution of the human casualties: J. Volcanol. Geotherm. Res., v. 126, no. 3\u20134, p. 169\u2013200.<\/li>\n \t<li class=\"csl-entry\">Mueller, S., and Phillips, R.J., 1991, On the initiation of [pb_glossary id=\"2602\"]subduction[\/pb_glossary]: J. Geophys. Res. [Solid Earth], v. 96, no. B1, p. 651\u2013665.<\/li>\n \t<li class=\"csl-entry\">Peacock, M.A., 1931, Classification of [pb_glossary id=\"2675\"]Igneous Rock[\/pb_glossary] Series: The Journal of Geology, v. 39, no. 1, p. 54\u201367.<\/li>\n \t<li class=\"csl-entry\">Perkins, S., 2011, 2010\u2019s [pb_glossary id=\"1181\"]Volcano[\/pb_glossary]-Induced Air Travel Shutdown Was Justified: Online, <a href=\"http:\/\/www.sciencemag.org\/news\/2011\/04\/2010s-volcano-induced-air-travel-shutdown-was-justified\">http:\/\/www.sciencemag.org\/news\/2011\/04\/2010s-volcano-induced-air-travel-shutdown-was-justified<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">Peterson, D.W., and Tilling, R.I., 1980, Transition of basaltic [pb_glossary id=\"2673\"]lava[\/pb_glossary] from [pb_glossary id=\"1192\"]pahoehoe[\/pb_glossary] to [pb_glossary id=\"1191\"]aa[\/pb_glossary], Kilauea [pb_glossary id=\"1181\"]Volcano[\/pb_glossary], Hawaii: Field observations and key factors - ScienceDirect: J. Volcanol. Geotherm. Res., v. 7, no. 3\u20134, p. 271\u2013293.<\/li>\n \t<li class=\"csl-entry\">Petrini and Podladchikov, 2000, Lithospheric pressure\u2013depth relationship in [pb_glossary id=\"1446\"]compressive[\/pb_glossary] regions of thickened [pb_glossary id=\"2580\"]crust[\/pb_glossary]: Journal of [pb_glossary id=\"2914\"]Metamorphic[\/pb_glossary] Geology, v. 18, no. 1, p. 67\u201377., doi: <a href=\"https:\/\/doi.org\/10.1046\/j.1525-1314.2000.00240.x\">10.1046\/j.1525-1314.2000.00240.x<\/a>.<\/li>\n \t<li class=\"csl-entry\">Reid, J.B., Evans, O.C., and Fates, D.G., 1983, [pb_glossary id=\"2672\"]Magma[\/pb_glossary] mixing in granitic rocks of the central Sierra Nevada, California: Earth and Planetary Science Letters, v. 66, p. 243\u2013261., doi: <a href=\"https:\/\/doi.org\/10.1016\/0012-821X(83)90139-5\">10.1016\/0012-821X(83)90139-5<\/a>.<\/li>\n \t<li class=\"csl-entry\">Rhodes, J.M., and Lockwood, J.P., 1995, Mauna Loa Revealed: Structure, [pb_glossary id=\"2831\"]Composition[\/pb_glossary], History, and Hazards: Washington DC American Geophysical Union Geophysical Monograph Series, v. 92.<\/li>\n \t<li class=\"csl-entry\">Scandone, R., Giacomelli, L., and Gasparini, P., 1993, Mount Vesuvius: 2000 years of volcanological observations: Journal of Volcanology and Geothermal Research, v. 58, p. 5\u201325.<\/li>\n \t<li class=\"csl-entry\">Stovall, W.K., Wilkins, A.M., Mandeville, C.W., and Driedger, C.L., 2016, Fact Sheet.:<\/li>\n \t<li class=\"csl-entry\">Thorarinsson, S., 1969, The Lakagigar eruption of 1783: Bull. Volcanol., v. 33, no. 3, p. 910\u2013929.<\/li>\n \t<li class=\"csl-entry\">Tilling, R.I., 2008, The critical role of [pb_glossary id=\"1181\"]volcano[\/pb_glossary] monitoring in risk reduction: Adv. Geosci., v. 14, p. 3\u201311.<\/li>\n \t<li class=\"csl-entry\">United States Geological Survey, 1999, Exploring the deep [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]: Online, <a href=\"http:\/\/pubs.usgs.gov\/gip\/dynamic\/exploring.html\">http:\/\/pubs.usgs.gov\/gip\/dynamic\/exploring.html<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">United States Geological Survey, 2012, Black Rock Desert [pb_glossary id=\"1181\"]Volcanic[\/pb_glossary] Field: Online, <a href=\"http:\/\/volcanoes.usgs.gov\/volcanoes\/black_rock_desert\/\">http:\/\/volcanoes.usgs.gov\/volcanoes\/black_rock_desert\/<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">USGS, 2001, Dual [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] tragedies in the Caribbean led to founding of HVO: Online, <a href=\"http:\/\/hvo.wr.usgs.gov\/volcanowatch\/archive\/2001\/01_05_03.html\">http:\/\/hvo.wr.usgs.gov\/volcanowatch\/archive\/2001\/01_05_03.html<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">USGS, 2011, [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary]: Principal Types of [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary]: Online, <a href=\"http:\/\/pubs.usgs.gov\/gip\/volc\/types.html\">http:\/\/pubs.usgs.gov\/gip\/volc\/types.html<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">USGS, 2012a, USGS: [pb_glossary id=\"1181\"]Volcano[\/pb_glossary] Hazards Program: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/vhp\/hazards.html\">https:\/\/volcanoes.usgs.gov\/vhp\/hazards.html<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">USGS, 2012b, Yellowstone [pb_glossary id=\"1181\"]Volcano[\/pb_glossary] Observatory: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/volcanoes\/yellowstone\/yellowstone_geo_hist_52.html\">https:\/\/volcanoes.usgs.gov\/volcanoes\/yellowstone\/yellowstone_geo_hist_52.html<\/a>, accessed July 2016.<\/li>\n \t<li class=\"csl-entry\">USGS, 2016, [pb_glossary id=\"1181\"]Volcanoes[\/pb_glossary] General - What are the different types of volcanoes? Online, <a href=\"https:\/\/www2.usgs.gov\/faq\/categories\/9819\/2730\">https:\/\/www2.usgs.gov\/faq\/categories\/9819\/2730<\/a>, accessed March 2017.<\/li>\n \t<li class=\"csl-entry\">USGS, 2017, The Volcanoes of Lewis and Clark - Mount St. Helens: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/observatories\/cvo\/Historical\/LewisClark\/Info\/summary_mount_st_helens.shtml\">https:\/\/volcanoes.usgs.gov\/observatories\/cvo\/Historical\/LewisClark\/Info\/summary_mount_st_helens.shtml<\/a>, accessed March 2017.<\/li>\n \t<li class=\"csl-entry\">Wallace, P.J., 2005, [pb_glossary id=\"2606\"]Volatiles[\/pb_glossary] in [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone [pb_glossary id=\"2672\"]magmas[\/pb_glossary]: concentrations and fluxes based on melt [pb_glossary id=\"2958\"]inclusion[\/pb_glossary] and [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] gas data: Journal of Volcanology and Geothermal Research, v. 140, no. 1\u20133, p. 217\u2013240., doi: <a href=\"https:\/\/doi.org\/10.1016\/j.jvolgeores.2004.07.023\">10.1016\/j.jvolgeores.2004.07.023<\/a>.<\/li>\n \t<li class=\"csl-entry\">Williams, H., 1942, The Geology of Crater Lake National Park, Oregon: With a Reconnaissance of the Cascade Range Southward to Mount Shasta: Carnegie institution.<\/li>\n<\/ol>\n<\/div>","rendered":"<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 2031px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/IT-PompeiiVesuvius.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-235 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius.jpg\" alt=\"The town in Italy is a ruin.\" width=\"2031\" height=\"1355\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius.jpg 2031w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-1024x683.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-768x512.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-1536x1025.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/12\/IT-PompeiiVesuvius-350x234.jpg 350w\" sizes=\"auto, (max-width: 2031px) 100vw, 2031px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mount Vesuvius towers over the ruins of Pompeii, a city destroyed by the eruption in 79 CE.<\/figcaption><\/figure>\n<h1>4 Igneous Processes and Volcanoes<\/h1>\n<p><b>KEY CONCEPTS<\/b><\/p>\n<p><strong>By the end of this chapter, students should be able to:<\/strong><\/p>\n<ul>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain the origin of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> it relates to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">plate tectonics<\/a><\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe how the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction Series<\/a>\u00a0relates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a> and melting temperatures<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain how cooling of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> leads to rock compositions and textures, and how these are used to classify <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Analyze the features of common <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> landforms and how they relate to their origin<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Explain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">fractionation<\/a>, and how they change <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> compositions<\/span><\/li>\n<li>Describe how silica content affects <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> and eruptive style of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> types, eruptive styles, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>, and their <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> settings<\/span><\/li>\n<li style=\"font-weight: 400\"><span style=\"font-weight: 400\">Describe <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> hazards<\/span><\/li>\n<\/ul>\n<p><span style=\"font-weight: 400\"><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous<\/a>\u00a0rock<\/strong> is\u00a0formed when liquid rock freezes into a solid rock. This molten material is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a><\/strong>\u00a0when it is in the ground and\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a><\/strong>\u00a0when it is on the surface. <\/span><span style=\"font-weight: 400\">Only the Earth&#8217;s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2595\">outer core<\/a> is liquid; the Earth&#8217;s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> is naturally solid. However, there are<\/span><span style=\"font-weight: 400\">\u00a0a few minor pockets of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that form near the surface where geologic processes cause melting. It is this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that becomes the source for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rocks<\/a>. This chapter will describe the classification of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks, the unique processes that form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a>, types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> processes, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> hazards, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a>\u00a0landforms.\u00a0<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-236\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-300x200.jpg\" alt=\"Pahoehoe lava flow in Hawaii\" width=\"300\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-1024x684.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-768x513.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-1536x1026.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii-350x234.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.1_Pahoehoe_and_Aa_flows_at_Hawaii.jpg 1920w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Lava flow in Hawaii<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> cools quickly on the surface of the earth and forms tiny microscopic crystals. These are known as fine-grained\u00a0<b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a><\/b>, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">Extrusive<\/a> rocks are often <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1945\">vesicular<\/a><\/strong>, filled with holes from escaping gas bubbles. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanism<\/a><\/strong> is the process in which <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> is erupted. Depending on the properties of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> that is erupted, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> can be drastically different, from smooth and gentle to dangerous and explosive. This leads to different types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> and different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> hazards.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Half-Dome-a-granitic-batholith-in-Yosemite.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-237\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-300x257.jpg\" alt=\"An intrusive igneous mass now exposed at the surface by erosion\" width=\"300\" height=\"257\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-300x257.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-768x657.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-65x56.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-225x193.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite-350x300.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Half-Dome-a-granitic-batholith-in-Yosemite.jpg 784w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Half Dome, an intrusive igneous batholith in Yosemite National Park<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">In contrast, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that cools slowly below the earth\u2019s surface forms larger crystals which can be seen with the naked eye. These are known as coarse-grained\u00a0<b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a><\/b>, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">plutonic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks. This relationship between cooling rates and grain sizes of the solidified <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks is important for interpreting the rock\u2019s geologic history.<\/span><\/p>\n<h2>\u00a0<span style=\"font-weight: 400\">4.1 Classification of Igneous Rocks<\/span><\/h2>\n<p><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous<\/a> rocks are classified based on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">Texture<\/a> describes the physical characteristics of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, such as grain size. This relates to the cooling history of the molten <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> from which it came. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a> refers to the rock&#8217;s specific mineralogy and chemical <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>. Cooling history is also related to changes that can occur to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks.<\/span><\/p>\n<h3><b>4.1.1 Texture<\/b><\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Fig-6.3-Granite-vs-Gneiss-e1495050932921.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-238\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-300x271.jpg\" alt=\"Image showing three or four distinct colors of clearly visible minerals.\" width=\"300\" height=\"271\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-300x271.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-65x59.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-225x203.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921-350x316.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig-6.3-Granite-vs-Gneiss-e1495050932921.jpg 552w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Granite is a classic coarse-grained (phaneritic) intrusive igneous rock. The different colors are unique minerals. The black colors are likely two or three different minerals.<\/figcaption><\/figure>\n<p>If <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> cools slowly, deep within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>, the resulting rock is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">plutonic<\/a>. The slow cooling process allows crystals to grow large, giving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> a coarse-grained or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1940\">phaneritic<\/a><\/strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a>. The individual crystals in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1940\">phaneritic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> are readily visible to the unaided eye.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 284px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/IMG_2623-e1495051966958-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-2840\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/IMG_2623-e1495051966958-scaled-1.jpg\" alt=\"Show dark rock with no visible minerals except for a few tiny green minerals that are olivine.\" width=\"284\" height=\"300\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Basalt is a classic fine-grained extrusive igneous rock. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine.<\/figcaption><\/figure>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">Extrusive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks have a fine-grained or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1941\">aphanitic<\/a><\/strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a>, in which the grains are too small to see with the unaided eye. The fine-grained <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> indicates the quickly cooling <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> did not have time to grow large crystals. These tiny crystals can be viewed under a petrographic microscope. In some cases, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> cools so rapidly it does not develop crystals at all. This non-crystalline material is not classified as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, but as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> glass<span style=\"font-weight: 400\">. This is a common component of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and rocks like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1947\">obsidian<\/a>.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.5_Porphyritic_texture.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-240\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-300x223.jpg\" alt=\"Porphyritic teture with large crystals in a finer grained groundmass\" width=\"300\" height=\"223\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-300x223.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-65x48.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-225x168.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture-350x261.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.5_Porphyritic_texture.jpg 419w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Porphyritic texture<\/figcaption><\/figure>\n<p>Some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks have a mix of coarse-grained <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> surrounded by a matrix of fine-grained material in a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1942\">porphyritic<\/a><\/strong>. The large crystals are called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1943\">phenocrysts<\/a><\/strong> and the fine-grained matrix is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1959\">groundmass<\/a><\/strong> or <strong>matrix<\/strong>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1942\">Porphyritic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> indicates the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.6_We-pegmatite.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-241\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-300x224.jpg\" alt=\"Pegmatic texture with large grains of minerals, mostly of felsic composition\" width=\"300\" height=\"224\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-300x224.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-768x574.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-225x168.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite-350x261.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.6_We-pegmatite.jpg 849w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Pegmatitic texture<\/figcaption><\/figure>\n<p>Residual molten material expelled from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> intrusions may form veins or masses containing very large crystals of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a>, beryl, tourmaline, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">mica<\/a>. This <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a>, which indicates a very slow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a>, is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1944\">pegmatitic<\/a><\/strong>. A rock that chiefly consists of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1944\">pegmatitic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> is known as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1944\">pegmatite<\/a><\/strong>. To give an example of how large these crystals can get, transparent cleavage sheets of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1944\">pegmatitic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">muscovite<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">mica<\/a> were used as windows during the Middle Ages.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.7_Scoria_Macro_Digon3-e1495227072616.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-242\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-300x286.jpg\" alt=\"A lava rock full of bubbles called scoria\" width=\"300\" height=\"286\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-300x286.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-1024x976.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-768x732.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-65x62.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-225x214.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616-350x334.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.7_Scoria_Macro_Digon3-e1495227072616.jpg 1342w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Scoria<\/figcaption><\/figure>\n<p>All <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> contain gases <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> in solution called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a><\/strong>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rises to the surface, the drop in pressure causes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a> to come bubbling out of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2705\">solution<\/a>, like the fizz in an opened bottle of soda. The gas bubbles become trapped in the solidifying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> to create a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1945\">vesicular<\/a><\/strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a>, with the holes specifically called vesicles. The type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">volcanic rock<\/a> with common vesicles is called <strong>scoria<\/strong>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.9_Pumice_stone-e1495052465796.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-243\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-298x300.jpg\" alt=\"A pumice stone, a hardened froth of volcanic glass\" width=\"250\" height=\"251\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-298x300.jpg 298w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-150x150.jpg 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-768x772.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-65x65.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-225x226.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796-350x352.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.9_Pumice_stone-e1495052465796.jpg 784w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Pumice<\/figcaption><\/figure>\n<p>An extreme version of scoria occurs when volatile-rich <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>\u00a0is very quickly quenched and becomes a meringue-like froth of glass called <b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1946\">pumice<\/a><\/b>. Some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1946\">pumice<\/a> is so full of vesicles that the density of the rock drops low enough that it will float.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 200px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.8_Lipari-Obsidienne_5.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-244\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-300x225.jpg\" alt=\"Photo of obsidian, a volcanic glass\" width=\"200\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.8_Lipari-Obsidienne_5.jpg 1920w\" sizes=\"auto, (max-width: 200px) 100vw, 200px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Obsidian (volcanic glass). Note conchoidal fracture.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> that cools extremely quickly may not form crystals at all, even microscopic ones. The resulting rock is called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> glass<\/strong>. <strong>O<\/strong><\/span><b>bsidian<\/b> is a rock consisting of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> glass.\u00a0<span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1947\">Obsidian<\/a> as a glassy rock shows an excellent example of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1935\">conchoidal<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1934\">fracture<\/a> similar to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/3-minerals\/\">Chapter 3<\/a>).<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.11_HoleInTheWallTuff.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-245\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-300x225.jpg\" alt=\"Tuff showing various size fragments of minerals and ash blown out of a volcano\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.11_HoleInTheWallTuff.jpg 1920w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Welded tuff<\/figcaption><\/figure>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> erupt explosively, vast amounts of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>, rock, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a>, and gases are thrown into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2667\">atmosphere<\/a>. The solid parts, called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a>, settle back to earth and cool into rocks with <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a><\/strong> textures. <em>Pyro,<\/em> meaning fire, refers to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> source of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> and <em><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2825\">clastic<\/a> <\/em>refers to the rock fragments. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">Tephra<\/a> fragments are named based on size\u2014<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a><\/strong> (&lt;2 mm), <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1950\">lapilli<\/a><\/strong> (2-64 mm), and <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1951\">bombs<\/a> or blocks<\/strong> (&gt;64 mm). <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">Pyroclastic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2919\">texture<\/a> is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. Rock formed from large deposits of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> fragments is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a><\/strong>. If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a>.<\/p>\n<h3><b>4.1.2 Composition<\/b><\/h3>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a> refers to a rock\u2019s chemical and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> make-up . For <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> is divided into four groups: <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a><\/strong>, <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a><\/strong>, <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a><\/strong>, and <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a><\/strong><em>.<\/em> These groups refer to differing amounts of silica, iron, and magnesium found in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> that make up the rocks. It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>.\u00a0<span style=\"font-weight: 400\">As an example, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a> is a commonly-used term, but has a very specific definition which includes exact quantities of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a>. Rocks labeled as &#8216;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a>&#8216; in laymen applications can be several other rocks, including\u00a0 syenite, tonalite, and monzonite.\u00a0<\/span>To avoid these complications, the following figure presents a simplified version of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> nomenclature focusing on the four main groups, which is adequate for an introductory student.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 1672px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-rock-composition-as-volume-percentages-of-minerals.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-246\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals.jpg\" alt=\"Diagram showing the mineral composition of the four classes of igneous rocks, ultramafic, mafic, intermediate, and felsic.\" width=\"1672\" height=\"1182\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals.jpg 1672w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-300x212.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-1024x724.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-768x543.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-1536x1086.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-65x46.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-225x159.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-rock-composition-as-volume-percentages-of-minerals-350x247.jpg 350w\" sizes=\"auto, (max-width: 1672px) 100vw, 1672px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mineral composition of common igneous rocks. Percentage of minerals is shown on the vertical axis. Percentage of silica is shown on the horizontal axis. Rock names at the top include a continuous spectrum of compositions grading from one into another.<\/figcaption><\/figure>\n<p><b>Fel<\/b><span style=\"font-weight: 400\">sic refers to a predominance of the light-colored (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>\u00a0<\/span><span style=\"text-decoration: underline\"><b>fel<\/b><\/span><span style=\"font-weight: 400\">dspar and <span style=\"text-decoration: underline\"><strong>si<\/strong><\/span>lica in the form of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a>. These light-colored <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> have more silica as a proportion of their overall chemical formula. Minor amounts of dark-colored (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a>) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2713\">amphibole<\/a> and biotite <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">mica<\/a> may be present as well. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">Felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks are rich in silica (in the 65-75% range, meaning the rock would be 65-75% weight percent SiO<sup>2<\/sup>) and poor in iron and magnesium.<\/span><\/p>\n<p><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">Intermediate<\/a><\/b> is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> between <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a>.\u00a0 It usually contains roughly-equal amounts of light and dark <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, including light grains of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a> and dark <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like amphibole.\u00a0 It is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> in silica in the 55-60% range.<\/p>\n<p><b>Maf<\/b><span style=\"font-weight: 400\">ic refers to a abundance of ferromagnesian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> (with magnesium and iron, chemical symbols <\/span><b>M<\/b>g<span style=\"font-weight: 400\">\u00a0and <\/span><b>F<\/b><span style=\"font-weight: 400\">e) plus <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a>. It is mostly made of dark <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2712\">pyroxene<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2711\">olivine<\/a>, which are rich in iron and magnesium and relatively poor in silica. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">Mafic<\/a> rocks are low in silica, in the 45-50% range.<\/span><\/p>\n<p><span style=\"font-weight: 400\"><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">Ultramafic<\/a><\/strong> refers to the extremely <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> rocks <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composed<\/a> of mostly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2711\">olivine<\/a> and some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2712\">pyroxene<\/a> which have even more magnesium and iron and even less silica. T<\/span><span style=\"font-weight: 400\">hese rocks are rare on the surface, but make up <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a>, the rock of the upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>. It is poor in silica, in the 40% or less range.<\/span><\/p>\n<p><span style=\"font-weight: 400\">On the figure above, the top row has both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">plutonic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks arranged in a continuous spectrum from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> on the left to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> toward the right.\u00a0<\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">Rhyolite<\/a><\/b><span style=\"font-weight: 400\">\u00a0thus refers to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks, and <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a><\/b><span style=\"font-weight: 400\"> thus refer to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks. <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1960\">Andesite<\/a><\/b><span style=\"font-weight: 400\"> and <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1963\">diorite<\/a><\/b><span style=\"font-weight: 400\">\u00a0likewise refer to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> rocks (with <\/span>dacite<span style=\"font-weight: 400\"> and <\/span>granodiorite<span style=\"font-weight: 400\"> applying to those rocks with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> between <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a>).\u00a0<\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1961\">Basalt<\/a><\/b><span style=\"font-weight: 400\"> and <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1964\">gabbro<\/a><\/b><span style=\"font-weight: 400\"> are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> names for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks, and <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a><\/b><span style=\"font-weight: 400\"> is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a>, with\u00a0<b>komatiite<\/b> as the fine-grained <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> equivalent. Komatiite is a rare rock because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> material that comes direct from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> is not common, although some examples can be found in ancient <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2205\">Archean<\/a> rocks<\/span><span style=\"font-weight: 400\">. Nature rarely has sharp boundaries and the classification and naming of rocks often imposes what appear to be sharp boundary names onto a continuous spectrum.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 2048px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-Classification-cropped-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-2848\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-Classification-cropped-scaled-1.jpg\" alt=\"Classification table of igneous rock.\" width=\"2048\" height=\"2560\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Igneous rock classification table with composition as vertical columns and texture as horizontal rows.<\/figcaption><\/figure>\n<h4><strong>Aphanitic\/Phaneritic Rock Types with images<\/strong><\/h4>\n<table style=\"height: 1296px\">\n<tbody>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;vertical-align: middle;height: 15px;width: 810.062px\" colspan=\"2\">\n<div class=\"mceTemp\"><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">Felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a><\/strong><\/div>\n<\/td>\n<\/tr>\n<tr style=\"height: 279px\">\n<td style=\"text-align: center;vertical-align: middle;height: 279px;width: 397.672px\">\n<figure id=\"attachment_2849\" aria-describedby=\"caption-attachment-2849\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/550px-Quartz_monzonite_36mw1037.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-248\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-300x262.jpg\" alt=\"Photograph of cut granite. showing a variety of visible minerals, including quartz and k-feldspar.\" width=\"300\" height=\"262\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-300x262.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-65x57.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-225x196.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037-350x305.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/550px-Quartz_monzonite_36mw1037.jpg 550w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2849\" class=\"wp-caption-text\">Granite from Cape Cod, Massachusetts.<\/figcaption><\/figure>\n<\/td>\n<td style=\"text-align: center;vertical-align: middle;height: 279px;width: 399.453px\">\n<figure id=\"attachment_2850\" aria-describedby=\"caption-attachment-2850\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/lossy-page1-640px-PinkRhyolite.tif_.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-249\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-300x214.jpg\" alt=\"Fine Crystalline rock with larger glassy fragments floating in the groundmass.\" width=\"300\" height=\"214\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-300x214.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-65x46.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-225x161.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_-350x250.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/lossy-page1-640px-PinkRhyolite.tif_.jpg 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2850\" class=\"wp-caption-text\">Rhyolite (source: Michael C. Rygel via Wikimedia Commons)<\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<tr style=\"height: 221px\">\n<td style=\"height: 221px;width: 397.672px\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">Granite<\/a> is a course-crystalline <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> rock. \u00a0The presence of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a> is a good indicator of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a>. \u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">Granite<\/a> commonly has large amounts of salmon pink potassium <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a> and white <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> crystals that have visible\u00a0cleavage planes. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">Granite<\/a> is a good approximation for the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental crust<\/a>, both in density and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>.<\/td>\n<td style=\"height: 221px;width: 399.453px\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">Rhyolite<\/a>\u00a0is a fine-crystalline <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> rock. \u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">Rhyolite<\/a> is commonly pink and will often have glassy <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1943\">phenocrysts<\/a>.\u00a0 Because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> lavas are less mobile, it is less common than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a>. Examples of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">rhyolite<\/a> include several <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows in Yellowstone National Park and the altered <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">rhyolite<\/a> that makes up the Grand Canyon of the Yellowstone.<\/td>\n<\/tr>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;height: 15px;width: 810.062px\" colspan=\"2\">\u00a0\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">Intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 257px\">\n<td style=\"height: 257px;width: 397.672px\">\n<figure id=\"attachment_2851\" aria-describedby=\"caption-attachment-2851\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Diorite_MA-e1496027879779.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-250\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-300x225.jpg\" alt=\"Rock with visible black and white crystals.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Diorite_MA-e1496027879779.jpg 808w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2851\" class=\"wp-caption-text\">Diorite<\/figcaption><\/figure>\n<\/td>\n<td style=\"height: 257px;width: 399.453px\">\n<figure id=\"attachment_2852\" aria-describedby=\"caption-attachment-2852\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Andesite2.tif_.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-251\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-300x240.jpg\" alt=\"Grey rock with fine crystals and black phenocrysts.\" width=\"300\" height=\"240\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-300x240.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-65x52.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-225x180.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_-350x280.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Andesite2.tif_.jpg 600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2852\" class=\"wp-caption-text\">Andesite<\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<tr style=\"height: 158px\">\n<td style=\"height: 158px;width: 397.672px\">\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1963\">Diorite<\/a> is a coarse-crystalline <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a>. Diorite is identifiable by it&#8217;s Dalmatian-like appearance of black hornblende and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">biotite<\/a>\u00a0and white <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">feldspar<\/a>. It is found in its namesake, the Andes Mountains as well as the Henry and Abajo mountains of Utah.<\/td>\n<td style=\"height: 158px;width: 399.453px\">\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1960\">Andesite<\/a> is a fine crystalline <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> rock. \u00a0It is commonly grey and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1942\">porphyritic<\/a>. \u00a0It can be found in the Andes Mountains and in some island arcs (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\">Chapter 2<\/a>). It is the fine grained compositional equivalent of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1963\">diorite<\/a>.<\/td>\n<\/tr>\n<tr style=\"height: 15px\">\n<td style=\"text-align: center;height: 15px;width: 810.062px\" colspan=\"2\"><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">Mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a><\/strong><\/td>\n<\/tr>\n<tr style=\"height: 242px\">\n<td style=\"height: 242px;width: 397.672px\">\n<figure id=\"attachment_2853\" aria-describedby=\"caption-attachment-2853\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/GabbroRockCreek1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-252\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-300x215.jpg\" alt=\"Dark rock with visible crystals.\" width=\"300\" height=\"215\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-300x215.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-1024x735.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-768x551.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-1536x1103.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-65x47.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-225x162.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1-350x251.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/GabbroRockCreek1.jpg 2000w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2853\" class=\"wp-caption-text\">Gabbro<\/figcaption><\/figure>\n<\/td>\n<td style=\"height: 242px;width: 399.453px\">\n<figure id=\"attachment_2854\" aria-describedby=\"caption-attachment-2854\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/VessicularBasalt1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-253\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-300x225.jpg\" alt=\"Dark grey rock with many visible holes and no visible crystals.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/VessicularBasalt1.jpg 704w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2854\" class=\"wp-caption-text\">Vesicular Basalt<\/figcaption><\/figure>\n<\/td>\n<\/tr>\n<tr style=\"height: 94px\">\n<td style=\"height: 94px;width: 397.672px\">\n<div class=\"mceTemp\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1964\">Gabbro<\/a> is a coarse-grained <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a>, made with mainly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2712\">pyroxene<\/a> and only minor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a>. Because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> is more mobile, it is less common than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1961\">basalt<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1964\">Gabbro<\/a> is a major component of the lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic crust<\/a>.<\/div>\n<\/td>\n<td style=\"height: 94px;width: 399.453px\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1961\">Basalt<\/a> is a fine-grained <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a>. It is commonly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1945\">vesicular<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1941\">aphanitic<\/a>. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1942\">porphyritic<\/a>, it often has either <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2711\">olivine<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1943\">phenocrysts<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1961\">Basalt<\/a> is the main rock which is formed at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a>, and is therefore the most common rock on the Earth&#8217;s surface, making up the entirety of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2885\">ocean floor<\/a> (except where covered by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2678\">sediment<\/a>).<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3><b>4.1.3 Igneous Rock Bodies<\/b><\/h3>\n<p><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous<\/a> rocks are common in the geologic record, but surprisingly, it is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> rocks that are more common. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">Extrusive<\/a> rocks, because of their small crystals and glass, are less durable. Plus, they are, by definition, exposed to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2700\">elements<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2677\">erosion<\/a> immediately. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">Intrusive<\/a> rocks, forming underground with larger, stronger crystals, are more likely to last. Therefore, most landforms and rock groups that owe their origin to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> bodies. A significant exception to this is active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>, which are discussed in a <a href=\"#4-5Volcanism\">later section on volcanism<\/a>. This section will <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3080\">focus<\/a> on the common <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> bodies which are found in many places within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">bedrock<\/a> of Earth.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-254\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-300x201.jpg\" alt=\"Igneous dike cuts across Baffin Island in the Canadian Arctic.\" width=\"300\" height=\"201\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-300x201.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-1024x687.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-768x515.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-65x44.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-225x151.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic-350x235.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olivine-gabbro-dike-cuts-across-Baffin-Island-in-the-Canadian-Arctic.jpg 1394w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Dike of olivine gabbro cuts across Baffin Island in the Canadian Arctic<\/figcaption><\/figure>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> intrudes into a weakness like a crack or fissure and solidifies, the resulting cross-cutting feature is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dike<\/a><\/strong>\u00a0(sometimes spelled <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dyke<\/a>)<span style=\"font-weight: 400\">. Because of this, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dikes<\/a> are often vertical or at an angle relative to the pre-existing rock layers that they intersect. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">Dikes<\/a> are therefore discordant intrusions, not following any layering that was present. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">Dikes<\/a> are important to geologists, not only for the study of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks themselves but also for dating rock sequences and interpreting the geologic history of an area. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dike<\/a> is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time (<a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/7-geologic-time\/\">Chapter 7<\/a>), may be used to assign actual numeric ages to sedimentary sequences, which are notoriously difficult to age date.\u00a0<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.14_Horton_Bluff_mid-Carboniferous_sill.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-255\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-300x225.jpg\" alt=\"Igneous sill intruding in between Paleozoic strata in Nova Scotia\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.14_Horton_Bluff_mid-Carboniferous_sill.jpg 1920w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Igneous sill intruding between Paleozoic strata in Nova Scotia<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1970\">Sills<\/a><\/strong> are another type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> structure. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1970\">sill<\/a> is a concordant intrusion that runs parallel to the sedimentary layers in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a>. They are formed when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> exploits a weakness between these layers, shouldering them apart and squeezing between them. As with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dikes<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1970\">sills<\/a> are younger than the surrounding layers and may be radioactively dated to study the age of sedimentary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2857\">strata<\/a>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-256\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-300x245.jpg\" alt=\"Exposure of Cottonwood Stock in Little Cottonwood Canyon, Utah\" width=\"300\" height=\"245\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-300x245.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-1024x837.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-768x627.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-65x53.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-225x184.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah-350x286.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Cottonwood-Stock-at-mouth-of-Little-Cottonwood-Canyon-Utah.jpg 1131w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Cottonwood Stock, a quartz monzonite pluton exposed at the mouth of Little Cottonwood Canyon, Utah<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a> is a large underground <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3341\">reservoir<\/a> of molten rock. The path of rising <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a><\/strong>. The processes by which a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a> intrudes into the surrounding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1924\">native<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a> are not well understood and are the subject of ongoing geological inquiry. For example, it is not known what happens to the pre-existing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a> as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a> intrudes. One <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2655\">theory<\/a> is the overriding rock gets shouldered aside, displaced by the increased volume of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. Another is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1924\">native<\/a> rock is melted and consumed into the rising <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> or broken into pieces that settle into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, a process known as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1967\">stoping<\/a><\/strong>. It has also been proposed that diapirs are not a real phenomenon, but just a series of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dikes<\/a> that blend into each other. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dikes<\/a> may be intruding over millions of years, but since they may be made of similar material, they would be appearing to be formed at the same time. Regardless, when a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a> cools, it forms an mass of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> rock called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">pluton<\/a><\/strong>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">Plutons<\/a> can have irregular shapes, but can often be somewhat round.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.2_Yosemite_Half-Dome.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-257\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-300x225.jpg\" alt=\"View showing an expansive area of a mountain range with exposed white granite in many places.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Yosemite_Half-Dome.jpg 1024w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Half-Dome in Yosemite National Park, California, is a part of the Sierra Nevada batholith which is mostly made of granite.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>When many <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">plutons<\/a> merge together in an extensive single feature, it is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">batholith<\/a><\/strong>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">Batholiths<\/a> are found in the cores of many mountain ranges, including the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1962\">granite<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formations<\/a> of Yosemite National Park in the Sierra Nevada of California. They are typically more than 100 km<sup>2<\/sup> in area, associated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zones, and mostly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>. A <strong>stock<\/strong> is a type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">pluton<\/a> with less surface exposure than a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">batholith<\/a>, and may represent a narrower neck of material emerging from the top of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">batholith<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">Batholiths<\/a> and stocks are discordant intrusions that cut across and through surrounding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a>.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Henry-Mountains-a-laccolith.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-258\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-300x198.jpg\" alt=\"Henry Mountains, Utah, interpreted to be a laccolith.\" width=\"250\" height=\"165\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-300x198.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-1024x676.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-768x507.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-225x149.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith-350x231.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Henry-Mountains-a-laccolith.jpg 1508w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">The Henry Mountains in Utah are interpreted to be a laccolith, exposed by erosion of the overlying layers.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.15_diagram_of_formation_of_laccolith.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-259\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-300x225.jpg\" alt=\"Laccolith forms as a blister in between sedimentary layers\" width=\"250\" height=\"188\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.15_diagram_of_formation_of_laccolith.jpg 767w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Laccolith forms as a blister in between sedimentary strata.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1172\">Laccoliths<\/a><\/strong> are blister-like, concordant intrusions of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that form between sedimentary layers. The Henry Mountains of Utah are a famous topographic landform formed by this process. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1172\">Laccoliths<\/a> bulge upwards; a similar downward-bulging intrusion is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1172\">lopolith<\/a><\/strong>.<\/p>\n<p>&nbsp;<\/p>\n<div id=\"h5p-23\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-23\" class=\"h5p-iframe\" data-content-id=\"23\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.2 Schematic diagram of plutonic and volcanic structures and processes (Source: Woudloper).\"><\/iframe><\/div>\n<\/div>\n<p><em>Click on the plus signs the illustration for descriptions of several <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\"><em>igneous<\/em><\/a> features.<\/em><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-260\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Schematic-diagram-of-plutonic-and-volcanic-structures-and-processes-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this interactive activity via this QR Code.<\/figcaption><\/figure>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-24\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-24\" class=\"h5p-iframe\" data-content-id=\"24\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.1 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.1-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-261\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.1-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 4.1 via this QR Code.<\/figcaption><\/figure>\n<h2><strong><a id=\"4-2BowensReaction\" href=\"\"><\/a>4.2 Bowen&#8217;s Reaction Series<\/strong><\/h2>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 696px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.4_Bowens_Reaction_Series.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-262\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series.png\" alt=\"Diagram of Bowen's Reaction Series, Y-shpaed with 8 minerals and a temperature scale\" width=\"696\" height=\"369\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series.png 696w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series-300x159.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series-65x34.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series-225x119.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.4_Bowens_Reaction_Series-350x186.png 350w\" sizes=\"auto, (max-width: 696px) 100vw, 696px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Bowen&#8217;s Reaction Series. Higher temperature minerals shown at top (olivine) and lower temperature minerals shown at bottom (quartz). (Source Colivine, modified from Bowen, 1922)<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Peridot2.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-209\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-272x300.jpg\" alt=\"The crystal is light green.\" width=\"150\" height=\"165\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-272x300.jpg 272w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-929x1024.jpg 929w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-768x847.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-65x72.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-225x248.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2-350x386.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Peridot2.jpg 1190w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Olivine, the first mineral to crystallize in a melt.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction Series<\/a><\/strong> describes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> at which <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> crystallize when cooled, or melt when heated. The low end of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> scale where all <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> crystallize into solid rock, is approximately 700\u00b0C (1292\u00b0F). The upper end of the range where all <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> exist in a molten state, is approximately 1,250\u00b0C (2,282\u00b0F). These numbers reference <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> that crystallize at standard sea-level pressure, 1 bar. The values will be different for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> located deep below the Earth\u2019s surface due to the increased pressure, which affects <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a> and melting temperatures (see <a href=\"#4-4PartialMelting\">Chapter 4.4<\/a>). However, the order and relationships are maintained.<\/p>\n<p>In the figure, the righthand column lists the four groups of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> from top to bottom: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>. The down-pointing arrow on the far right shows increasing amounts of silica, sodium, aluminum, and potassium as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> goes from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>. The up-pointing arrow shows increasing ferromagnesian components, specifically iron, magnesium, and calcium.\u00a0\u00a0 To the far left of the diagram is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> scale. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">Minerals<\/a> near the top of diagram, such as olivine and anorthite (a type of plagioclase), crystallize at higher temperatures. Minerals near the bottom, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1915\">quartz<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1914\">muscovite<\/a>, crystalize at lower temperatures.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 200px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/NormanLBowen_1909.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-263\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-200x300.jpg\" alt=\"Photo of Normal L. Bowen in 1909.\" width=\"200\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-200x300.jpg 200w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-65x98.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-225x338.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909-350x525.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/NormanLBowen_1909.jpg 400w\" sizes=\"auto, (max-width: 200px) 100vw, 200px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Normal L. Bowen<\/figcaption><\/figure>\n<p>The most important aspect of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen's Reaction Series<\/a> is to notice the relationships between <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a>. Norman L. Bowen (1887-1956) was an early 20th Century geologist who studied igneous rocks. He noticed that in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks, certain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> always occur together and these <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> assemblages exclude other <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. Curious as to why, and with the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2652\">hypothesis<\/a> in mind that it had to do with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> at which the rocks cooled, he set about conducting experiments on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks in the early 1900s.\u00a0He conducted experiments on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a>\u2014grinding combinations of rocks into powder, sealing the powders into metal capsules, heating them to various temperatures, and then cooling them.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 286px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/nlbowenexperimentingsm.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-264\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-286x300.jpg\" alt=\"Photo of Bowen working over his pertrographic microscope\" width=\"286\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-286x300.jpg 286w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-65x68.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-225x236.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm-350x368.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/nlbowenexperimentingsm.jpg 375w\" sizes=\"auto, (max-width: 286px) 100vw, 286px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Norman L. Bowen working with his petrographic microscope<\/figcaption><\/figure>\n<p>When he opened the quenched capsules, he found a glass surrounding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2192\">period<\/a> of several years, showed that the common <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> crystallize from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> at different temperatures. He also saw that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> occur together in rocks with others that crystallize within similar <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> ranges, and never crystallize with other <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. This relationship can explain the main difference between <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">Mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks contain more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, and therefore, crystallize at higher temperatures than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks. This is even seen in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows, with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> lavas erupting hundreds of degrees cooler than their <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> counterparts. Bowen\u2019s work laid the foundation for understanding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a><b> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1173\">petrology<\/a><\/b> (the study of rocks) and resulted in his book, <i>The Evolution of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous<\/a> Rocks<\/i> in 1928.<\/p>\n<h3><\/h3>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-25\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-25\" class=\"h5p-iframe\" data-content-id=\"25\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.2 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.2-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-265\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.2-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 4.2 via this QR Code.<\/figcaption><\/figure>\n<h2><a id=\"4-3MagmaGeneration\" href=\"\"><\/a>4.3 Magma Generation<\/h2>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> contain three components: melt, solids, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a>. The melt is made of ions from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> that have liquefied. The solids are made of crystallized <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> floating in the liquid melt. These may be <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> that have already cooled\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">Volatiles<\/a><\/strong> are gaseous components\u2014such as water vapor, carbon dioxide, sulfur, and chlorine\u2014<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. The presence and amount of these three components affect the physical behavior of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> and will be discussed more below.<\/p>\n<h3>4.3.1 Geothermal Gradient<\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.2_Temperature_schematic_of_inner_Earth.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-266\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-300x300.jpg\" alt=\"Diagram showing temperature increase with depth in the Earth\" width=\"300\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-300x300.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-150x150.jpg 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-65x65.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-225x224.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth-350x349.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.2_Temperature_schematic_of_inner_Earth.jpg 690w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Geothermal gradient<\/figcaption><\/figure>\n<p>Below the surface, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the Earth rises. This heat is caused by residual heat left from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formation<\/a> of Earth and ongoing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2966\">radioactive<\/a> decay. The rate at which <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> increases with depth is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a><\/strong>. The average <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> in the upper 100 km (62 mi) of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> is about 25\u00b0C per kilometer of depth. So for every kilometer of depth, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> increases by about 25\u00b0C.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 283px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Simple-P-T-Diagram-modified-from-Woudloper.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-267\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Simple-P-T-Diagram-modified-from-Woudloper.jpg\" alt=\"Diagram showing pressures and temperatures of the geothermal gradient increasing deeper in the earth. The solidus line shows that temperatures need to be much higher or pressure needs to be lower in order for rocks to start to melt.\" width=\"283\" height=\"256\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Simple-P-T-Diagram-modified-from-Woudloper.jpg 283w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Simple-P-T-Diagram-modified-from-Woudloper-65x59.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Simple-P-T-Diagram-modified-from-Woudloper-225x204.jpg 225w\" sizes=\"auto, (max-width: 283px) 100vw, 283px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Pressure-temperature diagram showing temperature in degrees Celsius on the x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and the green solidus line represents the temperature and pressure regime at which melting begins. Rocks at pressures and temperatures left of the green line are solid. If pressure\/temperature conditions change so that rocks pass to the right of the green line, then they will start to melt. (Source: Woudloper)<\/figcaption><\/figure>\n<p>The depth-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> graph (see figure) illustrates the relationship between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> (geotherm, red line) and the start of rock melting (solidus, green line). The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> changes with depth (which has a direct relationship to pressure) through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> into upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> with depth makes the depth of about 125 kilometers (78 miles) where the natural <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> is closest to the solidus.<\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> at 100 km (62 mi) deep is about 1,200\u00b0C (2,192\u00b0F). At bottom of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>, 35 km (22 mi) deep, the pressure is about 10,000 bars. A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> are solid. To a depth of 150 km (93 mi), the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> line stays to the left of the solidus line. This relationship continues through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2589\">core<\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> boundary, at 2,880 km (1,790 mi).<\/p>\n<p>The solidus line slopes to the right because the melting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of any substance depends on pressure. The higher pressure created at greater depth increases the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100\u00b0C. But if the pressure is lowered, as shown on the video below, water boils at a much lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Boiling-Water-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-268\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Boiling-Water-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"Boiling water at room temperature - science experiment\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/Ks4VuXTTKmo?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p>&nbsp;<\/p>\n<p>There are three principal ways rock behavior crosses to the right of the green solidus line to create molten <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>: 1) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a> caused by lowering the pressure, 2) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\">flux melting<\/a> caused by adding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a> (see more below), and 3) heat-induced melting caused by increasing the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction Series<\/a> shows that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> melt at different temperatures. Since <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is a mixture of different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, the solidus boundary is more of a fuzzy zone rather than a well-defined line; some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> are melted and some remain solid. This type of rock behavior is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a><\/strong> and represents real-world <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a>, which typically contain solid, liquid, and volatile components.<\/p>\n<p><span style=\"font-weight: 400\">The figure below uses P-T diagrams to show how melting can occur at three different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> settings.\u00a0 The green line is called the <strong>solidus<\/strong>, the melting point <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the rock at that pressure. Setting\u00a0<\/span>A is a situation (called &#8220;normal&#8221;) in the middle of a stable <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> in which no <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is generated. In the other three situations, rock at a lettered location with a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> is moved to a new P-T situation on the diagram. This shift is indicated by the arrow and its <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> relative to the solidus is shown by the red line. Partial melting occurs where the red line <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the rock crosses the green solidus on the diagram. Setting B is at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridge<\/a> <em>(<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\"><em>decompression melting<\/em><\/a>)<\/em> where reduction of pressure carries the rock at its <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> across the solidus. Setting C is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a> plus <em>addition of heat<\/em> carries the rock across the solidus, and setting D is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zone where a process called <em><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\"><em>flux melting<\/em><\/a><\/em> takes place where the solidus (melting point) is actually shifted to below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the rock.<\/p>\n<p>Graphs A-D below, along with the side view of the Earth\u2019s layers in various <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> settings (see figure), show how melting occurs in different situations. Graph A illustrates a normal situation, located in the middle of a stable <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a>, where no melted rock can be found. The remaining three graphs illustrate rock behavior relative to shifts in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> or solidus lines. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">Partial melting<\/a> occurs when the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> line crosses the solidus line. Graph B illustrates behavior of rock located at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridge<\/a>, labeled X in the graph and side view. Reduced pressure shifts the geotherm to the right of the solidus, causing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a>. Graph C and label Y illustrate a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> situation. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">Decompression melting<\/a>, plus an addition of heat, shifts the geotherm across the solidus. Graph D and label Z show a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zone, where an addition of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a> lowers the melting point, shifting the solidus to the left of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a>. B, C, and D all show different ways the Earth produces intersections of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> and the solidus, which results in melting each time.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_P-T-diagrams-in-mantle.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-large wp-image-269\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-1024x585.jpg\" alt=\"Pressure-Temperature diagrams showing temperture in the mantle plotted against pressure (depth)\" width=\"1024\" height=\"585\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-1024x585.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-300x171.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-768x439.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-65x37.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-225x129.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle-350x200.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_P-T-diagrams-in-mantle.jpg 1509w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Four P-T diagrams show temperature in degrees Celsius on x-axis and depth below the surface in kilometers (km) on the y-axis. The red line is the geothermal gradient and green solidus line represents at temperature and pressure regime at which melting begins. Each of the four P-T diagrams are associated a tectonic setting as shown by a side-view (cross-section) of the lithosphere and mantle.<\/figcaption><\/figure>\n<h3>4.3.2 Decompression Melting<\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 212px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ocean-birth.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-2867\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-1.png\" alt=\"The ocean starts as a valley and then gets wider and wider.\" width=\"212\" height=\"300\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Progression from rift to mid-ocean ridge, the divergent boundary types. Note the rising material in the center.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> is created at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a> via <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a><\/strong>. Strong <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2577\">convection<\/a> currents cause the solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2593\">asthenosphere<\/a> to slowly flow beneath the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a>. The upper part of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a> (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>) is a poor heat conductor, so the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> remains about the same throughout the underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material. Where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2577\">convection<\/a> currents cause <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material to rise, the pressure decreases, which causes the melting point to drop. In this situation, the rock at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> is rising toward the surface, thus hotter rock is now shallower, at a lower pressure, and the rock, still at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> at its old location, shifts past the its melting point (shown as the red line crossing over the solidus or green line in example B in previous figure) and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a> starts. As this magma continues to rise, it cools and crystallizes to form new lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>.<\/p>\n<h3>4.3.3 Flux Melting<\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 500px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Subduction-en.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-94\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-300x147.png\" alt=\"Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental\" width=\"500\" height=\"244\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-300x147.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-768x375.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-65x32.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-225x110.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_-350x171.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_.png 800w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Diagram of ocean-continent subduction. Note water vapor driven out of hydrated minerals in the descending oceanic slab.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\">Flux melting<\/a><\/strong> or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\">fluid-induced melting<\/a><\/strong> occurs in island arcs and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zones when volatile gases are added to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material (see figure: graph D, label Z). Flux-melted <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> produces many of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> in the circum-Pacific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zones, also known as the Ring of Fire. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subducting<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2605\">slab<\/a> contains <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a> and hydrated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. As covered in <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\">Chapter 2<\/a>, these hydrated forms are created when water ions <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2703\">bond<\/a> with the crystal structure of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2709\">silicate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2605\">slab<\/a> descends into the hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>, the increased <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> causes the hydrated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> to emit water vapor and other volatile gases, which are expelled from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2605\">slab<\/a> like water being squeezed out of a sponge. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolve<\/a> into the overlying asthenospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> and decrease its melting point. In this situation the applied pressure and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> have not changed, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>&#8216;s melting point has been lowered by the addition of volatile substances. The previous figure (graph D) shows the green solidus line shifting to the left of and below the red <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> line, and melting begins. This is analogous to adding salt to an icy roadway. The salt lowers the freezing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> of the solid ice so it turns into liquid water.<\/p>\n<h3>4.3.4 Heat-Induced Melting<\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/06.2-15-Mt-Blaca-Migmatite-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-48\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-300x225.jpg\" alt=\"Swirling bands of light and dark minerals.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/06.2-15-Mt-Blaca-Migmatite-1.jpg 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Migmatite, a rock which was partially molten. (Source: Peter Davis)<\/figcaption><\/figure>\n<p>Heat-induced melting, transforming solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> into liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> by simply applying heat, is the least common process for generating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> (see figure: graph C, label Y). Heat-induced melting occurs at a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> plumes or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspots<\/a>. The rock surrounding the plume is exposed to higher temperatures, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1175\">geothermal gradient<\/a> crosses to the right of the green solidus line, and the rock begins to melt. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2639\">mantle plume<\/a> includes rising <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material, meaning some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a> is occurring as well. A small amount of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is also generated by intense <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2946\">regional metamorphism<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/6-metamorphic-rocks\/\">Chapter 6<\/a>). This <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> becomes a hybrid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2914\">metamorphic<\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2933\">migmatite<\/a>.<\/p>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-26\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-26\" class=\"h5p-iframe\" data-content-id=\"26\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.3 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.3-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-271\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.3-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 4.3 via this QR Code.<\/figcaption><\/figure>\n<h2><b><a id=\"4-4PartialMelting\" href=\"\"><\/a><\/b><strong>4.4<\/strong>\u00a0<b>Partial Melting and Crystallization<\/b><\/h2>\n<p>Even though all <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> originate from similar <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rocks, and start out as similar <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, other things, like\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a> processes like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1179\">magmatic differentiation<\/a>, can change the chemistry of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. This explains the wide variety of resulting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks that are found all over Earth.<\/p>\n<h3>4.4.1 Partial Melting<\/h3>\n<p>Because the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composed<\/a> of many different minerals, it does not melt uniformly. As <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> with lower melting points turn into liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, those with higher melting points remain as solid crystals. This is known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a>. As <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> slowly rises and cools into solid rock, it undergoes physical and chemical changes in a process called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1179\">magmatic differentiation<\/a>.<\/p>\n<p>According to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction Series<\/a> (<a href=\"#4-2BowensReaction\">Section 4.2<\/a>), each <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> has a unique melting and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a>. Since most rocks are made of many different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>, when they start to melt, some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> begin melting sooner than others. This is known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a>, and creates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> with a different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> than the original <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material.<\/p>\n<p>The most important example occurs as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is generated from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rocks (as discussed in <a href=\"#4-3MagmaGeneration\">Section 4.3<\/a>). The chemistry of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rock (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a>) is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a>, low in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2709\">silicates<\/a> and high in iron and magnesium. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a> begins to melt, the silica-rich portions melt first due to their lower melting point. If this continues, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> becomes increasingly silica-rich, turning <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rises to the surface because it is more buoyant than the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 500px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/CratonGeolProv.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-92\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-300x159.jpg\" alt=\"The legend shows shields, platforms, orogens, basins, large igneous provinces, and extended crust.\" width=\"500\" height=\"265\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-300x159.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-1024x544.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-768x408.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-65x35.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-225x119.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv-350x186.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/CratonGeolProv.jpg 1200w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">Partial melting<\/a> also occurs as existing crustal rocks melt in the presence of heat from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a>. In this process, existing rocks melt, allowing the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> formed to be more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> and less <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> than the pre-existing rock. Early in the Earth\u2019s history when the continents were forming, silica-rich <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> formed and rose to the surface and solidified into granitic continents. In the figure, the old granitic cores of the continents, called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shields<\/a><\/strong>, are shown in orange.<\/p>\n<h3>4.4.2 Crystallization and Magmatic Differentiation<\/h3>\n<p>Liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is less dense than the surrounding solid rock, so it rises through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>. As <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> begins to cool and crystallize, a process known as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1179\">magmatic differentiation<\/a><\/strong> changes the chemistry of the resultant rock towards a more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>. This happens via two main methods: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1178\">assimilation<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">fractionation<\/a>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.16_Xenoliths_Little_Cottonwood_Canyon.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-272\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-300x211.jpg\" alt=\"Xenoliths are bits of surrounding counjtry rock incorporated in intrusive magma and solidified within it.\" width=\"300\" height=\"211\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-300x211.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-768x540.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-65x46.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-225x158.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon-350x246.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Xenoliths_Little_Cottonwood_Canyon.jpg 847w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Xenoliths in Little Cottonwood Stock, Utah<\/figcaption><\/figure>\n<p>During <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1178\">assimilation<\/a><\/strong>, pieces of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a> with a different, often more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> are added to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. These solid pieces may melt, which changes the composition of the original <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. At times, the solid fragments may remain intact within the cooling <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> and only partially melt. The unmelted country rocks within an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous rock<\/a> mass are called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2587\">xenoliths<\/a><\/strong>.<\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2587\">Xenoliths<\/a> are also common in the processes of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> mixing and rejuvenation, two other processes that can contribute to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1179\">magmatic differentiation<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> mixing occurs when two different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> come into contact and mix, though at times, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> can remain heterogeneous and create <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2587\">xenoliths<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1969\">dikes<\/a>, and other features. Magmatic rejuvenation happens when a cooled and crystallized body of rock is remelted and pieces of the original rock may remain as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2587\">xenoliths<\/a>.<\/p>\n<p>Much of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a> is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> (i.e. granitic), and normally more buoyant than the underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a>\/<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rises through thick <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental crust<\/a>, it does so slowly, more slowly than when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rises through <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plates<\/a>. This gives the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> lots of time to react with the surrounding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1971\">country rock<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> tends to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1178\">assimilate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> rock, becoming more silica-rich as it migrates through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a> and changing into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> by the time it reaches the surface. This is why <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> are much more common within continents.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Magmatism_and_volcanism_EN.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-273\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-300x235.png\" alt=\"Shows large pools of magma rising from the source in the mantle, up into the crust under a volcano.\" width=\"300\" height=\"235\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-300x235.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-65x51.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-225x176.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN-350x274.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Magmatism_and_volcanism_EN.png 766w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Rising magma diapirs in mantle and crust. Fractional crystallization assimilation occurs to the diapirs in the crust.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">Fractionation<\/a><\/strong> or <strong>fractional <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">crystallization<\/a><\/strong> is another process that increase <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> silica content, making it more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a> drops within a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a> rising through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>, some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> will crystallize and settle to the bottom of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma\u00a0chamber<\/a>, leaving the remaining melt depleted of those ions. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2711\">Olivine<\/a> is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> at the top of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction series<\/a> with a high melting point and a smaller percentage of silica verses other common <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> cools, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2711\">olivine<\/a> crystallizes first and settles to the bottom of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a> (see figure). This means the remaining melt becomes more silica-rich and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> further cools, the next <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen's Reaction Series<\/a> (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1916\">plagioclase<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2712\">pyroxene<\/a>) crystallize next, removing even more low-silica components from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, making it even more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a>. This crystal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">fractionation<\/a> can occur in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a>, but the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formation<\/a> of more differentiated, highly evolved <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a> is largely confined to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> regions where the longer time to the surface allows more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">fractionation<\/a> to occur.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 750px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.3_Fractional_crystallization.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-274\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization.png\" alt=\"Complicated diagram showing minerals settling out in the magma chamber and thus making the remaining liquid magma (the melt) more silica-rich in composition.\" width=\"750\" height=\"320\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization.png 750w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization-300x128.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization-65x28.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization-225x96.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.3_Fractional_crystallization-350x149.png 350w\" sizes=\"auto, (max-width: 750px) 100vw, 750px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Schematic diagram illustrating fractional crystallization. If magma at composition A is ultramafic, as the magma cools it changes composition as different minerals crystallize from the melt and settle to the bottom of the magma chamber. In section 1, olivine crystallizes; section 2: olivine and pyroxene crystallize; section 3: pyroxene and plagioclase crystallize; and section 4: plagioclase crystallizes. The crystals are separated from the melt and the remaining magma (composition B) is more silica-rich. (Source: Woudloper)<\/figcaption><\/figure>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-27\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-27\" class=\"h5p-iframe\" data-content-id=\"27\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.4 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.4-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-275\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.4-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 4.4 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-weight: 400\"><a id=\"4-5Volcanism\" href=\"\"><\/a>4.5 Volcanism<\/span><\/h2>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> emerges onto the Earth\u2019s surface, the molten rock is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>. A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a><\/strong> is a type of land <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formation<\/a> created when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> solidifies into rock. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a> have been an important part of human society for centuries, though their understanding has greatly increased as our understanding of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">plate tectonics<\/a> has made them less mysterious.\u00a0<span style=\"font-weight: 400\">This section\u00a0describes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> location, type, hazards, and monitoring.<\/span><\/p>\n<h3><b>4.5.1. Distribution and Tectonics<\/b><\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 775px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Main-types-of-plate-boundaries.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-276\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Main-types-of-plate-boundaries.gif\" alt=\"Diagram showing how volcanoes are associated with plate boundaries\" width=\"775\" height=\"429\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Association of volcanoes with plate boundaries. (Source: USGS)<\/figcaption><\/figure>\n<p>Most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1182\">interplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1182\">Interplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are located at active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> boundaries created by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zones, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">rifts<\/a>. The prefix &#8220;<em>inter-&#8220;<\/em> means between. Some volcanoes are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1183\">intraplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>. The prefix &#8220;<em>intra-&#8220;<\/em>\u00a0means within, and intraplate volcanoes are located within <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plates<\/a>, far removed from plate boundaries. Many <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1183\">intraplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are formed by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspots<\/a>.<\/p>\n<h4><span style=\"font-weight: 400\">Volcanoes at Mid-Ocean Ridges<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Spreading_ridges_volcanoes_map-en.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-277\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-300x151.png\" alt=\"Map showing spreading ridges throughout the world. These ridges are all over the world.\" width=\"300\" height=\"151\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-300x151.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-768x386.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-65x33.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-225x113.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_-350x176.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Spreading_ridges_volcanoes_map-en.svg_.png 1000w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Map of spreading ridges throughout the world.<\/figcaption><\/figure>\n<p>Most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> on Earth occurs on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2885\">ocean floor<\/a> along <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a>, a type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2599\">divergent<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2592\">plate boundary<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\" target=\"_blank\" rel=\"noopener\">Chapter 2<\/a>). These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1182\">interplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are also the least observed and famous, since most of them are located under 3,000-4,500 m (10,000-15,000 ft) of ocean and the eruptions are slow, gentle, and oozing. One exception is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1182\">interplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> of Iceland. The diverging and thinning <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">plates<\/a> allow hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rock to rise, releasing pressure and causing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">Ultramafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rock, consisting largely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a>, partially melts and generates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that is basaltic. Because of this, almost all <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2885\">ocean floor<\/a> are basaltic. In fact, most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2885\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a> is basaltic near the surface, with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1940\">phaneritic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1964\">gabbro<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1957\">ultramafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2588\">peridotite<\/a> underneath.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pillow-basalt.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-278\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-300x180.jpg\" alt=\"Pillow basalt on sea floor near Hawaii.\" width=\"300\" height=\"180\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-300x180.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-1024x613.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-768x460.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-1536x919.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-65x39.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-225x135.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt-350x209.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pillow-basalt.jpg 1669w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Pillow basalt on sea floor near Hawaii.<\/figcaption><\/figure>\n<p>When basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> erupts underwater it emerges in small explosions and\/or forms pillow-shaped structures called pillow basalts. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a>. This ecosystem exists around tall vents emitting black, hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a>-rich water called deep-sea <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2921\">hydrothermal<\/a> vents, also known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2922\">black smokers<\/a>.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 133px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/BlackSmoker.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-119\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-233x300.jpg\" alt=\"There is a large build up of minerals around the vent\" width=\"133\" height=\"171\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-233x300.jpg 233w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-795x1024.jpg 795w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-768x990.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-65x84.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-225x290.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker-350x451.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/BlackSmoker.jpg 929w\" sizes=\"auto, (max-width: 133px) 100vw, 133px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Black smoker hydrothermal vent with a colony of giant (6&#8217;+) tube worms.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 500px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/04.16_Distribution_of_hydrothermal_vent_fields.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-279\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-300x178.jpg\" alt=\"Map showing worldwide distgrbution of hydrothermal vent fields;\" width=\"500\" height=\"297\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-300x178.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-1024x608.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-768x456.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-65x39.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-225x134.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields-350x208.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/04.16_Distribution_of_hydrothermal_vent_fields.jpg 1198w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Distribution of hydrothermal vent fields<\/figcaption><\/figure>\n<p>Without sunlight to support photosynthesis, these organisms instead utilize a process called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1184\">chemosynthesis<\/a><\/strong>. Certain bacteria are able to turn hydrogen <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1921\">sulfide<\/a> (H<sub>2<\/sub>S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies. The three videos show some of the ecosystems found around deep-sea <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2921\">hydrothermal<\/a> vents.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-2\" title=\"The Alvin Submarine Part 1: Updating the Deep-Diving Submarine at 50 Years Old - WIRED\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/a5aQ4W9GbpU?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-1-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-280\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-1-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-3\" title=\"The Alvin Submarine Part 2: Incredible Views On-Board the Deep-Sea Vessel\u200b\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/dXOQFnU-49k?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-2-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-281\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-2-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-4\" title=\"The Alvin Submarine Part 3: Humans vs. Robots and the Future of Deep-Sea Research \u2013 WIRED\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/eUzz_ilsFa0?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Alvin-Submarine-Part-3-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-282\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alvin-Submarine-Part-3-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<h4><span style=\"font-weight: 400\">Volcanoes at Subduction Zones<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 500px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Map_plate_tectonics_world.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-283\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Map_plate_tectonics_world.gif\" alt=\"Map showing volcanoes follow the edges of tectonic plates.\" width=\"500\" height=\"314\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Distribution of volcanoes on the planet. Click here for an <a href=\"https:\/\/maps.ngdc.noaa.gov\/viewers\/hazards\/?layers=3\" target=\"_blank\" rel=\"noopener\">interactive map<\/a>\u00a0of volcano distributions.<\/figcaption><\/figure>\n<p>The second most commonly found location for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> is adjacent to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zones, a type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2600\">convergent<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2592\">plate boundary<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/\" target=\"_blank\" rel=\"noopener\">Chapter 2<\/a>). The process of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> expels water from hydrated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a> in the descending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2605\">slab<\/a>, which causes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\">flux melting<\/a> in the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> rock. Because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> occurs in a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2617\">volcanic arc<\/a>, the thickened <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> promotes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> differentiation. These evolve the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> from the mantle into more silica-rich <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a>-generated eruptions of mostly silica-rich <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>; the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">plutons<\/a> consist largely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a>-to-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> rock such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1960\">andesite<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">rhyolite<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1946\">pumice<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a>.<\/p>\n<h4><span style=\"font-weight: 400\">Volcanoes at Continental Rifts<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Black_Rock_Desert_volcanic_field.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-284\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-300x199.jpg\" alt=\"A barren landscape of lava flows in central Utah.\" width=\"300\" height=\"199\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-300x199.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-225x149.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field-350x232.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Black_Rock_Desert_volcanic_field.jpg 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Basaltic cinder cones of the Black Rock Desert near Beaver, Utah.<\/figcaption><\/figure>\n<p>Some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are created at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">rifts<\/a>, where crustal thinning is caused by diverging lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plates<\/a>, such as the East African <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">Rift<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1461\">Basin<\/a> in Africa. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanism<\/a> caused by crustal thinning without <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">rifting<\/a> is found in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2462\">Basin and Range<\/a> Province in North America. In this location, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> activity is produced by rising <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that stretches the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> (see figure). Lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> or upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material rises through the thinned <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>, releases pressure, and undergoes decompression-induced <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a>. This <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is less dense than the surrounding rock and continues to rise through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a> to the surface, erupting as basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>. These eruptions usually result in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1197\">flood basalts<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinder<\/a> cones, and basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows (see video). Relatively young <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinder<\/a> cones of basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> can be found in south-central Utah, in the Black Rock Desert <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanic<\/a> Field, which is part of the zone of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2462\">Basin and Range<\/a> crustal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1445\">extension<\/a>. These Utah <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinder<\/a> cones and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-5\" title=\"Basin &amp; Range Volcanic Processes  (Educational)\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/4VgMe-JXOAM?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcanic-Processes-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-285\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcanic-Processes-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<h4><span style=\"font-weight: 400\">Hotspots<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 193px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hotspotgeology-1.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-129\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-193x300.png\" alt=\"The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.\" width=\"193\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-193x300.png 193w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-660x1024.png 660w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-768x1192.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-65x101.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-225x349.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_-350x543.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hotspotgeology-1.svg_.png 771w\" sizes=\"auto, (max-width: 193px) 100vw, 193px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">Hotspots<\/a> are the main source of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1183\">intraplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a>. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">Hotspots<\/a><\/strong> occur when lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plates<\/a> glide over a hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2639\">mantle plume<\/a>, an ascending column of solid heated rock originating from deep within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2639\">mantle plume<\/a> generates melts as material rises, with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rising even more. When the ascending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/YellowstoneHotspot.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-133\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-300x206.jpg\" alt=\"The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.\" width=\"300\" height=\"206\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-300x206.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-1024x703.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-768x527.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-65x45.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-225x155.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot-350x240.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/YellowstoneHotspot.jpg 1063w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.<\/figcaption><\/figure>\n<p>Since most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> plumes are located beneath the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a>, the early stages of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1183\">intraplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> typically take place underwater. Over time, basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> may build up from the sea floor into islands, such as the Hawaiian Islands. Where a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> is found under a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a>, contact with the hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> magma may cause the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> rock to melt and mix with the mafic material below, forming <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>. Or the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> may continue to rise, and cool into a granitic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1968\">batholith<\/a> or erupt as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>. The Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> is an example of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> that resulted in an explosive eruption.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 296px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hawaii-Emperor_engl.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-131\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-296x300.png\" alt=\"There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.\" width=\"296\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-296x300.png 296w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-1010x1024.png 1010w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-768x778.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-65x66.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-225x228.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl-350x355.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii-Emperor_engl.png 1182w\" sizes=\"auto, (max-width: 296px) 100vw, 296px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">The Hawaii-Emperor seamount and island chain.<\/figcaption><\/figure>\n<p>A zone of actively erupting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> connected to a chain of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1708\">extinct<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> indicates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1183\">intraplate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> located over a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a>. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> chains are created by the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> slowly moving over a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2639\">mantle plume<\/a>. These chains are seen on the seafloor and continents and include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> that have been inactive for millions of years. The Hawaiian Islands on the Pacific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2581\">Oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> are the active end of a long <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> chain that extends from the northwest Pacific Ocean to the Emperor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2899\">Seamounts<\/a>, all the way to the to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zone beneath the Kamchatka Peninsula. The overriding North American <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> moved across a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2639\">mantle plume<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> for several million years, creating a chain of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> calderas that extends from Southwestern Idaho to the presently active Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> in Wyoming.<\/p>\n<p><span style=\"font-weight: 400\">Two three<\/span><span style=\"font-weight: 400\">-minute videos (below)<\/span><span style=\"font-weight: 400\"> illustrates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>.<\/span><\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-6\" title=\"What is a Volcanic Hotspot?    (Educational)\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/AhSaE0omw9o?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-286\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/What-is-a-Volcanic-Hotspot-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-7\" title=\"Life Of Hotspot Volcanic Island (Educational)\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/t5go-78gCJU?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-287\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Life-of-Hotspot-Volcanic-Island-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<h3><b>4.5.2 Volcano Features and Types<\/b><\/h3>\n<p><span style=\"font-weight: 400\">There are several different types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> based on their shape, eruption style, magmatic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>, and other aspects. <\/span><\/p>\n<p><span style=\"font-weight: 400\"><\/p>\n<div id=\"h5p-28\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-28\" class=\"h5p-iframe\" data-content-id=\"28\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.5 Overview of volcanic features and landforms.\"><\/iframe><\/div>\n<\/div>\n<p> <\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-288\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Overview-of-volcanic-features-and-landforms-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this interactive activity via this QR Code.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">The figure shows the main features of a typical <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">stratovolcano<\/a>:<\/span><span style=\"font-weight: 400\">\u00a01) <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a><\/strong>, 2) upper layers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2590\">lithosphere<\/a>, 3) the<\/span><span style=\"font-weight: 400\">\u00a0<\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1186\">conduit<\/a><\/b><span style=\"font-weight: 400\"> or narrow pipe through which the<\/span><span style=\"font-weight: 400\">\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> erupts, 4) the base or edge of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, 5) a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1970\">sill<\/a><\/strong> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> between layers of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, 6) a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1966\">diapir<\/a><\/strong> or feeder tube to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1970\">sill<\/a>, 7) layers of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a><\/strong> (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a>) from previous eruptions, 8 &amp; 9) layers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> erupting from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a> and flowing down the sides of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, 10) the <strong>crater<\/strong> at the top of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, 11) layers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> on (12), a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1189\">parasitic cone<\/a>. A\u00a0<b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1189\">parasitic cone<\/a><\/b> is a small <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1189\">parasitic cone<\/a> because it has its own separate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a>,\u00a0 13) the <strong>vents<\/strong> of the parasite and the main <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, 14) the rim of the crater, 15) clouds of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> blown into the sky by the eruption; this settles back onto the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> and surrounding land.<br \/>\n<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mt.-Shasta-and-Shastina-in-Washington.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-289\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-300x253.jpg\" alt=\"A smaller parasitic cone called Shastina on the flanks of Mt. Shasta in Washington\" width=\"300\" height=\"253\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-300x253.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-1024x863.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-768x648.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-65x55.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-225x190.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington-350x295.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Shasta-and-Shastina-in-Washington.jpg 1097w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mt. Shasta in Washington state with Shastina, its parasitic cone<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">The largest craters are called\u00a0<\/span><b>calderas<\/b><span style=\"font-weight: 400\">, such as the<\/span><span style=\"font-weight: 400\">\u00a0Crater Lake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">Caldera<\/a><\/span><span style=\"font-weight: 400\">\u00a0in Oregon. <\/span><span style=\"font-weight: 400\">Many <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> features are produced by\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a><\/strong>, a basic property of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">Viscosity<\/a> is the resistance to flowing by a fluid. Low <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> flows easily more like syrup, the basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> that occurs in Hawaii on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shield<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>.\u00a0High <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> means a thick and sticky <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, typically <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a>, that flows slowly, similar to toothpaste.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Crater_lake_oregon-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-45\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-300x200.jpg\" alt=\"The mountain has a large hole in the center that is filled with the lake.\" width=\"300\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1-350x233.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_lake_oregon-1.jpg 623w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Oregon&#8217;s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<h4><span style=\"font-weight: 400\">Shield Volcano<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Kilauea_Shield_Volcano_Hawaii_20071209A.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-290\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-300x200.jpg\" alt=\"The mountain has low-angle flanks\" width=\"300\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-1024x683.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-768x512.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-1536x1024.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A-350x233.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kilauea_Shield_Volcano_Hawaii_20071209A.jpg 1599w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Kilauea in Hawai&#8217;i.<\/figcaption><\/figure>\n<p>The largest <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shield<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a><\/strong>. They are characterized by broad low-angle flanks, small vents at the top, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> chambers. The name comes from the side view, which resembles a medieval warrior\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shield<\/a>. They are typically associated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspots<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\">mid-ocean ridges<\/a>, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">rifts<\/a> with rising upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material. The low-angle flanks are built up slowly from numerous low-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows that spread out over long distances. The basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> erupts effusively, meaning the eruptions are small, localized, and predictable.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Kiluea-eruption-2018.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-291\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-300x223.jpg\" alt=\"Lava from Kiluea destroying road in Hawaii.\" width=\"300\" height=\"223\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-300x223.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-1024x761.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-768x571.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-65x48.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-225x167.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018-350x260.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Kiluea-eruption-2018.jpg 1414w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Eruption of Kiluea in 2018 produced high viscosity lava shown here crossing a road. This eruption caused much property damage<\/figcaption><\/figure>\n<p>Typically, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1190\">shield volcano<\/a> eruptions are not much of a hazard to human life\u2014although non-explosive eruptions of Kilauea (Hawaii) in 2018 produced uncharacteristically large lavas that damaged roads and structures.\u00a0<span style=\"font-weight: 400\">Mauna Loa (see <a href=\"http:\/\/hvo.wr.usgs.gov\/maunaloa\/\">USGS page<\/a>) and Kilauea (see <a href=\"http:\/\/hvo.wr.usgs.gov\/kilauea\/\">USGS page<\/a>) in Hawaii are examples of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shield<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">Shield<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">Rift<\/a><\/span><span style=\"font-weight: 400\">. <\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 215px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Olympus-Mons-on-Mars.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-292\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-300x278.jpg\" alt=\"\" width=\"215\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-300x278.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-1024x950.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-768x713.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-65x60.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-225x209.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars-350x325.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Olympus-Mons-on-Mars.jpg 1043w\" sizes=\"auto, (max-width: 215px) 100vw, 215px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system, standing about two and a half times higher than Everest is above sea level.<\/figcaption><\/figure>\n<p>The largest <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> edifice in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2201\">Solar System<\/a> is Olympus Mons on Mars. This (possibly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1708\">extinct<\/a>) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1190\">shield volcano<\/a> covers an area the size of the state of Arizona. This may indicate the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> erupted over a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> for millions of years, which means Mars had little, if any, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> activity<span style=\"font-weight: 400\">.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2022\/01\/ReU_PtFournaise_Lavastrome.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-293\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-300x225.jpg\" alt=\"The lava is ropey\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ReU_PtFournaise_Lavastrome.jpg 1276w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Ropey pahoehoe lava<\/figcaption><\/figure>\n<p style=\"text-align: left\"><span style=\"font-weight: 400\">Basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> forms special landforms based on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2689\">temperature<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>, and content of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> gases and water vapor. The two main types of basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">volcanic rock<\/a> have Hawaiian names\u2014<em><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1192\"><em>pahoehoe<\/em><\/a><\/em> and <em><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1191\"><em>aa<\/em><\/a><\/em>. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1192\">Pahoehoe<\/a><\/strong> might come from low-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> lava that flows easily into ropey strands.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Etna_02aa.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-294\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-300x225.jpg\" alt=\"The lava is sharp and jagged\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Etna_02aa.jpg 1200w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Blocky a&#8217;a lava<\/figcaption><\/figure>\n<p style=\"text-align: left\"><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1191\">Aa<\/a><\/b><span style=\"font-weight: 400\"> (sometimes spelled a\u2019a or <\/span><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1191\">\u02bba\u02bb\u0101 <\/a>and pronounced &#8220;ah-ah&#8221;<\/span><span style=\"font-weight: 400\">) is more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscous<\/a> and has a crumbly blocky appearance. The exact details of what forms the two types of flows are still up for debate. <\/span><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">Felsic<\/a> lavas have lower temperatures and more silica, and thus are higher <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a>. These also form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1191\">aa<\/a>-style flows.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano_fissure_tube.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-295\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-300x199.jpg\" alt=\"The magma is sputtering outward\" width=\"300\" height=\"199\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-300x199.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-225x149.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube-350x232.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano_fissure_tube.jpg 678w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Volcanic fissure and flow, which could eventually form a lava tube.<\/figcaption><\/figure>\n<p>Low-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a>, fast-flowing basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> tends to harden on the outside into a tube and continue to flow internally. Once <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flow subsides, the empty outer shell may be left as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> tube. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> tubes, with or without collapsed roofs, make famous caves in Hawaii, Northern California, the Columbia <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3134\">River<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1961\">Basalt<\/a> Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho.<\/p>\n<p><strong>Fissures<\/strong> are cracks that commonly originate from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\">shield<\/a>-style eruptions. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> emerging from fissures is typically <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> and very fluid. The 2018 Kiluaea eruption included fissures associated with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows. Some fissures are caused by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3087\">seismic<\/a> activity rather than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows. Some fissures are influenced by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">plate tectonics<\/a>, such as the common fissures located parallel to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2599\">divergent<\/a> boundary in Iceland.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 200px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/DT2-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-2892\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/DT2-scaled-1.jpg\" alt=\"The rock is full of columns\" width=\"200\" height=\"150\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Devils Tower in Wyoming has columnar jointing.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 192px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Columnar-jointing-on-Giants-Causeway-in-Ireland.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-297\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-288x300.jpg\" alt=\"Columnar jointing on Giant's Causeway in Ireland.\" width=\"192\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-288x300.jpg 288w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-982x1024.jpg 982w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-768x800.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-65x68.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-225x235.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland-350x365.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Columnar-jointing-on-Giants-Causeway-in-Ireland.jpg 1017w\" sizes=\"auto, (max-width: 192px) 100vw, 192px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Columnar jointing on Giant&#8217;s Causeway in Ireland.<\/figcaption><\/figure>\n<p>Cooling <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> can contract into columns with semi-hexagonal cross sections called <strong>columnar jointing<\/strong>. This feature forms the famous Devils Tower in Wyoming, possibly an ancient <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a> from which the surrounding layers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> have been removed by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2677\">erosion<\/a>. Another well-known exposed example of columnar jointing is the Giant\u2019s Causeway in Ireland.<\/p>\n<h4><span style=\"font-weight: 400\">Stratovolcano<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 225px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_Rainier_over_Tacoma.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-298\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-300x200.jpg\" alt=\"The mountain is very tall, and looms over the city\" width=\"225\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-1024x683.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-768x512.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-1536x1024.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma-350x233.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Rainier_over_Tacoma.jpg 1600w\" sizes=\"auto, (max-width: 225px) 100vw, 225px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mount Rainier towers over Tacoma, Washington.<\/figcaption><\/figure>\n<p>A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">stratovolcano<\/a><\/strong>, also called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">composite cone<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>, has steep flanks, a symmetrical cone shape, distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> fragments like <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1951\">bombs<\/a>, and solidified <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows of varying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>. Examples include Mount Rainier in Washington state and Mount Fuji in Japan.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mt.-Fuji-300x190-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-299\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Fuji-300x190-1.jpg\" alt=\"\" width=\"250\" height=\"158\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Fuji-300x190-1.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Fuji-300x190-1-65x41.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt.-Fuji-300x190-1-225x143.jpg 225w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mt. Fuji in Japan, a typical stratovolcano, symmetrical, increasing slope, visible crater at the top.<\/figcaption><\/figure>\n<p>Stratovolcanoes usually have <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\">intermediate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> chambers, but can even produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> lavas. Stratovolcanoes have <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscous<\/a>\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">domes<\/a>, punctuated by explosive eruptions. This produces <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> with steep flanks.<\/p>\n<h4><span style=\"font-weight: 400\">Lava Domes<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSH06_aerial_crater_from_north_high_angle_09-12-06.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-300\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-300x200.jpg\" alt=\"The mountain has a hole, but the hole has filled in somewhat\" width=\"300\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-1024x681.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-768x511.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-1536x1022.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06-350x233.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH06_aerial_crater_from_north_high_angle_09-12-06.jpg 2048w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Lava domes have started the rebuilding process at Mount St. Helens, Washington.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">domes<\/a><\/strong> are accumulations of silica-rich <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">volcanic rock<\/a>, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">rhyolite<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1947\">obsidian<\/a>. Too <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscous<\/a> to flow easily, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> tends to pile up near the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a> in blocky masses. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">Lava<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">domes<\/a> often form in a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a> within the collapsed crater of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">stratovolcano<\/a>, and grow by internal expansion. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">dome<\/a> expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. Mount Saint Helens has a good example of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">dome<\/a> inside of a collapsed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">stratovolcano<\/a> crater. Examples of stand-alone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">domes<\/a> are Chaiten in Chile and Mammoth Mountain in California.<\/p>\n<h4><span style=\"font-weight: 400\">Caldera<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 149px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_Mazama_eruption_timeline.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-301\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline-89x300.png\" alt=\"It shows the eruption forming a caldera.\" width=\"149\" height=\"500\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline-89x300.png 89w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline-65x219.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline-225x757.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_Mazama_eruption_timeline.png 250w\" sizes=\"auto, (max-width: 149px) 100vw, 149px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Timeline of events at Mount Mazama.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/CL1-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-2898\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/CL1-scaled-1.jpg\" alt=\"The island is forested, as are the flanks\" width=\"250\" height=\"188\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Wizard Island sits in the caldera at Crater Lake.<\/figcaption><\/figure>\n<p><strong>Calderas<\/strong> are steep-walled, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1461\">basin<\/a>-shaped depressions formed by the collapse of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> edifice into an empty <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a>. Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> specifically refers to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a>; however, it is frequently used to describe a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> type. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">Caldera<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> are\u00a0typically formed by eruptions of high-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\">felsic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> having high <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a> content.<\/p>\n<p>Crater Lake, Yellowstone, and the Long Valley <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">Caldera<\/a> are good examples of this type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> at Crater Lake National Park in Oregon was created about 6,800 years ago when Mount Mazama, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1193\">composite volcano<\/a>, erupted in a huge explosive blast. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> ejected large amounts of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and rapidly drained the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a>, causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1194\">lava dome<\/a> that formed within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1461\">basin<\/a><span style=\"font-weight: 400\">.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Yellowstone_Caldera_map2.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-303\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-300x242.jpg\" alt=\"The map shows locations of calderas and rocks within Yellowstone\" width=\"300\" height=\"242\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-300x242.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-1024x826.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-768x620.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-65x52.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-225x182.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2-350x282.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_Caldera_map2.jpg 1486w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Map of calderas and related rocks around Yellowstone.<\/figcaption><\/figure>\n<p>The Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2664\">system<\/a> erupted three times in the recent geologic past\u20142.1, 1.3, and 0.64 million years ago\u2014leaving behind three <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> basins. Each eruption created large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1958\">rhyolite<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows as well as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows that solidified into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formations<\/a>. These extra-large eruptions rapidly emptied the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1185\">magma chamber<\/a>, causing the roof to collapse and form a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a>. The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">domes<\/a>. The calderas are difficult to see today due to the amount of time since their eruptions and subsequent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2677\">erosion<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1700\">glaciation<\/a>.<\/p>\n<p>Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> started about 17-million years ago as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> under the North American lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> near the Oregon\/Nevada border. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> moved to the southwest over the stationary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a>, it left behind a track of past <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> activities. Idaho\u2019s Snake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3134\">River<\/a> Plain was created from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> that produced a series of calderas and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> eventually arrived at its current location in northwestern Wyoming, where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2638\">hotspot<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> formed the Yellowstone calderas<span style=\"font-weight: 400\">.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Yellowstone_volcano_-_ash_beds.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-134\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-300x195.jpg\" alt=\"The eruptions trend eastward due to prevailing winds.\" width=\"300\" height=\"195\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-300x195.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-65x42.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-225x146.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds-350x228.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Yellowstone_volcano_-_ash_beds.jpg 580w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).<\/figcaption><\/figure>\n<p>The Long Valley <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">Caldera<\/a> near Mammoth, California, is the result of a large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> across the United States, in a manner similar to the Yellowstone eruptions. The Bishop <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">Tuff<\/a> deposit near Bishop, California, is made of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> from this eruption. The current <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">caldera<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1461\">basin<\/a> is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1460\">dome<\/a>, and several hot springs<span style=\"font-weight: 400\">.<\/span><\/p>\n<h4><span style=\"font-weight: 400\">Cinder Cone<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Sunset_Crater10.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-304\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-300x202.jpg\" alt=\"The cone is relatively small and red\" width=\"300\" height=\"202\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-300x202.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-1024x688.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-768x516.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-1536x1032.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-65x44.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-225x151.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10-350x235.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sunset_Crater10.jpg 1599w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Sunset Crater, Arizona is a cinder cone.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">Cinder<\/a> cones<\/strong> are small <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> with steep sides, and made of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> fragments that have been ejected from a pronounced central <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a>. The small fragments are called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinders<\/a><\/strong> and the largest are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1951\">bombs<\/a><\/strong>. The eruptions are usually short-lived events, typically consisting of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\">mafic<\/a> lavas with a high content of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">volatiles<\/a>. Hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">Cinder<\/a> cones are found throughout western North America<span style=\"font-weight: 400\">.<\/span><\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 200px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Paricutin_30_612.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-305\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-200x300.jpg\" alt=\"A person looks at the eruption of ash\" width=\"200\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-200x300.jpg 200w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-65x97.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-225x337.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612-350x525.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paricutin_30_612.jpg 667w\" sizes=\"auto, (max-width: 200px) 100vw, 200px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Soon after the birth of Par\u00edcutin in 1943.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Paracutin-lava-in-San-Juan-300x224-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-306\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Paracutin-lava-in-San-Juan-300x224-1.jpg\" alt=\"\" width=\"300\" height=\"224\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paracutin-lava-in-San-Juan-300x224-1.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paracutin-lava-in-San-Juan-300x224-1-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Paracutin-lava-in-San-Juan-300x224-1-225x168.jpg 225w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Lava from Par\u00edcutin covered the local church and destroyed the town of San Juan, Mexico<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">A recent and striking example of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1196\">cinder cone<\/a> is the eruption near the village of Par\u00edcutin, Mexico that started in 1943. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1196\">cinder cone<\/a> started explosively shooting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinders<\/a> out of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a> in the middle of a farmer\u2019s field. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> quickly built up the cone to a height of over 90 m (300 ft) within a week, and 365 m (1,200 ft) within the first 8 months. After the initial explosive eruption of gases and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinders<\/a>, basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> poured out from the base of the cone. This is a common order of events for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1195\">cinder<\/a> cones: violent eruption, cone and crater <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2960\">formation<\/a>, low-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flow from the base. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1196\">cinder cone<\/a> is not strong enough to support a column of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> rising to the top of the crater, so the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> breaks through and emerges near the bottom of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a>. During nine years of eruption activity, the ashfall covered about 260 km<sup>2<\/sup> (100 mi<sup>2<\/sup>) and destroyed the nearby town of San Juan<\/span><span style=\"font-weight: 400\">.<\/span><\/p>\n<h4><span style=\"font-weight: 400\">Flood Basalts<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 500px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/World-map-or-flood-basalts.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-307\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-300x168.jpg\" alt=\"World map of flood basalts. Note the largest is the Siberian Traps\" width=\"500\" height=\"280\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-300x168.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-1024x573.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-768x430.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-1536x860.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-65x36.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-225x126.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts-350x196.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/World-map-or-flood-basalts.jpg 1703w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">World map of flood basalts. Note the largest is the Siberian Traps<\/figcaption><\/figure>\n<p>A rare <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> eruption type,\u00a0unobserved in modern times, is the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1197\">flood basalt<\/a><\/strong>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1197\">Flood basalts<\/a> are some of the largest and lowest <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3134\">River<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1197\">Flood Basalts<\/a> in Washington, Oregon, and Idaho, the Deccan <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3342\">Traps<\/a>, which cover about 1\/3 of the country of India, and the Siberian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3342\">Traps<\/a>, which may have been involved in the Earth&#8217;s largest <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2223\">mass extinction<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/8-earth-history\/\" target=\"_blank\" rel=\"noopener\">chapter 8<\/a>).<\/p>\n<h4><span style=\"font-weight: 400\">Carbonatites<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Crater_of_Ol_Doinyo_Lengai_Jan_2011-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-3187\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Crater_of_Ol_Doinyo_Lengai_Jan_2011-1-300x169-1.jpg\" alt=\"The crater has white rocks in the walls\" width=\"300\" height=\"169\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Crater of Ol Doinyo Lengai in 2011. Note the white carbonatite in the walls of the crater.<\/figcaption><\/figure>\n<p>Arguably the most unusual <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> activity are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3365\">carbonatite<\/a><\/strong> eruptions. Only one actively erupting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3365\">carbonatite<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> exists on Earth today, Ol Doinyo Lengai, in the East African <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">Rift<\/a> Zone of Tanzania. While all other <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> on Earth originates from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2709\">silicate<\/a>-based <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3365\">carbonatites<\/a> are a product of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1917\">carbonate<\/a>-based <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> and produce <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> rocks containing greater than 50% <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1917\">carbonate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">minerals<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3365\">Carbonatite<\/a> lavas are very low <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> and relatively cold for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>. The erupting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> is black, and solidifies to brown\/grey rock that eventually turns white. These rocks are occasionally found in the geologic record and require special study to distinguish them from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2914\">metamorphic<\/a> marbles (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/6-metamorphic-rocks\/\">Chapter 6<\/a>). They are mostly associated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2624\">rifting<\/a>.<\/p>\n<p><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Igneous-e1546649689814.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-309\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814.png\" alt=\"Table of igneous rocks and related volcano types. Horizontal axis is arranged from low to high silica content (i.e. from ultramafic to felsic). First row shows the extrusive (surface) igneous rocks basalt, andesite, and rhyolite. Second row shows volcano types: mid-ocean ridge, shield, cinder cone, and strato (composite). Third row shows examples of each volcano: mid-atlantic ridge, Mauna Kea (Hawaii), Paricutin, and Mt. St. Helens. Forth row shows intrusive rocks from mafic to felsic: Dunite, gabbro, diorige, granite. Fifth row shows common plate-tectonic settings: divergent oceanic hot spot, and convergent boundaries. Sixth row is typical composition: ultramafic, mafic, intermediate, and felsic.\" width=\"759\" height=\"820\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814.png 759w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814-278x300.png 278w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814-65x70.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814-225x243.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Igneous-e1546649689814-350x378.png 350w\" sizes=\"auto, (max-width: 759px) 100vw, 759px\" \/><\/a><\/p>\n<blockquote>\n<p style=\"text-align: left\"><em><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\"><em>Igneous rock<\/em><\/a> types and related <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\"><em>volcano<\/em><\/a> types. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2630\"><em>Mid-ocean ridges<\/em><\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2207\"><em>shield<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\"><em>volcanoes<\/em><\/a> represent more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1956\"><em>mafic<\/em><\/a> compositions, and strato (composite) <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\"><em>volcanoes<\/em><\/a> generally represent a more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1955\"><em>intermediate<\/em><\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1954\"><em>felsic<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\"><em>composition<\/em><\/a> and a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2600\"><em>convergent<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\"><em>plate<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\"><em>tectonic<\/em><\/a> boundary. Note that there are exceptions to this generalized layout of volcano types and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\"><em>igneous rock<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\"><em>composition<\/em><\/a>.<\/em><\/p>\n<\/blockquote>\n<h3><b>4.5.3 Volcanic Hazards and Monitoring<\/b><\/h3>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 258px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Types_of_volcano_hazards_usgs.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-310 size-medium\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Types_of_volcano_hazards_usgs-258x300.gif\" alt=\"It shows many hazards\" width=\"258\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Types_of_volcano_hazards_usgs-258x300.gif 258w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Types_of_volcano_hazards_usgs-65x76.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Types_of_volcano_hazards_usgs-225x262.gif 225w\" sizes=\"auto, (max-width: 258px) 100vw, 258px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">General diagram of volcanic hazards.<\/figcaption><\/figure>\n<p>While the most obvious <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> hazard is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a>, the dangers posed by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> go far beyond <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> flows. For example, on May 18, 1980, Mount Saint Helens (Washington, United States) erupted with an explosion and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslide<\/a> that removed the upper 400 m (1,300 ft) of the mountain. The initial explosion was immediately followed by a lateral blast, which produced a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1198\">pyroclastic\u00a0flow<\/a> that covered nearly 600 km<sup>2<\/sup> (230 mi<sup>2<\/sup>) of forest with hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and debris. The pyroclastic flow moved at speeds of 80-130 kph (50-80 mph), flattening trees and ejecting clouds of ash into the air. The USGS video provides an account of this explosive eruption that killed 57 people.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-8\" title=\"Mount St. Helens: May 18, 1980\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/Ec30uU0G56U?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount-St.-helens-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-311\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount-St.-helens-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/DSC01727.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-312\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-300x225.jpg\" alt=\"The body is outlined with a cast, and the bones are seen.\" width=\"250\" height=\"188\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/DSC01727.jpg 2048w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Human remains from the 79 CE eruption of Vesuvius.<\/figcaption><\/figure>\n<p>In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1198\">pyroclastic flow<\/a> over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered in an archeological expedition in the 18th century. Pompeii famously contains the remains (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2179\">casts<\/a>) of people suffocated by ash and covered by 10 feet (3 m) of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1946\">pumice<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1950\">lapilli<\/a>, and collapsed roofs.<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Mount_St._Helens_one_day_before_the_devastating_eruption.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-313\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-300x203.jpg\" alt=\"The volcano is conical and forested.\" width=\"300\" height=\"203\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-300x203.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-1024x692.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-768x519.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-1536x1038.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-65x44.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-225x152.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption-350x237.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Mount_St._Helens_one_day_before_the_devastating_eruption.jpg 2048w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mount St. Helens, the day before the May 18th, 1980 eruption.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSH80_st_helens_from_johnston_ridge_09-10-80.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-314\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-300x201.jpg\" alt=\"The top of the mountain is gone\" width=\"300\" height=\"201\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-300x201.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-1024x687.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-768x515.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-1536x1031.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-65x44.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-225x151.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80-350x235.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSH80_st_helens_from_johnston_ridge_09-10-80.jpg 2048w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Picture 4 months after the major eruption of Mount St. Helens.<\/figcaption><\/figure>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 1500px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-315 size-full\" title=\"By Associated Press, via The Atlantic, https:\/\/www.theatlantic.com\/photo\/2015\/05\/the-eruption-of-mount-st-helens-in-1980\/393557\/\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Mt_Saint_Helens_Eruption_main_1500.gif\" alt=\"Series of images showing half of the mountain releasing as a giant landslide and ash billowing out from underneath.\" width=\"1500\" height=\"943\" \/><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Series of still images of the May 18, 1980, eruption of Mt. Saint Helens, Washington showing largest recorded landslide in history and subsequent eruption and pyroclastic flow (By The Associated Press via The Atlantic)<\/figcaption><\/figure>\n<h4><span style=\"font-weight: 400\">Pyroclastic flows<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pyroclastic_flows_at_Mayon_Volcano.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-316\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-300x188.jpg\" alt=\"Most of the material is heading up, but small portions of the eruption column head downward.\" width=\"300\" height=\"188\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-300x188.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-768x480.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-65x41.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-225x141.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano-350x219.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pyroclastic_flows_at_Mayon_Volcano.jpg 800w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">The material coming down from the eruption column is a pyroclastic flow.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">The most dangerous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> hazard are <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows<\/strong> (<\/span><a href=\"https:\/\/volcanoes.usgs.gov\/vsc\/movies\/movie_101\/PF_Animation.mp4\"><span style=\"font-weight: 400\">video<\/span><\/a><span style=\"font-weight: 400\">). These flows are a mix of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> blocks, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1946\">pumice<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a>, and hot gases between 200\u00b0C-700\u00b0C (400\u00b0F-1,300\u00b0F). The turbulent cloud of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and gas races down the steep flanks at high speeds up to 193 kph (120 mph) into the valleys around composite <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a><\/span><span style=\"font-weight: 400\">. Most explosive, silica-rich, high <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3368\">viscosity<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> such as composite cones usually have <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows. The rock <\/span><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a> <span style=\"font-weight: 400\">and <\/span>welded <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1953\">tuff<\/a><span style=\"font-weight: 400\"> is often formed from these <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 256px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pelee_1902_3.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-317\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3-256x300.jpg\" alt=\"A man is seen overlooking the destroyed city\" width=\"256\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3-256x300.jpg 256w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3-65x76.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3-225x264.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pelee_1902_3.jpg 322w\" sizes=\"auto, (max-width: 256px) 100vw, 256px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">The remains of St. Pierre.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">There are numerous examples of deadly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows. In 2014, the Mount Ontake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1198\">pyroclastic flow<\/a> in Japan killed 47 people. The flow was caused by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> heating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3129\">groundwater<\/a> into steam, which then rapidly ejected with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1951\">bombs<\/a>. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs<\/span><span style=\"font-weight: 400\">. <\/span><span style=\"font-weight: 400\">Two short videos below document<\/span><span style=\"font-weight: 400\">\u00a0eye-witness video of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows. In the early 1990s, Mount Unzen erupted several times with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1952\">pyroclastic<\/a> flows. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1198\">pyroclastic flow<\/a>\u00a0shown in this famous <\/span><span style=\"font-weight: 400\">short video<\/span><span style=\"font-weight: 400\"> killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee <\/span><span style=\"font-weight: 400\">erupted with a violent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1198\">pyroclastic flow<\/a> that destroyed the entire town of St. Pierre and killing 28,000 people in moments<\/span><span style=\"font-weight: 400\">.<\/span><\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-9\" title=\"Japan&#39;s Mount Ontake volcano erupted\/eruption, killing 34 people, report BBC (corrected aspect)\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/3ObsOj9Q2Do?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Japans-Mount-Ontake-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-318\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Japans-Mount-Ontake-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-10\" title=\"Dome collapse and pyroclastic flow at Unzen Volcano\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/Cvjwt9nnwXY?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-319\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Dome-collapse-and-pyroclastic-flow-at-Unzen-Volcano-YouTube-QR-Codes.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<h4><span style=\"font-weight: 400\">Landslides and Landslide-Generated Tsunamis<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Msh_may18_sequence.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-320\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-300x197.gif\" alt=\"The landslide opened an area for the eruption\" width=\"300\" height=\"197\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-300x197.gif 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-65x43.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-225x147.gif 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Msh_may18_sequence-350x229.gif 350w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Sequence of events for Mount St. Helens, May 18th, 1980. Note that an earthquake caused a landslide, which caused the &#8220;uncorking&#8221; of the mountain and started the eruption.<\/figcaption><\/figure>\n<p>The steep and unstable flanks of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> can lead to slope failure and dangerous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslides<\/a>. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslides<\/a> can be triggered by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> movement, explosive eruptions, large earthquakes, and\/or heavy rainfall. During the 1980 Mount St. Helens eruption, the entire north flank of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> collapsed and released a huge <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslide<\/a> that moved at speeds of 160-290 kph (100-180 mph).<\/p>\n<p>If enough <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslide<\/a> material reaches the ocean, it may cause a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3194\">tsunami<\/a>. In 1792, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslide<\/a> caused by the Mount Unzen eruption reached the Ariaka Sea, generating a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3194\">tsunami<\/a> that killed 15,000 people (see <a href=\"http:\/\/volcano.oregonstate.edu\/describe-1883-eruption-krakatau\" target=\"_blank\" rel=\"noopener\">USGS page<\/a>). When Mount Krakatau in Indonesia erupted in 1883, it generated ocean waves that towered 40 m (131 ft) above sea level. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3194\">tsunami<\/a> killed 36,000 people and destroyed 165 villages.<\/p>\n<h4><span style=\"font-weight: 400\">Tephra<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-321\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-300x204.jpg\" alt=\"The man is wearing a mask to prevent pneumonoultramicroscopicsilicovolvanoconiosis.\" width=\"300\" height=\"204\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-300x204.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-1024x695.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-768x521.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-1536x1043.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-65x44.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-225x153.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01-350x238.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ash_in_Yogyakarta_during_the_2014_eruption_of_Kelud_01.jpg 2048w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Aman sweeps ash from an eruption of Kelud, Indonesia.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a>, especially composite <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a>, eject large amounts of <\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> <\/b><span style=\"font-weight: 400\">(ejected rock materials), most notably\u00a0<\/span><b><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> <\/b><span style=\"font-weight: 400\">(<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> fragments less than 0.08 inches [2 mm]). Larger <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1948\">tephra<\/a> is heavier and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3119\">falls<\/a> closer to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1187\">vent<\/a>. Larger blocks and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1951\">bombs<\/a> pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier.<\/span><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 250px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Micrograph-of-volcanic-ash-particle.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-322\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Micrograph-of-volcanic-ash-particle.jpg\" alt=\"Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.\" width=\"250\" height=\"183\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Micrograph-of-volcanic-ash-particle.jpg 250w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Micrograph-of-volcanic-ash-particle-65x48.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Micrograph-of-volcanic-ash-particle-225x165.jpg 225w\" sizes=\"auto, (max-width: 250px) 100vw, 250px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Micrograph of silica particle in volcanic ash. A cloud of these is capable of destroying an aircraft or automobile engine.<\/figcaption><\/figure>\n<p>Hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> poses an immediate danger to people, animals, plants, machines, roads, and buildings located close to the eruption. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">Ash<\/a> is fine grained (&lt; 2mm) and can travel airborne long distances away from the eruption site. Heavy accumulations of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> can cause buildings to collapse. In people, it may cause respiratory issues like silicosis. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">Ash<\/a> is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services. In 2010, the Eyjafjallaj\u00f6kull <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> in Iceland emitted a large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> cloud into the upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2667\">atmosphere<\/a>, causing the largest air-travel disruption in northern Europe since World War II. No one was injured, but the service disruption was estimated to have cost the world economy billions of dollars.<\/p>\n<h4><span style=\"font-weight: 400\">Volcanic Gases<\/span><\/h4>\n<p>As <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> rises to the surface the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2917\">confining<\/a> pressure decreases, and allows <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> gases to escape into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2667\">atmosphere<\/a>. Even <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO<sub>2<\/sub>), sulfur dioxide (SO<sub>2<\/sub>), hydrogen <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1921\">sulfide<\/a> (H<sub>2<\/sub>S), and hydrogen <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1920\">halides<\/a> (HF, HCl, or HBr).<\/p>\n<p>Carbon dioxide tends to sink and accumulate in depressions and basins. In <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> areas known to emit carbon dioxide, low-lying areas may <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3342\">trap<\/a> hazardous concentrations of this colorless and odorless gas. The Mammoth Mountain Ski Resort in California, is located within the Long Valley <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1188\">Caldera<\/a>, is one such area of carbon dioxide-producing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a>. In 2006, three ski patrol members died of suffocation caused by carbon dioxide after falling into a snow depression near a fumarole\u00a0<span style=\"font-weight: 400\">(<\/span><a href=\"http:\/\/volcanoes.usgs.gov\/Imgs\/Jpg\/Unzen\/MayuyamaSlide_caption.html\" target=\"_blank\" rel=\"noopener\"><span style=\"font-weight: 400\">info<\/span><\/a><span style=\"font-weight: 400\">)<\/span>.<\/p>\n<p>In rare cases, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> may create a sudden emission of gases without warning. Limnic eruptions (<em>limne<\/em> is Greek for lake), occur in crater lakes associated with active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a>. The water in these lakes is supercharged with high concentrations of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> gases. If the water is physically jolted by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1199\">landslide<\/a> or earthquake, it may <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3117\">trigger<\/a> an immediate and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1933\">massive<\/a> release of gases out of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2705\">solution<\/a>. An analogous example would be what happens to vigorously shaken bottle of carbonated soda when the cap is opened. An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon. Almost 2,000 people were killed by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1933\">massive<\/a> release of carbon dioxide.<\/p>\n<h4><span style=\"font-weight: 400\">Lahars<\/span><\/h4>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/MSHlahar.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-323\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-300x200.jpg\" alt=\"The mud line is far up on the trees\" width=\"300\" height=\"200\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-300x200.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-65x43.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-225x150.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar-350x234.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MSHlahar.jpg 400w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Mud line shows the extent of lahars around Mount St. Helens.<\/figcaption><\/figure>\n<p><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1201\">Lahar<\/a><\/strong> is an Indonesian word and is used to describe a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> mudflow that forms from rapidly melting snow or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2464\">glaciers<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1201\">Lahars<\/a> are slurries resembling wet concrete, and consist of water, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a>, rock fragments, and other debris. These mudflows flow down the flanks of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanoes<\/a> or mountains covered with freshly-erupted <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1949\">ash<\/a> and on steep slopes can reach speeds of up to 80 kph (50 mph).<\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 235px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/LaharsRaineer.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-324\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-235x300.jpg\" alt=\"The cities are on top of old lahar deposits\" width=\"235\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-235x300.jpg 235w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-65x83.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-225x287.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer-350x446.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/LaharsRaineer.jpg 450w\" sizes=\"auto, (max-width: 235px) 100vw, 235px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">Old lahars around Tacoma, Washington.<\/figcaption><\/figure>\n<p><span style=\"font-weight: 400\">Several major cities, including Tacoma, are located on prehistoric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1201\">lahar<\/a> flows that extend for many kilometers across the flood plains surrounding Mount Rainier in Washington (see map). Mount Baker in Washington has a similar hazard for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1201\">lahar<\/a> flows. A tragic scenario played out recently, in 1985, when a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1201\">lahar<\/a> from the Nevado del Ruiz <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> in Colombia buried the town of Armero and killed an estimated 23,000 people.<\/span><\/p>\n<h4><span style=\"font-weight: 400\">Monitoring<\/span><\/h4>\n<p>Geologists use various instruments to detect changes or indications that an eruption is imminent. The three videos show different types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> monitoring used to predict eruptions 1) earthquake activity; 2) increases in gas emission; and 3) changes in land surface orientation and elevation.<\/p>\n<p>One video shows how monitoring earthquake frequency, especially special vibrational earthquakes called harmonic tremors, can detect <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> movement and possible eruption. Another video shows how gas monitoring may be used to predict an eruption. A rapid increase of gas emission may indicate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> that is actively rising to surface and releasing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2815\">dissolved<\/a> gases out of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2705\">solution<\/a>, and that an eruption is imminent. The last video shows how a GPS unit and tiltmeter can detect land surface changes, indicating the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> is moving underneath it.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-11\" title=\"Volcano Monitoring\u2014Earthquake signals (educational)\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/nlo-2JoNHrw?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-325\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Earthquake-Signals-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p><iframe loading=\"lazy\" id=\"oembed-12\" title=\"Volcano Monitoring\u2014Measuring Gas emmisions\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/owk4fWbw4qM?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-326\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Measuring-Gas-emmisions-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-13\" title=\"Volcano Monitoring_Deformation measured with tilt meter and GPS\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/sNYQkxxd_0Q?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-327\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Volcano-Monitoring-Deformation-measured-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this YouTube video via this QR Code.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-29\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-29\" class=\"h5p-iframe\" data-content-id=\"29\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4.5 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/4.5-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-328\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/4.5-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 4.5 via this QR Code.<\/figcaption><\/figure>\n<h2>Summary<\/h2>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous rock<\/a> is divided into two major groups: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">intrusive<\/a> rock that solidifies from underground <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1938\">extrusive<\/a> rock formed from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> that erupts and cools on the surface. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> is generated from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> material at several <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonics<\/a> situations by three types of melting: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1176\">decompression melting<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2608\">flux melting<\/a>, or heat-induced melting. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> is determined by differences in the melting temperatures of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2687\">mineral<\/a> components (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1174\">Bowen\u2019s Reaction Series<\/a>). The processes affecting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a> include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1177\">partial melting<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1179\">magmatic differentiation<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1178\">assimilation<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2620\">collision<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a> come in a wide variety of shapes and sizes, and are classified by a multiple factors, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">composition<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2576\">tectonic<\/a> activity. Because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a> presents serious hazards to human civilization, geologists carefully monitor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> activity to mitigate or avoid the dangers it presents.<\/p>\n<h3>Take this quiz to check your comprehension of this Chapter.<\/h3>\n<div id=\"h5p-30\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-30\" class=\"h5p-iframe\" data-content-id=\"30\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 4 Review\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_315\" aria-describedby=\"caption-attachment-315\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ch.4-Review-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-329\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.4-Review-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-315\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the review quiz for Chapter 4 via this QR Code.<\/figcaption><\/figure>\n<h2><strong>References<\/strong><\/h2>\n<div class=\"csl-bib-body\">\n<ol>\n<li class=\"csl-entry\">Arndt, N.T., 1994, Chapter 1 <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2205\">Archean<\/a> Komatiites, <i>in<\/i> K.C. Condie, editor, Developments in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2218\">Precambrian<\/a> Geology: Elsevier, p. 11\u201344.<\/li>\n<li class=\"csl-entry\">Bateman, P.C., and Chappell, B.W., 1979, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2674\">Crystallization<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1180\">fractionation<\/a>, and solidification of the Tuolumne <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1939\">Intrusive<\/a> Series, Yosemite National Park, California: Geological Society of America Bulletin, v. 90, no. 5, p. 465\u2013482., doi: &lt;a href=&#8221;https:\/\/doi.org\/10.1130\/0016-7606(1979)902.0.CO;2&#8243;&gt;10.1130\/0016-7606(1979)90&lt;465:CFASOT&gt;2.0.CO;2.<\/li>\n<li class=\"csl-entry\">Bell, K., and Keller, J., 2012, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_3365\">Carbonatite<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanism<\/a>: Oldoinyo Lengai and the petrogenesis of natrocarbonatites: Springer Science &amp; Business Media.<\/li>\n<li class=\"csl-entry\">Boehler, R., 1996, Melting temperatures of the Earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2586\">mantle<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2589\">core<\/a>: Earth\u2019s thermal structure: Annual Review of Earth and Planetary Sciences, v. 24, no. 1, p. 15\u201340., doi: <a href=\"https:\/\/doi.org\/10.1146\/annurev.earth.24.1.15\">10.1146\/annurev.earth.24.1.15<\/a>.<\/li>\n<li class=\"csl-entry\">Bowen, N.L., 1922, The Reaction Principle in Petrogenesis: J. Geol., v. 30, no. 3, p. 177\u2013198.<\/li>\n<li class=\"csl-entry\">Bowen, N.L., 1928, The evolution of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">igneous<\/a> rocks: Dover Publications, 334 p.<\/li>\n<li class=\"csl-entry\">Carr, M.H., 1975, Geologic map of the Tharsis Quadrangle of Mars: IMAP.<\/li>\n<li class=\"csl-entry\">Earle, S., 2015, Physical geology OER textbook: BC Campus OpenEd.<\/li>\n<li class=\"csl-entry\">EarthScope, 2014, Mount Ontake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanic<\/a> Eruption: Online, <a href=\"http:\/\/www.earthscope.org\/science\/geo-events\/mount-ontake-volcanic-eruption\">http:\/\/www.earthscope.org\/science\/geo-events\/mount-ontake-volcanic-eruption<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">Frankel, C., 2005, Worlds on Fire: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a> on the Earth, the Moon, Mars, Venus and Io: Cambridge University Press, 396 p.<\/li>\n<li class=\"csl-entry\">Glazner, A.F., Bartley, J.M., Coleman, D.S., Gray, W., and Taylor, R.Z., 2004, Are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1965\">plutons<\/a> assembled over millions of years by amalgamation from small <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magma<\/a> chambers? GSA Today, v. 14, no. 4, p. 4., doi: &lt;a href=&#8221;https:\/\/doi.org\/10.1130\/1052-5173(2004)0142.0.CO;2&#8243;&gt;10.1130\/1052-5173(2004)014&lt;0004:APAOMO&gt;2.0.CO;2.<\/li>\n<li class=\"csl-entry\">Luongo, G., Perrotta, A., Scarpati, C., De Carolis, E., Patricelli, G., and Ciarallo, A., 2003, Impact of the AD 79 explosive eruption on Pompeii, II. Causes of death of the inhabitants inferred by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2859\">stratigraphic<\/a> analysis and areal distribution of the human casualties: J. Volcanol. Geotherm. Res., v. 126, no. 3\u20134, p. 169\u2013200.<\/li>\n<li class=\"csl-entry\">Mueller, S., and Phillips, R.J., 1991, On the initiation of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a>: J. Geophys. Res. [Solid Earth], v. 96, no. B1, p. 651\u2013665.<\/li>\n<li class=\"csl-entry\">Peacock, M.A., 1931, Classification of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2675\">Igneous Rock<\/a> Series: The Journal of Geology, v. 39, no. 1, p. 54\u201367.<\/li>\n<li class=\"csl-entry\">Perkins, S., 2011, 2010\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcano<\/a>-Induced Air Travel Shutdown Was Justified: Online, <a href=\"http:\/\/www.sciencemag.org\/news\/2011\/04\/2010s-volcano-induced-air-travel-shutdown-was-justified\">http:\/\/www.sciencemag.org\/news\/2011\/04\/2010s-volcano-induced-air-travel-shutdown-was-justified<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">Peterson, D.W., and Tilling, R.I., 1980, Transition of basaltic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2673\">lava<\/a> from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1192\">pahoehoe<\/a> to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1191\">aa<\/a>, Kilauea <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcano<\/a>, Hawaii: Field observations and key factors &#8211; ScienceDirect: J. Volcanol. Geotherm. Res., v. 7, no. 3\u20134, p. 271\u2013293.<\/li>\n<li class=\"csl-entry\">Petrini and Podladchikov, 2000, Lithospheric pressure\u2013depth relationship in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1446\">compressive<\/a> regions of thickened <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2580\">crust<\/a>: Journal of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2914\">Metamorphic<\/a> Geology, v. 18, no. 1, p. 67\u201377., doi: <a href=\"https:\/\/doi.org\/10.1046\/j.1525-1314.2000.00240.x\">10.1046\/j.1525-1314.2000.00240.x<\/a>.<\/li>\n<li class=\"csl-entry\">Reid, J.B., Evans, O.C., and Fates, D.G., 1983, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">Magma<\/a> mixing in granitic rocks of the central Sierra Nevada, California: Earth and Planetary Science Letters, v. 66, p. 243\u2013261., doi: <a href=\"https:\/\/doi.org\/10.1016\/0012-821X(83)90139-5\">10.1016\/0012-821X(83)90139-5<\/a>.<\/li>\n<li class=\"csl-entry\">Rhodes, J.M., and Lockwood, J.P., 1995, Mauna Loa Revealed: Structure, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2831\">Composition<\/a>, History, and Hazards: Washington DC American Geophysical Union Geophysical Monograph Series, v. 92.<\/li>\n<li class=\"csl-entry\">Scandone, R., Giacomelli, L., and Gasparini, P., 1993, Mount Vesuvius: 2000 years of volcanological observations: Journal of Volcanology and Geothermal Research, v. 58, p. 5\u201325.<\/li>\n<li class=\"csl-entry\">Stovall, W.K., Wilkins, A.M., Mandeville, C.W., and Driedger, C.L., 2016, Fact Sheet.:<\/li>\n<li class=\"csl-entry\">Thorarinsson, S., 1969, The Lakagigar eruption of 1783: Bull. Volcanol., v. 33, no. 3, p. 910\u2013929.<\/li>\n<li class=\"csl-entry\">Tilling, R.I., 2008, The critical role of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcano<\/a> monitoring in risk reduction: Adv. Geosci., v. 14, p. 3\u201311.<\/li>\n<li class=\"csl-entry\">United States Geological Survey, 1999, Exploring the deep <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2885\">ocean floor<\/a>: Online, <a href=\"http:\/\/pubs.usgs.gov\/gip\/dynamic\/exploring.html\">http:\/\/pubs.usgs.gov\/gip\/dynamic\/exploring.html<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">United States Geological Survey, 2012, Black Rock Desert <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanic<\/a> Field: Online, <a href=\"http:\/\/volcanoes.usgs.gov\/volcanoes\/black_rock_desert\/\">http:\/\/volcanoes.usgs.gov\/volcanoes\/black_rock_desert\/<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">USGS, 2001, Dual <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> tragedies in the Caribbean led to founding of HVO: Online, <a href=\"http:\/\/hvo.wr.usgs.gov\/volcanowatch\/archive\/2001\/01_05_03.html\">http:\/\/hvo.wr.usgs.gov\/volcanowatch\/archive\/2001\/01_05_03.html<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">USGS, 2011, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a>: Principal Types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a>: Online, <a href=\"http:\/\/pubs.usgs.gov\/gip\/volc\/types.html\">http:\/\/pubs.usgs.gov\/gip\/volc\/types.html<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">USGS, 2012a, USGS: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcano<\/a> Hazards Program: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/vhp\/hazards.html\">https:\/\/volcanoes.usgs.gov\/vhp\/hazards.html<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">USGS, 2012b, Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcano<\/a> Observatory: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/volcanoes\/yellowstone\/yellowstone_geo_hist_52.html\">https:\/\/volcanoes.usgs.gov\/volcanoes\/yellowstone\/yellowstone_geo_hist_52.html<\/a>, accessed July 2016.<\/li>\n<li class=\"csl-entry\">USGS, 2016, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">Volcanoes<\/a> General &#8211; What are the different types of volcanoes? Online, <a href=\"https:\/\/www2.usgs.gov\/faq\/categories\/9819\/2730\">https:\/\/www2.usgs.gov\/faq\/categories\/9819\/2730<\/a>, accessed March 2017.<\/li>\n<li class=\"csl-entry\">USGS, 2017, The Volcanoes of Lewis and Clark &#8211; Mount St. Helens: Online, <a href=\"https:\/\/volcanoes.usgs.gov\/observatories\/cvo\/Historical\/LewisClark\/Info\/summary_mount_st_helens.shtml\">https:\/\/volcanoes.usgs.gov\/observatories\/cvo\/Historical\/LewisClark\/Info\/summary_mount_st_helens.shtml<\/a>, accessed March 2017.<\/li>\n<li class=\"csl-entry\">Wallace, P.J., 2005, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2606\">Volatiles<\/a> in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2602\">subduction<\/a> zone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2672\">magmas<\/a>: concentrations and fluxes based on melt <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_2958\">inclusion<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_330_1181\">volcanic<\/a> gas data: Journal of Volcanology and Geothermal Research, v. 140, no. 1\u20133, p. 217\u2013240., doi: <a href=\"https:\/\/doi.org\/10.1016\/j.jvolgeores.2004.07.023\">10.1016\/j.jvolgeores.2004.07.023<\/a>.<\/li>\n<li class=\"csl-entry\">Williams, H., 1942, The Geology of Crater Lake National Park, Oregon: With a Reconnaissance of the Cascade Range Southward to Mount Shasta: Carnegie institution.<\/li>\n<\/ol>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_330_2672\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2672\"><div tabindex=\"-1\"><p>Liquid rock within the Earth.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2576\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2576\"><div tabindex=\"-1\"><p>The theory that the outer layer of the Earth (the lithosphere) is broken in several plates, and these plates move relative to one another, causing the major topographic features of Earth (e.g. mountains, oceans) and most earthquakes and volcanoes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1174\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1174\"><div tabindex=\"-1\"><p>A series of mineral formation temperatures that can explain the minerals that form in specific igneous rocks. For example, pyroxene will form with olivine and amphibole, but not quartz.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2687\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2687\"><div tabindex=\"-1\"><p>A natural substance that is typically solid, has a crystalline structure, and is typically formed by inorganic processes. Minerals are the building blocks of most rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2674\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2674\"><div tabindex=\"-1\"><p>The process of liquid rock freezing into solid rock. Because liquid rock is made of many components, the process is complex as different components freeze at different temperatures.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2675\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2675\"><div tabindex=\"-1\"><p>Rocks that are formed from liquid rock, i.e. from volcanic processes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1177\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1177\"><div tabindex=\"-1\"><p>The process of some material being derived from a heterogenous mixture when melting (e.g. rocks). Because all rocks are made of many different components, they have many different melting points. As they are heated, certain easy-to-melt components will be melted first.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1180\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1180\"><div tabindex=\"-1\"><p>The process of a magma changing from mafic to felsic via cooling. As the magma cools, higher temperature, mafic minerals crystalize, and a more felsic magma is left.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3368\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3368\"><div tabindex=\"-1\"><p>The resistance of a fluid to flow, where a high value means a fluid which does not like to flow (like toothpaste), and a low value means a fluid which flows easily (like water).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1181\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1181\"><div tabindex=\"-1\"><p>Place where lava is erupted at the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2831\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2831\"><div tabindex=\"-1\"><p>The mineral make up of a rock, i.e. which minerals are found within a rock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2591\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2591\"><div tabindex=\"-1\"><p>A solid part&nbsp;of the lithosphere which moves as a unit, i.e. the entire plate generally moves the same direction at the same speed.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2673\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2673\"><div tabindex=\"-1\"><p>Liquid rock on the surface of the Earth.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2595\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2595\"><div tabindex=\"-1\"><p>The outer physical&nbsp;layer of the core, which is liquid. Movement within the outer core is believed to be responsible for Earth's magnetic field and flips of the magnetic field.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2586\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2586\"><div tabindex=\"-1\"><p>Middle chemical layer of the Earth, made of mainly iron and magnesium silicates. It is generally denser than the crust (except for older oceanic crust) and less dense than the core.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2580\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2580\"><div tabindex=\"-1\"><p>The outermost chemical layer of the Earth, defined by its low density and higher concentrations of lighter elements. The crust has two types: <strong>continental<\/strong>, which is the thick, more ductile, and lowest density, and <strong>oceanic<\/strong>, which is higher density, more brittle, and thinner.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1938\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1938\"><div tabindex=\"-1\"><p>Igneous rock cooling, and thus forming, outside of the Earth, i.e. on the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1945\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1945\"><div tabindex=\"-1\"><p>An extrusive rock filled with small bubble structures, frozen in place as gases escaped from the cooling lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1939\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1939\"><div tabindex=\"-1\"><p>Igneous rock cooling, and thus forming, inside of the Earth, i.e. under the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2919\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2919\"><div tabindex=\"-1\"><p>Arrangement of minerals within a rock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1940\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1940\"><div tabindex=\"-1\"><p>Large, easy-to-see crystals within an igneous rock. This is common in intrusive rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1941\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1941\"><div tabindex=\"-1\"><p>Small, microscopic, hard-to-see crystals (i.e. no visible crystals) within an igneous rock. This is common in extrusive rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1949\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1949\"><div tabindex=\"-1\"><p>Volcanic tephra that is less than 2 mm in diameter.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1947\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1947\"><div tabindex=\"-1\"><p>Dark colored volcanic glass, with extremely small microscopic crystals or no crystals. Typically form from felsic volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1942\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1942\"><div tabindex=\"-1\"><p>An igneous rock with two distinctive crystal sizes. This is common in intrusive or extrusive rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1943\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1943\"><div tabindex=\"-1\"><p>A large crystal within an igneous rock. These can be seen within phaneritic and porphyritic rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1959\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1959\"><div tabindex=\"-1\"><p>General term from the fine-grained, not discernible part of a rock. In igneous rocks, this is the part of the rock that is not phenocrysts, and can help in determining the composition of extrusive rocks. In sedimentary rocks, it typically refers to the fine-grained components, namely mud. In metamorphic rocks, it is usually referring to material between porphyroblasts or a low-grade rock with only microscopic mineralization.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1916\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1916\"><div tabindex=\"-1\"><p>Consisting of three end members:&nbsp;potassium feldspar (K-spar, KAlSi3O8), plagioclase with calcium (CaAl2Si2O8, called anorthite), and plagioclase with sodium (NaAlSi3O8, called albite). Commonly blocky, with two cleavages as ~90\u00b0. Plagioclase is typically more dull white and grey, and K-spar is more vibrant white, orange, or red. The most common mineral found within the crust, and a major component of almost all igneous rocks, some sedimentary rocks, and some metamorphic rocks. Structure is a three-dimensional framework of silica tetrahedra, with locations open for cations (K, Na, Ca).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1915\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1915\"><div tabindex=\"-1\"><p>SiO2. Transparent, but can be any color imaginable with impurities. No cleavage, hard, and commonly forms equant masses. Perfect crystals are hexagonal prisms topped with pyramidal shapes. One of the most common minerals, and is found in many different geologic settings, including the dominant component of sand on the surface of Earth. Structure is a three-dimensional network of silica tetrahedra, connected as much as possible to each other.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1914\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1914\"><div tabindex=\"-1\"><p>X1A2-3Z4O10(OH,\u202fF)2, where commonly X=K, Na, Ca; A=Al, Mg, Fe; Z=Si, Al. Has two more-common occurrences, light-colored (translucent and pearly tan) muscovite, and dark colored biotite. Has one strong cleavage, and is typically seen as sheets, in stacks or \"books.\" Common in many igneous and metamorphic rocks. Structure is two-dimensional sheets of silica tetrahedra in a hexagonal netowork.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1944\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1944\"><div tabindex=\"-1\"><p>A rock (or texture within a rock) with unusually-large crystals, minerals with rare trace element concentrations, and\/or unusual minerals, typically forming in veins as the last dredges of magma crystallize.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2815\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2815\"><div tabindex=\"-1\"><p>The process in which solids (like minerals) are disassociated and the ionic components are dispersed in a liquid (usually water).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2606\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2606\"><div tabindex=\"-1\"><p>Components of magma which are dissolved until it reaches the surface, where they expand. Examples include water and carbon dioxide. Volatiles also cause flux melting in the mantle, causing volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2705\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2705\"><div tabindex=\"-1\"><p>The act of taking a solid and dissolving it into a liquid. This commonly occurs with salts and other minerals in water.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1946\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1946\"><div tabindex=\"-1\"><p>Low density, highly vesiculated, usually white to tan volcanic rock. Typically arises from felsic volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1935\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1935\"><div tabindex=\"-1\"><p>Fractures that have a circular appearance.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1934\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1934\"><div tabindex=\"-1\"><p>A break within a rock that has no relative movement between the sides. Caused by cooling, pressure release, tectonic forces, etc.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2667\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2667\"><div tabindex=\"-1\"><p>The gases that are part of the Earth, which are mainly nitrogen and oxygen.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1948\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1948\"><div tabindex=\"-1\"><p>General term for solid, but fragmented, material erupted from a volcano. Has three subcomponents:&nbsp;ash (&lt;2mm), lapilli (2-64 mm), blocks and bombs (&gt;64mm).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1952\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1952\"><div tabindex=\"-1\"><p>Rocks (or rock textures) that are formed from explosive volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2825\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2825\"><div tabindex=\"-1\"><p>Sedimentary rocks that are made of sediment, weathered pieces of bedrock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1950\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1950\"><div tabindex=\"-1\"><p>Volcanic tephra that has a diameter between 2 mm and 64 mm. Many cinders are within the category of lapilli.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1951\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1951\"><div tabindex=\"-1\"><p>Large volcanic tephra greater than 64 mm in diameter.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1953\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1953\"><div tabindex=\"-1\"><p>Rocks made from pyroclastic tephra: either ash, lapilli, and\/or bombs. Tephra type can be used as an adjective, i.e. ash-fall tuff. If deposited hot, where material can fuse together while hot, the rock is then called a welded tuff.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1954\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1954\"><div tabindex=\"-1\"><p>Can refer to a volcanic rock with higher silica composition, or the minerals that make up those rocks, namely quartz, feldspar (both potassium feldspar and plagioclase feldspar), and muscovite mica. Felsic rocks are lighter in color and contain more minerals that are light in color, but can contain some biotite and amphibole. Primary felsic rocks are rhyolite (extrusive) and granite (intrusive).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1955\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1955\"><div tabindex=\"-1\"><p>A volcanic rock with medium silica composition, equally rich in felsic minerals (feldspar) and mafic minerals (amphibole, biotite, pyroxene). Intermediate rocks are grey in color and contain somewhat equal amounts of minerals that are light and dark in color. Primary intermediate rocks are andesite (extrusive) and diorite (intrusive).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1956\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1956\"><div tabindex=\"-1\"><p>Can refer to a volcanic rock with lower silica composition, or the minerals that make up those rocks, namely olivine, pyroxene, amphibole, and biotite. Mafic rocks are darker in color and contain more minerals that are dark in color, but can contain some plagioclase feldspar. Primary mafic rocks are basalt (extrusive) and gabbro (intrusive).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1957\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1957\"><div tabindex=\"-1\"><p>An igneous rock with extremely low silica composition, being made of almost all olivine and pyroxene. Ultramafic rocks contain very low amount of silica and are common in the mantle. Primary ultramafic rocks are komatiite (extrusive) and peridotite (intrusive).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1962\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1962\"><div tabindex=\"-1\"><p>General name of a felsic rock that is intrusive. Has more felsic minerals than mafic minerals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2713\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2713\"><div tabindex=\"-1\"><p>(RSi4O11)2, where R is a large number of different cations that can sub in. Can be many colors, but the common form, hornblende, is dark brown to black. Has two cleavages at 54\u00b0 and 126\u00b0. Crystals are typically elongated needles or diamond shapes. Common in many igneous rocks and some metamorphic rocks. Structure is a double chain of silica tetrahedra.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_4444\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_4444\"><div tabindex=\"-1\"><\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2712\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2712\"><div tabindex=\"-1\"><p>XY(Al,Si)2O6, in which X typically equals Na, Ca, Mg, or Fe and Y typically equals Mg, Fe, or Al. Typically black to dark green, blocky, with two cleavages at ~90\u00b0. Common in mafic igneous rocks and some metamorphic rocks. Structure is a single chain of silica tetrahedra.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2711\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2711\"><div tabindex=\"-1\"><p>(Fe,Mg)2SiO4.&nbsp;Typically translucent olive green and equant, with no cleavage. Common in mafic igneous rocks and in the mantle, but easily weathered in surface conditions. Structure is isolated silica tetrahedra. Known as peridot when a gem.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2588\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2588\"><div tabindex=\"-1\"><p>An intrusive ultramafic rock, which is the main component of the mantle. The minerals in peridotite are typically olivine with some pyroxene.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1958\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1958\"><div tabindex=\"-1\"><p>General name of a felsic rock that is extrusive. Generally has a white, tan, or pink groundmass color.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1960\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1960\"><div tabindex=\"-1\"><p>General name of a intermediate rock that is extrusive. Generally has a grey groundmass color.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1963\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1963\"><div tabindex=\"-1\"><p>General name of an intermediate rock that is intrusive. Has about the same amount of felsic minerals and mafic minerals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1961\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1961\"><div tabindex=\"-1\"><p>General name of a mafic rock that is extrusive. Generally has a black groundmass color.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1964\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1964\"><div tabindex=\"-1\"><p>General name of a mafic rock that is intrusive. Has more mafic minerals than felsic minerals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2205\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2205\"><div tabindex=\"-1\"><p>Eon defined as the time between 4 billion years ago to 2.5 billion years ago. Most of the oldest rocks on Earth, including large portions of the continents, formed at this time.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2575\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2575\"><div tabindex=\"-1\"><p>The layers of igneous, sedimentary, and metamorphic rocks that form the continents. Continental crust is much thicker than oceanic crust. Continental crust is defined as having higher concentrations of&nbsp;very light elements like K, Na, and Ca, and is the lowest density rocky layer of Earth. Its average composition is similar to granite.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2581\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2581\"><div tabindex=\"-1\"><p>The thin, outer layer of the Earth which makes up the rocky bottom of the ocean basins. It is made of rocks similar to basalt, and as it cools, even become more dense than the upper mantle below.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2630\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2630\"><div tabindex=\"-1\"><p>A divergent boundary within an oceanic plate, where new lithosphere and crust is created as the two plates spread apart. Mid-ocean ridge and spreading center are synonyms.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2885\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2885\"><div tabindex=\"-1\"><p>Relatively flat ocean floor, which accumulates very fine grained detrital and chemical sediments.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2678\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2678\"><div tabindex=\"-1\"><p>Pieces of rock that have been weathered and possibly eroded.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2700\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2700\"><div tabindex=\"-1\"><p>A group of all atoms with a specific number of protons, having specific, universal, and unique properties.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2677\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2677\"><div tabindex=\"-1\"><p>The transport and movement of weathered sediments.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3080\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3080\"><div tabindex=\"-1\"><p>Initiation point of an earthquake or fault movement.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1971\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1971\"><div tabindex=\"-1\"><p>Term for the underlying lithified rocks that make up the geologic record in an area. This term can sometimes refer to only the deeper, crystalline (non-layered) rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1969\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1969\"><div tabindex=\"-1\"><p>A narrow igneous intrusion that cuts through existing rock, not along bedding planes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1970\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1970\"><div tabindex=\"-1\"><p>A type of dike that is parallel to bedding planes within the bedrock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2857\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2857\"><div tabindex=\"-1\"><p>Discernible layers of rock, typically from a sedimentary rock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1185\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1185\"><div tabindex=\"-1\"><p>A reservoir of magma below a volcano.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3341\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3341\"><div tabindex=\"-1\"><p>Rocks which allow petroleum resources to collect or move.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1966\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1966\"><div tabindex=\"-1\"><p>A ductile material that moves toward the surface of Earth. Can be used to describe salt domes and intrusions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1924\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1924\"><div tabindex=\"-1\"><p>Minerals made from just a single element, bonded to itself. Examples include gold, silver, copper, and diamond, which is a native version of carbon.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2655\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2655\"><div tabindex=\"-1\"><p>An accepted scientific idea that explains a process using the best available information.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1967\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1967\"><div tabindex=\"-1\"><p>The process of surrounding bedrock being broken off and passed through a magma.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1965\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1965\"><div tabindex=\"-1\"><p>A coherent body of intrusive rock (which formed underground) which is now at (or near) the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1968\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1968\"><div tabindex=\"-1\"><p>Used to describe a large mass or chain of many plutons and intrusive rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2960\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2960\"><div tabindex=\"-1\"><p>An extensive, distinct, and mapped set of geologic layers.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2602\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2602\"><div tabindex=\"-1\"><p>A process where an oceanic plate descends bellow a less dense plate, causing the removal of the plate from the surface. Subduction causes the largest earthquakes possible, as the subducting plate can lock as it goes down. Volcanism is also caused as the plate releases volatiles into the mantle, causing melting.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1172\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1172\"><div tabindex=\"-1\"><p>Large igneous intrusion that is wedged between sedimentary layers, bulging upwards. Called a lopolith if bulging downward.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2689\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2689\"><div tabindex=\"-1\"><p>The measure of the vibrational (kinetic) energy of a substance.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2652\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2652\"><div tabindex=\"-1\"><p>A proposed explanation for an observation that can be tested.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2192\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2192\"><div tabindex=\"-1\"><p>The third largest span of time recognized by geologists; smaller than a era, larger than a epoch. We are currently in the Quaternary period. Rocks of a specific period are called systems.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1173\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1173\"><div tabindex=\"-1\"><p>The study of rocks, either macroscopically or microscopically. This study is typically divided into one of the three rock types (e.g. igneous petrology).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2966\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2966\"><div tabindex=\"-1\"><p>The process of atoms breaking down randomly and spontaneously.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1175\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1175\"><div tabindex=\"-1\"><p>The average change in temperature that is experienced as material moves into the Earth. Near the surface, this rate is about 25\u00b0C\/km.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2589\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2589\"><div tabindex=\"-1\"><p>The innermost chemical layer of the Earth, made chiefly of iron and nickel. It has both liquid and solid components.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1176\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1176\"><div tabindex=\"-1\"><p>Melting that occurs as material is moved upward and pressure is released, typically found at divergent plate boundaries or hot spots.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2608\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2608\"><div tabindex=\"-1\"><p>The process in which volatiles enter the mantle wedge, and the volatiles lower the melting temperature, causing volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2638\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2638\"><div tabindex=\"-1\"><p>Rising stationary magma, forming a succession of volcanism. This is reflected as islands on oceanic plates, and volcanic mountains or craters on land.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2577\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2577\"><div tabindex=\"-1\"><p>The property of unevenly-heated (heated from one direction) fluids (like water, air, ductile solids) in which warmer, less dense parts within the fluid rise while cooler, denser parts sink. This typically creates convection cells: round loops of rising and sinking material.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2593\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2593\"><div tabindex=\"-1\"><p>A ductile physical layer of the Earth, below the lithosphere. Movement within the asthenosphere is the main driver of plate motion, as the overriding lithosphere is pushed by this.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2590\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2590\"><div tabindex=\"-1\"><p>The outermost physical layer of the Earth, made of the entire crust and upper mantle. It is brittle and broken into a series of plates, and these plates move in various ways (relative to one another), causing the features of the theory of plate tectonics.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2605\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2605\"><div tabindex=\"-1\"><p>Name given to the subducting plate, where volatiles are driven out at depth, causing volcanism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2703\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2703\"><div tabindex=\"-1\"><p>Two or more atoms or ions that are connected chemically.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2709\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2709\"><div tabindex=\"-1\"><p>Mineral group in which the silica tetrahedra, SiO4-4, is the building block.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2639\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2639\"><div tabindex=\"-1\"><p>Rising material and heat derived from the mantle. These may be responsible for hot spots.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2946\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2946\"><div tabindex=\"-1\"><p>Metamorphism that occurs with large-scale tectonic processes, like collision zones.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2914\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2914\"><div tabindex=\"-1\"><p>Rocks and minerals that change within the Earth are called metamorphic, changed by heat and pressure. Metamorphism is the name of the process.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2933\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2933\"><div tabindex=\"-1\"><p>A rock transitional between metamorphic and igneous rock, i.e. rocks so metamorphosed that they begin the process of melting.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1179\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1179\"><div tabindex=\"-1\"><p>The process of changing a magma's composition, usually through assimilation or fractionation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2207\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2207\"><div tabindex=\"-1\"><p>An exposed part of a craton.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1178\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1178\"><div tabindex=\"-1\"><p>Bedrock around the magma chamber being incorporated into the magma, sometimes changing the composition of the magma.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2587\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2587\"><div tabindex=\"-1\"><p>A piece of foreign rock that has been incorporated into a magma body. This can be a different type of magma, or a mantle xenolith, a rock from the mantle brought up near the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1182\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1182\"><div tabindex=\"-1\"><p>Activity that occurs at the boundaries between plates.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2624\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2624\"><div tabindex=\"-1\"><p>Area of extended continental lithosphere, forming a depression. Rifts can be narrow (focused in one place) or broad (spread out over a large area with&nbsp;many faults).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1183\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1183\"><div tabindex=\"-1\"><p>Activities that occur within plates, away from plate boundaries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2599\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2599\"><div tabindex=\"-1\"><p>Place where two plates are moving apart, creating either a rift (continental lithosphere) or a mid-ocean ridge (oceanic lithosphere).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2592\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2592\"><div tabindex=\"-1\"><p>Location where two plates are in contact, allowing a relative motion between the two plates. These are the locations where&nbsp;most earthquakes and volcanoes are found.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2921\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2921\"><div tabindex=\"-1\"><p>Metamorphism which occurs with hot fluids going within rocks, altering and changing the rocks.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2922\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2922\"><div tabindex=\"-1\"><p>Mineral chimneys that form at hydrothermal vents.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1184\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1184\"><div tabindex=\"-1\"><p>A biologic process of gaining energy from chemicals from within the Earth, similar to using the energy of the sun in photosynthesis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1921\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1921\"><div tabindex=\"-1\"><p>Minerals bonded via a sulfur (S-2) atom.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2600\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2600\"><div tabindex=\"-1\"><p>Place where two plates come together, casing subduction or collision.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2617\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2617\"><div tabindex=\"-1\"><p>Place with a chain of mountain volcanism on a continent, from oceanic-continental subduction.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1461\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1461\"><div tabindex=\"-1\"><p>A down-warped feature in the crust.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2462\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2462\"><div tabindex=\"-1\"><p>Term for the extensional tectonic province that extends from California's Sierra Nevada Mountains in the west, to Utah's Wasatch Mountains to the east, to southern Oregon and Idaho to the north, to northern Mexico to the south. Known as a wide rift, as each graben 'basin,' bounded by horst 'ranges.' Each set of horsts with a graben has some individual extension, adding up to the overall rifting.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1197\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1197\"><div tabindex=\"-1\"><p>Rare very low viscosity eruption that covers vast areas. None have been observed in human history.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1195\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1195\"><div tabindex=\"-1\"><p>A type of tephra which forms as blobs of magma splatter out of a volcanic vent (e.g. cinder cone) and cool and harden quickly.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1445\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1445\"><div tabindex=\"-1\"><p>Stresses that pull objects apart into a larger surface area or volume; stretching forces.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1188\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1188\"><div tabindex=\"-1\"><p>Hole left behind after a large volume of material erupts out of a volcano. This depression is often tuned into a valley or lake after the eruption is over.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1708\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1708\"><div tabindex=\"-1\"><p>When a species no longer exists.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2899\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2899\"><div tabindex=\"-1\"><p>An eroded island. Since wave and weather action does not extend deep into the ocean, the root of the island is preserved as a seamount. Reefs can grow around seamounts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1193\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1193\"><div tabindex=\"-1\"><p>Volcano with steep sides, made of a composite of many types of eruption styles, from low viscosity mafic magma, higher viscosity felsic lava, but most commonly, intermediate andesite lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1186\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1186\"><div tabindex=\"-1\"><p>Pipe that connects the magma chamber to the volcanic vent.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1187\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1187\"><div tabindex=\"-1\"><p>Opening of a volcano where lava can erupt.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1189\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1189\"><div tabindex=\"-1\"><p>Small side vent of a stratovolcano where secondary eruption can occur.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1190\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1190\"><div tabindex=\"-1\"><p>Volcano with a gentle slope, formed from low viscosity, low volatile, mafic, basaltic lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2201\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2201\"><div tabindex=\"-1\"><p>A collection of planets orbiting around a star.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1192\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1192\"><div tabindex=\"-1\"><p>Rope-like, flowing basaltic lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1191\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1191\"><div tabindex=\"-1\"><p>A blocky, stubby, rubble-like lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3134\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3134\"><div tabindex=\"-1\"><p>A channelled body of water.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3087\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3087\"><div tabindex=\"-1\"><p>Energy that radiates from fault movement via earthquakes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1460\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1460\"><div tabindex=\"-1\"><p>A rock up-warping of symmetrical anticlines.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1194\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1194\"><div tabindex=\"-1\"><p>Very steep sided volcanic feature formed by higher viscosity, higher-silica lava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2664\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2664\"><div tabindex=\"-1\"><p>An interconnected set of parts that combine and make up a whole.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1700\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1700\"><div tabindex=\"-1\"><p>A period of cooler temperatures on Earth in which ice sheets can grow on continents.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1196\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1196\"><div tabindex=\"-1\"><p>Volcano formed from piles of cinders and tephra. Forms with low viscosity lava with high volatile content.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3342\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3342\"><div tabindex=\"-1\"><p>A geologic circumstance (such as a fold, fault, change in lithology, etc.) which allows petroleum resources to collect.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2223\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2223\"><div tabindex=\"-1\"><p>A pronounced increase in the extinction rate, typically caused by significant environmental change. There have been 5 mass extinctions in geologic history, and a sixth that has been suggested to be currently occurring.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3365\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3365\"><div tabindex=\"-1\"><p>An igneous composition or rock containing more than 50% carbonate minerals (e.g. calcite). Magma of this composition is very low temperature (500-600 C) relative to other magmas.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1917\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1917\"><div tabindex=\"-1\"><p>Mineral group in which the carbonate ion, CO3-2, is the building block. This can also refer to the rocks that are made from these minerals, namely limestone and dolomite (dolostone).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1199\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1199\"><div tabindex=\"-1\"><p>General term for sudden material falling down a slope due to gravity. The term can sometimes cover a wide range of events, including debris flows, rock falls, and mudslides.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1198\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1198\"><div tabindex=\"-1\"><p>A collapsed part of the eruption column that travels down at very hot temperatures and very fast speeds. They are the most dangerous immediate volcanic hazard.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2179\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2179\"><div tabindex=\"-1\"><p>Material filling in a cavity left by a organism that has dissolved away.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3129\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3129\"><div tabindex=\"-1\"><p>Water that is below the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3194\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3194\"><div tabindex=\"-1\"><p>Formally known as a tidal wave, it is a large wave produced from a sudden movement of the floor of a ocean basin, caused by events such as earthquakes, volcanic eruptions, landslides, and bolide impacts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3119\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3119\"><div tabindex=\"-1\"><p>Detached, free-falling rocks from very steep slopes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2917\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2917\"><div tabindex=\"-1\"><p>Non-directional pressure resulting from burial.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1920\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1920\"><div tabindex=\"-1\"><p>Minerals based on bonds to column 17 halogens, such as chlorine and fluorine.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_3117\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_3117\"><div tabindex=\"-1\"><p>An event that causes a landslide event. Water is a common trigger.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1933\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1933\"><div tabindex=\"-1\"><p>A feature with no internal structure, habit, or layering.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1201\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1201\"><div tabindex=\"-1\"><p>A type of volcanic mudslide, in which rain or snowmelt accumulates volcanic ash of the slopes of steep volcanoes or other mountains and then wash downhill, causing damaging flooding.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2464\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2464\"><div tabindex=\"-1\"><p>A body of ice that moves downhill under its own mass.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2620\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2620\"><div tabindex=\"-1\"><p>When two continents crash, with no subduction (and thus little to no volcanism), since each continent is to buoyant. Many of the largest mountain ranges and broadest zones of seismic activity come from collisions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2218\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2218\"><div tabindex=\"-1\"><p>A term for the collective time before the Phanerozoic (pre-541 million years ago), including the Hadean, Archean, and Proterozoic. Known for a lack of easy-to-find fossils.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2859\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2859\"><div tabindex=\"-1\"><p>The study of rock layers and their relationships to each other within a specific area.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_1446\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_1446\"><div tabindex=\"-1\"><p>Stresses that push objects together into a smaller surface area or volume; contracting forces.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_330_2958\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_330_2958\"><div tabindex=\"-1\"><p>A piece of a rock that is caught up inside of another rock.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":83,"menu_order":4,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-330","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":19,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/330","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/users\/83"}],"version-history":[{"count":3,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/330\/revisions"}],"predecessor-version":[{"id":3405,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/330\/revisions\/3405"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/parts\/19"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/330\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/media?parent=330"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapter-type?post=330"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/contributor?post=330"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/license?post=330"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}