{"id":138,"date":"2021-09-16T19:46:12","date_gmt":"2021-09-16T19:46:12","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/"},"modified":"2025-01-21T01:03:27","modified_gmt":"2025-01-21T01:03:27","slug":"2-plate-tectonics","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/2-plate-tectonics\/","title":{"raw":"2 Plate Tectonics","rendered":"2 Plate Tectonics"},"content":{"raw":"[caption id=\"attachment_2498\" align=\"aligncenter\" width=\"2048\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/MT3.jpg\"><img class=\"wp-image-67 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3.jpg\" alt=\"The rock is getting thinner farther away.\" width=\"2048\" height=\"1536\"><\/a> A layer of shallow ocean limestone (white) has been brought to the top of a mountain by the convergent forces of the Sevier Orogeny. Near Sun River Canyon, Montana.[\/caption]\n<h1>2 Plate Tectonics<\/h1>\n<b>KEY CONCEPTS<\/b>\n\n<b>At the end of this chapter, students should be able to:<\/b>\n<ul>\n \t<li>Describe how the ideas behind [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] started with Alfred Wegener\u2019s [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] of [pb_glossary id=\"2575\"]continental[\/pb_glossary]\u00a0drift<\/li>\n \t<li>Describe the physical and chemical layers of the Earth and how they affect [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement<\/li>\n \t<li>Explain how movement at the three types of [pb_glossary id=\"2591\"]plate[\/pb_glossary] boundaries causes earthquakes, [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary], and mountain building<\/li>\n \t<li>Identify [pb_glossary id=\"2600\"]convergent[\/pb_glossary] boundaries, including [pb_glossary id=\"2602\"]subduction[\/pb_glossary] and collisions, as places where [pb_glossary id=\"2591\"]plates[\/pb_glossary]\u00a0come together<\/li>\n \t<li>Identify [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries, including [pb_glossary id=\"2624\"]rifts[\/pb_glossary]\u00a0and [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], as places where [pb_glossary id=\"2591\"]plates[\/pb_glossary] separate<\/li>\n \t<li>Explain [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries as places where adjacent [pb_glossary id=\"2591\"]plates[\/pb_glossary] [pb_glossary id=\"1447\"]shear[\/pb_glossary] past each other<\/li>\n \t<li>Describe the [pb_glossary id=\"2637\"]Wilson Cycle[\/pb_glossary], beginning with [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary], ocean [pb_glossary id=\"1461\"]basin[\/pb_glossary] creation, [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2602\"]subduction[\/pb_glossary], and ending with ocean [pb_glossary id=\"1461\"]basin[\/pb_glossary] closure<\/li>\n \t<li>Explain how the tracks of [pb_glossary id=\"2638\"]hotspots[\/pb_glossary], places that have continually rising [pb_glossary id=\"2672\"]magma[\/pb_glossary], is used to calculate [pb_glossary id=\"2591\"]plate[\/pb_glossary] motion<\/li>\n<\/ul>\n[caption id=\"attachment_2499\" align=\"aligncenter\" width=\"4898\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Tectonic_plates_boundaries_detailed-en.svg_.png\"><img class=\"size-full wp-image-68\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_.png\" alt=\"The map shows many plates.\" width=\"4898\" height=\"2461\"><\/a> Detailed map of all known plates, their boundaries, and movements.[\/caption]\n\nRevolution is a word usually reserved for significant political or social changes. Several of these idea revolutions forced scientists to re-examine their entire field, triggering a paradigm shift that shook up their conventionally held knowledge. Charles Darwin\u2019s book on evolution, <em>On the Origin of Species<\/em>, published in 1859; Gregor Mendel\u2019s discovery of the genetic principles of inheritance in 1866; and James Watson, Francis Crick, and Rosalind Franklin\u2019s model for the structure of DNA in 1953 did that for biology. Albert Einstein\u2019s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics.\n\nThe concept of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] was just as revolutionary for geology. The [pb_glossary id=\"2655\"]theory[\/pb_glossary] of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] attributes the movement of [pb_glossary id=\"1933\"]massive[\/pb_glossary] sections of the Earth\u2019s outer layers with creating earthquakes, mountains, and [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary]. Many earth processes make more sense when viewed through the lens of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary]. Because it is so important in understanding how the world works, [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] is the first topic of discussion in this textbook.\n<h2><span style=\"font-weight: 400;\">2.1 Alfred Wegener\u2019s Continental Drift Hypothesis<\/span><\/h2>\n[caption id=\"attachment_2500\" align=\"alignleft\" width=\"195\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Alfred_Wegener_ca.1924-30-2.jpg\"><img class=\"size-full wp-image-28\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alfred_Wegener_ca.1924-30-2.jpg\" alt=\"He is a male in a suit.\" width=\"195\" height=\"240\"><\/a> Wegener later in his life, ca. 1924-1930.[\/caption]\n\nAlfred Wegener (1880-1930) was a German scientist who specialized in meteorology and climatology. His knack for questioning accepted ideas started in 1910 when he disagreed with the explanation that the Bering Land Bridge was formed by [pb_glossary id=\"1706\"]isostasy[\/pb_glossary], and that similar land bridges once connected the continents. After reviewing the scientific literature, he published a [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] stating the continents were originally connected, and then drifted apart. While he did not have the precise mechanism worked out, his [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] was backed up by a long list of evidence.\n<h3><\/h3>\n<h3><b>2.1.1 Early Evidence for Continental Drift Hypothesis<\/b><\/h3>\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic.jpg\"><img class=\"size-medium wp-image-69\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-300x177.jpg\" alt=\"It shows South America and Africa connected, then apart.\" width=\"300\" height=\"177\"><\/a> Snider-Pellegrini's map showing the continental fit and separation, 1858.[\/caption]\n\nWegener\u2019s first piece of evidence was that the coastlines of some continents fit together like pieces of a jigsaw puzzle. People noticed the similarities in the coastlines of South America and Africa on the first world maps, and some suggested the continents had been ripped apart. Antonio Snider-Pellegrini did preliminary work on [pb_glossary id=\"2575\"]continental[\/pb_glossary] separation and matching [pb_glossary id=\"2176\"]fossils[\/pb_glossary] in 1858.\n\n[caption id=\"attachment_2502\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/ElevationWorld.jpg\"><img class=\"size-medium wp-image-70\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-300x150.jpg\" alt=\"The shape of the continents is different than what is seen by just coastlines.\" width=\"300\" height=\"150\"><\/a> Map of world elevations. Note the light blue, which are continental shelves flooded by shallow ocean water. These show the true shapes of the continents.[\/caption]\n\nWhat Wegener did differently was synthesize a large amount of data in one place. He used true edges of the continents, based on the shapes of the [pb_glossary id=\"2891\"]continental[\/pb_glossary] shelves. This resulted in a better fit than previous efforts that traced the existing coastlines.\n\n[caption id=\"attachment_3259\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Wegener_fossil_map.svg_.png\"><img class=\"size-medium wp-image-71\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-300x231.png\" alt=\"There are four different fossil organisms that connect South America, Africa, India, Antartica, and Australia.\" width=\"300\" height=\"231\"><\/a> Image showing fossils that connect the continents of Gondwana (the southern continents of Pangea).[\/caption]\n\n&nbsp;\n\nWegener also compiled evidence by comparing similar rocks, mountains, [pb_glossary id=\"2176\"]fossils[\/pb_glossary], and [pb_glossary id=\"2910\"]glacial[\/pb_glossary] [pb_glossary id=\"2960\"]formations[\/pb_glossary] across oceans. For example, the [pb_glossary id=\"2176\"]fossils[\/pb_glossary] of the primitive aquatic reptile <em>Mesosaurus<\/em> were found on the separate coastlines of Africa and South America. [pb_glossary id=\"2176\"]Fossils[\/pb_glossary] of another reptile, <em>Lystrosaurus,<\/em> were found on Africa, India, and Antarctica. He pointed out these were land-dwelling creatures could not have swum across an entire ocean.\n\nOpponents of [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift insisted trans-[pb_glossary id=\"2581\"]oceanic [\/pb_glossary] land bridges allowed animals and plants to move between continents. The land bridges eventually eroded away, leaving the continents permanently separated. The problem with this [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] is the improbability of a land bridge being tall and long enough to stretch across a broad, deep ocean.\n\nMore support for [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift came from the puzzling evidence that [pb_glossary id=\"2464\"]glaciers[\/pb_glossary] once existed in normally very warm areas in southern Africa, India, Australia, and Arabia. These [pb_glossary id=\"1710\"]climate[\/pb_glossary] [pb_glossary id=\"1719\"]anomalies[\/pb_glossary] could not be explained by land bridges. Wegener found similar evidence when he discovered tropical plant [pb_glossary id=\"2176\"]fossils[\/pb_glossary] in the frozen region of the Arctic Circle. As Wegener collected more data, he realized the explanation that best fit all the [pb_glossary id=\"1710\"]climate[\/pb_glossary], rock, and [pb_glossary id=\"2176\"]fossil[\/pb_glossary] observations involved moving continents.\n<h3><b>2.1.2 Proposed Mechanism for Continental Drift<\/b><\/h3>\n[caption id=\"attachment_2504\" align=\"alignleft\" width=\"400\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Convection.gif\"><img class=\"wp-image-72 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Convection.gif\" alt=\"The rising material is drawn red. The cool material is blue.\" width=\"400\" height=\"300\"><\/a> Animation of the basic idea of convection: an uneven heat source in a fluid causes rising material next to the heat and sinking material far from the heat.[\/caption]\n\n&nbsp;\n\nWegener\u2019s work was considered a fringe science [pb_glossary id=\"2655\"]theory[\/pb_glossary] for his entire life. One of the biggest flaws in his [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] was an inability to provide a mechanism for how the continents moved. Obviously, the continents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that supported a credible mechanism. Other pro-[pb_glossary id=\"2575\"]continental[\/pb_glossary] drift followers used expansion, contraction, or even the moon\u2019s origin to explain how the continents moved. Wegener used centrifugal forces and [pb_glossary id=\"1702\"]precession[\/pb_glossary], but this model was proven wrong. He also speculated about seafloor spreading, with hints of [pb_glossary id=\"2577\"]convection[\/pb_glossary], but could not substantiate these proposals. As it turns out, current scientific knowledge reveals [pb_glossary id=\"2577\"]convection[\/pb_glossary] is one the major forces in driving [pb_glossary id=\"2591\"]plate[\/pb_glossary] movements, along with gravity and density.\n<h3><b>2.1.3 Development of Plate Tectonic Theory<\/b><\/h3>\n[caption id=\"attachment_2505\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Global_plate_motion_2008-04-17.jpg\"><img class=\"size-medium wp-image-73\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-300x212.jpg\" alt=\"The map shows many data points all over the world.\" width=\"300\" height=\"212\"><\/a> GPS measurements of plate motions.[\/caption]\n\n&nbsp;\n\nWegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science. However, in the 1950s, evidence started to trickle in that made [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism\u2014namely, seafloor spreading\u2014for Wegener\u2019s [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] of [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift to be accepted as the [pb_glossary id=\"2655\"]theory[\/pb_glossary] of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary]. Ongoing GPS and earthquake data analyses continue to support this [pb_glossary id=\"2655\"]theory[\/pb_glossary]. The next section provides the pieces of evidence that helped [pb_glossary id=\"2601\"]transform[\/pb_glossary] one man\u2019s wild notion into a scientific [pb_glossary id=\"2655\"]theory[\/pb_glossary].\n<h4><span style=\"font-weight: 400;\">Mapping of the Ocean Floors<\/span><\/h4>\n[caption id=\"attachment_2506\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Deep_sea_vent_chemistry_diagram.jpg\"><img class=\"size-medium wp-image-74\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-300x174.jpg\" alt=\"The diagram shows water going into the ground and coming out, with many different reactions.\" width=\"300\" height=\"174\"><\/a> The complex chemistry around mid-ocean ridges.[\/caption]\n\n&nbsp;\n\nIn 1947 researchers started using an adaptation of [pb_glossary id=\"3364\"]SONAR[\/pb_glossary] to map a region in the middle of the Atlantic Ocean with poorly-understood topographic and thermal properties. Using this information, Bruce Heezen and Marie Tharp created the first detailed map of the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] to reveal the Mid-Atlantic Ridge, a basaltic mountain range that spanned the length of the Atlantic Ocean, with rock chemistry and dimensions unlike the mountains found on the continents. Initially scientists thought the ridge was part of a mechanism that explained the expanding Earth or ocean-[pb_glossary id=\"1461\"]basin[\/pb_glossary] growth [pb_glossary id=\"2652\"]hypotheses[\/pb_glossary]. In 1959, Harry Hess proposed the [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] of seafloor spreading \u2013 that the [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] represented [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] factories, where new [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] was issuing from these long [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] ridges. Scientists later included [pb_glossary id=\"2601\"]transform[\/pb_glossary] [pb_glossary id=\"3916\"]faults[\/pb_glossary] perpendicular to the ridges to better account for varying rates of movement between the newly formed [pb_glossary id=\"2591\"]plates[\/pb_glossary]. When earthquake [pb_glossary id=\"3085\"]epicenters[\/pb_glossary] were discovered along the ridges, the idea that earthquakes were linked to [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement took hold.\n<div style=\"height: 0; padding-bottom: 56.25%;\">\n\n[embed]https:\/\/youtu.be\/TgfYjS0OTWw[\/embed]\n\n<\/div>\n\n[caption id=\"attachment_3739\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Marie-Tharp-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-75\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-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\nSeafloor [pb_glossary id=\"2678\"]sediment[\/pb_glossary], measured by dredging and drilling, provided another clue. Scientists once believed [pb_glossary id=\"2678\"]sediment[\/pb_glossary] accumulated on the ocean floors over a very long time in a static environment. When some studies showed less [pb_glossary id=\"2678\"]sediment[\/pb_glossary] than expected, these results were initially used to argue against [pb_glossary id=\"2575\"]continental[\/pb_glossary] movement. With more time, researchers discovered these thinner [pb_glossary id=\"2678\"]sediment[\/pb_glossary] layers were located close to [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], indicating the ridges were younger than the surrounding [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]. This finding supported the idea that the sea floor was not fixed in one place.\n<h4><span style=\"font-weight: 400;\">Paleomagnetism<\/span><\/h4>\n[caption id=\"attachment_2507\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earths_magnetic_field_schematic.svg_.png\"><img class=\"size-medium wp-image-76\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-300x250.png\" alt=\"The north end of the magnet is south topographically, and vice versa.\" width=\"300\" height=\"250\"><\/a> The magnetic field of Earth, simplified as a bar magnet.[\/caption]\n\nThe seafloor was also mapped magnetically. Scientists had long known of strange magnetic [pb_glossary id=\"1719\"]anomalies[\/pb_glossary] that formed a striped pattern of symmetrical rows on both sides of mid-[pb_glossary id=\"2581\"]oceanic[\/pb_glossary] ridges. What made these features unusual was the north and south magnetic poles within each stripe was reversed in alternating rows. By 1963, Harry Hess and other scientists used these magnetic reversal patterns to support their model for seafloor spreading (see also Lawrence W. Morley).\n\n[caption id=\"attachment_2508\" align=\"alignleft\" width=\"351\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earth_Magnetic_Field_Declination_from_1590_to_1990.gif\"><img class=\"wp-image-77 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth_Magnetic_Field_Declination_from_1590_to_1990.gif\" alt=\"The poles shift slightly every year.\" width=\"351\" height=\"293\"><\/a> This animation shows how the magnetic poles have moved over 400 years.[\/caption]\n\n[pb_glossary id=\"2579\"]Paleomagnetism[\/pb_glossary] is the study of magnetic fields frozen within rocks, basically a fossilized compass. In fact, the first hard evidence to support [pb_glossary id=\"2591\"]plate[\/pb_glossary] motion came from [pb_glossary id=\"2579\"]paleomagnetism[\/pb_glossary].\n\n[pb_glossary id=\"2675\"]Igneous[\/pb_glossary] rocks containing magnetic [pb_glossary id=\"2687\"]minerals[\/pb_glossary] like magnetite typically provide the most useful data. In their liquid state as [pb_glossary id=\"2672\"]magma[\/pb_glossary] or [pb_glossary id=\"2673\"]lava[\/pb_glossary], the magnetic poles of the [pb_glossary id=\"2687\"]minerals[\/pb_glossary] align themselves with the Earth\u2019s magnetic field. When the rock cools and solidifies, this alignment is frozen into place, creating a permanent paleomagnetic record that includes magnetic inclination related to global [pb_glossary id=\"3372\"]latitude[\/pb_glossary], and declination related to magnetic north.\n\n[caption id=\"attachment_2509\" align=\"alignright\" width=\"240\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.1-magnetic_stripes.gif\"><img class=\"size-full wp-image-78\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.1-magnetic_stripes.gif\" alt=\"Animated gif depicting a mid-ocean ridge with two oceanic plates moving away from the center of the ridge. As the movement progresses, symettrical magnetic stripes appear on each side of the ridge.\" width=\"240\" height=\"180\"><\/a> The iron in the solidifying rock preserves the current magnetic polarity as new oceanic plates form at mid ocean ridges[\/caption]\n\nScientists had noticed for some time the alignment of magnetic north in many rocks was nowhere close to the earth\u2019s current magnetic north. Some explained this away are part of the normal movement of earth\u2019s magnetic north pole. Eventually, scientists realized adding the idea of [pb_glossary id=\"2575\"]continental[\/pb_glossary] movement explained the data better than pole movement alone.\n<h4><\/h4>\n<h4><\/h4>\n<h4><span style=\"font-weight: 400;\">Wadati-Benioff Zones<\/span><\/h4>\n[caption id=\"attachment_2510\" align=\"alignleft\" width=\"297\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/benioff_zone.gif\"><img class=\"size-full wp-image-79\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/benioff_zone.gif\" alt=\"The earthquakes descend at an angle into the Earth.\" width=\"297\" height=\"243\"><\/a> The Wadati-Benioff zone, showing earthquakes following the subducting slab down.[\/caption]\n\nAround the same time [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] were being investigated, other scientists linked the creation of ocean trenches and island arcs to [pb_glossary id=\"3087\"]seismic[\/pb_glossary] activity and [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement. Several independent research groups recognized earthquake [pb_glossary id=\"3085\"]epicenters[\/pb_glossary] traced the shapes of [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] sinking into the [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. These deep earthquake zones congregated in planes that started near the surface around ocean trenches and angled beneath the continents and island arcs. Today these earthquake zones called Wadati-Benioff zones.\n\n[caption id=\"attachment_2511\" align=\"alignright\" width=\"244\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/John_Tuzo_Wilson_in_1992-2.jpg\"><img class=\"size-medium wp-image-39\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-244x300.jpg\" alt=\"He is an older man in this 1992 image.\" width=\"244\" height=\"300\"><\/a> J. Tuzo Wilson[\/caption]\n\n&nbsp;\n\nBased on the mounting evidence, the [pb_glossary id=\"2655\"]theory[\/pb_glossary] [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] continued to take shape. J. Tuzo Wilson was the first scientist to put the entire picture together by proposing that the opening and closing of the ocean basins. Before long, scientists proposed other models showing [pb_glossary id=\"2591\"]plates[\/pb_glossary] moving with respect to each other, with clear boundaries between them. Others started piecing together complicated histories of [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement. The [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] revolution had taken hold.\n\n&nbsp;\n\n&nbsp;\n\n[h5p id=\"8\"]\n\n[caption id=\"attachment_3738\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Continental-Drive-Activity-QR-Code.png\"><img class=\"size-thumbnail wp-image-80\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-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=\"9\"]\n\n[caption id=\"attachment_3737\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.1-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-81\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-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 2.1 via this QR Code.[\/caption]\n<h2><span style=\"font-weight: 400;\">2.2 Layers of the Earth<\/span><\/h2>\n&nbsp;\n\n[caption id=\"attachment_2512\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earth-cutaway-schematic-english.svg_-1.png\"><img class=\"size-medium wp-image-51\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-300x211.png\" alt=\"The crust and lithosphere are on the outside of the Earth and are thin. Below the crust is the mantle and core. Below the lithosphere is the asthenosphere.\" width=\"300\" height=\"211\"><\/a> The layers of the Earth. Physical layers include lithosphere and asthenosphere; chemical layers are crust, mantle, and core.[\/caption]\n\nIn order to understand the details of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary], it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing [pb_glossary id=\"3087\"]seismic wave[\/pb_glossary] data and [pb_glossary id=\"2202\"]meteorite[\/pb_glossary] materials. In general, the Earth can be divided into layers based on chemical [pb_glossary id=\"2831\"]composition[\/pb_glossary] and physical characteristics.\n<h3><b>2.2.1 Chemical Layers<\/b><\/h3>\nCertainly the earth is [pb_glossary id=\"2831\"]composed[\/pb_glossary] of a countless combination of [pb_glossary id=\"2700\"]elements[\/pb_glossary]. Regardless of what [pb_glossary id=\"2700\"]elements[\/pb_glossary] are involved two major factors\u2014[pb_glossary id=\"2689\"]temperature[\/pb_glossary] and pressure\u2014are responsible for creating three distinct chemical layers.\n<h4><span style=\"font-weight: 400;\">Crust<\/span><\/h4>\nThe outermost chemical layer and the one we currently reside on, is the [pb_glossary id=\"2580\"]crust[\/pb_glossary]. There are two types of [pb_glossary id=\"2580\"]crust[\/pb_glossary]. [pb_glossary id=\"2575\"]Continental crust[\/pb_glossary] has a relatively low density and [pb_glossary id=\"2831\"]composition[\/pb_glossary] similar to [pb_glossary id=\"1962\"]granite[\/pb_glossary]. [pb_glossary id=\"2581\"]Oceanic crust[\/pb_glossary] has a relatively high density, especially when cold and old, and [pb_glossary id=\"2831\"]composition[\/pb_glossary] similar to [pb_glossary id=\"1961\"]basalt[\/pb_glossary]. The surface levels of [pb_glossary id=\"2580\"]crust[\/pb_glossary] are relatively [pb_glossary id=\"2583\"]brittle[\/pb_glossary]. The deeper parts of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] are subjected to higher temperatures and pressure, which makes them more [pb_glossary id=\"2582\"]ductile[\/pb_glossary]. [pb_glossary id=\"2582\"]Ductile[\/pb_glossary] materials are like soft plastics or putty, they move under force. [pb_glossary id=\"2583\"]Brittle[\/pb_glossary] materials are like solid glass or pottery, they break under force, especially when it is applied quickly. Earthquakes, generally occur in the upper [pb_glossary id=\"2580\"]crust[\/pb_glossary] and are caused by the rapid movement of relatively [pb_glossary id=\"2583\"]brittle[\/pb_glossary] materials.\n\n&nbsp;\n\n[caption id=\"attachment_2513\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/MohoDepth-1.png\"><img class=\"size-medium wp-image-50\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-300x167.png\" alt=\"Places with mountain building have a deeper moho.\" width=\"300\" height=\"167\"><\/a> The global map of the depth of the moho.[\/caption]\n\nThe base of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] is characterized by a large increase in [pb_glossary id=\"3087\"]seismic[\/pb_glossary] velocity, which measures how fast earthquake waves travel through solid matter. Called the Mohorovi\u010di\u0107 Discontinuity, or [pb_glossary id=\"2584\"]Moho[\/pb_glossary] for short, this zone was discovered by Andrija Mohorovi\u010di\u0107 (pronounced mo-ho-ro-vee-cheech; <a href=\"https:\/\/www.merriam-webster.com\/dictionary\/Mohorovicic%20discontinuity\">audio pronunciation<\/a>) in 1909 after studying earthquake wave paths in his [pb_glossary id=\"1924\"]native[\/pb_glossary] Croatia. The change in wave direction and speed is caused by dramatic chemical differences of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] and [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. Underneath the oceans, the [pb_glossary id=\"2584\"]Moho[\/pb_glossary] is found roughly 5 km below the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]. Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events known as orogenies, the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2584\"]Moho[\/pb_glossary] depth is doubled.\n<h4><span style=\"font-weight: 400;\">Mantle<\/span><\/h4>\n[caption id=\"attachment_2514\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Iddingsite.jpg\"><img class=\"size-medium wp-image-82\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-300x225.jpg\" alt=\"The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.\" width=\"300\" height=\"225\"><\/a> This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.[\/caption]\n\nThe [pb_glossary id=\"2586\"]mantle[\/pb_glossary] sits below the [pb_glossary id=\"2580\"]crust[\/pb_glossary] and above the [pb_glossary id=\"2589\"]core[\/pb_glossary]. It is the largest chemical layer by volume, extending from the base of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] to a depth of about 2900 km. Most of what we know about the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] comes from [pb_glossary id=\"3087\"]seismic wave[\/pb_glossary] analysis, though information is gathered by studying [pb_glossary id=\"2622\"]ophiolites[\/pb_glossary] and [pb_glossary id=\"2587\"]xenoliths[\/pb_glossary]. [pb_glossary id=\"2622\"]Ophiolites[\/pb_glossary] are pieces of [pb_glossary id=\"2586\"]mantle[\/pb_glossary] that have risen through the [pb_glossary id=\"2580\"]crust[\/pb_glossary] until they are exposed as part of the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]. [pb_glossary id=\"2587\"]Xenoliths[\/pb_glossary] are carried within [pb_glossary id=\"2672\"]magma[\/pb_glossary] and brought to the Earth\u2019s surface by volcanic eruptions. Most [pb_glossary id=\"2587\"]xenoliths[\/pb_glossary] are made of [pb_glossary id=\"2588\"]peridotite[\/pb_glossary], an [pb_glossary id=\"1957\"]ultramafic[\/pb_glossary] class of [pb_glossary id=\"2675\"]igneous rock[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes#4-2BowensReaction\" target=\"_blank\" rel=\"noopener\">chapter 4.2<\/a> for explanation). Because of this, scientists hypothesize most of the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] is made of [pb_glossary id=\"2588\"]peridotite[\/pb_glossary].\n<h4><span style=\"font-weight: 400;\">Core<\/span><\/h4>\n[caption id=\"attachment_2515\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.2_TolucaMeteorite.jpg\"><img class=\"size-medium wp-image-83\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-300x225.jpg\" alt=\"The meteorite is polished showing the Widmanst\u00e4tten Pattern.\" width=\"300\" height=\"225\"><\/a> A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanst\u00e4tten Pattern.[\/caption]\n\n&nbsp;\n\nThe [pb_glossary id=\"2589\"]core[\/pb_glossary] of the Earth, which has both liquid and solid layers, and consists mostly of iron, nickel, and possibly some oxygen. Scientists looking at [pb_glossary id=\"3087\"]seismic[\/pb_glossary] data first discovered this innermost chemical layer in 1906. Through a union of hypothetical modeling, astronomical insight, and hard [pb_glossary id=\"3087\"]seismic[\/pb_glossary] data, they concluded the [pb_glossary id=\"2589\"]core[\/pb_glossary] is mostly [pb_glossary id=\"3345\"]metallic[\/pb_glossary] iron. Scientists studying [pb_glossary id=\"2202\"]meteorites[\/pb_glossary], which typically contain more iron than surface rocks, have proposed the earth was formed from meteoric material. They believe the liquid component of the [pb_glossary id=\"2589\"]core[\/pb_glossary] was created as the iron and nickel sank into the center of the planet, where it was liquefied by intense pressure.\n<h3><b>2.2.2 Physical Layers<\/b><\/h3>\n<span style=\"font-weight: 400;\">The Earth can also be broken down into five distinct physical layers based on how each layer responds to [pb_glossary id=\"2915\"]stress[\/pb_glossary]. While there is some overlap in the chemical and physical designations of layers, specifically the [pb_glossary id=\"2589\"]core[\/pb_glossary]-[pb_glossary id=\"2586\"]mantle[\/pb_glossary] boundary, there are significant differences between the two systems.<\/span>\n<h4><span style=\"font-weight: 400;\">Lithosphere<\/span><\/h4>\n[caption id=\"attachment_2516\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Plates_tect2_en.svg_-2.png\"><img class=\"size-medium wp-image-49\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-300x205.png\" alt=\"There are about 10 major plates\" width=\"300\" height=\"205\"><\/a> Map of the major plates and their motions along boundaries.[\/caption]\n\n<em>Lithos<\/em> is Greek for stone, and the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is the outermost physical layer of the Earth. It is grouped into two types: [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] and [pb_glossary id=\"2575\"]continental[\/pb_glossary]. [pb_glossary id=\"2581\"]Oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is thin and relatively rigid. It ranges in thickness from nearly zero in new [pb_glossary id=\"2591\"]plates[\/pb_glossary] found around [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], to an average of 140 km in most other locations. [pb_glossary id=\"2575\"]Continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is generally thicker and considerably more plastic, especially at the deeper levels. Its thickness ranges from 40 to 280 km. The [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is not continuous. It is broken into segments called [pb_glossary id=\"2591\"]plates[\/pb_glossary]. A [pb_glossary id=\"2592\"]plate boundary[\/pb_glossary] is where two [pb_glossary id=\"2591\"]plates[\/pb_glossary] meet and move relative to each other. [pb_glossary id=\"2591\"]Plate[\/pb_glossary] boundaries are where we see [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] in action\u2014mountain building, triggering earthquakes, and generating [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activity.\n<h4><span style=\"font-weight: 400;\">Asthenosphere<\/span><\/h4>\n[caption id=\"attachment_2517\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earths_Inner_Layers_denoting_the_LAB.png\"><img class=\"size-medium wp-image-84\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-300x207.png\" alt=\"It is thin at a mid-ocean ridge, thick under collisions\" width=\"300\" height=\"207\"><\/a> The lithosphere-asthenosphere boundary changes with certain tectonic situations.[\/caption]\n\n&nbsp;\n\nThe [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] is the layer below the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary]. <em>Astheno-<\/em> means lacking strength, and the most distinctive property of the [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] is movement. Because it is mechanically weak, this layer moves and flows due to [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents created by heat coming from the earth\u2019s [pb_glossary id=\"2589\"]core[\/pb_glossary] cause. Unlike the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] that consists of multiple [pb_glossary id=\"2591\"]plates[\/pb_glossary], the [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] is relatively unbroken. Scientists have determined this by analyzing [pb_glossary id=\"3087\"]seismic[\/pb_glossary] waves that pass through the layer. The depth of at which the [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] is found is [pb_glossary id=\"2689\"]temperature[\/pb_glossary]-dependent. It tends to lie closer to the earth\u2019s surface around [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] and much deeper underneath mountains and the centers of lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary].\n<h4><span style=\"font-weight: 400;\">Mesosphere<\/span><\/h4>\n&nbsp;\n\n[caption id=\"attachment_2518\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Perovskite.jpg\"><img class=\"size-medium wp-image-85\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-300x288.jpg\" alt=\"The atoms are arranged.\" width=\"300\" height=\"288\"><\/a> General perovskite structure. Perovskite silicates (i.e.<br>Bridgmenite,<br>(Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth.[\/caption]\n\nThe [pb_glossary id=\"2594\"]mesosphere[\/pb_glossary], sometimes known as the lower [pb_glossary id=\"2586\"]mantle[\/pb_glossary], is more rigid and immobile than the [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary]. Located at a depth of approximately 410 and 660 km below the earth\u2019s surface, the [pb_glossary id=\"2594\"]mesosphere[\/pb_glossary] is subjected to very high pressures and temperatures. These extreme conditions create a transition zone in the upper [pb_glossary id=\"2594\"]mesosphere[\/pb_glossary] where [pb_glossary id=\"2687\"]minerals[\/pb_glossary] continuously change into various forms, or pseudomorphs. Scientists identify this zone by changes in [pb_glossary id=\"3087\"]seismic[\/pb_glossary] velocity and sometimes physical barriers to movement. Below this transitional zone, the [pb_glossary id=\"2594\"]mesosphere[\/pb_glossary] is relatively uniform until it reaches the [pb_glossary id=\"2589\"]core[\/pb_glossary].\n<h4><span style=\"font-weight: 400;\">Inner and Outer Core<\/span><\/h4>\n[caption id=\"attachment_2519\" align=\"alignleft\" width=\"206\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Inge_Lehmann_1932.jpg\"><img class=\"size-medium wp-image-86\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-206x300.jpg\" alt=\"Is shows her as a young woman\" width=\"206\" height=\"300\"><\/a> Lehmann in 1932[\/caption]\n\nThe [pb_glossary id=\"2595\"]outer core[\/pb_glossary] is the only entirely liquid layer within the Earth. It starts at a depth of 2,890 km and extends to 5,150 km, making it about 2,300 km thick. In 1936, the Danish geophysicist Inge Lehmann analyzed [pb_glossary id=\"3087\"]seismic[\/pb_glossary] data and was the first to prove a solid [pb_glossary id=\"2596\"]inner core[\/pb_glossary] existed within a liquid [pb_glossary id=\"2595\"]outer core[\/pb_glossary] . The solid [pb_glossary id=\"2596\"]inner core[\/pb_glossary] is about 1,220 km thick, and the [pb_glossary id=\"2595\"]outer core[\/pb_glossary] is about 2,300 km thick.\n\nIt seems like a contradiction that the hottest part of the Earth is solid, as the [pb_glossary id=\"2687\"]minerals[\/pb_glossary] making up the [pb_glossary id=\"2589\"]core[\/pb_glossary] should be liquified or vaporized at this [pb_glossary id=\"2689\"]temperature[\/pb_glossary]. Immense pressure keeps the [pb_glossary id=\"2687\"]minerals[\/pb_glossary] of the [pb_glossary id=\"2596\"]inner core[\/pb_glossary] in a solid phase. The [pb_glossary id=\"2596\"]inner core[\/pb_glossary] grows slowly from the lower [pb_glossary id=\"2595\"]outer core[\/pb_glossary] solidifying as heat escapes the interior of the Earth and is dispersed to the outer layers.\n\n[caption id=\"attachment_2520\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.2_SpinningOuterCore.gif\"><img class=\"size-medium wp-image-87\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-300x289.gif\" alt=\"The Earth is cut out with the core being shown.\" width=\"300\" height=\"289\"><\/a> The outer core's spin causes our protective magnetic field.[\/caption]\n\nThe earth\u2019s liquid [pb_glossary id=\"2595\"]outer core[\/pb_glossary] is critically important in maintaining a breathable [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary] and other environmental conditions favorable for life. Scientists believe the earth\u2019s magnetic field is generated by the circulation of molten iron and nickel within the [pb_glossary id=\"2595\"]outer core[\/pb_glossary]. If the [pb_glossary id=\"2595\"]outer core[\/pb_glossary] were to stop circulating or become solid, the loss of the magnetic field would result in Earth getting stripped of life-supporting gases and water. This is what happened, and continues to happen, on Mars.\n\n&nbsp;\n\n&nbsp;\n\n&nbsp;\n\n&nbsp;\n\n[h5p id=\"10\"]\n\n[caption id=\"attachment_3736\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Layers-of-Earth-Practice-QR-Code.png\"><img class=\"size-thumbnail wp-image-88\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-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><b>2.2.3 Plate Tectonic Boundaries<\/b><\/h3>\n[caption id=\"attachment_2521\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Passive_Contiental_Margin.jpg\"><img class=\"size-medium wp-image-89\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-300x143.jpg\" alt=\"The plate thins from continent to ocean\" width=\"300\" height=\"143\"><\/a> Passive margin[\/caption]\n\n&nbsp;\n\nAt passive margins the [pb_glossary id=\"2591\"]plates[\/pb_glossary] don\u2019t move\u2014the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] transitions into oceanic lithosphere and forms [pb_glossary id=\"2591\"]plates[\/pb_glossary] made of both types. A [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] may be made of both [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] connected by a [pb_glossary id=\"2598\"]passive margin[\/pb_glossary]. North and South America\u2019s eastern coastlines are examples of passive margins. Active margins are places where the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] and [pb_glossary id=\"2575\"]continental[\/pb_glossary] lithospheric [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] meet and move relative to each other, such as the western coasts of North and South America. This movement is caused by frictional drag created between the [pb_glossary id=\"2591\"]plates[\/pb_glossary] and differences in [pb_glossary id=\"2591\"]plate[\/pb_glossary] densities. The majority of mountain-building events, earthquake activity and active [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] on the Earth\u2019s surface can be attributed to [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement at active margins.\n\n&nbsp;\n\n[caption id=\"attachment_2522\" align=\"aligncenter\" width=\"775\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Tectonic_plate_boundaries.png\"><img class=\"size-full wp-image-90\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries.png\" alt=\"It shows all the types\" width=\"775\" height=\"429\"><\/a> Schematic of plate boundary types.[\/caption]\n\nIn a simplified model, there are three categories of [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] boundaries. [pb_glossary id=\"2600\"]Convergent[\/pb_glossary] boundaries are places where [pb_glossary id=\"2591\"]plates[\/pb_glossary] move toward each other. At [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries, the [pb_glossary id=\"2591\"]plates[\/pb_glossary] move apart. At [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries, the [pb_glossary id=\"2591\"]plates[\/pb_glossary] slide past each other.\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"11\"]\n\n[caption id=\"attachment_3735\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.2-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-91\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-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 2.2 via this QR Code.[\/caption]\n<h2><span style=\"font-size: 28px;\">2.3 Convergent Boundaries<\/span><\/h2>\n&nbsp;\n\n[caption id=\"attachment_2523\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/CratonGeolProv.jpg\"><img class=\"size-medium 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=\"300\" height=\"159\"><\/a> Geologic provinces with the Shield (orange) and Platform (pink) comprising the Craton, the stable interior of continents.[\/caption]\n\n[pb_glossary id=\"2600\"]Convergent[\/pb_glossary] boundaries, also called destructive boundaries, are places where two or more [pb_glossary id=\"2591\"]plates[\/pb_glossary] move toward each other. . [pb_glossary id=\"2600\"]Convergent[\/pb_glossary] boundary movement is divided into two types, [pb_glossary id=\"2602\"]subduction[\/pb_glossary] and [pb_glossary id=\"2620\"]collision[\/pb_glossary], depending on the density of the involved [pb_glossary id=\"2591\"]plates[\/pb_glossary]. [pb_glossary id=\"2575\"]Continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is of lower density and thus more buoyant than the underlying [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary]. [pb_glossary id=\"2581\"]Oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is more dense than [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], and, when old and cold, may even be more dense than [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary].\n\nWhen [pb_glossary id=\"2591\"]plates[\/pb_glossary] of different densities converge, the higher density [pb_glossary id=\"2591\"]plate[\/pb_glossary] is pushed beneath the more buoyant [pb_glossary id=\"2591\"]plate[\/pb_glossary] in a process called [pb_glossary id=\"2602\"]subduction[\/pb_glossary]. When [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] converge without [pb_glossary id=\"2602\"]subduction[\/pb_glossary] occurring, this process is called [pb_glossary id=\"2620\"]collision[\/pb_glossary].\n<h3><b>2.3.1. Subduction<\/b><\/h3>\n[video width=\"720\" height=\"540\" mp4=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/Subduction.mp4\"][\/video]\n\n<em><a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP1GTect\/cSubduction.html\">Video<\/a> showing [pb_glossary id=\"2575\"]<em>continental<\/em>[\/pb_glossary]-[pb_glossary id=\"2581\"]<em>oceanic <\/em>[\/pb_glossary] [pb_glossary id=\"2602\"]<em>subduction<\/em>[\/pb_glossary], causing [pb_glossary id=\"1181\"]<em>volcanism<\/em>[\/pb_glossary]. By Tanya Atwater and John Iwerks.<\/em>\n\n[caption id=\"attachment_3734\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Subduction-Animation-QR-Code.png\"><img class=\"size-thumbnail wp-image-93\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this video via this QR Code.[\/caption]\n\n[pb_glossary id=\"2602\"]Subduction[\/pb_glossary] occurs when a dense [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] meets a more buoyant [pb_glossary id=\"2591\"]plate[\/pb_glossary], like a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] or warmer\/younger [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary], and descends into the [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. The worldwide average rate of [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2602\"]subduction[\/pb_glossary] is 25 miles per million years, about a half-inch per year. As an [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] descends, it pulls the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] down into a [pb_glossary id=\"2603\"]trench[\/pb_glossary]. These trenches can be more than twice as deep as the average depth of the adjacent ocean [pb_glossary id=\"1461\"]basin[\/pb_glossary], which is usually three to four km. The Mariana [pb_glossary id=\"2603\"]Trench[\/pb_glossary], for example, approaches a staggering 11 km.\n\n&nbsp;\n\n[caption id=\"attachment_2524\" align=\"aligncenter\" width=\"800\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Subduction-en.svg_.png\"><img class=\"size-full wp-image-94\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_.png\" alt=\"Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental\" width=\"800\" height=\"391\"><\/a> Diagram of ocean-continent subduction.[\/caption]\n\n[caption id=\"attachment_2525\" align=\"alignright\" width=\"212\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/drawing.png\"><img class=\"size-medium wp-image-95\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-212x300.png\" alt=\"This drawing depicts a microcontinent riding with a subducting plate, and not being subductable, becoming accreted to the melange.\" width=\"212\" height=\"300\"><\/a> Microcontinents can become part of the accretionary prism of a subduction zone.[\/caption]\n\nWithin the [pb_glossary id=\"2603\"]trench[\/pb_glossary], [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] [pb_glossary id=\"2678\"]sediments[\/pb_glossary] are scraped together and compressed between the [pb_glossary id=\"2602\"]subducting[\/pb_glossary] and overriding [pb_glossary id=\"2591\"]plates[\/pb_glossary]. This feature is called the [pb_glossary id=\"2604\"]accretionary wedge[\/pb_glossary], m\u00e9lange, or accretionary prism. Fragments of [pb_glossary id=\"2575\"]continental[\/pb_glossary] material, including microcontinents, riding atop the [pb_glossary id=\"2602\"]subducting[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] may become sutured to the [pb_glossary id=\"2604\"]accretionary wedge [\/pb_glossary] and accumulate into a large area of land called a [pb_glossary id=\"2686\"]terrane[\/pb_glossary]. Vast portions of California are comprised of accreted terranes.\n\n[caption id=\"attachment_2526\" align=\"alignleft\" width=\"179\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/usgs_terranes.gif\"><img class=\"size-medium wp-image-96\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/usgs_terranes-179x300.gif\" alt=\"Map showing large areas of the western North American continent that are accreted.\" width=\"179\" height=\"300\"><\/a> Accreted terranes of western North America. Everything that is not the \"Ancient continental interior (craton)\" has been smeared onto the side of the continent by accretion from subduction.[\/caption]\n\nWhen the [pb_glossary id=\"2602\"]subducting[\/pb_glossary] [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary], or [pb_glossary id=\"2605\"]slab[\/pb_glossary], sinks into the [pb_glossary id=\"2586\"]mantle[\/pb_glossary], the immense heat and pressure pushes volatile materials like water and carbon dioxide into an area below the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] and above the descending [pb_glossary id=\"2591\"]plate[\/pb_glossary] called the [pb_glossary id=\"2607\"]mantle wedge[\/pb_glossary]. The [pb_glossary id=\"2606\"]volatiles[\/pb_glossary] are released mostly by hydrated [pb_glossary id=\"2687\"]minerals[\/pb_glossary] that revert to non-hydrated [pb_glossary id=\"2687\"]minerals[\/pb_glossary] in these higher [pb_glossary id=\"2689\"]temperature[\/pb_glossary] and pressure conditions. When mixed with asthenospheric material above the [pb_glossary id=\"2591\"]plate[\/pb_glossary], the volatile lower the melting point of the [pb_glossary id=\"2607\"]mantle wedge[\/pb_glossary], and through a process called [pb_glossary id=\"2608\"]flux melting[\/pb_glossary] it becomes liquid [pb_glossary id=\"2672\"]magma[\/pb_glossary]. The molten [pb_glossary id=\"2672\"]magma[\/pb_glossary] is more buoyant than the lithospheric [pb_glossary id=\"2591\"]plate[\/pb_glossary] above it and migrates to the Earth\u2019s surface where it emerges as [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. The resulting [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] frequently appear as curved mountain chains, [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] arcs, due to the curvature of the earth. Both [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] can contain [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] arcs.\n\n[caption id=\"attachment_2527\" align=\"alignright\" width=\"236\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/1755_Lisbon_Earthquake_Location.png\"><img class=\"size-medium wp-image-97\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location-236x300.png\" alt=\"It is large and offshore.\" width=\"236\" height=\"300\"><\/a> Location of the large (Mw 8.5-9.0) 1755 Lisbon Earthquake.[\/caption]\n\nHow [pb_glossary id=\"2602\"]subduction[\/pb_glossary] is initiated is still a matter of scientific debate. It is generally accepted that [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones start as passive margins, where [pb_glossary id=\"2581\"]oceanic[\/pb_glossary]\u00a0and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] come together, and then gravity initiates [pb_glossary id=\"2602\"]subduction[\/pb_glossary] and converts the [pb_glossary id=\"2598\"]passive margin[\/pb_glossary] into an active one. One [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] is gravity pulls the denser [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] down or the [pb_glossary id=\"2591\"]plate[\/pb_glossary] can start to flow ductility at a low angle. Scientists seeking to answer this question have collected evidence that suggests a new [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone is forming off the [pb_glossary id=\"2890\"]coast[\/pb_glossary]\u00a0of Portugal. Some scientists have proposed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone, although the evidence is not definitive. Another [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] proposes [pb_glossary id=\"2602\"]subduction[\/pb_glossary] happens at [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries involving [pb_glossary id=\"2591\"]plates[\/pb_glossary] of different densities.\n\nSome [pb_glossary id=\"2591\"]plate[\/pb_glossary] boundaries look like they should be active, but show no evidence of [pb_glossary id=\"2602\"]subduction[\/pb_glossary]. The [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary] on either side of the Atlantic Ocean for example, are denser than the underlying [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] and are not [pb_glossary id=\"2602\"]subducting[\/pb_glossary] beneath the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary]. One [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] is the [pb_glossary id=\"2703\"]bond[\/pb_glossary] holding the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] together is stronger than the downwards force created by the difference in [pb_glossary id=\"2591\"]plate[\/pb_glossary] densities.\n\n[caption id=\"attachment_2528\" align=\"alignleft\" width=\"234\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/SundaMegathrustSeismicity.png\"><img class=\"size-medium wp-image-98\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-234x300.png\" alt=\"The earthquakes follow the slab down.\" width=\"234\" height=\"300\"><\/a> Earthquakes along the Sunda megathrust subduction zone, along the island of Sumatra, showing the 2006 Mw 9.1-9.3 Indian Ocean Earthquake as a star.[\/caption]\n\n[pb_glossary id=\"2602\"]Subduction[\/pb_glossary] zones are known for having the largest earthquakes and [pb_glossary id=\"3194\"]tsunamis[\/pb_glossary]; they are the only places with [pb_glossary id=\"3065\"]fault[\/pb_glossary] surfaces large enough to create [pb_glossary id=\"3098\"]magnitude[\/pb_glossary]-9 earthquakes. These [pb_glossary id=\"2602\"]subduction[\/pb_glossary]-zone earthquakes not only are very large, but also are very deep. When a [pb_glossary id=\"2602\"]subducting[\/pb_glossary] [pb_glossary id=\"2605\"]slab[\/pb_glossary] becomes stuck and cannot descend, a [pb_glossary id=\"1933\"]massive[\/pb_glossary] amount of energy builds up between the stuck [pb_glossary id=\"2591\"]plates[\/pb_glossary]. If this energy is not gradually dispersed, it may force the [pb_glossary id=\"2591\"]plates[\/pb_glossary] to suddenly release along several hundred kilometers of the [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone. Because [pb_glossary id=\"2602\"]subduction[\/pb_glossary]-zone [pb_glossary id=\"3065\"]faults[\/pb_glossary] are located on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary], this [pb_glossary id=\"1933\"]massive[\/pb_glossary] amount of movement can generate giant [pb_glossary id=\"3194\"]tsunamis[\/pb_glossary] such as those that followed the 2004 Indian Ocean Earthquake and 2011 T\u014dhoku Earthquake in Japan.\n\n[caption id=\"attachment_2529\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Forearc.gif\"><img class=\"size-medium wp-image-99\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Forearc-300x258.gif\" alt=\"It shows backarc, forearc, and arc.\" width=\"300\" height=\"258\"><\/a> Various parts of a subduction zone. This subduction zone is ocean-ocean subduction, though the same features can apply to continent-ocean subduction.[\/caption]\n\nAll [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones have a [pb_glossary id=\"2614\"]forearc basin[\/pb_glossary], a feature of the overriding [pb_glossary id=\"2591\"]plate[\/pb_glossary] found between the [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary] and [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2603\"]trench[\/pb_glossary]. The [pb_glossary id=\"2614\"]forearc basin[\/pb_glossary] experiences a lot of [pb_glossary id=\"3065\"]faulting[\/pb_glossary]\u00a0and [pb_glossary id=\"1448\"]deformation[\/pb_glossary] activity, particularly within the [pb_glossary id=\"2604\"]accretionary wedge[\/pb_glossary].\n\nIn some [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, [pb_glossary id=\"1445\"]tensional[\/pb_glossary] forces\u00a0working on the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] create a backarc [pb_glossary id=\"1461\"]basin[\/pb_glossary] on the interior side of the [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary]. Some scientists have proposed a [pb_glossary id=\"2602\"]subduction[\/pb_glossary] mechanism called [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2605\"]slab[\/pb_glossary] rollback creates [pb_glossary id=\"1445\"]extension[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] in the overriding [pb_glossary id=\"2591\"]plates[\/pb_glossary]. In this model, the descending [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2605\"]slab[\/pb_glossary] does not slide directly under the overriding [pb_glossary id=\"2591\"]plate[\/pb_glossary] but instead rolls back, pulling the overlying [pb_glossary id=\"2591\"]plate[\/pb_glossary] seaward. The [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] behind the [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary] gets stretched like pizza dough until the surface cracks and collapses to form a backarc [pb_glossary id=\"1461\"]basin[\/pb_glossary]. If the [pb_glossary id=\"1445\"]extension[\/pb_glossary] activity is extensive and deep enough, a backarc [pb_glossary id=\"1461\"]basin[\/pb_glossary] can develop into a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary] zone. These [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries may be less symmetrical than their [pb_glossary id=\"2630\"]mid-ocean ridge[\/pb_glossary] counterparts.\n\nIn places where numerous young buoyant [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] are converging and [pb_glossary id=\"2602\"]subducting[\/pb_glossary] at a relatively high velocity, they may force the overlying [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] to buckle and crack. This is called [pb_glossary id=\"2610\"]back-arc[\/pb_glossary] [pb_glossary id=\"3065\"]faulting[\/pb_glossary]. [pb_glossary id=\"1445\"]Extensional[\/pb_glossary] [pb_glossary id=\"2610\"]back-arc[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] pull rocks and chunks of [pb_glossary id=\"2591\"]plates[\/pb_glossary] apart. [pb_glossary id=\"1446\"]Compressional[\/pb_glossary] [pb_glossary id=\"2610\"]back-arc[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary], also known as thrust [pb_glossary id=\"3065\"]faults[\/pb_glossary], push them together.\n\nThe dual spines of the Andes Mountain range include a example of [pb_glossary id=\"1446\"]compressional[\/pb_glossary] thrust [pb_glossary id=\"3065\"]faulting[\/pb_glossary]. The western spine is part of a [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary]. Thrust [pb_glossary id=\"3065\"]faults[\/pb_glossary] have deformed the non-[pb_glossary id=\"1181\"]volcanic[\/pb_glossary] eastern spine, \u00a0pushing rocks and pieces of [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] on top of each other.\n\nThere are two styles of [pb_glossary id=\"3074\"]thrust fault[\/pb_glossary] [pb_glossary id=\"1448\"]deformation[\/pb_glossary]: [pb_glossary id=\"2611\"]thin-skinned[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] that occur in superficial rocks lying on top of the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] and [pb_glossary id=\"2612\"]thick-skinned[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] that reach deeper into the [pb_glossary id=\"2580\"]crust[\/pb_glossary]. The Sevier [pb_glossary id=\"2585\"]Orogeny[\/pb_glossary] in the western U.S. is a notable [pb_glossary id=\"2611\"]thin-skinned[\/pb_glossary] type of [pb_glossary id=\"1448\"]deformation[\/pb_glossary] created during the [pb_glossary id=\"1440\"]Cretaceous[\/pb_glossary] [pb_glossary id=\"2192\"]Period[\/pb_glossary]. The Laramide [pb_glossary id=\"2585\"]Orogeny[\/pb_glossary], a [pb_glossary id=\"2612\"]thick-skinned[\/pb_glossary] type of [pb_glossary id=\"1448\"]deformation[\/pb_glossary], occurred near the end of and slightly after the Sevier [pb_glossary id=\"2585\"]Orogeny[\/pb_glossary] \u00a0in the same region.\n\n[caption id=\"attachment_2530\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Shallow_subduction_Laramide_orogeny.png\"><img class=\"size-medium wp-image-100\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny-300x142.png\" alt=\"The subducting plate goes right under the overriding plate\" width=\"300\" height=\"142\"><\/a> Shallow subduction during the Laramide Orogeny.[\/caption]\n\nFlat-[pb_glossary id=\"2605\"]slab[\/pb_glossary], or shallow, [pb_glossary id=\"2602\"]subduction[\/pb_glossary] caused the Laramide [pb_glossary id=\"2585\"]Orogeny[\/pb_glossary]. When the descending [pb_glossary id=\"2605\"]slab[\/pb_glossary] [pb_glossary id=\"2602\"]subducts[\/pb_glossary] at a low angle, there is more contact between the [pb_glossary id=\"2605\"]slab[\/pb_glossary] and the overlying [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] than in a typical [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone. The shallowly-[pb_glossary id=\"2602\"]subducting[\/pb_glossary] [pb_glossary id=\"2605\"]slab[\/pb_glossary] pushes against the overriding [pb_glossary id=\"2591\"]plate[\/pb_glossary] and creates an area of [pb_glossary id=\"1448\"]deformation[\/pb_glossary] on the overriding [pb_glossary id=\"2591\"]plate[\/pb_glossary] many kilometers away from the [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone.\n<h4><span style=\"font-weight: 400;\">Oceanic-Continental subduction<\/span><\/h4>\n[caption id=\"attachment_2531\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/OceanContSub.gif\"><img class=\"size-medium wp-image-101\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/OceanContSub-300x177.gif\" alt=\"The thinner ocean plate is going under the thicker continental plate.\" width=\"300\" height=\"177\"><\/a> Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc.[\/caption]\n\n[pb_glossary id=\"2616\"]Oceanic-continental subduction[\/pb_glossary] occurs when an [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] dives below a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary]. This [pb_glossary id=\"2600\"]convergent[\/pb_glossary] boundary has a [pb_glossary id=\"2603\"]trench[\/pb_glossary] and [pb_glossary id=\"2607\"]mantle wedge[\/pb_glossary] and frequently, a [pb_glossary id=\"2617\"]volcanic arc[\/pb_glossary]. Well-known examples of [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2617\"]volcanic arcs[\/pb_glossary] are the Cascade Mountains in the Pacific Northwest and western Andes Mountains in South America.\n<h4><span style=\"font-weight: 400;\">Oceanic-Oceanic Subduction<\/span><\/h4>\n[caption id=\"attachment_2532\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Fig21oceanocean.gif\"><img class=\"size-medium wp-image-102\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig21oceanocean-300x173.gif\" alt=\"The ocean plate subducts beneath a different ocean plate.\" width=\"300\" height=\"173\"><\/a> Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc.[\/caption]\n\nThe boundaries of [pb_glossary id=\"2618\"]oceanic-oceanic subduction[\/pb_glossary] zones show very different activity from those involving [pb_glossary id=\"2581\"]oceanic[\/pb_glossary]-[pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary]. Since both [pb_glossary id=\"2591\"]plates[\/pb_glossary] are made of [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], it is usually the older [pb_glossary id=\"2591\"]plate[\/pb_glossary] that [pb_glossary id=\"2602\"]subducts[\/pb_glossary] because it is colder and denser. The [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] on the overlying [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] may remain hidden underwater.. If the [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] rise high enough the reach the ocean surface, the chain of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] forms an [pb_glossary id=\"2619\"]island arc[\/pb_glossary]. Examples of these island arcs include the Aleutian Islands in the northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean.\n<h3><b>2.3.2. Collisions<\/b><\/h3>\n[caption id=\"attachment_2533\" align=\"alignleft\" width=\"301\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/contcontCollision.gif\"><img class=\"size-full wp-image-103\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/contcontCollision.gif\" alt=\"The two continental plates stay up.\" width=\"301\" height=\"181\"><\/a> Two continental plates colliding.[\/caption]\n\nWhen [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] converge, during the closing of an ocean [pb_glossary id=\"1461\"]basin[\/pb_glossary] for example, [pb_glossary id=\"2602\"]subduction[\/pb_glossary] is not possible between the equally buoyant [pb_glossary id=\"2591\"]plates[\/pb_glossary]. Instead of one [pb_glossary id=\"2591\"]plate[\/pb_glossary] descending beneath another, the two masses of [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] slam together in a process known as [pb_glossary id=\"2620\"]collision[\/pb_glossary]. Without [pb_glossary id=\"2602\"]subduction[\/pb_glossary], there is no [pb_glossary id=\"2672\"]magma[\/pb_glossary] [pb_glossary id=\"2960\"]formation[\/pb_glossary] and no [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. [pb_glossary id=\"2620\"]Collision[\/pb_glossary] zones are characterized by tall, non-[pb_glossary id=\"1181\"]volcanic[\/pb_glossary] mountains; a broad zone of frequent, large earthquakes; and very little [pb_glossary id=\"1181\"]volcanism[\/pb_glossary].\n\nWhen [pb_glossary id=\"2581\"]oceanic crust[\/pb_glossary] connected by a [pb_glossary id=\"2598\"]passive margin[\/pb_glossary] to [pb_glossary id=\"2575\"]continental crust[\/pb_glossary] completely [pb_glossary id=\"2602\"]subducts[\/pb_glossary] beneath a [pb_glossary id=\"2575\"]continent[\/pb_glossary], an ocean [pb_glossary id=\"1461\"]basin[\/pb_glossary] closes, and [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2620\"]collision[\/pb_glossary] begins. \u00a0Eventually, as ocean basins close, continents join together to form a [pb_glossary id=\"1933\"]massive[\/pb_glossary] accumulation of continents called a [pb_glossary id=\"2623\"]supercontinent[\/pb_glossary], a process that has taken place in ~500 million year old cycles over earth\u2019s history.\n\n[caption id=\"attachment_2534\" align=\"alignright\" width=\"267\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Pangaea_continents.png\"><img class=\"size-medium wp-image-104\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-267x300.png\" alt=\"Pangaea has a crescent shape.\" width=\"267\" height=\"300\"><\/a> A reconstruction of Pangaea, showing approximate positions of modern continents.[\/caption]\n\n&nbsp;\n\nThe process of [pb_glossary id=\"2620\"]collision[\/pb_glossary] created [pb_glossary id=\"3366\"]Pangea[\/pb_glossary], the supercontinent envisioned by Wegener as the key component of his [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary]. Geologists now have evidence that [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] have been continuously converging into [pb_glossary id=\"2623\"]supercontinents[\/pb_glossary] and splitting into smaller [pb_glossary id=\"1461\"]basin[\/pb_glossary]-separated continents throughout Earth\u2019s existence, calling this process the [pb_glossary id=\"2623\"]supercontinent[\/pb_glossary] cycle, a process that takes place in approximately 500 million years. For example, they estimate [pb_glossary id=\"3366\"]Pangea[\/pb_glossary] began separating 200 million years ago. [pb_glossary id=\"3366\"]Pangea[\/pb_glossary] was preceded by an earlier [pb_glossary id=\"2623\"]supercontinents[\/pb_glossary], one of which being [pb_glossary id=\"2211\"]Rodinia[\/pb_glossary], which existed 1.1 billion years ago and started breaking apart 800 million to 600 million years ago.\n\n[caption id=\"attachment_2535\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/ZagrosFTB.png\"><img class=\"size-medium wp-image-105\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-300x231.png\" alt=\"The mountains are loading the crust down, leading to a depressed basin, which is the Persian Gulf\" width=\"300\" height=\"231\"><\/a> The tectonics of the Zagros Mountains. Note the Persian Gulf foreland basin.[\/caption]\n\n&nbsp;\n\nA foreland [pb_glossary id=\"1461\"]basin[\/pb_glossary] is a feature that develops near mountain belts, as the combined mass of the mountains forms a depression in the lithospheric [pb_glossary id=\"2591\"]plate[\/pb_glossary]. While foreland basins may occur at [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, they are most commonly found at [pb_glossary id=\"2620\"]collision[\/pb_glossary] boundaries. The Persian Gulf is possibly the best modern example, created entirely by the weight of the nearby Zagros Mountains.\n\n[caption id=\"attachment_2536\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.3_ItalyPillowBasalt.jpg\"><img class=\"size-medium wp-image-106\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-300x225.jpg\" alt=\"The rock is cray with many circles inside\" width=\"300\" height=\"225\"><\/a> Pillow lavas, which only form under water, from an ophiolite in the Apennine Mountains of central Italy.[\/caption]\n\nIf [pb_glossary id=\"2575\"]continental[\/pb_glossary] and [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] are fused on the same [pb_glossary id=\"2591\"]plate[\/pb_glossary], it can partially subduct but its buoyancy prevents it from fully descending. In very rare cases, part of a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] may become trapped beneath a descending [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] in a process called [pb_glossary id=\"2621\"]obduction[\/pb_glossary]. When a portion of the [pb_glossary id=\"2575\"]continental crust[\/pb_glossary] is driven down into the [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zone, due to its buoyancy it returns to the surface relatively quickly.\n\nAs pieces of the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] break loose and migrate upward through the [pb_glossary id=\"2621\"]obduction[\/pb_glossary] zone, they bring along bits of the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] and amend them on top of the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary]. Rocks[pb_glossary id=\"2831\"] composed[\/pb_glossary] of this [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and ocean-floor material are called [pb_glossary id=\"2622\"]ophiolites[\/pb_glossary] and they provide valuable information about the [pb_glossary id=\"2831\"]composition[\/pb_glossary] of the [pb_glossary id=\"2586\"]mantle[\/pb_glossary].\n\nThe area of [pb_glossary id=\"2620\"]collision[\/pb_glossary]-zone [pb_glossary id=\"1448\"]deformation[\/pb_glossary] and [pb_glossary id=\"3087\"]seismic[\/pb_glossary] activity usually covers a broader area because [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] is plastic and malleable. Unlike [pb_glossary id=\"2602\"]subduction[\/pb_glossary]-zone earthquakes, which tend to be located along a narrow swath near the [pb_glossary id=\"2600\"]convergent[\/pb_glossary] boundary, [pb_glossary id=\"2620\"]collision[\/pb_glossary]-zone earthquakes may occur hundreds of kilometers from the boundary between the [pb_glossary id=\"2591\"]plates[\/pb_glossary].\n\nThe Eurasian [pb_glossary id=\"2575\"]continent[\/pb_glossary] has many examples of [pb_glossary id=\"2620\"]collision[\/pb_glossary]-zone deformations covering vast areas. The Pyrenees mountains begin in the Iberian Peninsula and cross into France. Also, there are the Alps stretching from Italy to central Europe; the Zagros mountains from Arabia to Iran; and Himalaya mountains from the Indian subcontinent to central Asia.\n\n[video width=\"720\" height=\"478\" mp4=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/IndiaAsiaCollision.mp4\"][\/video]\n\n<em>Animation of India crashing into Asia, by Tanya Atwater.<\/em>\n\n[caption id=\"attachment_3733\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/India-Asia-Collision-Animation-QR-Code.png\"><img class=\"size-thumbnail wp-image-107\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this animation via this QR Code.[\/caption]\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"12\"]\n\n[caption id=\"attachment_3732\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.3-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-108\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-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 2.3 via this QR Code.[\/caption]\n<h2><span style=\"font-weight: 400;\">2.4 Divergent Boundaries<\/span><\/h2>\nAt [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries, sometimes called constructive boundaries, lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary] move away from each other. There are two types of [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries, categorized by where they occur: [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rift[\/pb_glossary] zones and [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary]. [pb_glossary id=\"2575\"]Continental[\/pb_glossary] [pb_glossary id=\"2624\"]rift[\/pb_glossary] zones occur in weak spots in the [pb_glossary id=\"2575\"]continental[\/pb_glossary] lithospheric [pb_glossary id=\"2591\"]plate[\/pb_glossary]. A [pb_glossary id=\"2630\"]mid-ocean ridge[\/pb_glossary] usually originates in a [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] as a [pb_glossary id=\"2624\"]rift[\/pb_glossary] zone that expands to the point of splitting the [pb_glossary id=\"2591\"]plate[\/pb_glossary] apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents. This process is known as [pb_glossary id=\"2624\"]rift[\/pb_glossary]-to-drift.\n<h3><b>2.4.1. Continental Rifting<\/b><\/h3>\n[caption id=\"attachment_2537\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Horst-Graben.svg_.png\"><img class=\"size-medium wp-image-109\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-300x154.png\" alt=\"While the area extends, individual grabens drop down relative to the horsts.\" width=\"300\" height=\"154\"><\/a> Faulting that occurs in divergent boundaries.[\/caption]\n\nIn places where the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] are very thick, they reflect so much heat back into the [pb_glossary id=\"2586\"]mantle[\/pb_glossary] it develops strong [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents that push super-heated [pb_glossary id=\"2586\"]mantle[\/pb_glossary] material up against the overlying [pb_glossary id=\"2591\"]plate[\/pb_glossary], softening it. [pb_glossary id=\"1445\"]Tensional[\/pb_glossary] forces created by this convective upwelling begin to pull the weakened [pb_glossary id=\"2591\"]plate[\/pb_glossary] apart. As it stretches, it becomes thinner and develops deep cracks called [pb_glossary id=\"1445\"]extension[\/pb_glossary] or normal [pb_glossary id=\"3065\"]faults[\/pb_glossary]. Eventually plate sections located between large faults drop into deep depressions known as [pb_glossary id=\"2624\"]rift[\/pb_glossary] valleys, which often contain keystone-shaped blocks of down-dropped [pb_glossary id=\"2580\"]crust[\/pb_glossary] known as [pb_glossary id=\"2626\"]grabens[\/pb_glossary]. The shoulders of these [pb_glossary id=\"2626\"]grabens[\/pb_glossary] are called [pb_glossary id=\"2625\"]horsts[\/pb_glossary]. If only one side of a section drops, it is called a [pb_glossary id=\"3071\"]half-graben[\/pb_glossary]. Depending on the conditions, [pb_glossary id=\"2624\"]rifts[\/pb_glossary] can grow into very large lakes and even oceans.\n\n[caption id=\"attachment_2538\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/TopographicAfar.png\"><img class=\"size-medium wp-image-110\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-300x300.png\" alt=\"The branches of the plate boundaries are 120 degrees apart.\" width=\"300\" height=\"300\"><\/a> The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120\u00b0 apart.[\/caption]\n\nWhile seemingly occurring at random, [pb_glossary id=\"2624\"]rifting[\/pb_glossary] is dictated by two factors. [pb_glossary id=\"2624\"]Rifting[\/pb_glossary] does not occur in continents with older and more stable interiors, known as [pb_glossary id=\"2640\"]cratons[\/pb_glossary]. When [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary] does occur, the break-up pattern resembles the seams of a soccer ball, also called a truncated icosahedron. This is the most common surface-[pb_glossary id=\"1934\"]fracture[\/pb_glossary] pattern to develop on an evenly expanding sphere because it uses the least amount of energy.\n\nUsing the soccer ball model, [pb_glossary id=\"2624\"]rifting[\/pb_glossary] tends to lengthen and expand along a particular seam while fizzling out in the other directions. These seams with little or no [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] activity are called failed rift arms. A [pb_glossary id=\"2627\"]failed rift arm[\/pb_glossary] is still a weak spot in the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary]; even without the presence of active [pb_glossary id=\"1445\"]extension[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary], it may develop into a called an [pb_glossary id=\"2628\"]aulacogen[\/pb_glossary]. One example of a [pb_glossary id=\"2627\"]failed rift arm[\/pb_glossary] is the Mississippi Valley Embayment, a depression through which the upper end of the Mississippi [pb_glossary id=\"3134\"]River[\/pb_glossary] flows. Occasionally connected [pb_glossary id=\"2624\"]rift[\/pb_glossary] arms do develop concurrently, creating multiple boundaries of active [pb_glossary id=\"2624\"]rifting[\/pb_glossary]. In places where the [pb_glossary id=\"2624\"]rift[\/pb_glossary] arms do not fail, for example the Afar Triangle, three [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries can develop near each other forming a [pb_glossary id=\"2629\"]triple junction[\/pb_glossary].\n\n[caption id=\"attachment_2539\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Basin_range_province.jpg\"><img class=\"size-medium wp-image-111\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-300x240.jpg\" alt=\"There is a series of mountains and valleys\" width=\"300\" height=\"240\"><\/a> NASA image of the Basin and Range horsts and grabens across central Nevada.[\/caption]\n\n[pb_glossary id=\"2624\"]Rifts[\/pb_glossary] come in two types: narrow and broad. Narrow [pb_glossary id=\"2624\"]rifts[\/pb_glossary] are characterized by a high density of highly active [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries. The East African [pb_glossary id=\"2624\"]Rift[\/pb_glossary] Zone, where the [pb_glossary id=\"2479\"]horn[\/pb_glossary] of Africa is pulling away from the mainland, is an excellent example of an active narrow [pb_glossary id=\"2624\"]rift[\/pb_glossary]. Lake Baikal in Russia is another. Broad [pb_glossary id=\"2624\"]rifts[\/pb_glossary] also have numerous [pb_glossary id=\"3065\"]fault[\/pb_glossary] zones, but they are distributed over wide areas of [pb_glossary id=\"1448\"]deformation[\/pb_glossary]. The [pb_glossary id=\"2462\"]Basin and Range[\/pb_glossary] region located in the western United States is a type of broad [pb_glossary id=\"2624\"]rift[\/pb_glossary]. The Wasatch [pb_glossary id=\"3065\"]Fault[\/pb_glossary], which also created the Wasatch Mountain Range in the state of Utah, forms the eastern [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundary of this broad [pb_glossary id=\"2624\"]rift[\/pb_glossary]\u00a0 (<a href=\"https:\/\/youtu.be\/TvvWqAdNV84\">Animation 1<\/a> and <a href=\"https:\/\/youtu.be\/7DxcAMmNeZk\">Animation 2<\/a>).\n\n&nbsp;\n\n[pb_glossary id=\"2624\"]Rifts[\/pb_glossary] have earthquakes, although not of the [pb_glossary id=\"3098\"]magnitude[\/pb_glossary] and frequency of other boundaries. They may also exhibit [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. Unlike the flux-melted [pb_glossary id=\"2672\"]magma[\/pb_glossary] found in [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones, [pb_glossary id=\"2624\"]rift[\/pb_glossary]-zone [pb_glossary id=\"2672\"]magma[\/pb_glossary] is created by [pb_glossary id=\"1176\"]decompression melting[\/pb_glossary]. As the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] are pulled apart, they create a region of low pressure that melts the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] and draws it upwards. When this molten [pb_glossary id=\"2672\"]magma[\/pb_glossary] reaches the weakened and [pb_glossary id=\"3065\"]fault[\/pb_glossary]-riddled [pb_glossary id=\"2624\"]rift[\/pb_glossary] zone, it migrates to surface by breaking through the [pb_glossary id=\"2591\"]plate[\/pb_glossary] or escaping via an open [pb_glossary id=\"3065\"]fault[\/pb_glossary]. Examples of young [pb_glossary id=\"2624\"]rift[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] dot the [pb_glossary id=\"2462\"]Basin and Range[\/pb_glossary] region in the United States. [pb_glossary id=\"2624\"]Rift[\/pb_glossary]-zone activity is responsible for generating some unique [pb_glossary id=\"1181\"]volcanism[\/pb_glossary], such as the Ol Doinyo Lengai in Tanzania. This [pb_glossary id=\"1181\"]volcano[\/pb_glossary] erupts [pb_glossary id=\"2673\"]lava[\/pb_glossary] consisting largely of [pb_glossary id=\"3365\"]carbonatite[\/pb_glossary], a relatively cold, liquid [pb_glossary id=\"1917\"]carbonate[\/pb_glossary] [pb_glossary id=\"2687\"]mineral[\/pb_glossary].\n\n[video width=\"720\" height=\"474\" mp4=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/SoAtlantic_CutwithConvect.mp4\"][\/video]\n\nSouth America and Africa [pb_glossary id=\"2624\"]rift[\/pb_glossary], forming the Atlantic. <a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP1GTect\/eSoAtlantic_CutGlobe.html\">Video<\/a> by Tanya Atwater.\n\n[caption id=\"attachment_3731\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Rift-Atlantic-Animation-QR-Code.png\"><img class=\"size-thumbnail wp-image-112\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this video via this QR Code.[\/caption]\n<h3><b>2.4.2. Mid-ocean ridges<\/b><\/h3>\n[caption id=\"attachment_2541\" align=\"alignleft\" width=\"212\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Ocean-birth.svg_.png\"><img class=\"size-medium wp-image-113\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-212x300.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.[\/caption]\n\n&nbsp;\n\n<span style=\"font-weight: 400;\">As [pb_glossary id=\"2624\"]rifting[\/pb_glossary] and [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] activity progress, the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] becomes more [pb_glossary id=\"1956\"]mafic[\/pb_glossary] (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes\/\" target=\"_blank\" rel=\"noopener\">Chapter 4<\/a>) and thinner, with the eventual result transforming the [pb_glossary id=\"2591\"]plate[\/pb_glossary] under the [pb_glossary id=\"2624\"]rifting[\/pb_glossary] area into [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary]. This is the process that gives birth to a new ocean, much like the narrow Red Sea emerged with the movement of Arabia away from Africa. As the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] continues to diverge, a [pb_glossary id=\"2630\"]mid-ocean ridge[\/pb_glossary] is formed.<\/span>\n\n[pb_glossary id=\"2630\"]Mid-ocean ridges[\/pb_glossary], also known as [pb_glossary id=\"2630\"]spreading centers[\/pb_glossary], have several distinctive features. They are the only places on earth that create new [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary]. [pb_glossary id=\"1176\"]Decompression melting[\/pb_glossary] in the [pb_glossary id=\"2624\"]rift[\/pb_glossary] zone changes [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary] material into new [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary], which oozes up through cracks in [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary]. The amount of new [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] being created at [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] is highly significant. These undersea [pb_glossary id=\"2624\"]rift[\/pb_glossary] [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] produce more [pb_glossary id=\"2673\"]lava[\/pb_glossary] than all other types of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] combined. Despite this, most mid-[pb_glossary id=\"2581\"]oceanic[\/pb_glossary] ridge [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] remains unmapped because the volcanoes are located deep on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary].\n\nIn rare cases, such as a few locations in Iceland, [pb_glossary id=\"2624\"]rift[\/pb_glossary] zones display the type of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary], spreading, and ridge [pb_glossary id=\"2960\"]formation[\/pb_glossary] found on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary].\n\n[caption id=\"attachment_2542\" align=\"alignright\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/age_oceanic_lith.jpg\"><img class=\"size-medium wp-image-114\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-300x191.jpg\" alt=\"The map shoes colors that represent different ages.\" width=\"300\" height=\"191\"><\/a> Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.[\/caption]\n\nThe ridge feature is created by the accumulation of hot [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] material, which is lighter than the dense underlying [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary]. This chunk of isostatically buoyant [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] sits partially submerged and partially exposed on the [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary], like an ice cube floating in a glass of water.\n\nAs the ridge continues to spread, the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] material is pulled away from the area of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] and becomes colder and denser. As it continues to spread and cool, the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] settles into wide swathes of relatively featureless topography called [pb_glossary id=\"2884\"]abyssal[\/pb_glossary] plains with lower topography.\n\nThis model of ridge [pb_glossary id=\"2960\"]formation[\/pb_glossary] suggests the sections of [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary] furthest away from the [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] will be the oldest. Scientists have tested this idea by comparing the age of rocks located in various locations on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]. Rocks found near ridges are younger than those found far away from any ridges. [pb_glossary id=\"2678\"]Sediment[\/pb_glossary] accumulation patterns also confirm the idea of sea-floor spreading. [pb_glossary id=\"2678\"]Sediment[\/pb_glossary] layers tend to be thinner near [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary], indicating it has had less time to build up.\n\n&nbsp;\n\n[caption id=\"attachment_2543\" align=\"aligncenter\" width=\"600\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/atwater_mag_reversal_mid_ocean_ridge.gif\"><img class=\"wp-image-115 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/atwater_mag_reversal_mid_ocean_ridge.gif\" alt=\"animation showing the mid ocean ridges. As new oceanic plate is made at the ridge, it cools and preserves the current magnetic field at the time of cooling. When the poles reverse, the magnetic polarity flip is preserved in the oceanic plate record.\" width=\"600\" height=\"400\"><\/a> Spreading along several mid-ocean ridges, showing magnetic striping symmetry. By Tanya Atwater.[\/caption]\n\n[caption id=\"attachment_3743\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Atwater-Spreading-GIF.png\"><img class=\"size-thumbnail wp-image-116\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this animation via this QR Code.[\/caption]\n\nAs mentioned in the section on [pb_glossary id=\"2579\"]paleomagnetism[\/pb_glossary] and the development of [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2655\"]theory[\/pb_glossary], scientists noticed [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] contained unique magnetic [pb_glossary id=\"1719\"]anomalies[\/pb_glossary] that show up as symmetrical striping on both sides of the ridge. The Vine-Matthews-Morley [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] proposes these alternating reversals are created by the earth\u2019s magnetic field being imprinted into [pb_glossary id=\"2672\"]magma[\/pb_glossary]\n\n[caption id=\"attachment_2544\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_.png\"><img class=\"size-medium wp-image-117\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-300x212.png\" alt=\"The older stripes are farther from the ridge.\" width=\"300\" height=\"212\"><\/a> A time progression (with \"a\" being youngest and \"c\" being oldest) showing a spreading center getting wider while recording changes in the magnetic field of the Earth.[\/caption]\n\nafter it emerges from the ridge. Very hot [pb_glossary id=\"2672\"]magma[\/pb_glossary] has no magnetic field. As the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] get pulled apart, the [pb_glossary id=\"2672\"]magma[\/pb_glossary] cools below the Curie point, the [pb_glossary id=\"2689\"]temperature[\/pb_glossary] below which a magnetic field gets locked into magnetic [pb_glossary id=\"2687\"]minerals[\/pb_glossary]. The alternating magnetic reversals in the rocks reflects the periodic swapping of earth\u2019s magnetic north and south poles. This paleomagnetic pattern provides a great historical record of ocean-floor movement, and is used to reconstruct past [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] activity and determine rates of ridge spreading.\n\n[embed]https:\/\/youtu.be\/6o1HawAOTEI[\/embed]\n\n<em><a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP3RegTect\/bNoCentAtlantic.html\">Video<\/a> of the breakup of [pb_glossary id=\"3366\"]<em>Pangea<\/em>[\/pb_glossary] and [pb_glossary id=\"2960\"]<em>formation<\/em>[\/pb_glossary] of the northern Atlantic Ocean. By Tanya Atwater.<\/em>\n\n[caption id=\"attachment_3729\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pangea-Animation-QR-Code.png\"><img class=\"size-thumbnail wp-image-118\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\"><\/a> If you are using the printed version of this OER, access this video via this QR Code.[\/caption]\n\n&nbsp;\n\n[caption id=\"attachment_2545\" align=\"alignright\" width=\"233\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/BlackSmoker.jpg\"><img class=\"wp-image-119 size-medium\" 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=\"233\" height=\"300\"><\/a> Black smoker hydrothermal vent with a colony of giant (6'+) tube worms.[\/caption]\n\nThanks to their distinctive geology, [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] are home to some of the most unique ecosystems ever discovered. The ridges are often studded with [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] vents, deep fissures that allow seawater to circulate through the upper portions of the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] and interact with hot rock. The super-heated seawater rises back up to the surface of the [pb_glossary id=\"2591\"]plate[\/pb_glossary], carrying [pb_glossary id=\"2815\"]dissolved[\/pb_glossary] gasses and [pb_glossary id=\"2687\"]minerals[\/pb_glossary], and small particulates.\u00a0 The resulting emitted [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] water looks like black underwater smoke.\n\nScientists had known about these geothermal areas on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary] for some time. However, it was not until 1977, when scientists piloting a deep submergence vehicle, the Alvin, discovered a thriving community of organisms clustered around these [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] vents. These unique organisms, which include 10-foot-long tube worms taller than people, live in the complete darkness of the [pb_glossary id=\"2885\"]ocean floor [\/pb_glossary] deprived of oxygen and sunlight. They use geothermal energy provided by the vents and a process called bacterial [pb_glossary id=\"1184\"]chemosynthesis[\/pb_glossary] to feed on sulfur compounds. Before this discovery, scientists believed life on earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest this type of environment could have been the origin of life on Earth, and perhaps even extraterrestrial life elsewhere in the galaxy, such as on Jupiter\u2019s moon Europa.\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"13\"]\n\n[caption id=\"attachment_3728\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.4-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-120\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-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 2.4 via this QR Code.[\/caption]\n<h2><span style=\"font-weight: 400;\">2.5 Transform Boundaries<\/span><\/h2>\n[caption id=\"attachment_2546\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Strike_slip_fault.png\"><img class=\"size-medium wp-image-121\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-300x137.png\" alt=\"Sinistral moves to the left, dextral moves to the right.\" width=\"300\" height=\"137\"><\/a> The two types of transform\/strike slip faults.[\/caption]\n\n&nbsp;\n\nA [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundary, sometimes called a [pb_glossary id=\"3076\"]strike-slip[\/pb_glossary] or conservative boundary, is where the lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary] slide past each other in the horizontal plane. This movement is described based on the perspective of an observer standing on one of the [pb_glossary id=\"2591\"]plates[\/pb_glossary], looking across the boundary at the opposing [pb_glossary id=\"2591\"]plate[\/pb_glossary]. [pb_glossary id=\"3077\"]Dextral[\/pb_glossary], also known as [pb_glossary id=\"3077\"]right-lateral[\/pb_glossary], movement describes the opposing [pb_glossary id=\"2591\"]plate[\/pb_glossary] moving to the right. [pb_glossary id=\"2632\"]Sinistral[\/pb_glossary], also known as [pb_glossary id=\"2632\"]left lateral[\/pb_glossary], movement describe the opposing [pb_glossary id=\"2591\"]plate[\/pb_glossary] moving to the left.\n\nMost [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries are found on the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary], around [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary]. These boundaries form [pb_glossary id=\"3367\"]aseismic[\/pb_glossary] [pb_glossary id=\"1934\"]fracture[\/pb_glossary] zones, filled with earthquake-free [pb_glossary id=\"2601\"]transform[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary], to accommodate different rates of spreading occurring at the ridge.\n\n[caption id=\"attachment_2547\" align=\"alignright\" width=\"217\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Sanandreas.jpg\"><img class=\"size-medium wp-image-122\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-217x300.jpg\" alt=\"The fault runs through California.\" width=\"217\" height=\"300\"><\/a> Map of the San Andreas fault, showing relative motion.[\/caption]\n\n&nbsp;\n\nSome [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries produce significant [pb_glossary id=\"3087\"]seismic[\/pb_glossary] activity, primarily as earthquakes, with very little mountain-building or [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. This type of [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundary may contain a single [pb_glossary id=\"3065\"]fault[\/pb_glossary] or series of [pb_glossary id=\"3065\"]faults[\/pb_glossary], which develop in places where [pb_glossary id=\"2591\"]plate[\/pb_glossary] [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2915\"]stresses[\/pb_glossary] are transferred to the surface. As with other types of active boundaries, if the [pb_glossary id=\"2591\"]plates[\/pb_glossary] are unable to [pb_glossary id=\"1447\"]shear[\/pb_glossary] past each other the [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] forces will continue to build up. If the built up energy between the [pb_glossary id=\"2591\"]plates[\/pb_glossary] is suddenly released, the result is an earthquake.\n\nIn the eyes of humanity, the most significant [pb_glossary id=\"2601\"]transform[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] occur within [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary], and have a [pb_glossary id=\"1447\"]shearing[\/pb_glossary] motion that frequently produces moderate-to-large [pb_glossary id=\"3098\"]magnitude[\/pb_glossary] earthquakes. Notable examples include the San Andreas [pb_glossary id=\"3065\"]Fault[\/pb_glossary] in California, Northern and Eastern Anatolian [pb_glossary id=\"3065\"]Faults[\/pb_glossary]\u00a0in Turkey, Altyn Tagh [pb_glossary id=\"3065\"]Fault[\/pb_glossary] in central Asia, and Alpine [pb_glossary id=\"3065\"]Fault[\/pb_glossary] in New Zealand.\n<h3><b>2.5.1. Transpression and Transtension<\/b><\/h3>\n[caption id=\"attachment_2548\" align=\"alignleft\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Transpression.png\"><img class=\"wp-image-123 size-thumbnail\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Transpression-150x150.png\" alt=\"The fault is dextral, and has a leftward bend, causing uplift.\" width=\"150\" height=\"150\"><\/a> A transpressional strike-slip fault, causing uplift called a restraining bend.[\/caption]\n\n&nbsp;\n\nBends along [pb_glossary id=\"2601\"]transform[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] may create [pb_glossary id=\"1446\"]compressional[\/pb_glossary] or [pb_glossary id=\"1445\"]extensional[\/pb_glossary] forces that cause secondary [pb_glossary id=\"3065\"]faulting[\/pb_glossary] zones. [pb_glossary id=\"2634\"]Transpression[\/pb_glossary] occurs where there is a component of [pb_glossary id=\"1446\"]compression[\/pb_glossary] in addition to the [pb_glossary id=\"1447\"]shearing[\/pb_glossary] motion. These forces build up around the area of the bend, where the opposing [pb_glossary id=\"2591\"]plates[\/pb_glossary] are restricted from sliding past each other. As the forces continue to build up, they create mountains in the restraining bend around the [pb_glossary id=\"3065\"]fault[\/pb_glossary]. The Big Bend area, located in the southern part of the San Andreas [pb_glossary id=\"3065\"]Fault[\/pb_glossary] includes a large area of [pb_glossary id=\"2634\"]transpression[\/pb_glossary] where many mountains have been built, moved, and even rotated.\n\n&nbsp;\n\n[caption id=\"attachment_2549\" align=\"alignright\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Transtension.png\"><img class=\"wp-image-124 size-thumbnail\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Transtension-150x150.png\" alt=\"The fault is dextral, and has a rightward bend, causing a valley.\" width=\"150\" height=\"150\"><\/a> A transtensional strike-slip fault, causing a restraining bend. In the center of the fault, a depression with extension would be found.[\/caption]\n\n[pb_glossary id=\"2635\"]Transtension[\/pb_glossary] zones require a [pb_glossary id=\"3065\"]fault[\/pb_glossary] that includes a releasing bend, where the [pb_glossary id=\"2591\"]plates[\/pb_glossary] are being pulled apart by [pb_glossary id=\"1445\"]extensional[\/pb_glossary] forces. Depressions and sometimes [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] develop in the releasing bend, along the [pb_glossary id=\"3065\"]fault[\/pb_glossary]. The Dead Sea found between Israel and Jordan, and the Salton Sea of California are examples of basins formed by transtensional forces.\n<h3><\/h3>\n<h3><b>2.5.2. Piercing Points<\/b><\/h3>\n[caption id=\"attachment_2550\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Wallace_Creek_offset_across_the_San_Andreas_Fault.png\"><img class=\"size-medium wp-image-125\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-300x205.png\" alt=\"The offset is to the left.\" width=\"300\" height=\"205\"><\/a> Wallace (dry) Creek on the Cariso Plain, California. Note as the creek flows from the northern mountainous part of the image, it takes a sharp right (as viewed from the flow of water), then a sharp left. This is caused by the San Andreas Fault cutting roughly perpendicular to the creek, and shifting the location of the creek over time. The fault can be seen about halfway down, trending left to right, as a change in the topography.[\/caption]\n\n&nbsp;\n\nWhen a geological feature is cut by a [pb_glossary id=\"3065\"]fault[\/pb_glossary], it is called a [pb_glossary id=\"2636\"]piercing point[\/pb_glossary]. Piercing points are very useful for recreating past [pb_glossary id=\"3065\"]fault[\/pb_glossary] movement, especially along [pb_glossary id=\"2601\"]transform[\/pb_glossary] boundaries. [pb_glossary id=\"2601\"]Transform[\/pb_glossary] [pb_glossary id=\"3065\"]faults[\/pb_glossary] are unique because their horizontal motion keeps a geological feature relatively intact, preserving the record of what happened. Other types of [pb_glossary id=\"3065\"]faults[\/pb_glossary]\u2014normal and reverse \u2014tend to be more destructive, obscuring or destroying these features. The best type of [pb_glossary id=\"2636\"]piercing point[\/pb_glossary] includes unique patterns that are used to match the parts of a geological feature separated by [pb_glossary id=\"3065\"]fault[\/pb_glossary] movement. Detailed studies of piercing points show the San Andreas [pb_glossary id=\"3065\"]Fault[\/pb_glossary] has experienced over 225 km of movement in the last 20 million years, and this movement occurred at three different [pb_glossary id=\"3065\"]fault[\/pb_glossary] traces.\n\n[embed]https:\/\/www.youtube.com\/embed\/IDTBY5WDELg[\/embed]\n\n<em>Video of the origin of the San Andreas [pb_glossary id=\"3065\"]<em>fault<\/em>[\/pb_glossary]. As the [pb_glossary id=\"2630\"]<em>mid-ocean ridge<\/em>[\/pb_glossary] [pb_glossary id=\"2602\"]<em>subducts<\/em>[\/pb_glossary], the relative motion between the remaining [pb_glossary id=\"2591\"]<em>plates<\/em>[\/pb_glossary] become [pb_glossary id=\"2601\"]<em>transform<\/em>[\/pb_glossary], forming the [pb_glossary id=\"3065\"]<em>fault<\/em>[\/pb_glossary] [pb_glossary id=\"2664\"]<em>system<\/em>[\/pb_glossary]. Note that because the motion of the [pb_glossary id=\"2591\"]<em>plates<\/em>[\/pb_glossary] is not exactly parallel to the [pb_glossary id=\"3065\"]<em>fault<\/em>[\/pb_glossary], it causes [pb_glossary id=\"2599\"]<em>divergent<\/em>[\/pb_glossary] motion in the interior of North America. By Tanya Atwater.<\/em>\n\n[caption id=\"attachment_3727\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Plate-Tectonics-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-126\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-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>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"14\"]\n\n[caption id=\"attachment_3726\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.5-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-127\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-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 2.5 via this QR Code.[\/caption]\n<h2><span style=\"font-weight: 400;\">2.6 The Wilson Cycle<\/span><\/h2>\n[caption id=\"attachment_2551\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Wilson-cycle_hg.svg_.png\"><img class=\"size-medium wp-image-128\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-300x164.png\" alt=\"The diagram shows the last 1000 million years.\" width=\"300\" height=\"164\"><\/a> Diagram of the Wilson Cycle, showing rifting and collision phases.[\/caption]\n\n&nbsp;\n\nThe [pb_glossary id=\"2637\"]Wilson Cycle[\/pb_glossary] is named for J. Tuzo Wilson who first described it in 1966, and it outlines the ongoing origin and breakup of [pb_glossary id=\"2623\"]supercontinents[\/pb_glossary], such as [pb_glossary id=\"3366\"]Pangea[\/pb_glossary] and [pb_glossary id=\"2211\"]Rodinia[\/pb_glossary]. Scientists have determined this cycle has been operating for at least three billion years and possibly earlier.\n\nThere are a number of [pb_glossary id=\"2652\"]hypotheses[\/pb_glossary] about how the [pb_glossary id=\"2637\"]Wilson Cycle[\/pb_glossary] works. One mechanism proposes that [pb_glossary id=\"2624\"]rifting[\/pb_glossary] happens because [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary] reflect the heat much better than [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary]. When continents congregate together, they reflect more of the Earth\u2019s heat back into the [pb_glossary id=\"2586\"]mantle[\/pb_glossary], generating more vigorous [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents that then start the [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary] process. Some geologists believe [pb_glossary id=\"2586\"]mantle[\/pb_glossary] plumes are remnants of these [pb_glossary id=\"2192\"]periods[\/pb_glossary] of increased [pb_glossary id=\"2586\"]mantle[\/pb_glossary] [pb_glossary id=\"2689\"]temperature[\/pb_glossary] and [pb_glossary id=\"2577\"]convection[\/pb_glossary] upwelling, and study them for clues about the origin of [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary].\n\nThe mechanism behind how [pb_glossary id=\"2623\"]supercontinents[\/pb_glossary] are created is still largely a mystery. There are three schools of thought about what continues to drive the continents further apart and eventually bring them together. The ridge-push [pb_glossary id=\"2652\"]hypothesis[\/pb_glossary] suggests after the initial [pb_glossary id=\"2624\"]rifting[\/pb_glossary] event, [pb_glossary id=\"2591\"]plates[\/pb_glossary] continue to be pushed apart by mid-ocean [pb_glossary id=\"2630\"]spreading centers[\/pb_glossary] and their underlying [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents. Slab-pull proposes the [pb_glossary id=\"2591\"]plates[\/pb_glossary] are pulled apart by descending slabs in the [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones of the [pb_glossary id=\"2581\"]oceanic[\/pb_glossary]-[pb_glossary id=\"2575\"]continental[\/pb_glossary] margins. A third idea, gravitational sliding, attributes the movement to gravitational forces pulling the lithospheric [pb_glossary id=\"2591\"]plates[\/pb_glossary] down from the elevated [pb_glossary id=\"2630\"]mid-ocean ridges[\/pb_glossary] and across the underlying [pb_glossary id=\"2593\"]asthenosphere[\/pb_glossary]. Current evidence seems to support [pb_glossary id=\"2605\"]slab[\/pb_glossary] pull more than ridge push or gravitational sliding.\n<h2><span style=\"font-weight: 400;\">2.7 Hotspots<\/span><\/h2>\n[caption id=\"attachment_2552\" align=\"alignright\" 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&nbsp;\n\nThe [pb_glossary id=\"2637\"]Wilson Cycle[\/pb_glossary] provides a broad overview of [pb_glossary id=\"2576\"]tectonic[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement. To analyze [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement more precisely, scientists study [pb_glossary id=\"2638\"]hotspots[\/pb_glossary]. First postulated by J. Tuzo Wilson in 1963, a [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is an area in the lithospheric [pb_glossary id=\"2591\"]plate[\/pb_glossary] where molten [pb_glossary id=\"2672\"]magma[\/pb_glossary] breaks through and creates a [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] center, islands in the ocean and mountains on land. As the [pb_glossary id=\"2591\"]plate[\/pb_glossary] moves across the [pb_glossary id=\"2638\"]hotspot[\/pb_glossary], the [pb_glossary id=\"1181\"]volcano[\/pb_glossary] center becomes [pb_glossary id=\"1708\"]extinct[\/pb_glossary] because it is no longer over an active [pb_glossary id=\"2672\"]magma[\/pb_glossary] source. Instead, the [pb_glossary id=\"2672\"]magma[\/pb_glossary] emerges through another area in the [pb_glossary id=\"2591\"]plate[\/pb_glossary] to create a new active [pb_glossary id=\"1181\"]volcano[\/pb_glossary]. Over time, the combination of moving [pb_glossary id=\"2591\"]plate[\/pb_glossary] and stationary [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] creates a chain of islands or mountains. The classic definition of [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] states they do not move, although recent evidence suggests that there may be exceptions.\n\n[caption id=\"attachment_2553\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/HotspotsWorld.png\"><img class=\"size-medium wp-image-130\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-300x175.png\" alt=\"Hotspots are scattered around the world.\" width=\"300\" height=\"175\"><\/a> Map of world hotspots. Larger circles indicate more active hotspots.[\/caption]\n\n&nbsp;\n\n[pb_glossary id=\"2638\"]Hotspots[\/pb_glossary] are the only types of [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] not associated with [pb_glossary id=\"2602\"]subduction[\/pb_glossary] or [pb_glossary id=\"2624\"]rifting[\/pb_glossary] zones at [pb_glossary id=\"2591\"]plate[\/pb_glossary] boundaries; they seem totally disconnected from any [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] processes, such as earthquakes. However, there are\u00a0 relationships between [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] and [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary]. There are several [pb_glossary id=\"2638\"]hotspots[\/pb_glossary], current and former, that are believed to have begun at the time of [pb_glossary id=\"2624\"]rifting[\/pb_glossary]. Also, scientists use the age of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] eruptions and shape of the chain to quantify the rate and direction of [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement relative to the [pb_glossary id=\"2638\"]hotspot[\/pb_glossary].\n\nScientists are divided over how [pb_glossary id=\"2672\"]magma[\/pb_glossary] is generated in [pb_glossary id=\"2638\"]hotspots[\/pb_glossary]. Some suggest that [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] originate from super-heated material from as deep as the [pb_glossary id=\"2589\"]core[\/pb_glossary] that reaches the Earth\u2019s [pb_glossary id=\"2580\"]crust[\/pb_glossary] as a [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary]. Others argue the molten material that feeds [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] is sourced from the [pb_glossary id=\"2586\"]mantle[\/pb_glossary]. Of course, it is difficult to collect data from these deep-Earth features due to the extremely high pressure and [pb_glossary id=\"2689\"]temperature[\/pb_glossary].\n\nHow [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] are initiated is another highly debated subject. The prevailing mechanism has [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] starting in [pb_glossary id=\"2599\"]divergent[\/pb_glossary] boundaries during [pb_glossary id=\"2623\"]supercontinent[\/pb_glossary] [pb_glossary id=\"2624\"]rifting[\/pb_glossary]. Scientists have identified a number of current and past [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] believed to have begun this way. [pb_glossary id=\"2602\"]Subducting[\/pb_glossary] slabs have also been named as causing [pb_glossary id=\"2586\"]mantle[\/pb_glossary] plumes and hot-spot [pb_glossary id=\"1181\"]volcanism[\/pb_glossary]. Some geologists have suggested another geological process not involving [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] may be involved, such as a large space objects crashing into Earth. Regardless of how they are formed, dozens are on the Earth. Some well-known examples include the Tahiti Islands, Afar Triangle, Easter Island, Iceland, Galapagos Islands, and Samoan Islands. The United States is home to two of the largest and best-studied [pb_glossary id=\"2638\"]hotspots[\/pb_glossary]: Hawaii and Yellowstone.\n<h3><b>2.7.1 Hawaiian hotspot<\/b><\/h3>\n[caption id=\"attachment_2554\" align=\"alignright\" 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\n&nbsp;\n\nThe active [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] in Hawaii represent one of the most active [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] sites on earth. Scientific evidence indicates the Hawaiian [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is at least 80 million years old. Geologists believe it is actually much older; however any rocks with proof of this have been [pb_glossary id=\"2602\"]subducted[\/pb_glossary] under the [pb_glossary id=\"2885\"]ocean floor[\/pb_glossary]. The big island of Hawaii sits atop a large [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary] that marks the active [pb_glossary id=\"2638\"]hotspot[\/pb_glossary]. The Kilauea [pb_glossary id=\"1181\"]volcano[\/pb_glossary] is the main [pb_glossary id=\"1187\"]vent[\/pb_glossary] for this [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] and has been actively erupting since 1983.\n\nThis enormous [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] island chain, much of which is underwater, stretches across the Pacific for almost 6,000 km. The [pb_glossary id=\"2899\"]seamount[\/pb_glossary] chain\u2019s most striking feature is a sharp 60-degree bend located at the midpoint, which marks a significant change in [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement direction that occurred 50 million years ago. The change in direction has been more often linked to a [pb_glossary id=\"2591\"]plate[\/pb_glossary] reconfiguration, but also to other things like plume migration.\n\n[caption id=\"attachment_2555\" align=\"alignleft\" width=\"300\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hawaii_hotspot_cross-sectional_diagram.jpg\"><img class=\"size-medium wp-image-132\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-300x159.jpg\" alt=\"The islands get older to the left.\" width=\"300\" height=\"159\"><\/a> Diagram of the Hawaiian hotspot and islands that it formed.[\/caption]\n\n&nbsp;\n\nIn an attempt to map the Hawaiian [pb_glossary id=\"2639\"]mantle plume[\/pb_glossary] as far down as the lower [pb_glossary id=\"2586\"]mantle[\/pb_glossary], scientists have used [pb_glossary id=\"3096\"]tomography[\/pb_glossary], a type of three-dimensional [pb_glossary id=\"3087\"]seismic[\/pb_glossary] imaging. This information\u2014along with other evidence gathered from rock ages, vegetation types, and island size\u2014indicate the oldest islands in the chain are located the furthest away from the active [pb_glossary id=\"2638\"]hotspot[\/pb_glossary].\n<h4><b>2.7.2 Yellowstone hotspot<\/b><\/h4>\n[caption id=\"attachment_2556\" 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\n&nbsp;\n\nLike the Hawaiian version, the Yellowstone [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is formed by [pb_glossary id=\"2672\"]magma[\/pb_glossary] rising through the [pb_glossary id=\"2590\"]lithosphere[\/pb_glossary]. What makes it different is this [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is located under a thick, [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary]. Hawaii sits on a thin [pb_glossary id=\"2581\"]oceanic[\/pb_glossary]\u00a0[pb_glossary id=\"2591\"]plate[\/pb_glossary], which is easily breached by [pb_glossary id=\"2672\"]magma[\/pb_glossary] coming to the surface. At Yellowstone, the thick [pb_glossary id=\"2575\"]continental[\/pb_glossary] [pb_glossary id=\"2591\"]plate[\/pb_glossary] presents a much more difficult barrier for [pb_glossary id=\"2672\"]magma[\/pb_glossary] to penetrate. When it does emerge, the eruptions are generally much more violent. Thankfully they are also less frequent.\n\nOver 15 million years of eruptions by this [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] have carved a curved path across the western United States. It has been suggested the Yellowstone [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] is connected to the much older Columbia [pb_glossary id=\"3134\"]River[\/pb_glossary] [pb_glossary id=\"1197\"]flood basalts[\/pb_glossary] and even to 70 million-year-old [pb_glossary id=\"1181\"]volcanism[\/pb_glossary] found in the Yukon region of Canada.\n\n[caption id=\"attachment_2557\" align=\"alignleft\" 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 most recent major eruption of this [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] created the Yellowstone [pb_glossary id=\"1188\"]Caldera[\/pb_glossary] and [pb_glossary id=\"2673\"]Lava[\/pb_glossary] Creek [pb_glossary id=\"1953\"]tuff[\/pb_glossary] [pb_glossary id=\"2960\"]formation[\/pb_glossary] approximately 631,000 years ago. The eruption threw 1,000 cubic kilometers of [pb_glossary id=\"1949\"]ash[\/pb_glossary] and [pb_glossary id=\"2672\"]magma[\/pb_glossary] into the [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary], some of which was found as far away as Mississippi. Should the [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] erupt again, scientists predict it will be another [pb_glossary id=\"1933\"]massive[\/pb_glossary] event. This would be a calamity reaching far beyond the western United States. These super [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] eruptions fill the earth\u2019s [pb_glossary id=\"2667\"]atmosphere[\/pb_glossary] with so much gas and [pb_glossary id=\"1949\"]ash[\/pb_glossary], they block sunlight from reaching the earth. Not only would this drastically alter climates and environments around the globe, it could affect worldwide food production.\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n[h5p id=\"15\"]\n\n[caption id=\"attachment_3725\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.6-7-Did-I-Get-It-QR-Code.png\"><img class=\"size-thumbnail wp-image-135\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-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 sections 2.6 and 2.7 via this QR Code.[\/caption]\n<h2><\/h2>\n<h2>Summary<\/h2>\n[embed]https:\/\/youtu.be\/6wJBOk9xjto[\/embed]\n\n[caption id=\"attachment_3724\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Plate-Tectonics-Basics-YouTube-QR-Code.png\"><img class=\"size-thumbnail wp-image-136\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-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[pb_glossary id=\"2576\"]Plate tectonics[\/pb_glossary] is a unifying [pb_glossary id=\"2655\"]theory[\/pb_glossary]; it explains nearly all of the major geologic processes on Earth. Since its early inception in the 1950s and 1960s, geologists have been guided by this revolutionary perception of the world. The [pb_glossary id=\"2655\"]theory[\/pb_glossary] of [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary] states the surface layer of the Earth is broken into a network of solid, relatively [pb_glossary id=\"2583\"]brittle[\/pb_glossary] [pb_glossary id=\"2591\"]plates[\/pb_glossary]. Underneath the [pb_glossary id=\"2591\"]plates[\/pb_glossary] is a much hotter and more [pb_glossary id=\"2582\"]ductile[\/pb_glossary] layer that contains zones of convective upwelling generated by the interior heat of Earth. These [pb_glossary id=\"2577\"]convection[\/pb_glossary] currents move the surface [pb_glossary id=\"2591\"]plates[\/pb_glossary] around\u2014bringing them together, pulling them apart, and [pb_glossary id=\"1447\"]shearing[\/pb_glossary] them side-by-side. Earthquakes and [pb_glossary id=\"1181\"]volcanoes[\/pb_glossary] form at the boundaries where the [pb_glossary id=\"2591\"]plates[\/pb_glossary] interact, with the exception of [pb_glossary id=\"1181\"]volcanic[\/pb_glossary] [pb_glossary id=\"2638\"]hotspots[\/pb_glossary], which are not caused by [pb_glossary id=\"2591\"]plate[\/pb_glossary] movement.\n\n&nbsp;\n<h3>Take this quiz to check your comprehension of this Chapter.<\/h3>\n[h5p id=\"16\"]\n\n[caption id=\"attachment_3723\" align=\"aligncenter\" width=\"150\"]<a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ch.2-Review-QR-Code.png\"><img class=\"size-thumbnail wp-image-137\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-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 2 via this QR Code.[\/caption]\n<h2>References<\/h2>\n<div class=\"csl-bib-body\">\n<ol>\n \t<li class=\"csl-entry\">Aitta, A., 2006, Iron melting curve with a tricritical point: J. Stat. Mech., v. 2006, no. 12, p. P12015.<\/li>\n \t<li class=\"csl-entry\">Alfe, D., Gillan, M.J., and Price, G.D., 2002, [pb_glossary id=\"2831\"]Composition[\/pb_glossary] and [pb_glossary id=\"2689\"]temperature[\/pb_glossary] of the Earth\u2019s [pb_glossary id=\"2589\"]core[\/pb_glossary] constrained by combining ab initio calculations and [pb_glossary id=\"3087\"]seismic[\/pb_glossary] data: Earth Planet. Sci. Lett., v. 195, no. 1, p. 91\u201398.<\/li>\n \t<li class=\"csl-entry\">Atwater, T., 1970, Implications of [pb_glossary id=\"2576\"]Plate Tectonics[\/pb_glossary] for the [pb_glossary id=\"1441\"]Cenozoic[\/pb_glossary] [pb_glossary id=\"2576\"]Tectonic[\/pb_glossary] Evolution of Western North America: Geol. Soc. Am. Bull., v. 81, no. 12, p. 3513\u20133536., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2\">10.1130\/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Bacon, F., and Montagu, B., 1848, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author: The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author, Parry &amp; McMillan, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author.<\/li>\n \t<li class=\"csl-entry\">Benioff, H., 1949, [pb_glossary id=\"3087\"]Seismic[\/pb_glossary] evidence for the [pb_glossary id=\"3065\"]fault[\/pb_glossary] origin of [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] deeps: Geological Society of America Bulletin, v. 60, no. 12, p. 1837\u20131856., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1949)60[1837:SEFTFO]2.0.CO;2\">10.1130\/0016-7606(1949)60[1837:SEFTFO]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Birch, F., 1952, Elasticity and constitution of the Earth\u2019s interior: J. Geophys. Res., v. 57, no. 2, p. 227\u2013286., doi: <a href=\"https:\/\/doi.org\/10.1029\/JZ057i002p00227\">10.1029\/JZ057i002p00227<\/a>.<\/li>\n \t<li class=\"csl-entry\">Birch, F., 1964, Density and [pb_glossary id=\"2831\"]composition[\/pb_glossary] of [pb_glossary id=\"2586\"]mantle[\/pb_glossary] and [pb_glossary id=\"2589\"]core[\/pb_glossary]: J. Geophys. Res., v. 69, no. 20, p. 4377\u20134388.<\/li>\n \t<li class=\"csl-entry\">Bott, M.H.P., 1993, Modelling the [pb_glossary id=\"2591\"]plate[\/pb_glossary]-driving mechanism: Journal of the Geological Society, v. 150, no. 5, p. 941\u2013951., doi: <a href=\"https:\/\/doi.org\/10.1144\/gsjgs.150.5.0941\">10.1144\/gsjgs.150.5.0941<\/a>.<\/li>\n \t<li class=\"csl-entry\">Coats, R.R., 1962, [pb_glossary id=\"2672\"]Magma[\/pb_glossary] type and crustal structure in the Aleutian [pb_glossary id=\"2609\"]Arc[\/pb_glossary], <i>in<\/i> The [pb_glossary id=\"2580\"]Crust[\/pb_glossary] of the Pacific [pb_glossary id=\"1461\"]Basin[\/pb_glossary]: American Geophysical Union, p. 92\u2013109., doi: <a href=\"https:\/\/doi.org\/10.1029\/GM006p0092\">10.1029\/GM006p0092<\/a>.<\/li>\n \t<li class=\"csl-entry\">Conrad, C.P., and Lithgow-Bertelloni, C., 2002, How [pb_glossary id=\"2586\"]mantle[\/pb_glossary] slabs drive [pb_glossary id=\"2576\"]plate tectonics[\/pb_glossary]: Science (New York, N.Y.), v. 298, no. 5591, p. 207\u2013209., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.1074161\">10.1126\/science.1074161<\/a>.<\/li>\n \t<li class=\"csl-entry\">Corliss, J.B., Dymond, J.G., Gordon, L.I., Edmond, J.M., von Heezen, R.P., Ballard, R.D., Green, K., Williams, D.L., Bainbridge, A., Crane, K., and van Andel, T.H., 1979, Submarine thermal springs on the Galapagos [pb_glossary id=\"2624\"]Rift[\/pb_glossary]: Science, v. 203, p. 107321083.<\/li>\n \t<li class=\"csl-entry\">Davis, E.E., and Lister, C.R.B., 1974, Fundamentals of ridge crest topography: Earth Planet. Sci. Lett., v. 21, no. 4, p. 405\u2013413.<\/li>\n \t<li class=\"csl-entry\">Dawson, J.B., Pinkerton, H., Norton, G.E., and Pyle, D.M., 1990, Physicochemical properties of alkali [pb_glossary id=\"3365\"]carbonatite[\/pb_glossary] lavas:Data from the 1988 eruption of Oldoinyo Lengai, Tanzania: Geology, v. 18, no. 3, p. 260\u2013263.<\/li>\n \t<li class=\"csl-entry\">Drake, E.T., 1976, Alfred Wegener\u2019s reconstruction of [pb_glossary id=\"3366\"]Pangea[\/pb_glossary]: Geology, v. 4, no. 1, p. 41\u201344., doi: &lt;a href=\"https:\/\/doi.org\/10.1130\/0091-7613(1976)42.0.CO;2\"&gt;10.1130\/0091-7613(1976)4&lt;41:AWROP&gt;2.0.CO;2.<\/li>\n \t<li class=\"csl-entry\">Engdahl, E.R., Flynn, E.A., and Masse, R.P., 1974, Differential PkiKP travel times and the radius of the [pb_glossary id=\"2589\"]core[\/pb_glossary]: Geophysical J Royal Astro Soc, v. 40, p. 457\u2013463.<\/li>\n \t<li class=\"csl-entry\">Ewing, M., Ewing, J.I., and Talwani, M., 1964, [pb_glossary id=\"2678\"]Sediment[\/pb_glossary] distribution in the oceans: The Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 75, no. 1, p. 17\u201336., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1964)75[17:SDITOT]2.0.CO;2\">10.1130\/0016-7606(1964)75[17:SDITOT]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Ewing, M., Houtz, R., and Ewing, J., 1969, South Pacific [pb_glossary id=\"2678\"]sediment[\/pb_glossary] distribution: J. Geophys. Res., v. 74, no. 10, p. 2477\u20132493., doi: <a href=\"https:\/\/doi.org\/10.1029\/JB074i010p02477\">10.1029\/JB074i010p02477<\/a>.<\/li>\n \t<li class=\"csl-entry\">Fernandez, L.M., and Careaga, J., 1968, The thickness of the [pb_glossary id=\"2580\"]crust[\/pb_glossary] in central United States and La Paz, Bolivia, from the spectrum of longitudinal [pb_glossary id=\"3087\"]seismic[\/pb_glossary] waves: Bull. Seismol. Soc. Am., v. 58, no. 2, p. 711\u2013741.<\/li>\n \t<li class=\"csl-entry\">Fluegel, von H.W., 1980, Wegener-Ampferer-Schwinner. Ein Beitrag zur Geschichte der Geologie in \u00d6sterreich: Mitt. Oesterr. Geol. Ges., v. 73, p. 237\u2013254.<\/li>\n \t<li class=\"csl-entry\">Forsyth, D.W., 1975, The Early Structural Evolution and Anisotropy of the [pb_glossary id=\"2581\"]Oceanic[\/pb_glossary] Upper [pb_glossary id=\"2586\"]Mantle[\/pb_glossary]: Geophys. J. Int., v. 43, no. 1, p. 103\u2013162., doi: <a href=\"https:\/\/doi.org\/10.1111\/j.1365-246X.1975.tb00630.x\">10.1111\/j.1365-246X.1975.tb00630.x<\/a>.<\/li>\n \t<li class=\"csl-entry\">Frankel, H., 1982, The Development, Reception, and Acceptance of the Vine-Matthews-Morley [pb_glossary id=\"2652\"]Hypothesis[\/pb_glossary]: Hist. Stud. Phys. Biol. Sci., v. 13, no. 1, p. 1\u201339.<\/li>\n \t<li class=\"csl-entry\">Fukao, Y., and Obayashi, M., 2013, [pb_glossary id=\"2602\"]Subducted[\/pb_glossary] slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity: J. Geophys. Res. [Solid Earth], v. 118, no. 11, p. 2013JB010466.<\/li>\n \t<li class=\"csl-entry\">Hagstrum, J.T., 2005, Antipodal [pb_glossary id=\"2638\"]hotspots[\/pb_glossary] and bipolar catastrophes: Were [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] large-body impacts the cause? Earth Planet. Sci. Lett., v. 236, no. 1\u20132, p. 13\u201327.<\/li>\n \t<li class=\"csl-entry\">Hanks, T.C., and Anderson, D.L., 1969, The early thermal history of the earth: Phys. Earth Planet. Inter., v. 2, no. 1, p. 19\u201329.<\/li>\n \t<li class=\"csl-entry\">Heezen, B.C., and Tharp, M., 1965, [pb_glossary id=\"2576\"]Tectonic[\/pb_glossary] Fabric of the Atlantic and Indian Oceans and [pb_glossary id=\"2575\"]Continental[\/pb_glossary] Drift: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 90\u2013106., doi: <a href=\"https:\/\/doi.org\/10.1098\/rsta.1965.0024\">10.1098\/rsta.1965.0024<\/a>.<\/li>\n \t<li class=\"csl-entry\">Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, E.S., Shuster, M.W., Winslow, N.S., and Lawton, T.F., 1986, Time of initial [pb_glossary id=\"3074\"]thrusting[\/pb_glossary] in the Sevier orogenic belt, Idaho-Wyoming and Utah: Geology, v. 14, no. 5, p. 388\u2013391.<\/li>\n \t<li class=\"csl-entry\">Herak, D., and Herak, M., 2007, Andrija Mohorovi\u010di\u0107 (1857-1936)\u2014On the occasion of the 150th anniversary of his birth: Seismol. Res. Lett., v. 78, no. 6, p. 671\u2013674.<\/li>\n \t<li class=\"csl-entry\">Hess, H.H., 1962, History of ocean basins: [pb_glossary id=\"1173\"]Petrologic[\/pb_glossary] studies, v. 4, p. 599\u2013620.<\/li>\n \t<li class=\"csl-entry\">Hutson, P., Middleton, J., and Miller, D., 2003, [pb_glossary id=\"2620\"]Collision[\/pb_glossary] Zones: Online, <a href=\"http:\/\/www.geosci.usyd.edu.au\/users\/prey\/ACSGT\/EReports\/eR.2003\/GroupD\/Report1\/web%20pages\/contents.html\">http:\/\/www.geosci.usyd.edu.au\/users\/prey\/ACSGT\/EReports\/eR.2003\/GroupD\/Report1\/web%20pages\/contents.html<\/a>, accessed June 2017.<\/li>\n \t<li class=\"csl-entry\">Isacks, B., Oliver, J., and Sykes, L.R., 1968, Seismology and the new global [pb_glossary id=\"2576\"]tectonics[\/pb_glossary]: J. Geophys. Res., v. 73, no. 18, p. 5855\u20135899.<\/li>\n \t<li class=\"csl-entry\">Ito, E., and Takahashi, E., 1989, Postspinel transformations in the [pb_glossary id=\"2664\"]system[\/pb_glossary] Mg2SiO4-Fe2SiO4 and some geophysical implications: J. Geophys. Res. [Solid Earth], v. 94, no. B8, p. 10637\u201310646.<\/li>\n \t<li class=\"csl-entry\">Jacoby, W.R., 1981, Modern concepts of Earth dynamics anticipated by Alfred Wegener in 1912: Geology, v. 9, no. 1, p. 25\u201327., doi: &lt;a href=\"https:\/\/doi.org\/10.1130\/0091-7613(1981)92.0.CO;2\"&gt;10.1130\/0091-7613(1981)9&lt;25:MCOEDA&gt;2.0.CO;2.<\/li>\n \t<li class=\"csl-entry\">Jakosky, B.M., Grebowsky, J.M., Luhmann, J.G., Connerney, J., Eparvier, F., Ergun, R., Halekas, J., Larson, D., Mahaffy, P., McFadden, J., Mitchell, D.F., Schneider, N., Zurek, R., Bougher, S., and others, 2015, MAVEN observations of the response of Mars to an interplanetary coronal mass ejection: Science, v. 350, no. 6261, p. aad0210.<\/li>\n \t<li class=\"csl-entry\">James, D.E., Fouch, M.J., Carlson, R.W., and Roth, J.B., 2011, [pb_glossary id=\"2605\"]Slab[\/pb_glossary] fragmentation, edge flow and the origin of the Yellowstone [pb_glossary id=\"2638\"]hotspot[\/pb_glossary] track: Earth Planet. Sci. Lett., v. 311, no. 1\u20132, p. 124\u2013135.<\/li>\n \t<li class=\"csl-entry\">Ji, Y., and Nataf, H.-C., 1998, Detection of [pb_glossary id=\"2586\"]mantle[\/pb_glossary] plumes in the lower [pb_glossary id=\"2586\"]mantle[\/pb_glossary] by diffraction [pb_glossary id=\"3096\"]tomography[\/pb_glossary]: Hawaii: Earth Planet. Sci. Lett., v. 159, no. 3\u20134, p. 99\u2013115.<\/li>\n \t<li class=\"csl-entry\">Johnston, S.T., Jane Wynne, P., Francis, D., Hart, C.J.R., Enkin, R.J., and Engebretson, D.C., 1996, Yellowstone in Yukon: The Late [pb_glossary id=\"1440\"]Cretaceous[\/pb_glossary] Carmacks Group: Geology, v. 24, no. 11, p. 997\u20131000.<\/li>\n \t<li class=\"csl-entry\">Kearey, P., Klepeis, K.A., and Vine, F.J., 2009, Global [pb_glossary id=\"2576\"]Tectonics[\/pb_glossary]: Oxford\u202f; Chichester, West Sussex\u202f; Hoboken, NJ, Wiley-Blackwell, 496 p.<\/li>\n \t<li class=\"csl-entry\">Le Pichon, X., 1968, Sea-floor spreading and [pb_glossary id=\"2575\"]continental[\/pb_glossary] drift: J. Geophys. Res., v. 73, no. 12, p. 3661\u20133697.<\/li>\n \t<li class=\"csl-entry\">Lehmann, I., 1936, P\u2019, Publ: Bur. Centr. Seism. Internat. Serie A, v. 14, p. 87\u2013115.<\/li>\n \t<li class=\"csl-entry\">Mantovani, R., 1889, Les [pb_glossary id=\"1934\"]fractures[\/pb_glossary] de l\u2019\u00e9corce terrestre et la th\u00e9orie de Laplace: Bull. Soc. Sc. et Arts R\u00e9union, p. 41\u201353.<\/li>\n \t<li class=\"csl-entry\">Mason, R.G., 1958, A magnetic survey off the west [pb_glossary id=\"2890\"]coast[\/pb_glossary] of the United-States between latitudes 32-degrees-N and 36-degrees-N longitudes 121-degrees-W and 128-degrees-W: Geophysical Journal of the Royal Astronomical Society, v. 1, no. 4, p. 320.<\/li>\n \t<li class=\"csl-entry\">Mason, R.G., and Raff, A.D., 1961, Magnetic Survey Off the West [pb_glossary id=\"2890\"]Coast[\/pb_glossary] of North America, 32\u00b0 N. [pb_glossary id=\"3372\"]Latitude[\/pb_glossary] to 42\u00b0 N. [pb_glossary id=\"3372\"]Latitude[\/pb_glossary]: Geological Society of America Bulletin, v. 72, no. 8, p. 1259\u20131265., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2\">10.1130\/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">McCollom, T.M., 1999, Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in [pb_glossary id=\"2921\"]hydrothermal[\/pb_glossary] systems on Europa: J. Geophys. Res., v. 104, no. E12, p. 30729\u201330742., doi: <a href=\"https:\/\/doi.org\/10.1029\/1999JE001126\">10.1029\/1999JE001126<\/a>.<\/li>\n \t<li class=\"csl-entry\">McKenzie, D.P., and Parker, R.L., 1967, The North Pacific: an Example of [pb_glossary id=\"2576\"]Tectonics[\/pb_glossary] on a Sphere: Nature, v. 216, p. 1276\u20131280., doi: <a href=\"https:\/\/doi.org\/10.1038\/2161276a0\">10.1038\/2161276a0<\/a>.<\/li>\n \t<li class=\"csl-entry\">Miller, A.R., Densmore, C.D., Degens, E.T., Hathaway, J.C., Manheim, F.T., McFarlin, P.F., Pocklington, R., and Jokela, A., 1966, Hot brines and recent iron deposits in deeps of the Red Sea: Geochimica et Cosmochimica Acta, v. 30, no. 3, p. 341\u2013359., doi: <a href=\"https:\/\/doi.org\/10.1016\/0016-7037(66)90007-X\">10.1016\/0016-7037(66)90007-X<\/a>.<\/li>\n \t<li class=\"csl-entry\">Morgan, W.J., 1968, Rises, trenches, great [pb_glossary id=\"3065\"]faults[\/pb_glossary], and crustal blocks: J. Geophys. Res., v. 73, no. 6, p. 1959\u20131982., doi: <a href=\"https:\/\/doi.org\/10.1029\/JB073i006p01959\">10.1029\/JB073i006p01959<\/a>.<\/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\">Oldham, R.D., 1906, The constitution of the interior of the Earth, as revealed by earthquakes: Q. J. Geol. Soc. London, v. 62, no. 1\u20134, p. 456\u2013475.<\/li>\n \t<li class=\"csl-entry\">Pasyanos, M.E., 2010, Lithospheric thickness modeled from long-[pb_glossary id=\"2192\"]period[\/pb_glossary] [pb_glossary id=\"3089\"]surface wave[\/pb_glossary] dispersion: Tectonophysics, v. 481, no. 1\u20134, p. 38\u201350.<\/li>\n \t<li class=\"csl-entry\">Powell, R.E., and Weldon, R.J., 1992, Evolution of the San Andreas [pb_glossary id=\"3065\"]fault[\/pb_glossary]: Annu. Rev. Earth Planet. Sci., v. 20, p. 431.<\/li>\n \t<li class=\"csl-entry\">Raff, A.D., and Mason, R.G., 1961, Magnetic Survey Off the West [pb_glossary id=\"2890\"]Coast[\/pb_glossary] of North America, 40 N. [pb_glossary id=\"3372\"]Latitude[\/pb_glossary] to 52 N. [pb_glossary id=\"3372\"]Latitude[\/pb_glossary]: Geological Society of America Bulletin, v. 72, no. 8, p. 1267\u20131270., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2\">10.1130\/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Runcorn, S.K., 1965, Palaeomagnetic comparisons between Europe and North America: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 1\u201311.<\/li>\n \t<li class=\"csl-entry\">Saito, T., Ewing, M., and Burckle, L.H., 1966, Tertiary [pb_glossary id=\"2678\"]sediment[\/pb_glossary] from the mid-atlantic ridge: Science, v. 151, no. 3714, p. 1075\u20131079., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.151.3714.1075\">10.1126\/science.151.3714.1075<\/a>.<\/li>\n \t<li class=\"csl-entry\">Satake, K., and Atwater, B.F., 2007, Long-term perspectives on giant earthquakes and [pb_glossary id=\"3194\"]tsunamis[\/pb_glossary] at [pb_glossary id=\"2602\"]subduction[\/pb_glossary] zones*: Annu. Rev. Earth Planet. Sci., v. 35, p. 349\u2013374.<\/li>\n \t<li class=\"csl-entry\">Scheidegger, A.E., 1953, Examination of the physics of theories of orogenesis: Geol. Soc. Am. Bull., v. 64, no. 2, p. 127\u2013150., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1953)64[127:EOTPOT]2.0.CO;2\">10.1130\/0016-7606(1953)64[127:EOTPOT]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Simpson, G.G., 1943, Mammals and the nature of continents: Am. J. Sci., v. 241, no. 1, p. 1\u201331.<\/li>\n \t<li class=\"csl-entry\">Starr, A.M., 2015, Ambient [pb_glossary id=\"3103\"]resonance[\/pb_glossary] of rock arches: Salt Lake City, Utah, University of Utah, 134 p.<\/li>\n \t<li class=\"csl-entry\">Stern, R.J., 1998, A [pb_glossary id=\"2602\"]subduction[\/pb_glossary] primer for instructors of introductory geology courses and authors of introductory-geology textbooks: J. Geosci. Educ., v. 46, p. 221.<\/li>\n \t<li class=\"csl-entry\">Stern, R.J., 2004, [pb_glossary id=\"2602\"]Subduction[\/pb_glossary] initiation: spontaneous and induced: Earth Planet. Sci. Lett., v. 226, no. 3\u20134, p. 275\u2013292.<\/li>\n \t<li class=\"csl-entry\">Stich, D., Mancilla, F. de L., Pondrelli, S., and Morales, J., 2007, Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake: Geophys. Res. Lett., v. 34, no. 12, p. L12308.<\/li>\n \t<li class=\"csl-entry\">Tatsumi, Y., 2005, The [pb_glossary id=\"2602\"]subduction[\/pb_glossary]\u00a0factory: how it operates in the evolving Earth: GSA Today, v. 15, no. 7, p. 4.<\/li>\n \t<li class=\"csl-entry\">Todo, Y., Kitazato, H., Hashimoto, J., and Gooday, A.J., 2005, Simple foraminifera flourish at the ocean\u2019s deepest point: Science, v. 307, no. 5710, p. 689., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.1105407\">10.1126\/science.1105407<\/a>.<\/li>\n \t<li class=\"csl-entry\">Tolstoy, I., and Ewing, M., 1949, North Atlantic hydrography and the Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 60, no. 10, p. 1527\u20131540., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1949)60[1527:NAHATM]2.0.CO;2\">10.1130\/0016-7606(1949)60[1527:NAHATM]2.0.CO;2<\/a>.<\/li>\n \t<li class=\"csl-entry\">Vine, F.J., and Matthews, D.H., 1963, Magnetic [pb_glossary id=\"1719\"]anomalies[\/pb_glossary] over [pb_glossary id=\"2581\"]oceanic[\/pb_glossary] ridges: Nature, v. 199, no. 4897, p. 947\u2013949.<\/li>\n \t<li class=\"csl-entry\">W\u00e4chtersh\u00e4user, G., 1990, Evolution of the first metabolic cycles: Proc. Natl. Acad. Sci. U. S. A., v. 87, no. 1, p. 200\u2013204.<\/li>\n \t<li class=\"csl-entry\">Wadati, K., 1935, On the activity of deep-[pb_glossary id=\"3080\"]focus[\/pb_glossary] earthquakes in the Japan Islands and neighbourhoods: Geophys. Mag., v. 8, no. 3\u20134, p. 305\u2013325.<\/li>\n \t<li class=\"csl-entry\">Waszek, L., Irving, J., and Deuss, A., 2011, Reconciling the hemispherical structure of Earth\/\u2019s [pb_glossary id=\"2596\"]inner core[\/pb_glossary] with its super-rotation: Nat. Geosci., v. 4, no. 4, p. 264\u2013267., doi: <a href=\"https:\/\/doi.org\/10.1038\/ngeo1083\">10.1038\/ngeo1083<\/a>.<\/li>\n \t<li class=\"csl-entry\">Wegener, A., 1912, Die Entstehung der Kontinente: Geol. Rundsch., v. 3, no. 4, p. 276\u2013292., doi: <a href=\"https:\/\/doi.org\/10.1007\/BF02202896\">10.1007\/BF02202896<\/a>.<\/li>\n \t<li class=\"csl-entry\">Wegener, A., 1920, Die entstehung der kontinente und ozeane: \u0420\u0438\u043f\u043e\u043b \u041a\u043b\u0430\u0441\u0441\u0438\u043a.<\/li>\n \t<li class=\"csl-entry\">Wells, H.G., Huxley, J., and Wells, G.P., 1931, The Science of Life: Philosophy, v. 6, no. 24, p. 506\u2013507.<\/li>\n \t<li class=\"csl-entry\">White, I.C., and Moreira, C., 1908, Commiss\u00e3o de estudos das minas de Carv\u00e3o de Pedra do Brazil:<\/li>\n \t<li class=\"csl-entry\">de Wijs, G.A., Kresse, G., Vo\u010dadlo, L., Dobson, D., Alf\u00e8, D., Gillan, M.J., and Price, G.D., 1998, The [pb_glossary id=\"3368\"]viscosity[\/pb_glossary] of liquid iron at the physical conditions of the Earth\u2019s [pb_glossary id=\"2589\"]core[\/pb_glossary]: Nature, v. 392, no. 6678, p. 805\u2013807., doi: <a href=\"https:\/\/doi.org\/10.1038\/33905\">10.1038\/33905<\/a>.<\/li>\n \t<li class=\"csl-entry\">Wilson, J.T., 1966, Did the Atlantic close and then re-open? Nature.<\/li>\n \t<li class=\"csl-entry\">Wilson, M., 1993, [pb_glossary id=\"2591\"]Plate[\/pb_glossary]-moving mechanisms: constraints and controversies: Journal of the Geological Society, v. 150, no. 5, p. 923\u2013926., doi: <a href=\"https:\/\/doi.org\/10.1144\/gsjgs.150.5.0923\">10.1144\/gsjgs.150.5.0923<\/a>.<\/li>\n \t<li class=\"csl-entry\">Wyllie, P.J., 1970, [pb_glossary id=\"1957\"]Ultramafic[\/pb_glossary] rocks and the upper [pb_glossary id=\"2586\"]mantle[\/pb_glossary], <i>in<\/i> Morgan, B.A., editor, Fiftieth anniversary symposia: Mineralogy and [pb_glossary id=\"1173\"]petrology[\/pb_glossary] of the Upper [pb_glossary id=\"2586\"]Mantle[\/pb_glossary]; [pb_glossary id=\"1921\"]Sulfides[\/pb_glossary]; Mineralogy and geochemistry of non-[pb_glossary id=\"2883\"]marine[\/pb_glossary] [pb_glossary id=\"2842\"]evaporites[\/pb_glossary]: Washington, DC, Mineralogical Society of America, p. 3\u201332.<\/li>\n \t<li class=\"csl-entry\">Zhou, Z., 2004, The origin and early evolution of birds: discoveries, disputes, and perspectives from [pb_glossary id=\"2176\"]fossil[\/pb_glossary] evidence: Naturwissenschaften, v. 91, no. 10, p. 455\u2013471.<\/li>\n<\/ol>\n<\/div>","rendered":"<figure id=\"attachment_2498\" aria-describedby=\"caption-attachment-2498\" style=\"width: 2048px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/MT3.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-67 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3.jpg\" alt=\"The rock is getting thinner farther away.\" width=\"2048\" height=\"1536\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3.jpg 2048w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2021\/09\/MT3-350x263.jpg 350w\" sizes=\"auto, (max-width: 2048px) 100vw, 2048px\" \/><\/a><figcaption id=\"caption-attachment-2498\" class=\"wp-caption-text\">A layer of shallow ocean limestone (white) has been brought to the top of a mountain by the convergent forces of the Sevier Orogeny. Near Sun River Canyon, Montana.<\/figcaption><\/figure>\n<h1>2 Plate Tectonics<\/h1>\n<p><b>KEY CONCEPTS<\/b><\/p>\n<p><b>At the end of this chapter, students should be able to:<\/b><\/p>\n<ul>\n<li>Describe how the ideas behind <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> started with Alfred Wegener\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a>\u00a0drift<\/li>\n<li>Describe the physical and chemical layers of the Earth and how they affect <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement<\/li>\n<li>Explain how movement at the three types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> boundaries causes earthquakes, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a>, and mountain building<\/li>\n<li>Identify <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2600\">convergent<\/a> boundaries, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> and collisions, as places where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>\u00a0come together<\/li>\n<li>Identify <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries, including <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifts<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>, as places where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> separate<\/li>\n<li>Explain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries as places where adjacent <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1447\">shear<\/a> past each other<\/li>\n<li>Describe the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2637\">Wilson Cycle<\/a>, beginning with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a>, ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> creation, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>, and ending with ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> closure<\/li>\n<li>Explain how the tracks of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>, places that have continually rising <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a>, is used to calculate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> motion<\/li>\n<\/ul>\n<figure id=\"attachment_2499\" aria-describedby=\"caption-attachment-2499\" style=\"width: 4898px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Tectonic_plates_boundaries_detailed-en.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-68\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_.png\" alt=\"The map shows many plates.\" width=\"4898\" height=\"2461\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_.png 4898w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-300x151.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-1024x515.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-768x386.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-1536x772.png 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-2048x1029.png 2048w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-65x33.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-225x113.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plates_boundaries_detailed-en.svg_-350x176.png 350w\" sizes=\"auto, (max-width: 4898px) 100vw, 4898px\" \/><\/a><figcaption id=\"caption-attachment-2499\" class=\"wp-caption-text\">Detailed map of all known plates, their boundaries, and movements.<\/figcaption><\/figure>\n<p>Revolution is a word usually reserved for significant political or social changes. Several of these idea revolutions forced scientists to re-examine their entire field, triggering a paradigm shift that shook up their conventionally held knowledge. Charles Darwin\u2019s book on evolution, <em>On the Origin of Species<\/em>, published in 1859; Gregor Mendel\u2019s discovery of the genetic principles of inheritance in 1866; and James Watson, Francis Crick, and Rosalind Franklin\u2019s model for the structure of DNA in 1953 did that for biology. Albert Einstein\u2019s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics.<\/p>\n<p>The concept of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> was just as revolutionary for geology. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> attributes the movement of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1933\">massive<\/a> sections of the Earth\u2019s outer layers with creating earthquakes, mountains, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a>. Many earth processes make more sense when viewed through the lens of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a>. Because it is so important in understanding how the world works, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> is the first topic of discussion in this textbook.<\/p>\n<h2><span style=\"font-weight: 400;\">2.1 Alfred Wegener\u2019s Continental Drift Hypothesis<\/span><\/h2>\n<figure id=\"attachment_2500\" aria-describedby=\"caption-attachment-2500\" style=\"width: 195px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Alfred_Wegener_ca.1924-30-2.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-28\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Alfred_Wegener_ca.1924-30-2.jpg\" alt=\"He is a male in a suit.\" width=\"195\" height=\"240\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alfred_Wegener_ca.1924-30-2.jpg 195w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Alfred_Wegener_ca.1924-30-2-65x80.jpg 65w\" sizes=\"auto, (max-width: 195px) 100vw, 195px\" \/><\/a><figcaption id=\"caption-attachment-2500\" class=\"wp-caption-text\">Wegener later in his life, ca. 1924-1930.<\/figcaption><\/figure>\n<p>Alfred Wegener (1880-1930) was a German scientist who specialized in meteorology and climatology. His knack for questioning accepted ideas started in 1910 when he disagreed with the explanation that the Bering Land Bridge was formed by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1706\">isostasy<\/a>, and that similar land bridges once connected the continents. After reviewing the scientific literature, he published a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> stating the continents were originally connected, and then drifted apart. While he did not have the precise mechanism worked out, his <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> was backed up by a long list of evidence.<\/p>\n<h3><\/h3>\n<h3><b>2.1.1 Early Evidence for Continental Drift Hypothesis<\/b><\/h3>\n<figure id=\"attachment_2501\" aria-describedby=\"caption-attachment-2501\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-69\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-300x177.jpg\" alt=\"It shows South America and Africa connected, then apart.\" width=\"300\" height=\"177\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-300x177.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-65x38.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-225x133.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic-350x206.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Antonio_Snider-Pellegrini_Opening_of_the_Atlantic.jpg 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2501\" class=\"wp-caption-text\">Snider-Pellegrini&#8217;s map showing the continental fit and separation, 1858.<\/figcaption><\/figure>\n<p>Wegener\u2019s first piece of evidence was that the coastlines of some continents fit together like pieces of a jigsaw puzzle. People noticed the similarities in the coastlines of South America and Africa on the first world maps, and some suggested the continents had been ripped apart. Antonio Snider-Pellegrini did preliminary work on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> separation and matching <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossils<\/a> in 1858.<\/p>\n<figure id=\"attachment_2502\" aria-describedby=\"caption-attachment-2502\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/ElevationWorld.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-70\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-300x150.jpg\" alt=\"The shape of the continents is different than what is seen by just coastlines.\" width=\"300\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-300x150.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-1024x512.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-768x384.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-1536x768.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-65x33.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-225x113.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld-350x175.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ElevationWorld.jpg 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2502\" class=\"wp-caption-text\">Map of world elevations. Note the light blue, which are continental shelves flooded by shallow ocean water. These show the true shapes of the continents.<\/figcaption><\/figure>\n<p>What Wegener did differently was synthesize a large amount of data in one place. He used true edges of the continents, based on the shapes of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2891\">continental<\/a> shelves. This resulted in a better fit than previous efforts that traced the existing coastlines.<\/p>\n<figure id=\"attachment_3259\" aria-describedby=\"caption-attachment-3259\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Wegener_fossil_map.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-71\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-300x231.png\" alt=\"There are four different fossil organisms that connect South America, Africa, India, Antartica, and Australia.\" width=\"300\" height=\"231\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-300x231.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-1024x787.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-768x591.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-65x50.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-225x173.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_-350x269.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wegener_fossil_map.svg_.png 1160w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-3259\" class=\"wp-caption-text\">Image showing fossils that connect the continents of Gondwana (the southern continents of Pangea).<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Wegener also compiled evidence by comparing similar rocks, mountains, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossils<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2910\">glacial<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\">formations<\/a> across oceans. For example, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossils<\/a> of the primitive aquatic reptile <em>Mesosaurus<\/em> were found on the separate coastlines of Africa and South America. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">Fossils<\/a> of another reptile, <em>Lystrosaurus,<\/em> were found on Africa, India, and Antarctica. He pointed out these were land-dwelling creatures could not have swum across an entire ocean.<\/p>\n<p>Opponents of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift insisted trans-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic <\/a> land bridges allowed animals and plants to move between continents. The land bridges eventually eroded away, leaving the continents permanently separated. The problem with this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> is the improbability of a land bridge being tall and long enough to stretch across a broad, deep ocean.<\/p>\n<p>More support for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift came from the puzzling evidence that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2464\">glaciers<\/a> once existed in normally very warm areas in southern Africa, India, Australia, and Arabia. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1710\">climate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1719\">anomalies<\/a> could not be explained by land bridges. Wegener found similar evidence when he discovered tropical plant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossils<\/a> in the frozen region of the Arctic Circle. As Wegener collected more data, he realized the explanation that best fit all the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1710\">climate<\/a>, rock, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossil<\/a> observations involved moving continents.<\/p>\n<h3><b>2.1.2 Proposed Mechanism for Continental Drift<\/b><\/h3>\n<figure id=\"attachment_2504\" aria-describedby=\"caption-attachment-2504\" style=\"width: 400px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Convection.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-72 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Convection.gif\" alt=\"The rising material is drawn red. The cool material is blue.\" width=\"400\" height=\"300\" \/><\/a><figcaption id=\"caption-attachment-2504\" class=\"wp-caption-text\">Animation of the basic idea of convection: an uneven heat source in a fluid causes rising material next to the heat and sinking material far from the heat.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Wegener\u2019s work was considered a fringe science <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a> for his entire life. One of the biggest flaws in his <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> was an inability to provide a mechanism for how the continents moved. Obviously, the continents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that supported a credible mechanism. Other pro-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift followers used expansion, contraction, or even the moon\u2019s origin to explain how the continents moved. Wegener used centrifugal forces and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1702\">precession<\/a>, but this model was proven wrong. He also speculated about seafloor spreading, with hints of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a>, but could not substantiate these proposals. As it turns out, current scientific knowledge reveals <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> is one the major forces in driving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movements, along with gravity and density.<\/p>\n<h3><b>2.1.3 Development of Plate Tectonic Theory<\/b><\/h3>\n<figure id=\"attachment_2505\" aria-describedby=\"caption-attachment-2505\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Global_plate_motion_2008-04-17.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-73\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-300x212.jpg\" alt=\"The map shows many data points all over the world.\" width=\"300\" height=\"212\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-300x212.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-65x46.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-225x159.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17-350x248.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Global_plate_motion_2008-04-17.jpg 700w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2505\" class=\"wp-caption-text\">GPS measurements of plate motions.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Wegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science. However, in the 1950s, evidence started to trickle in that made <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism\u2014namely, seafloor spreading\u2014for Wegener\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift to be accepted as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a>. Ongoing GPS and earthquake data analyses continue to support this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a>. The next section provides the pieces of evidence that helped <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> one man\u2019s wild notion into a scientific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a>.<\/p>\n<h4><span style=\"font-weight: 400;\">Mapping of the Ocean Floors<\/span><\/h4>\n<figure id=\"attachment_2506\" aria-describedby=\"caption-attachment-2506\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Deep_sea_vent_chemistry_diagram.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-74\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-300x174.jpg\" alt=\"The diagram shows water going into the ground and coming out, with many different reactions.\" width=\"300\" height=\"174\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-300x174.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-1024x595.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-768x446.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-1536x893.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-65x38.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-225x131.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram-350x203.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Deep_sea_vent_chemistry_diagram.jpg 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2506\" class=\"wp-caption-text\">The complex chemistry around mid-ocean ridges.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>In 1947 researchers started using an adaptation of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3364\">SONAR<\/a> to map a region in the middle of the Atlantic Ocean with poorly-understood topographic and thermal properties. Using this information, Bruce Heezen and Marie Tharp created the first detailed map of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a> to reveal the Mid-Atlantic Ridge, a basaltic mountain range that spanned the length of the Atlantic Ocean, with rock chemistry and dimensions unlike the mountains found on the continents. Initially scientists thought the ridge was part of a mechanism that explained the expanding Earth or ocean-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> growth <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypotheses<\/a>. In 1959, Harry Hess proposed the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> of seafloor spreading \u2013 that the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> represented <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> factories, where new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> was issuing from these long <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> ridges. Scientists later included <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> faults perpendicular to the ridges to better account for varying rates of movement between the newly formed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. When earthquake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3085\">epicenters<\/a> were discovered along the ridges, the idea that earthquakes were linked to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement took hold.<\/p>\n<div style=\"height: 0; padding-bottom: 56.25%;\">\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"Marie Tharp: Uncovering the Secrets of the Ocean Floor - with Helen Czerski\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/TgfYjS0OTWw?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<\/div>\n<figure id=\"attachment_3739\" aria-describedby=\"caption-attachment-3739\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Marie-Tharp-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-75\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Marie-Tharp-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3739\" 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>Seafloor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a>, measured by dredging and drilling, provided another clue. Scientists once believed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a> accumulated on the ocean floors over a very long time in a static environment. When some studies showed less <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a> than expected, these results were initially used to argue against <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> movement. With more time, researchers discovered these thinner <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a> layers were located close to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>, indicating the ridges were younger than the surrounding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>. This finding supported the idea that the sea floor was not fixed in one place.<\/p>\n<h4><span style=\"font-weight: 400;\">Paleomagnetism<\/span><\/h4>\n<figure id=\"attachment_2507\" aria-describedby=\"caption-attachment-2507\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earths_magnetic_field_schematic.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-76\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-300x250.png\" alt=\"The north end of the magnet is south topographically, and vice versa.\" width=\"300\" height=\"250\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-300x250.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-1024x853.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-768x640.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-65x54.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-225x188.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_-350x292.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_magnetic_field_schematic.svg_.png 1200w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2507\" class=\"wp-caption-text\">The magnetic field of Earth, simplified as a bar magnet.<\/figcaption><\/figure>\n<p>The seafloor was also mapped magnetically. Scientists had long known of strange magnetic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1719\">anomalies<\/a> that formed a striped pattern of symmetrical rows on both sides of mid-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> ridges. What made these features unusual was the north and south magnetic poles within each stripe was reversed in alternating rows. By 1963, Harry Hess and other scientists used these magnetic reversal patterns to support their model for seafloor spreading (see also Lawrence W. Morley).<\/p>\n<figure id=\"attachment_2508\" aria-describedby=\"caption-attachment-2508\" style=\"width: 351px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earth_Magnetic_Field_Declination_from_1590_to_1990.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-77 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth_Magnetic_Field_Declination_from_1590_to_1990.gif\" alt=\"The poles shift slightly every year.\" width=\"351\" height=\"293\" \/><\/a><figcaption id=\"caption-attachment-2508\" class=\"wp-caption-text\">This animation shows how the magnetic poles have moved over 400 years.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2579\">Paleomagnetism<\/a> is the study of magnetic fields frozen within rocks, basically a fossilized compass. In fact, the first hard evidence to support <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> motion came from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2579\">paleomagnetism<\/a>.<\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2675\">Igneous<\/a> rocks containing magnetic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> like magnetite typically provide the most useful data. In their liquid state as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2673\">lava<\/a>, the magnetic poles of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> align themselves with the Earth\u2019s magnetic field. When the rock cools and solidifies, this alignment is frozen into place, creating a permanent paleomagnetic record that includes magnetic inclination related to global <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3372\">latitude<\/a>, and declination related to magnetic north.<\/p>\n<figure id=\"attachment_2509\" aria-describedby=\"caption-attachment-2509\" style=\"width: 240px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.1-magnetic_stripes.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-78\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.1-magnetic_stripes.gif\" alt=\"Animated gif depicting a mid-ocean ridge with two oceanic plates moving away from the center of the ridge. As the movement progresses, symettrical magnetic stripes appear on each side of the ridge.\" width=\"240\" height=\"180\" \/><\/a><figcaption id=\"caption-attachment-2509\" class=\"wp-caption-text\">The iron in the solidifying rock preserves the current magnetic polarity as new oceanic plates form at mid ocean ridges<\/figcaption><\/figure>\n<p>Scientists had noticed for some time the alignment of magnetic north in many rocks was nowhere close to the earth\u2019s current magnetic north. Some explained this away are part of the normal movement of earth\u2019s magnetic north pole. Eventually, scientists realized adding the idea of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> movement explained the data better than pole movement alone.<\/p>\n<h4><\/h4>\n<h4><\/h4>\n<h4><span style=\"font-weight: 400;\">Wadati-Benioff Zones<\/span><\/h4>\n<figure id=\"attachment_2510\" aria-describedby=\"caption-attachment-2510\" style=\"width: 297px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/benioff_zone.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-79\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/benioff_zone.gif\" alt=\"The earthquakes descend at an angle into the Earth.\" width=\"297\" height=\"243\" \/><\/a><figcaption id=\"caption-attachment-2510\" class=\"wp-caption-text\">The Wadati-Benioff zone, showing earthquakes following the subducting slab down.<\/figcaption><\/figure>\n<p>Around the same time <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> were being investigated, other scientists linked the creation of ocean trenches and island arcs to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> activity and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement. Several independent research groups recognized earthquake <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3085\">epicenters<\/a> traced the shapes of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> sinking into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>. These deep earthquake zones congregated in planes that started near the surface around ocean trenches and angled beneath the continents and island arcs. Today these earthquake zones called Wadati-Benioff zones.<\/p>\n<figure id=\"attachment_2511\" aria-describedby=\"caption-attachment-2511\" style=\"width: 244px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/John_Tuzo_Wilson_in_1992-2.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-39\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-244x300.jpg\" alt=\"He is an older man in this 1992 image.\" width=\"244\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-244x300.jpg 244w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-65x80.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-225x277.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2-350x431.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/John_Tuzo_Wilson_in_1992-2.jpg 440w\" sizes=\"auto, (max-width: 244px) 100vw, 244px\" \/><\/a><figcaption id=\"caption-attachment-2511\" class=\"wp-caption-text\">J. Tuzo Wilson<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Based on the mounting evidence, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> continued to take shape. J. Tuzo Wilson was the first scientist to put the entire picture together by proposing that the opening and closing of the ocean basins. Before long, scientists proposed other models showing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> moving with respect to each other, with clear boundaries between them. Others started piecing together complicated histories of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> revolution had taken hold.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<div id=\"h5p-8\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-8\" class=\"h5p-iframe\" data-content-id=\"8\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Continental Drift vs. Plate Tectonics\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3738\" aria-describedby=\"caption-attachment-3738\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Continental-Drive-Activity-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-80\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Continental-Drive-Activity-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3738\" 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-9\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-9\" class=\"h5p-iframe\" data-content-id=\"9\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.1 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3737\" aria-describedby=\"caption-attachment-3737\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.1-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-81\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-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\/2.1-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.1-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3737\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 2.1 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-weight: 400;\">2.2 Layers of the Earth<\/span><\/h2>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2512\" aria-describedby=\"caption-attachment-2512\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earth-cutaway-schematic-english.svg_-1.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-51\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-300x211.png\" alt=\"The crust and lithosphere are on the outside of the Earth and are thin. Below the crust is the mantle and core. Below the lithosphere is the asthenosphere.\" width=\"300\" height=\"211\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-300x211.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-65x46.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-225x158.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1-350x246.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earth-cutaway-schematic-english.svg_-1.png 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2512\" class=\"wp-caption-text\">The layers of the Earth. Physical layers include lithosphere and asthenosphere; chemical layers are crust, mantle, and core.<\/figcaption><\/figure>\n<p>In order to understand the details of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a>, it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic wave<\/a> data and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2202\">meteorite<\/a> materials. In general, the Earth can be divided into layers based on chemical <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composition<\/a> and physical characteristics.<\/p>\n<h3><b>2.2.1 Chemical Layers<\/b><\/h3>\n<p>Certainly the earth is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composed<\/a> of a countless combination of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2700\">elements<\/a>. Regardless of what <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2700\">elements<\/a> are involved two major factors\u2014<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a> and pressure\u2014are responsible for creating three distinct chemical layers.<\/p>\n<h4><span style=\"font-weight: 400;\">Crust<\/span><\/h4>\n<p>The outermost chemical layer and the one we currently reside on, is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a>. There are two types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">Continental crust<\/a> has a relatively low density and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composition<\/a> similar to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1962\">granite<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">Oceanic crust<\/a> has a relatively high density, especially when cold and old, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composition<\/a> similar to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1961\">basalt<\/a>. The surface levels of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> are relatively <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2583\">brittle<\/a>. The deeper parts of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> are subjected to higher temperatures and pressure, which makes them more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2582\">ductile<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2582\">Ductile<\/a> materials are like soft plastics or putty, they move under force. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2583\">Brittle<\/a> materials are like solid glass or pottery, they break under force, especially when it is applied quickly. Earthquakes, generally occur in the upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> and are caused by the rapid movement of relatively <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2583\">brittle<\/a> materials.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2513\" aria-describedby=\"caption-attachment-2513\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/MohoDepth-1.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-50\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-300x167.png\" alt=\"Places with mountain building have a deeper moho.\" width=\"300\" height=\"167\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-300x167.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-1024x568.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-768x426.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-1536x852.png 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-65x36.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-225x125.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1-350x194.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/MohoDepth-1.png 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2513\" class=\"wp-caption-text\">The global map of the depth of the moho.<\/figcaption><\/figure>\n<p>The base of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> is characterized by a large increase in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> velocity, which measures how fast earthquake waves travel through solid matter. Called the Mohorovi\u010di\u0107 Discontinuity, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2584\">Moho<\/a> for short, this zone was discovered by Andrija Mohorovi\u010di\u0107 (pronounced mo-ho-ro-vee-cheech; <a href=\"https:\/\/www.merriam-webster.com\/dictionary\/Mohorovicic%20discontinuity\">audio pronunciation<\/a>) in 1909 after studying earthquake wave paths in his <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1924\">native<\/a> Croatia. The change in wave direction and speed is caused by dramatic chemical differences of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>. Underneath the oceans, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2584\">Moho<\/a> is found roughly 5 km below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>. Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events known as orogenies, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2584\">Moho<\/a> depth is doubled.<\/p>\n<h4><span style=\"font-weight: 400;\">Mantle<\/span><\/h4>\n<figure id=\"attachment_2514\" aria-describedby=\"caption-attachment-2514\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Iddingsite.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-82\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-300x225.jpg\" alt=\"The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Iddingsite.jpg 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2514\" class=\"wp-caption-text\">This mantle xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.<\/figcaption><\/figure>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> sits below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> and above the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>. It is the largest chemical layer by volume, extending from the base of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> to a depth of about 2900 km. Most of what we know about the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> comes from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic wave<\/a> analysis, though information is gathered by studying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2622\">ophiolites<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2587\">xenoliths<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2622\">Ophiolites<\/a> are pieces of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> that have risen through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> until they are exposed as part of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2587\">Xenoliths<\/a> are carried within <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> and brought to the Earth\u2019s surface by volcanic eruptions. Most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2587\">xenoliths<\/a> are made of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2588\">peridotite<\/a>, an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1957\">ultramafic<\/a> class of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2675\">igneous rock<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes#4-2BowensReaction\" target=\"_blank\" rel=\"noopener\">chapter 4.2<\/a> for explanation). Because of this, scientists hypothesize most of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> is made of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2588\">peridotite<\/a>.<\/p>\n<h4><span style=\"font-weight: 400;\">Core<\/span><\/h4>\n<figure id=\"attachment_2515\" aria-describedby=\"caption-attachment-2515\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.2_TolucaMeteorite.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-83\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-300x225.jpg\" alt=\"The meteorite is polished showing the Widmanst\u00e4tten Pattern.\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_TolucaMeteorite.jpg 1024w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2515\" class=\"wp-caption-text\">A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanst\u00e4tten Pattern.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> of the Earth, which has both liquid and solid layers, and consists mostly of iron, nickel, and possibly some oxygen. Scientists looking at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> data first discovered this innermost chemical layer in 1906. Through a union of hypothetical modeling, astronomical insight, and hard <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> data, they concluded the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> is mostly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3345\">metallic<\/a> iron. Scientists studying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2202\">meteorites<\/a>, which typically contain more iron than surface rocks, have proposed the earth was formed from meteoric material. They believe the liquid component of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> was created as the iron and nickel sank into the center of the planet, where it was liquefied by intense pressure.<\/p>\n<h3><b>2.2.2 Physical Layers<\/b><\/h3>\n<p><span style=\"font-weight: 400;\">The Earth can also be broken down into five distinct physical layers based on how each layer responds to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2915\">stress<\/a>. While there is some overlap in the chemical and physical designations of layers, specifically the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> boundary, there are significant differences between the two systems.<\/span><\/p>\n<h4><span style=\"font-weight: 400;\">Lithosphere<\/span><\/h4>\n<figure id=\"attachment_2516\" aria-describedby=\"caption-attachment-2516\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Plates_tect2_en.svg_-2.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-49\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-300x205.png\" alt=\"There are about 10 major plates\" width=\"300\" height=\"205\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-300x205.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-1024x699.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-768x524.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-1536x1049.png 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-65x44.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-225x154.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2-350x239.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plates_tect2_en.svg_-2.png 2048w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2516\" class=\"wp-caption-text\">Map of the major plates and their motions along boundaries.<\/figcaption><\/figure>\n<p><em>Lithos<\/em> is Greek for stone, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is the outermost physical layer of the Earth. It is grouped into two types: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">Oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is thin and relatively rigid. It ranges in thickness from nearly zero in new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> found around <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>, to an average of 140 km in most other locations. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">Continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is generally thicker and considerably more plastic, especially at the deeper levels. Its thickness ranges from 40 to 280 km. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is not continuous. It is broken into segments called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2592\">plate boundary<\/a> is where two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> meet and move relative to each other. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">Plate<\/a> boundaries are where we see <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> in action\u2014mountain building, triggering earthquakes, and generating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> activity.<\/p>\n<h4><span style=\"font-weight: 400;\">Asthenosphere<\/span><\/h4>\n<figure id=\"attachment_2517\" aria-describedby=\"caption-attachment-2517\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Earths_Inner_Layers_denoting_the_LAB.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-84\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-300x207.png\" alt=\"It is thin at a mid-ocean ridge, thick under collisions\" width=\"300\" height=\"207\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-300x207.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-65x45.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-225x155.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB-350x242.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Earths_Inner_Layers_denoting_the_LAB.png 766w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2517\" class=\"wp-caption-text\">The lithosphere-asthenosphere boundary changes with certain tectonic situations.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> is the layer below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>. <em>Astheno-<\/em> means lacking strength, and the most distinctive property of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> is movement. Because it is mechanically weak, this layer moves and flows due to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> currents created by heat coming from the earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> cause. Unlike the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> that consists of multiple <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> is relatively unbroken. Scientists have determined this by analyzing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> waves that pass through the layer. The depth of at which the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> is found is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a>-dependent. It tends to lie closer to the earth\u2019s surface around <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> and much deeper underneath mountains and the centers of lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>.<\/p>\n<h4><span style=\"font-weight: 400;\">Mesosphere<\/span><\/h4>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2518\" aria-describedby=\"caption-attachment-2518\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Perovskite.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-85\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-300x288.jpg\" alt=\"The atoms are arranged.\" width=\"300\" height=\"288\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-300x288.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-65x63.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-225x216.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite-350x337.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Perovskite.jpg 755w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2518\" class=\"wp-caption-text\">General perovskite structure. Perovskite silicates (i.e.<br \/>Bridgmenite,<br \/>(Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth.<\/figcaption><\/figure>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2594\">mesosphere<\/a>, sometimes known as the lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>, is more rigid and immobile than the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>. Located at a depth of approximately 410 and 660 km below the earth\u2019s surface, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2594\">mesosphere<\/a> is subjected to very high pressures and temperatures. These extreme conditions create a transition zone in the upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2594\">mesosphere<\/a> where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> continuously change into various forms, or pseudomorphs. Scientists identify this zone by changes in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> velocity and sometimes physical barriers to movement. Below this transitional zone, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2594\">mesosphere<\/a> is relatively uniform until it reaches the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>.<\/p>\n<h4><span style=\"font-weight: 400;\">Inner and Outer Core<\/span><\/h4>\n<figure id=\"attachment_2519\" aria-describedby=\"caption-attachment-2519\" style=\"width: 206px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Inge_Lehmann_1932.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-86\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-206x300.jpg\" alt=\"Is shows her as a young woman\" width=\"206\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-206x300.jpg 206w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-65x95.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-225x328.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932-350x511.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Inge_Lehmann_1932.jpg 460w\" sizes=\"auto, (max-width: 206px) 100vw, 206px\" \/><\/a><figcaption id=\"caption-attachment-2519\" class=\"wp-caption-text\">Lehmann in 1932<\/figcaption><\/figure>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> is the only entirely liquid layer within the Earth. It starts at a depth of 2,890 km and extends to 5,150 km, making it about 2,300 km thick. In 1936, the Danish geophysicist Inge Lehmann analyzed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> data and was the first to prove a solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2596\">inner core<\/a> existed within a liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> . The solid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2596\">inner core<\/a> is about 1,220 km thick, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> is about 2,300 km thick.<\/p>\n<p>It seems like a contradiction that the hottest part of the Earth is solid, as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> making up the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> should be liquified or vaporized at this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a>. Immense pressure keeps the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2596\">inner core<\/a> in a solid phase. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2596\">inner core<\/a> grows slowly from the lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> solidifying as heat escapes the interior of the Earth and is dispersed to the outer layers.<\/p>\n<figure id=\"attachment_2520\" aria-describedby=\"caption-attachment-2520\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.2_SpinningOuterCore.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-87\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-300x289.gif\" alt=\"The Earth is cut out with the core being shown.\" width=\"300\" height=\"289\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-300x289.gif 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-65x63.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-225x216.gif 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.2_SpinningOuterCore-350x337.gif 350w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2520\" class=\"wp-caption-text\">The outer core&#8217;s spin causes our protective magnetic field.<\/figcaption><\/figure>\n<p>The earth\u2019s liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> is critically important in maintaining a breathable <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2667\">atmosphere<\/a> and other environmental conditions favorable for life. Scientists believe the earth\u2019s magnetic field is generated by the circulation of molten iron and nickel within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a>. If the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2595\">outer core<\/a> were to stop circulating or become solid, the loss of the magnetic field would result in Earth getting stripped of life-supporting gases and water. This is what happened, and continues to happen, on Mars.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<div id=\"h5p-10\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-10\" class=\"h5p-iframe\" data-content-id=\"10\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Layers of the Earth practice\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3736\" aria-describedby=\"caption-attachment-3736\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Layers-of-Earth-Practice-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-88\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Layers-of-Earth-Practice-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3736\" 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><b>2.2.3 Plate Tectonic Boundaries<\/b><\/h3>\n<figure id=\"attachment_2521\" aria-describedby=\"caption-attachment-2521\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Passive_Contiental_Margin.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-89\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-300x143.jpg\" alt=\"The plate thins from continent to ocean\" width=\"300\" height=\"143\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-300x143.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-1024x490.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-768x367.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-65x31.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-225x108.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin-350x167.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Passive_Contiental_Margin.jpg 1146w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2521\" class=\"wp-caption-text\">Passive margin<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>At passive margins the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> don\u2019t move\u2014the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> transitions into oceanic lithosphere and forms <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> made of both types. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> may be made of both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> connected by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2598\">passive margin<\/a>. North and South America\u2019s eastern coastlines are examples of passive margins. Active margins are places where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> meet and move relative to each other, such as the western coasts of North and South America. This movement is caused by frictional drag created between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> and differences in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> densities. The majority of mountain-building events, earthquake activity and active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> on the Earth\u2019s surface can be attributed to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement at active margins.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2522\" aria-describedby=\"caption-attachment-2522\" style=\"width: 775px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Tectonic_plate_boundaries.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-90\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries.png\" alt=\"It shows all the types\" width=\"775\" height=\"429\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries.png 775w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries-300x166.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries-768x425.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries-65x36.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries-225x125.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Tectonic_plate_boundaries-350x194.png 350w\" sizes=\"auto, (max-width: 775px) 100vw, 775px\" \/><\/a><figcaption id=\"caption-attachment-2522\" class=\"wp-caption-text\">Schematic of plate boundary types.<\/figcaption><\/figure>\n<p>In a simplified model, there are three categories of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> boundaries. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2600\">Convergent<\/a> boundaries are places where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> move toward each other. At <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> move apart. At <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> slide past each other.<\/p>\n<p>&nbsp;<\/p>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-11\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-11\" class=\"h5p-iframe\" data-content-id=\"11\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.2 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3735\" aria-describedby=\"caption-attachment-3735\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.2-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-91\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-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\/2.2-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.2-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3735\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 2.2 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-size: 28px;\">2.3 Convergent Boundaries<\/span><\/h2>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2523\" aria-describedby=\"caption-attachment-2523\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/CratonGeolProv.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium 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=\"300\" height=\"159\" 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: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2523\" 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_138_2600\">Convergent<\/a> boundaries, also called destructive boundaries, are places where two or more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> move toward each other. . <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2600\">Convergent<\/a> boundary movement is divided into two types, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>, depending on the density of the involved <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">Continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is of lower density and thus more buoyant than the underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">Oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is more dense than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>, and, when old and cold, may even be more dense than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>.<\/p>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> of different densities converge, the higher density <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> is pushed beneath the more buoyant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> in a process called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> converge without <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> occurring, this process is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>.<\/p>\n<h3><b>2.3.1. Subduction<\/b><\/h3>\n<div style=\"width: 720px;\" class=\"wp-video\"><video class=\"wp-video-shortcode\" id=\"video-138-1\" width=\"720\" height=\"540\" preload=\"metadata\" controls=\"controls\"><source type=\"video\/mp4\" src=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/Subduction.mp4?_=1\" \/><a href=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/Subduction.mp4\">http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/Subduction.mp4<\/a><\/video><\/div>\n<p><em><a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP1GTect\/cSubduction.html\">Video<\/a> showing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\"><em>continental<\/em><\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\"><em>oceanic <\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\"><em>subduction<\/em><\/a>, causing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\"><em>volcanism<\/em><\/a>. By Tanya Atwater and John Iwerks.<\/em><\/p>\n<figure id=\"attachment_3734\" aria-describedby=\"caption-attachment-3734\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Subduction-Animation-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-93\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-Animation-QR-Code.png 1148w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3734\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this video via this QR Code.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">Subduction<\/a> occurs when a dense <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> meets a more buoyant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, like a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> or warmer\/younger <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, and descends into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>. The worldwide average rate of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> is 25 miles per million years, about a half-inch per year. As an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> descends, it pulls the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a> down into a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2603\">trench<\/a>. These trenches can be more than twice as deep as the average depth of the adjacent ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a>, which is usually three to four km. The Mariana <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2603\">Trench<\/a>, for example, approaches a staggering 11 km.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2524\" aria-describedby=\"caption-attachment-2524\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Subduction-en.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-94\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_.png\" alt=\"Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental\" width=\"800\" height=\"391\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Subduction-en.svg_.png 800w, 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\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><\/a><figcaption id=\"caption-attachment-2524\" class=\"wp-caption-text\">Diagram of ocean-continent subduction.<\/figcaption><\/figure>\n<figure id=\"attachment_2525\" aria-describedby=\"caption-attachment-2525\" style=\"width: 212px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/drawing.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-95\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-212x300.png\" alt=\"This drawing depicts a microcontinent riding with a subducting plate, and not being subductable, becoming accreted to the melange.\" width=\"212\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-212x300.png 212w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-724x1024.png 724w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-65x92.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-225x318.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing-350x495.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/drawing.png 744w\" sizes=\"auto, (max-width: 212px) 100vw, 212px\" \/><\/a><figcaption id=\"caption-attachment-2525\" class=\"wp-caption-text\">Microcontinents can become part of the accretionary prism of a subduction zone.<\/figcaption><\/figure>\n<p>Within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2603\">trench<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediments<\/a> are scraped together and compressed between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> and overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. This feature is called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2604\">accretionary wedge<\/a>, m\u00e9lange, or accretionary prism. Fragments of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> material, including microcontinents, riding atop the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> may become sutured to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2604\">accretionary wedge <\/a> and accumulate into a large area of land called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2686\">terrane<\/a>. Vast portions of California are comprised of accreted terranes.<\/p>\n<figure id=\"attachment_2526\" aria-describedby=\"caption-attachment-2526\" style=\"width: 179px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/usgs_terranes.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-96\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/usgs_terranes-179x300.gif\" alt=\"Map showing large areas of the western North American continent that are accreted.\" width=\"179\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/usgs_terranes-179x300.gif 179w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/usgs_terranes-65x109.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/usgs_terranes-225x377.gif 225w\" sizes=\"auto, (max-width: 179px) 100vw, 179px\" \/><\/a><figcaption id=\"caption-attachment-2526\" class=\"wp-caption-text\">Accreted terranes of western North America. Everything that is not the &#8220;Ancient continental interior (craton)&#8221; has been smeared onto the side of the continent by accretion from subduction.<\/figcaption><\/figure>\n<p>When the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a>, sinks into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>, the immense heat and pressure pushes volatile materials like water and carbon dioxide into an area below the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> and above the descending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2607\">mantle wedge<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2606\">volatiles<\/a> are released mostly by hydrated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> that revert to non-hydrated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a> in these higher <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a> and pressure conditions. When mixed with asthenospheric material above the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, the volatile lower the melting point of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2607\">mantle wedge<\/a>, and through a process called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2608\">flux melting<\/a> it becomes liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a>. The molten <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> is more buoyant than the lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> above it and migrates to the Earth\u2019s surface where it emerges as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>. The resulting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> frequently appear as curved mountain chains, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> arcs, due to the curvature of the earth. Both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> can contain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> arcs.<\/p>\n<figure id=\"attachment_2527\" aria-describedby=\"caption-attachment-2527\" style=\"width: 236px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/1755_Lisbon_Earthquake_Location.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-97\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location-236x300.png\" alt=\"It is large and offshore.\" width=\"236\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location-236x300.png 236w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location-65x82.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location-225x285.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/1755_Lisbon_Earthquake_Location.png 331w\" sizes=\"auto, (max-width: 236px) 100vw, 236px\" \/><\/a><figcaption id=\"caption-attachment-2527\" class=\"wp-caption-text\">Location of the large (Mw 8.5-9.0) 1755 Lisbon Earthquake.<\/figcaption><\/figure>\n<p>How <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> is initiated is still a matter of scientific debate. It is generally accepted that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones start as passive margins, where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> come together, and then gravity initiates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> and converts the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2598\">passive margin<\/a> into an active one. One <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> is gravity pulls the denser <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> down or the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> can start to flow ductility at a low angle. Scientists seeking to answer this question have collected evidence that suggests a new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone is forming off the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2890\">coast<\/a>\u00a0of Portugal. Some scientists have proposed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone, although the evidence is not definitive. Another <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> proposes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> happens at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries involving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> of different densities.<\/p>\n<p>Some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> boundaries look like they should be active, but show no evidence of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> on either side of the Atlantic Ocean for example, are denser than the underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> and are not <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> beneath the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. One <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2703\">bond<\/a> holding the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> together is stronger than the downwards force created by the difference in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> densities.<\/p>\n<figure id=\"attachment_2528\" aria-describedby=\"caption-attachment-2528\" style=\"width: 234px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/SundaMegathrustSeismicity.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-98\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-234x300.png\" alt=\"The earthquakes follow the slab down.\" width=\"234\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-234x300.png 234w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-65x83.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-225x289.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity-350x449.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/SundaMegathrustSeismicity.png 468w\" sizes=\"auto, (max-width: 234px) 100vw, 234px\" \/><\/a><figcaption id=\"caption-attachment-2528\" class=\"wp-caption-text\">Earthquakes along the Sunda megathrust subduction zone, along the island of Sumatra, showing the 2006 Mw 9.1-9.3 Indian Ocean Earthquake as a star.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">Subduction<\/a> zones are known for having the largest earthquakes and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3194\">tsunamis<\/a>; they are the only places with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> surfaces large enough to create <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3098\">magnitude<\/a>-9 earthquakes. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>-zone earthquakes not only are very large, but also are very deep. When a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> becomes stuck and cannot descend, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1933\">massive<\/a> amount of energy builds up between the stuck <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. If this energy is not gradually dispersed, it may force the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> to suddenly release along several hundred kilometers of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone. Because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>-zone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> are located on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>, this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1933\">massive<\/a> amount of movement can generate giant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3194\">tsunamis<\/a> such as those that followed the 2004 Indian Ocean Earthquake and 2011 T\u014dhoku Earthquake in Japan.<\/p>\n<figure id=\"attachment_2529\" aria-describedby=\"caption-attachment-2529\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Forearc.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-99\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Forearc-300x258.gif\" alt=\"It shows backarc, forearc, and arc.\" width=\"300\" height=\"258\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Forearc-300x258.gif 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Forearc-65x56.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Forearc-225x194.gif 225w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2529\" class=\"wp-caption-text\">Various parts of a subduction zone. This subduction zone is ocean-ocean subduction, though the same features can apply to continent-ocean subduction.<\/figcaption><\/figure>\n<p>All <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones have a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2614\">forearc basin<\/a>, a feature of the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> found between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arc<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2603\">trench<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2614\">forearc basin<\/a> experiences a lot of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faulting<\/a>\u00a0and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a> activity, particularly within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2604\">accretionary wedge<\/a>.<\/p>\n<p>In some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">tensional<\/a> forces\u00a0working on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> create a backarc <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> on the interior side of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arc<\/a>. Some scientists have proposed a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> mechanism called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> rollback creates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extension<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> in the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. In this model, the descending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> does not slide directly under the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> but instead rolls back, pulling the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> seaward. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> behind the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arc<\/a> gets stretched like pizza dough until the surface cracks and collapses to form a backarc <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a>. If the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extension<\/a> activity is extensive and deep enough, a backarc <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> can develop into a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> zone. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries may be less symmetrical than their <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridge<\/a> counterparts.<\/p>\n<p>In places where numerous young buoyant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are converging and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> at a relatively high velocity, they may force the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> to buckle and crack. This is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2610\">back-arc<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faulting<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">Extensional<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2610\">back-arc<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> pull rocks and chunks of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> apart. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1446\">Compressional<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2610\">back-arc<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, also known as thrust <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, push them together.<\/p>\n<p>The dual spines of the Andes Mountain range include a example of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1446\">compressional<\/a> thrust <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faulting<\/a>. The western spine is part of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arc<\/a>. Thrust <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> have deformed the non-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> eastern spine, \u00a0pushing rocks and pieces of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> on top of each other.<\/p>\n<p>There are two styles of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3074\">thrust fault<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a>: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2611\">thin-skinned<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> that occur in superficial rocks lying on top of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2612\">thick-skinned<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> that reach deeper into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a>. The Sevier <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2585\">Orogeny<\/a> in the western U.S. is a notable <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2611\">thin-skinned<\/a> type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a> created during the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1440\">Cretaceous<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2192\">Period<\/a>. The Laramide <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2585\">Orogeny<\/a>, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2612\">thick-skinned<\/a> type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a>, occurred near the end of and slightly after the Sevier <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2585\">Orogeny<\/a> \u00a0in the same region.<\/p>\n<figure id=\"attachment_2530\" aria-describedby=\"caption-attachment-2530\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Shallow_subduction_Laramide_orogeny.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-100\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny-300x142.png\" alt=\"The subducting plate goes right under the overriding plate\" width=\"300\" height=\"142\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny-300x142.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny-65x31.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny-225x106.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Shallow_subduction_Laramide_orogeny.png 336w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2530\" class=\"wp-caption-text\">Shallow subduction during the Laramide Orogeny.<\/figcaption><\/figure>\n<p>Flat-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a>, or shallow, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> caused the Laramide <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2585\">Orogeny<\/a>. When the descending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducts<\/a> at a low angle, there is more contact between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> and the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> than in a typical <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone. The shallowly-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducting<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> pushes against the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> and creates an area of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a> on the overriding <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> many kilometers away from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone.<\/p>\n<h4><span style=\"font-weight: 400;\">Oceanic-Continental subduction<\/span><\/h4>\n<figure id=\"attachment_2531\" aria-describedby=\"caption-attachment-2531\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/OceanContSub.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-101\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/OceanContSub-300x177.gif\" alt=\"The thinner ocean plate is going under the thicker continental plate.\" width=\"300\" height=\"177\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/OceanContSub-300x177.gif 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/OceanContSub-65x38.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/OceanContSub-225x133.gif 225w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2531\" class=\"wp-caption-text\">Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2616\">Oceanic-continental subduction<\/a> occurs when an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> dives below a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. This <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2600\">convergent<\/a> boundary has a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2603\">trench<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2607\">mantle wedge<\/a> and frequently, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arc<\/a>. Well-known examples of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2617\">volcanic arcs<\/a> are the Cascade Mountains in the Pacific Northwest and western Andes Mountains in South America.<\/p>\n<h4><span style=\"font-weight: 400;\">Oceanic-Oceanic Subduction<\/span><\/h4>\n<figure id=\"attachment_2532\" aria-describedby=\"caption-attachment-2532\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Fig21oceanocean.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-102\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig21oceanocean-300x173.gif\" alt=\"The ocean plate subducts beneath a different ocean plate.\" width=\"300\" height=\"173\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig21oceanocean-300x173.gif 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig21oceanocean-65x37.gif 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Fig21oceanocean-225x130.gif 225w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2532\" class=\"wp-caption-text\">Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island arc.<\/figcaption><\/figure>\n<p>The boundaries of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2618\">oceanic-oceanic subduction<\/a> zones show very different activity from those involving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. Since both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are made of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>, it is usually the older <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducts<\/a> because it is colder and denser. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> on the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> may remain hidden underwater.. If the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> rise high enough the reach the ocean surface, the chain of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> forms an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2619\">island arc<\/a>. Examples of these island arcs include the Aleutian Islands in the northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean.<\/p>\n<h3><b>2.3.2. Collisions<\/b><\/h3>\n<figure id=\"attachment_2533\" aria-describedby=\"caption-attachment-2533\" style=\"width: 301px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/contcontCollision.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-103\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/contcontCollision.gif\" alt=\"The two continental plates stay up.\" width=\"301\" height=\"181\" \/><\/a><figcaption id=\"caption-attachment-2533\" class=\"wp-caption-text\">Two continental plates colliding.<\/figcaption><\/figure>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> converge, during the closing of an ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> for example, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> is not possible between the equally buoyant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. Instead of one <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> descending beneath another, the two masses of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> slam together in a process known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>. Without <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>, there is no <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\">formation<\/a> and no <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">Collision<\/a> zones are characterized by tall, non-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> mountains; a broad zone of frequent, large earthquakes; and very little <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>.<\/p>\n<p>When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic crust<\/a> connected by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2598\">passive margin<\/a> to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental crust<\/a> completely <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducts<\/a> beneath a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continent<\/a>, an ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> closes, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a> begins. \u00a0Eventually, as ocean basins close, continents join together to form a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1933\">massive<\/a> accumulation of continents called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinent<\/a>, a process that has taken place in ~500 million year old cycles over earth\u2019s history.<\/p>\n<figure id=\"attachment_2534\" aria-describedby=\"caption-attachment-2534\" style=\"width: 267px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Pangaea_continents.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-104\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-267x300.png\" alt=\"Pangaea has a crescent shape.\" width=\"267\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-267x300.png 267w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-65x73.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-225x253.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents-350x394.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangaea_continents.png 758w\" sizes=\"auto, (max-width: 267px) 100vw, 267px\" \/><\/a><figcaption id=\"caption-attachment-2534\" class=\"wp-caption-text\">A reconstruction of Pangaea, showing approximate positions of modern continents.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The process of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a> created <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\">Pangea<\/a>, the supercontinent envisioned by Wegener as the key component of his <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a>. Geologists now have evidence that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> have been continuously converging into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinents<\/a> and splitting into smaller <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a>-separated continents throughout Earth\u2019s existence, calling this process the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinent<\/a> cycle, a process that takes place in approximately 500 million years. For example, they estimate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\">Pangea<\/a> began separating 200 million years ago. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\">Pangea<\/a> was preceded by an earlier <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinents<\/a>, one of which being <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2211\">Rodinia<\/a>, which existed 1.1 billion years ago and started breaking apart 800 million to 600 million years ago.<\/p>\n<figure id=\"attachment_2535\" aria-describedby=\"caption-attachment-2535\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/ZagrosFTB.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-105\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-300x231.png\" alt=\"The mountains are loading the crust down, leading to a depressed basin, which is the Persian Gulf\" width=\"300\" height=\"231\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-300x231.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-1024x789.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-768x592.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-65x50.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-225x173.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB-350x270.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/ZagrosFTB.png 1184w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2535\" class=\"wp-caption-text\">The tectonics of the Zagros Mountains. Note the Persian Gulf foreland basin.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>A foreland <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">basin<\/a> is a feature that develops near mountain belts, as the combined mass of the mountains forms a depression in the lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. While foreland basins may occur at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones, they are most commonly found at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a> boundaries. The Persian Gulf is possibly the best modern example, created entirely by the weight of the nearby Zagros Mountains.<\/p>\n<figure id=\"attachment_2536\" aria-describedby=\"caption-attachment-2536\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/02.3_ItalyPillowBasalt.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-106\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-300x225.jpg\" alt=\"The rock is cray with many circles inside\" width=\"300\" height=\"225\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-300x225.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-1024x768.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-768x576.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-1536x1152.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-65x49.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-225x169.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt-350x263.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/02.3_ItalyPillowBasalt.jpg 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2536\" class=\"wp-caption-text\">Pillow lavas, which only form under water, from an ophiolite in the Apennine Mountains of central Italy.<\/figcaption><\/figure>\n<p>If <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> are fused on the same <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, it can partially subduct but its buoyancy prevents it from fully descending. In very rare cases, part of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> may become trapped beneath a descending <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> in a process called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2621\">obduction<\/a>. When a portion of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental crust<\/a> is driven down into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zone, due to its buoyancy it returns to the surface relatively quickly.<\/p>\n<p>As pieces of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> break loose and migrate upward through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2621\">obduction<\/a> zone, they bring along bits of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a> and amend them on top of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. Rocks<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\"> composed<\/a> of this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> and ocean-floor material are called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2622\">ophiolites<\/a> and they provide valuable information about the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composition<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>.<\/p>\n<p>The area of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>-zone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> activity usually covers a broader area because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> is plastic and malleable. Unlike <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>-zone earthquakes, which tend to be located along a narrow swath near the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2600\">convergent<\/a> boundary, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>-zone earthquakes may occur hundreds of kilometers from the boundary between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>.<\/p>\n<p>The Eurasian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continent<\/a> has many examples of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">collision<\/a>-zone deformations covering vast areas. The Pyrenees mountains begin in the Iberian Peninsula and cross into France. Also, there are the Alps stretching from Italy to central Europe; the Zagros mountains from Arabia to Iran; and Himalaya mountains from the Indian subcontinent to central Asia.<\/p>\n<div style=\"width: 720px;\" class=\"wp-video\"><video class=\"wp-video-shortcode\" id=\"video-138-2\" width=\"720\" height=\"478\" preload=\"metadata\" controls=\"controls\"><source type=\"video\/mp4\" src=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/IndiaAsiaCollision.mp4?_=2\" \/><a href=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/IndiaAsiaCollision.mp4\">http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/IndiaAsiaCollision.mp4<\/a><\/video><\/div>\n<p><em>Animation of India crashing into Asia, by Tanya Atwater.<\/em><\/p>\n<figure id=\"attachment_3733\" aria-describedby=\"caption-attachment-3733\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/India-Asia-Collision-Animation-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-107\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/India-Asia-Collision-Animation-QR-Code.png 1125w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3733\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this animation 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-12\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-12\" class=\"h5p-iframe\" data-content-id=\"12\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.3 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3732\" aria-describedby=\"caption-attachment-3732\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.3-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-108\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-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\/2.3-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.3-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3732\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 2.3 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-weight: 400;\">2.4 Divergent Boundaries<\/span><\/h2>\n<p>At <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries, sometimes called constructive boundaries, lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> move away from each other. There are two types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries, categorized by where they occur: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zones and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">Continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zones occur in weak spots in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridge<\/a> usually originates in a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zone that expands to the point of splitting the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents. This process is known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>-to-drift.<\/p>\n<h3><b>2.4.1. Continental Rifting<\/b><\/h3>\n<figure id=\"attachment_2537\" aria-describedby=\"caption-attachment-2537\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Horst-Graben.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-109\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-300x154.png\" alt=\"While the area extends, individual grabens drop down relative to the horsts.\" width=\"300\" height=\"154\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-300x154.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-1024x525.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-768x394.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-65x33.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-225x115.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_-350x180.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Horst-Graben.svg_.png 1068w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2537\" class=\"wp-caption-text\">Faulting that occurs in divergent boundaries.<\/figcaption><\/figure>\n<p>In places where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are very thick, they reflect so much heat back into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> it develops strong <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> currents that push super-heated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> material up against the overlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, softening it. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">Tensional<\/a> forces created by this convective upwelling begin to pull the weakened <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> apart. As it stretches, it becomes thinner and develops deep cracks called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extension<\/a> or normal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>. Eventually plate sections located between large faults drop into deep depressions known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> valleys, which often contain keystone-shaped blocks of down-dropped <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2626\">grabens<\/a>. The shoulders of these <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2626\">grabens<\/a> are called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2625\">horsts<\/a>. If only one side of a section drops, it is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3071\">half-graben<\/a>. Depending on the conditions, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifts<\/a> can grow into very large lakes and even oceans.<\/p>\n<figure id=\"attachment_2538\" aria-describedby=\"caption-attachment-2538\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/TopographicAfar.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-110\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-300x300.png\" alt=\"The branches of the plate boundaries are 120 degrees apart.\" width=\"300\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/TopographicAfar.png 1536w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2538\" class=\"wp-caption-text\">The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120\u00b0 apart.<\/figcaption><\/figure>\n<p>While seemingly occurring at random, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> is dictated by two factors. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rifting<\/a> does not occur in continents with older and more stable interiors, known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2640\">cratons<\/a>. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> does occur, the break-up pattern resembles the seams of a soccer ball, also called a truncated icosahedron. This is the most common surface-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1934\">fracture<\/a> pattern to develop on an evenly expanding sphere because it uses the least amount of energy.<\/p>\n<p>Using the soccer ball model, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> tends to lengthen and expand along a particular seam while fizzling out in the other directions. These seams with little or no <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> activity are called failed rift arms. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2627\">failed rift arm<\/a> is still a weak spot in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>; even without the presence of active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extension<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, it may develop into a called an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2628\">aulacogen<\/a>. One example of a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2627\">failed rift arm<\/a> is the Mississippi Valley Embayment, a depression through which the upper end of the Mississippi <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3134\">River<\/a> flows. Occasionally connected <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> arms do develop concurrently, creating multiple boundaries of active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a>. In places where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> arms do not fail, for example the Afar Triangle, three <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries can develop near each other forming a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2629\">triple junction<\/a>.<\/p>\n<figure id=\"attachment_2539\" aria-describedby=\"caption-attachment-2539\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Basin_range_province.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-111\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-300x240.jpg\" alt=\"There is a series of mountains and valleys\" width=\"300\" height=\"240\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-300x240.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-65x52.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-225x180.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province-350x280.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Basin_range_province.jpg 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2539\" class=\"wp-caption-text\">NASA image of the Basin and Range horsts and grabens across central Nevada.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rifts<\/a> come in two types: narrow and broad. Narrow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifts<\/a> are characterized by a high density of highly active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries. The East African <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rift<\/a> Zone, where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2479\">horn<\/a> of Africa is pulling away from the mainland, is an excellent example of an active narrow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>. Lake Baikal in Russia is another. Broad <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifts<\/a> also have numerous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> zones, but they are distributed over wide areas of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1448\">deformation<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2462\">Basin and Range<\/a> region located in the western United States is a type of broad <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>. The Wasatch <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a>, which also created the Wasatch Mountain Range in the state of Utah, forms the eastern <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundary of this broad <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>\u00a0 (<a href=\"https:\/\/youtu.be\/TvvWqAdNV84\">Animation 1<\/a> and <a href=\"https:\/\/youtu.be\/7DxcAMmNeZk\">Animation 2<\/a>).<\/p>\n<p>&nbsp;<\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rifts<\/a> have earthquakes, although not of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3098\">magnitude<\/a> and frequency of other boundaries. They may also exhibit <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>. Unlike the flux-melted <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> found in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>-zone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> is created by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1176\">decompression melting<\/a>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are pulled apart, they create a region of low pressure that melts the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> and draws it upwards. When this molten <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> reaches the weakened and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>-riddled <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zone, it migrates to surface by breaking through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> or escaping via an open <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>. Examples of young <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> dot the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2462\">Basin and Range<\/a> region in the United States. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rift<\/a>-zone activity is responsible for generating some unique <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>, such as the Ol Doinyo Lengai in Tanzania. This <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcano<\/a> erupts <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2673\">lava<\/a> consisting largely of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3365\">carbonatite<\/a>, a relatively cold, liquid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1917\">carbonate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">mineral<\/a>.<\/p>\n<div style=\"width: 720px;\" class=\"wp-video\"><video class=\"wp-video-shortcode\" id=\"video-138-3\" width=\"720\" height=\"474\" preload=\"metadata\" controls=\"controls\"><source type=\"video\/mp4\" src=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/SoAtlantic_CutwithConvect.mp4?_=3\" \/><a href=\"http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/SoAtlantic_CutwithConvect.mp4\">http:\/\/opengeology.org\/textbook\/wp-content\/uploads\/2016\/07\/SoAtlantic_CutwithConvect.mp4<\/a><\/video><\/div>\n<p>South America and Africa <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a>, forming the Atlantic. <a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP1GTect\/eSoAtlantic_CutGlobe.html\">Video<\/a> by Tanya Atwater.<\/p>\n<figure id=\"attachment_3731\" aria-describedby=\"caption-attachment-3731\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Rift-Atlantic-Animation-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-112\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Rift-Atlantic-Animation-QR-Code.png 1125w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3731\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this video via this QR Code.<\/figcaption><\/figure>\n<h3><b>2.4.2. Mid-ocean ridges<\/b><\/h3>\n<figure id=\"attachment_2541\" aria-describedby=\"caption-attachment-2541\" style=\"width: 212px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Ocean-birth.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-113\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-212x300.png\" alt=\"The ocean starts as a valley and then gets wider and wider.\" width=\"212\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-212x300.png 212w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-724x1024.png 724w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-768x1086.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-65x92.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-225x318.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_-350x495.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ocean-birth.svg_.png 848w\" sizes=\"auto, (max-width: 212px) 100vw, 212px\" \/><\/a><figcaption id=\"caption-attachment-2541\" class=\"wp-caption-text\">Progression from rift to mid-ocean ridge.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><span style=\"font-weight: 400;\">As <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> activity progress, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> becomes more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1956\">mafic<\/a> (see <a href=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/chapter\/4-igneous-processes-and-volcanoes\/\" target=\"_blank\" rel=\"noopener\">Chapter 4<\/a>) and thinner, with the eventual result transforming the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> under the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> area into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>. This is the process that gives birth to a new ocean, much like the narrow Red Sea emerged with the movement of Arabia away from Africa. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> continues to diverge, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridge<\/a> is formed.<\/span><\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">Mid-ocean ridges<\/a>, also known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">spreading centers<\/a>, have several distinctive features. They are the only places on earth that create new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1176\">Decompression melting<\/a> in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zone changes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a> material into new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>, which oozes up through cracks in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. The amount of new <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> being created at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> is highly significant. These undersea <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> produce more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2673\">lava<\/a> than all other types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> combined. Despite this, most mid-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> ridge <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> remains unmapped because the volcanoes are located deep on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>.<\/p>\n<p>In rare cases, such as a few locations in Iceland, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rift<\/a> zones display the type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>, spreading, and ridge <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\">formation<\/a> found on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>.<\/p>\n<figure id=\"attachment_2542\" aria-describedby=\"caption-attachment-2542\" style=\"width: 300px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/age_oceanic_lith.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-114\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-300x191.jpg\" alt=\"The map shoes colors that represent different ages.\" width=\"300\" height=\"191\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-300x191.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-1024x653.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-768x490.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-1536x980.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-65x41.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-225x144.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith-350x223.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/age_oceanic_lith.jpg 2001w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2542\" class=\"wp-caption-text\">Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.<\/figcaption><\/figure>\n<p>The ridge feature is created by the accumulation of hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> material, which is lighter than the dense underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>. This chunk of isostatically buoyant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> sits partially submerged and partially exposed on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>, like an ice cube floating in a glass of water.<\/p>\n<p>As the ridge continues to spread, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> material is pulled away from the area of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> and becomes colder and denser. As it continues to spread and cool, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> settles into wide swathes of relatively featureless topography called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2884\">abyssal<\/a> plains with lower topography.<\/p>\n<p>This model of ridge <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\">formation<\/a> suggests the sections of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a> furthest away from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> will be the oldest. Scientists have tested this idea by comparing the age of rocks located in various locations on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>. Rocks found near ridges are younger than those found far away from any ridges. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">Sediment<\/a> accumulation patterns also confirm the idea of sea-floor spreading. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">Sediment<\/a> layers tend to be thinner near <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>, indicating it has had less time to build up.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2543\" aria-describedby=\"caption-attachment-2543\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/atwater_mag_reversal_mid_ocean_ridge.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-115 size-full\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/atwater_mag_reversal_mid_ocean_ridge.gif\" alt=\"animation showing the mid ocean ridges. As new oceanic plate is made at the ridge, it cools and preserves the current magnetic field at the time of cooling. When the poles reverse, the magnetic polarity flip is preserved in the oceanic plate record.\" width=\"600\" height=\"400\" \/><\/a><figcaption id=\"caption-attachment-2543\" class=\"wp-caption-text\">Spreading along several mid-ocean ridges, showing magnetic striping symmetry. By Tanya Atwater.<\/figcaption><\/figure>\n<figure id=\"attachment_3743\" aria-describedby=\"caption-attachment-3743\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Atwater-Spreading-GIF.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-116\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Atwater-Spreading-GIF.png 1125w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3743\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this animation via this QR Code.<\/figcaption><\/figure>\n<p>As mentioned in the section on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2579\">paleomagnetism<\/a> and the development of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a>, scientists noticed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> contained unique magnetic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1719\">anomalies<\/a> that show up as symmetrical striping on both sides of the ridge. The Vine-Matthews-Morley <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> proposes these alternating reversals are created by the earth\u2019s magnetic field being imprinted into <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a><\/p>\n<figure id=\"attachment_2544\" aria-describedby=\"caption-attachment-2544\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-117\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-300x212.png\" alt=\"The older stripes are farther from the ridge.\" width=\"300\" height=\"212\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-300x212.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-65x46.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-225x159.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_-350x248.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg_.png 640w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2544\" class=\"wp-caption-text\">A time progression (with &#8220;a&#8221; being youngest and &#8220;c&#8221; being oldest) showing a spreading center getting wider while recording changes in the magnetic field of the Earth.<\/figcaption><\/figure>\n<p>after it emerges from the ridge. Very hot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> has no magnetic field. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> get pulled apart, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> cools below the Curie point, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a> below which a magnetic field gets locked into magnetic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a>. The alternating magnetic reversals in the rocks reflects the periodic swapping of earth\u2019s magnetic north and south poles. This paleomagnetic pattern provides a great historical record of ocean-floor movement, and is used to reconstruct past <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> activity and determine rates of ridge spreading.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-2\" title=\"PANGEA Breakup\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/6o1HawAOTEI?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><em><a href=\"http:\/\/emvc.geol.ucsb.edu\/2_infopgs\/IP3RegTect\/bNoCentAtlantic.html\">Video<\/a> of the breakup of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\"><em>Pangea<\/em><\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\"><em>formation<\/em><\/a> of the northern Atlantic Ocean. By Tanya Atwater.<\/em><\/p>\n<figure id=\"attachment_3729\" aria-describedby=\"caption-attachment-3729\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Pangea-Animation-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-118\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Pangea-Animation-QR-Code.png 1148w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3729\" class=\"wp-caption-text\">If you are using the printed version of this OER, access this video via this QR Code.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2545\" aria-describedby=\"caption-attachment-2545\" style=\"width: 233px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/BlackSmoker.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-119 size-medium\" 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=\"233\" height=\"300\" 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: 233px) 100vw, 233px\" \/><\/a><figcaption id=\"caption-attachment-2545\" class=\"wp-caption-text\">Black smoker hydrothermal vent with a colony of giant (6&#8217;+) tube worms.<\/figcaption><\/figure>\n<p>Thanks to their distinctive geology, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> are home to some of the most unique ecosystems ever discovered. The ridges are often studded with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2921\">hydrothermal<\/a> vents, deep fissures that allow seawater to circulate through the upper portions of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> and interact with hot rock. The super-heated seawater rises back up to the surface of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, carrying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2815\">dissolved<\/a> gasses and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2687\">minerals<\/a>, and small particulates.\u00a0 The resulting emitted <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2921\">hydrothermal<\/a> water looks like black underwater smoke.<\/p>\n<p>Scientists had known about these geothermal areas on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a> for some time. However, it was not until 1977, when scientists piloting a deep submergence vehicle, the Alvin, discovered a thriving community of organisms clustered around these <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2921\">hydrothermal<\/a> vents. These unique organisms, which include 10-foot-long tube worms taller than people, live in the complete darkness of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor <\/a> deprived of oxygen and sunlight. They use geothermal energy provided by the vents and a process called bacterial <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1184\">chemosynthesis<\/a> to feed on sulfur compounds. Before this discovery, scientists believed life on earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest this type of environment could have been the origin of life on Earth, and perhaps even extraterrestrial life elsewhere in the galaxy, such as on Jupiter\u2019s moon Europa.<\/p>\n<p>&nbsp;<\/p>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-13\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-13\" class=\"h5p-iframe\" data-content-id=\"13\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.4 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3728\" aria-describedby=\"caption-attachment-3728\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.4-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-120\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-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\/2.4-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.4-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3728\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 2.4 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-weight: 400;\">2.5 Transform Boundaries<\/span><\/h2>\n<figure id=\"attachment_2546\" aria-describedby=\"caption-attachment-2546\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Strike_slip_fault.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-121\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-300x137.png\" alt=\"Sinistral moves to the left, dextral moves to the right.\" width=\"300\" height=\"137\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-300x137.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-65x30.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-225x103.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault-350x160.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Strike_slip_fault.png 437w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2546\" class=\"wp-caption-text\">The two types of transform\/strike slip faults.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundary, sometimes called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3076\">strike-slip<\/a> or conservative boundary, is where the lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> slide past each other in the horizontal plane. This movement is described based on the perspective of an observer standing on one of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>, looking across the boundary at the opposing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3077\">Dextral<\/a>, also known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3077\">right-lateral<\/a>, movement describes the opposing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> moving to the right. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2632\">Sinistral<\/a>, also known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2632\">left lateral<\/a>, movement describe the opposing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> moving to the left.<\/p>\n<p>Most <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries are found on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>, around <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a>. These boundaries form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3367\">aseismic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1934\">fracture<\/a> zones, filled with earthquake-free <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, to accommodate different rates of spreading occurring at the ridge.<\/p>\n<figure id=\"attachment_2547\" aria-describedby=\"caption-attachment-2547\" style=\"width: 217px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Sanandreas.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-122\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-217x300.jpg\" alt=\"The fault runs through California.\" width=\"217\" height=\"300\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-217x300.jpg 217w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-65x90.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-225x311.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas-350x483.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Sanandreas.jpg 362w\" sizes=\"auto, (max-width: 217px) 100vw, 217px\" \/><\/a><figcaption id=\"caption-attachment-2547\" class=\"wp-caption-text\">Map of the San Andreas fault, showing relative motion.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Some <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries produce significant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> activity, primarily as earthquakes, with very little mountain-building or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>. This type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundary may contain a single <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> or series of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, which develop in places where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2915\">stresses<\/a> are transferred to the surface. As with other types of active boundaries, if the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are unable to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1447\">shear<\/a> past each other the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> forces will continue to build up. If the built up energy between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> is suddenly released, the result is an earthquake.<\/p>\n<p>In the eyes of humanity, the most significant <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> occur within <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>, and have a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1447\">shearing<\/a> motion that frequently produces moderate-to-large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3098\">magnitude<\/a> earthquakes. Notable examples include the San Andreas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a> in California, Northern and Eastern Anatolian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Faults<\/a>\u00a0in Turkey, Altyn Tagh <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a> in central Asia, and Alpine <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a> in New Zealand.<\/p>\n<h3><b>2.5.1. Transpression and Transtension<\/b><\/h3>\n<figure id=\"attachment_2548\" aria-describedby=\"caption-attachment-2548\" style=\"width: 150px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Transpression.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-123 size-thumbnail\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Transpression-150x150.png\" alt=\"The fault is dextral, and has a leftward bend, causing uplift.\" width=\"150\" height=\"150\" \/><\/a><figcaption id=\"caption-attachment-2548\" class=\"wp-caption-text\">A transpressional strike-slip fault, causing uplift called a restraining bend.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>Bends along <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> may create <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1446\">compressional<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extensional<\/a> forces that cause secondary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faulting<\/a> zones. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2634\">Transpression<\/a> occurs where there is a component of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1446\">compression<\/a> in addition to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1447\">shearing<\/a> motion. These forces build up around the area of the bend, where the opposing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are restricted from sliding past each other. As the forces continue to build up, they create mountains in the restraining bend around the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>. The Big Bend area, located in the southern part of the San Andreas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a> includes a large area of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2634\">transpression<\/a> where many mountains have been built, moved, and even rotated.<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_2549\" aria-describedby=\"caption-attachment-2549\" style=\"width: 150px\" class=\"wp-caption alignright\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Transtension.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-124 size-thumbnail\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Transtension-150x150.png\" alt=\"The fault is dextral, and has a rightward bend, causing a valley.\" width=\"150\" height=\"150\" \/><\/a><figcaption id=\"caption-attachment-2549\" class=\"wp-caption-text\">A transtensional strike-slip fault, causing a restraining bend. In the center of the fault, a depression with extension would be found.<\/figcaption><\/figure>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2635\">Transtension<\/a> zones require a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> that includes a releasing bend, where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are being pulled apart by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1445\">extensional<\/a> forces. Depressions and sometimes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> develop in the releasing bend, along the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>. The Dead Sea found between Israel and Jordan, and the Salton Sea of California are examples of basins formed by transtensional forces.<\/p>\n<h3><\/h3>\n<h3><b>2.5.2. Piercing Points<\/b><\/h3>\n<figure id=\"attachment_2550\" aria-describedby=\"caption-attachment-2550\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Wallace_Creek_offset_across_the_San_Andreas_Fault.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-125\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-300x205.png\" alt=\"The offset is to the left.\" width=\"300\" height=\"205\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-300x205.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-768x524.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-65x44.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-225x153.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault-350x239.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wallace_Creek_offset_across_the_San_Andreas_Fault.png 962w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2550\" class=\"wp-caption-text\">Wallace (dry) Creek on the Cariso Plain, California. Note as the creek flows from the northern mountainous part of the image, it takes a sharp right (as viewed from the flow of water), then a sharp left. This is caused by the San Andreas Fault cutting roughly perpendicular to the creek, and shifting the location of the creek over time. The fault can be seen about halfway down, trending left to right, as a change in the topography.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>When a geological feature is cut by a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>, it is called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2636\">piercing point<\/a>. Piercing points are very useful for recreating past <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> movement, especially along <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">transform<\/a> boundaries. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\">Transform<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a> are unique because their horizontal motion keeps a geological feature relatively intact, preserving the record of what happened. Other types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>\u2014normal and reverse \u2014tend to be more destructive, obscuring or destroying these features. The best type of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2636\">piercing point<\/a> includes unique patterns that are used to match the parts of a geological feature separated by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> movement. Detailed studies of piercing points show the San Andreas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">Fault<\/a> has experienced over 225 km of movement in the last 20 million years, and this movement occurred at three different <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> traces.<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-3\" title=\"Plate Tectonics in a Nutshell (Tanya Atwater)\" width=\"500\" height=\"375\" src=\"https:\/\/www.youtube.com\/embed\/IDTBY5WDELg?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><em>Video of the origin of the San Andreas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\"><em>fault<\/em><\/a>. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\"><em>mid-ocean ridge<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\"><em>subducts<\/em><\/a>, the relative motion between the remaining <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\"><em>plates<\/em><\/a> become <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2601\"><em>transform<\/em><\/a>, forming the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\"><em>fault<\/em><\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2664\"><em>system<\/em><\/a>. Note that because the motion of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\"><em>plates<\/em><\/a> is not exactly parallel to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\"><em>fault<\/em><\/a>, it causes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\"><em>divergent<\/em><\/a> motion in the interior of North America. By Tanya Atwater.<\/em><\/p>\n<figure id=\"attachment_3727\" aria-describedby=\"caption-attachment-3727\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Plate-Tectonics-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-126\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3727\" 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>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-14\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-14\" class=\"h5p-iframe\" data-content-id=\"14\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.5 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3726\" aria-describedby=\"caption-attachment-3726\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.5-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-127\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-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\/2.5-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.5-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3726\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for section 2.5 via this QR Code.<\/figcaption><\/figure>\n<h2><span style=\"font-weight: 400;\">2.6 The Wilson Cycle<\/span><\/h2>\n<figure id=\"attachment_2551\" aria-describedby=\"caption-attachment-2551\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Wilson-cycle_hg.svg_.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-128\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-300x164.png\" alt=\"The diagram shows the last 1000 million years.\" width=\"300\" height=\"164\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-300x164.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-1024x559.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-768x419.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-1536x838.png 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-65x35.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-225x123.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_-350x191.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Wilson-cycle_hg.svg_.png 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2551\" class=\"wp-caption-text\">Diagram of the Wilson Cycle, showing rifting and collision phases.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2637\">Wilson Cycle<\/a> is named for J. Tuzo Wilson who first described it in 1966, and it outlines the ongoing origin and breakup of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinents<\/a>, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\">Pangea<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2211\">Rodinia<\/a>. Scientists have determined this cycle has been operating for at least three billion years and possibly earlier.<\/p>\n<p>There are a number of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypotheses<\/a> about how the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2637\">Wilson Cycle<\/a> works. One mechanism proposes that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> happens because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> reflect the heat much better than <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. When continents congregate together, they reflect more of the Earth\u2019s heat back into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>, generating more vigorous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> currents that then start the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> process. Some geologists believe <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> plumes are remnants of these <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2192\">periods<\/a> of increased <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> upwelling, and study them for clues about the origin of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a>.<\/p>\n<p>The mechanism behind how <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinents<\/a> are created is still largely a mystery. There are three schools of thought about what continues to drive the continents further apart and eventually bring them together. The ridge-push <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">hypothesis<\/a> suggests after the initial <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> event, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> continue to be pushed apart by mid-ocean <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">spreading centers<\/a> and their underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> currents. Slab-pull proposes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> are pulled apart by descending slabs in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a>&#8211;<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> margins. A third idea, gravitational sliding, attributes the movement to gravitational forces pulling the lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> down from the elevated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2630\">mid-ocean ridges<\/a> and across the underlying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2593\">asthenosphere<\/a>. Current evidence seems to support <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">slab<\/a> pull more than ridge push or gravitational sliding.<\/p>\n<h2><span style=\"font-weight: 400;\">2.7 Hotspots<\/span><\/h2>\n<figure id=\"attachment_2552\" aria-describedby=\"caption-attachment-2552\" style=\"width: 193px\" class=\"wp-caption alignright\"><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-2552\" class=\"wp-caption-text\">Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2637\">Wilson Cycle<\/a> provides a broad overview of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement. To analyze <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement more precisely, scientists study <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>. First postulated by J. Tuzo Wilson in 1963, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> is an area in the lithospheric <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> where molten <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> breaks through and creates a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> center, islands in the ocean and mountains on land. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> moves across the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcano<\/a> center becomes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1708\">extinct<\/a> because it is no longer over an active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> source. Instead, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> emerges through another area in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> to create a new active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcano<\/a>. Over time, the combination of moving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> and stationary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> creates a chain of islands or mountains. The classic definition of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> states they do not move, although recent evidence suggests that there may be exceptions.<\/p>\n<figure id=\"attachment_2553\" aria-describedby=\"caption-attachment-2553\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/HotspotsWorld.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-130\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-300x175.png\" alt=\"Hotspots are scattered around the world.\" width=\"300\" height=\"175\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-300x175.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-65x38.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-225x131.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld-350x204.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/HotspotsWorld.png 682w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2553\" class=\"wp-caption-text\">Map of world hotspots. Larger circles indicate more active hotspots.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">Hotspots<\/a> are the only types of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> not associated with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a> zones at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> boundaries; they seem totally disconnected from any <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> processes, such as earthquakes. However, there are\u00a0 relationships between <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a>. There are several <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>, current and former, that are believed to have begun at the time of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a>. Also, scientists use the age of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> eruptions and shape of the chain to quantify the rate and direction of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement relative to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a>.<\/p>\n<p>Scientists are divided over how <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> is generated in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>. Some suggest that <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> originate from super-heated material from as deep as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> that reaches the Earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2639\">mantle plume<\/a>. Others argue the molten material that feeds <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> is sourced from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>. Of course, it is difficult to collect data from these deep-Earth features due to the extremely high pressure and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a>.<\/p>\n<p>How <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> are initiated is another highly debated subject. The prevailing mechanism has <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> starting in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2599\">divergent<\/a> boundaries during <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2623\">supercontinent<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">rifting<\/a>. Scientists have identified a number of current and past <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> believed to have begun this way. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">Subducting<\/a> slabs have also been named as causing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> plumes and hot-spot <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a>. Some geologists have suggested another geological process not involving <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> may be involved, such as a large space objects crashing into Earth. Regardless of how they are formed, dozens are on the Earth. Some well-known examples include the Tahiti Islands, Afar Triangle, Easter Island, Iceland, Galapagos Islands, and Samoan Islands. The United States is home to two of the largest and best-studied <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>: Hawaii and Yellowstone.<\/p>\n<h3><b>2.7.1 Hawaiian hotspot<\/b><\/h3>\n<figure id=\"attachment_2554\" aria-describedby=\"caption-attachment-2554\" style=\"width: 296px\" class=\"wp-caption alignright\"><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-2554\" class=\"wp-caption-text\">The Hawaii-Emperor seamount and island chain.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>The active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> in Hawaii represent one of the most active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> sites on earth. Scientific evidence indicates the Hawaiian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> is at least 80 million years old. Geologists believe it is actually much older; however any rocks with proof of this have been <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subducted<\/a> under the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2885\">ocean floor<\/a>. The big island of Hawaii sits atop a large <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2639\">mantle plume<\/a> that marks the active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a>. The Kilauea <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcano<\/a> is the main <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1187\">vent<\/a> for this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> and has been actively erupting since 1983.<\/p>\n<p>This enormous <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> island chain, much of which is underwater, stretches across the Pacific for almost 6,000 km. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2899\">seamount<\/a> chain\u2019s most striking feature is a sharp 60-degree bend located at the midpoint, which marks a significant change in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement direction that occurred 50 million years ago. The change in direction has been more often linked to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> reconfiguration, but also to other things like plume migration.<\/p>\n<figure id=\"attachment_2555\" aria-describedby=\"caption-attachment-2555\" style=\"width: 300px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/09\/Hawaii_hotspot_cross-sectional_diagram.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-132\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-300x159.jpg\" alt=\"The islands get older to the left.\" width=\"300\" height=\"159\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-300x159.jpg 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-1024x543.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-768x407.jpg 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-1536x814.jpg 1536w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-65x34.jpg 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-225x119.jpg 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram-350x186.jpg 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Hawaii_hotspot_cross-sectional_diagram.jpg 1600w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-2555\" class=\"wp-caption-text\">Diagram of the Hawaiian hotspot and islands that it formed.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>In an attempt to map the Hawaiian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2639\">mantle plume<\/a> as far down as the lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>, scientists have used <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3096\">tomography<\/a>, a type of three-dimensional <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> imaging. This information\u2014along with other evidence gathered from rock ages, vegetation types, and island size\u2014indicate the oldest islands in the chain are located the furthest away from the active <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a>.<\/p>\n<h4><b>2.7.2 Yellowstone hotspot<\/b><\/h4>\n<figure id=\"attachment_2556\" aria-describedby=\"caption-attachment-2556\" 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-2556\" 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>&nbsp;<\/p>\n<p>Like the Hawaiian version, the Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> is formed by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> rising through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2590\">lithosphere<\/a>. What makes it different is this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> is located under a thick, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>. Hawaii sits on a thin <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a>\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>, which is easily breached by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> coming to the surface. At Yellowstone, the thick <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> presents a much more difficult barrier for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> to penetrate. When it does emerge, the eruptions are generally much more violent. Thankfully they are also less frequent.<\/p>\n<p>Over 15 million years of eruptions by this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> have carved a curved path across the western United States. It has been suggested the Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> is connected to the much older Columbia <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3134\">River<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1197\">flood basalts<\/a> and even to 70 million-year-old <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanism<\/a> found in the Yukon region of Canada.<\/p>\n<figure id=\"attachment_2557\" aria-describedby=\"caption-attachment-2557\" style=\"width: 300px\" class=\"wp-caption alignleft\"><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-2557\" 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 most recent major eruption of this <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> created the Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1188\">Caldera<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2673\">Lava<\/a> Creek <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1953\">tuff<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2960\">formation<\/a> approximately 631,000 years ago. The eruption threw 1,000 cubic kilometers of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1949\">ash<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">magma<\/a> into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2667\">atmosphere<\/a>, some of which was found as far away as Mississippi. Should the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> erupt again, scientists predict it will be another <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1933\">massive<\/a> event. This would be a calamity reaching far beyond the western United States. These super <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> eruptions fill the earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2667\">atmosphere<\/a> with so much gas and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1949\">ash<\/a>, they block sunlight from reaching the earth. Not only would this drastically alter climates and environments around the globe, it could affect worldwide food production.<\/p>\n<p>&nbsp;<\/p>\n<h3>Take this quiz to check your comprehension of this section.<\/h3>\n<div id=\"h5p-15\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-15\" class=\"h5p-iframe\" data-content-id=\"15\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"2.6\/7 Did I Get It?\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3725\" aria-describedby=\"caption-attachment-3725\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/2.6-7-Did-I-Get-It-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-135\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-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\/2.6-7-Did-I-Get-It-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/2.6-7-Did-I-Get-It-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3725\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the quiz for sections 2.6 and 2.7 via this QR Code.<\/figcaption><\/figure>\n<h2><\/h2>\n<h2>Summary<\/h2>\n<p><iframe loading=\"lazy\" id=\"oembed-4\" title=\"Plate Tectonics Basics 1\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/6wJBOk9xjto?feature=oembed&#38;rel=0&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<figure id=\"attachment_3724\" aria-describedby=\"caption-attachment-3724\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Plate-Tectonics-Basics-YouTube-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-136\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Plate-Tectonics-Basics-YouTube-QR-Code.png 1155w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3724\" 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><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Plate tectonics<\/a> is a unifying <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a>; it explains nearly all of the major geologic processes on Earth. Since its early inception in the 1950s and 1960s, geologists have been guided by this revolutionary perception of the world. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2655\">theory<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a> states the surface layer of the Earth is broken into a network of solid, relatively <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2583\">brittle<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a>. Underneath the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> is a much hotter and more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2582\">ductile<\/a> layer that contains zones of convective upwelling generated by the interior heat of Earth. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2577\">convection<\/a> currents move the surface <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> around\u2014bringing them together, pulling them apart, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1447\">shearing<\/a> them side-by-side. Earthquakes and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanoes<\/a> form at the boundaries where the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plates<\/a> interact, with the exception of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1181\">volcanic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a>, which are not caused by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a> movement.<\/p>\n<p>&nbsp;<\/p>\n<h3>Take this quiz to check your comprehension of this Chapter.<\/h3>\n<div id=\"h5p-16\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-16\" class=\"h5p-iframe\" data-content-id=\"16\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 2 Review\"><\/iframe><\/div>\n<\/div>\n<figure id=\"attachment_3723\" aria-describedby=\"caption-attachment-3723\" style=\"width: 150px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/35\/2021\/12\/Ch.2-Review-QR-Code.png\"><img loading=\"lazy\" decoding=\"async\" class=\"size-thumbnail wp-image-137\" src=\"https:\/\/pressbooks.ccconline.org\/accdigitalmarketing\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-150x150.png\" alt=\"\" width=\"150\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-150x150.png 150w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-300x300.png 300w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-1024x1024.png 1024w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-768x768.png 768w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-65x65.png 65w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-225x225.png 225w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code-350x350.png 350w, https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-content\/uploads\/sites\/222\/2025\/01\/Ch.2-Review-QR-Code.png 1147w\" sizes=\"auto, (max-width: 150px) 100vw, 150px\" \/><\/a><figcaption id=\"caption-attachment-3723\" class=\"wp-caption-text\">If you are using the printed version of this OER, access the review quiz for Chapter 2 via this QR Code.<\/figcaption><\/figure>\n<h2>References<\/h2>\n<div class=\"csl-bib-body\">\n<ol>\n<li class=\"csl-entry\">Aitta, A., 2006, Iron melting curve with a tricritical point: J. Stat. Mech., v. 2006, no. 12, p. P12015.<\/li>\n<li class=\"csl-entry\">Alfe, D., Gillan, M.J., and Price, G.D., 2002, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">Composition<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2689\">temperature<\/a> of the Earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a> constrained by combining ab initio calculations and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> data: Earth Planet. Sci. Lett., v. 195, no. 1, p. 91\u201398.<\/li>\n<li class=\"csl-entry\">Atwater, T., 1970, Implications of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Plate Tectonics<\/a> for the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1441\">Cenozoic<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Tectonic<\/a> Evolution of Western North America: Geol. Soc. Am. Bull., v. 81, no. 12, p. 3513\u20133536., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2\">10.1130\/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Bacon, F., and Montagu, B., 1848, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author: The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author, Parry &amp; McMillan, The Works of Francis Bacon, Lord Chancellor of England: With a Life of the Author.<\/li>\n<li class=\"csl-entry\">Benioff, H., 1949, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">Seismic<\/a> evidence for the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a> origin of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> deeps: Geological Society of America Bulletin, v. 60, no. 12, p. 1837\u20131856., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1949)60[1837:SEFTFO]2.0.CO;2\">10.1130\/0016-7606(1949)60[1837:SEFTFO]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Birch, F., 1952, Elasticity and constitution of the Earth\u2019s interior: J. Geophys. Res., v. 57, no. 2, p. 227\u2013286., doi: <a href=\"https:\/\/doi.org\/10.1029\/JZ057i002p00227\">10.1029\/JZ057i002p00227<\/a>.<\/li>\n<li class=\"csl-entry\">Birch, F., 1964, Density and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2831\">composition<\/a> of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>: J. Geophys. Res., v. 69, no. 20, p. 4377\u20134388.<\/li>\n<li class=\"csl-entry\">Bott, M.H.P., 1993, Modelling the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">plate<\/a>-driving mechanism: Journal of the Geological Society, v. 150, no. 5, p. 941\u2013951., doi: <a href=\"https:\/\/doi.org\/10.1144\/gsjgs.150.5.0941\">10.1144\/gsjgs.150.5.0941<\/a>.<\/li>\n<li class=\"csl-entry\">Coats, R.R., 1962, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2672\">Magma<\/a> type and crustal structure in the Aleutian <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2609\">Arc<\/a>, <i>in<\/i> The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">Crust<\/a> of the Pacific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1461\">Basin<\/a>: American Geophysical Union, p. 92\u2013109., doi: <a href=\"https:\/\/doi.org\/10.1029\/GM006p0092\">10.1029\/GM006p0092<\/a>.<\/li>\n<li class=\"csl-entry\">Conrad, C.P., and Lithgow-Bertelloni, C., 2002, How <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> slabs drive <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">plate tectonics<\/a>: Science (New York, N.Y.), v. 298, no. 5591, p. 207\u2013209., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.1074161\">10.1126\/science.1074161<\/a>.<\/li>\n<li class=\"csl-entry\">Corliss, J.B., Dymond, J.G., Gordon, L.I., Edmond, J.M., von Heezen, R.P., Ballard, R.D., Green, K., Williams, D.L., Bainbridge, A., Crane, K., and van Andel, T.H., 1979, Submarine thermal springs on the Galapagos <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2624\">Rift<\/a>: Science, v. 203, p. 107321083.<\/li>\n<li class=\"csl-entry\">Davis, E.E., and Lister, C.R.B., 1974, Fundamentals of ridge crest topography: Earth Planet. Sci. Lett., v. 21, no. 4, p. 405\u2013413.<\/li>\n<li class=\"csl-entry\">Dawson, J.B., Pinkerton, H., Norton, G.E., and Pyle, D.M., 1990, Physicochemical properties of alkali <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3365\">carbonatite<\/a> lavas:Data from the 1988 eruption of Oldoinyo Lengai, Tanzania: Geology, v. 18, no. 3, p. 260\u2013263.<\/li>\n<li class=\"csl-entry\">Drake, E.T., 1976, Alfred Wegener\u2019s reconstruction of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3366\">Pangea<\/a>: Geology, v. 4, no. 1, p. 41\u201344., doi: &lt;a href=&#8221;https:\/\/doi.org\/10.1130\/0091-7613(1976)42.0.CO;2&#8243;&gt;10.1130\/0091-7613(1976)4&lt;41:AWROP&gt;2.0.CO;2.<\/li>\n<li class=\"csl-entry\">Engdahl, E.R., Flynn, E.A., and Masse, R.P., 1974, Differential PkiKP travel times and the radius of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>: Geophysical J Royal Astro Soc, v. 40, p. 457\u2013463.<\/li>\n<li class=\"csl-entry\">Ewing, M., Ewing, J.I., and Talwani, M., 1964, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">Sediment<\/a> distribution in the oceans: The Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 75, no. 1, p. 17\u201336., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1964)75[17:SDITOT]2.0.CO;2\">10.1130\/0016-7606(1964)75[17:SDITOT]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Ewing, M., Houtz, R., and Ewing, J., 1969, South Pacific <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a> distribution: J. Geophys. Res., v. 74, no. 10, p. 2477\u20132493., doi: <a href=\"https:\/\/doi.org\/10.1029\/JB074i010p02477\">10.1029\/JB074i010p02477<\/a>.<\/li>\n<li class=\"csl-entry\">Fernandez, L.M., and Careaga, J., 1968, The thickness of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2580\">crust<\/a> in central United States and La Paz, Bolivia, from the spectrum of longitudinal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3087\">seismic<\/a> waves: Bull. Seismol. Soc. Am., v. 58, no. 2, p. 711\u2013741.<\/li>\n<li class=\"csl-entry\">Fluegel, von H.W., 1980, Wegener-Ampferer-Schwinner. Ein Beitrag zur Geschichte der Geologie in \u00d6sterreich: Mitt. Oesterr. Geol. Ges., v. 73, p. 237\u2013254.<\/li>\n<li class=\"csl-entry\">Forsyth, D.W., 1975, The Early Structural Evolution and Anisotropy of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">Oceanic<\/a> Upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">Mantle<\/a>: Geophys. J. Int., v. 43, no. 1, p. 103\u2013162., doi: <a href=\"https:\/\/doi.org\/10.1111\/j.1365-246X.1975.tb00630.x\">10.1111\/j.1365-246X.1975.tb00630.x<\/a>.<\/li>\n<li class=\"csl-entry\">Frankel, H., 1982, The Development, Reception, and Acceptance of the Vine-Matthews-Morley <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2652\">Hypothesis<\/a>: Hist. Stud. Phys. Biol. Sci., v. 13, no. 1, p. 1\u201339.<\/li>\n<li class=\"csl-entry\">Fukao, Y., and Obayashi, M., 2013, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">Subducted<\/a> slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity: J. Geophys. Res. [Solid Earth], v. 118, no. 11, p. 2013JB010466.<\/li>\n<li class=\"csl-entry\">Hagstrum, J.T., 2005, Antipodal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspots<\/a> and bipolar catastrophes: Were <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> large-body impacts the cause? Earth Planet. Sci. Lett., v. 236, no. 1\u20132, p. 13\u201327.<\/li>\n<li class=\"csl-entry\">Hanks, T.C., and Anderson, D.L., 1969, The early thermal history of the earth: Phys. Earth Planet. Inter., v. 2, no. 1, p. 19\u201329.<\/li>\n<li class=\"csl-entry\">Heezen, B.C., and Tharp, M., 1965, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Tectonic<\/a> Fabric of the Atlantic and Indian Oceans and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">Continental<\/a> Drift: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 90\u2013106., doi: <a href=\"https:\/\/doi.org\/10.1098\/rsta.1965.0024\">10.1098\/rsta.1965.0024<\/a>.<\/li>\n<li class=\"csl-entry\">Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, E.S., Shuster, M.W., Winslow, N.S., and Lawton, T.F., 1986, Time of initial <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3074\">thrusting<\/a> in the Sevier orogenic belt, Idaho-Wyoming and Utah: Geology, v. 14, no. 5, p. 388\u2013391.<\/li>\n<li class=\"csl-entry\">Herak, D., and Herak, M., 2007, Andrija Mohorovi\u010di\u0107 (1857-1936)\u2014On the occasion of the 150th anniversary of his birth: Seismol. Res. Lett., v. 78, no. 6, p. 671\u2013674.<\/li>\n<li class=\"csl-entry\">Hess, H.H., 1962, History of ocean basins: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1173\">Petrologic<\/a> studies, v. 4, p. 599\u2013620.<\/li>\n<li class=\"csl-entry\">Hutson, P., Middleton, J., and Miller, D., 2003, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2620\">Collision<\/a> Zones: Online, <a href=\"http:\/\/www.geosci.usyd.edu.au\/users\/prey\/ACSGT\/EReports\/eR.2003\/GroupD\/Report1\/web%20pages\/contents.html\">http:\/\/www.geosci.usyd.edu.au\/users\/prey\/ACSGT\/EReports\/eR.2003\/GroupD\/Report1\/web%20pages\/contents.html<\/a>, accessed June 2017.<\/li>\n<li class=\"csl-entry\">Isacks, B., Oliver, J., and Sykes, L.R., 1968, Seismology and the new global <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">tectonics<\/a>: J. Geophys. Res., v. 73, no. 18, p. 5855\u20135899.<\/li>\n<li class=\"csl-entry\">Ito, E., and Takahashi, E., 1989, Postspinel transformations in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2664\">system<\/a> Mg2SiO4-Fe2SiO4 and some geophysical implications: J. Geophys. Res. [Solid Earth], v. 94, no. B8, p. 10637\u201310646.<\/li>\n<li class=\"csl-entry\">Jacoby, W.R., 1981, Modern concepts of Earth dynamics anticipated by Alfred Wegener in 1912: Geology, v. 9, no. 1, p. 25\u201327., doi: &lt;a href=&#8221;https:\/\/doi.org\/10.1130\/0091-7613(1981)92.0.CO;2&#8243;&gt;10.1130\/0091-7613(1981)9&lt;25:MCOEDA&gt;2.0.CO;2.<\/li>\n<li class=\"csl-entry\">Jakosky, B.M., Grebowsky, J.M., Luhmann, J.G., Connerney, J., Eparvier, F., Ergun, R., Halekas, J., Larson, D., Mahaffy, P., McFadden, J., Mitchell, D.F., Schneider, N., Zurek, R., Bougher, S., and others, 2015, MAVEN observations of the response of Mars to an interplanetary coronal mass ejection: Science, v. 350, no. 6261, p. aad0210.<\/li>\n<li class=\"csl-entry\">James, D.E., Fouch, M.J., Carlson, R.W., and Roth, J.B., 2011, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2605\">Slab<\/a> fragmentation, edge flow and the origin of the Yellowstone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2638\">hotspot<\/a> track: Earth Planet. Sci. Lett., v. 311, no. 1\u20132, p. 124\u2013135.<\/li>\n<li class=\"csl-entry\">Ji, Y., and Nataf, H.-C., 1998, Detection of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> plumes in the lower <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a> by diffraction <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3096\">tomography<\/a>: Hawaii: Earth Planet. Sci. Lett., v. 159, no. 3\u20134, p. 99\u2013115.<\/li>\n<li class=\"csl-entry\">Johnston, S.T., Jane Wynne, P., Francis, D., Hart, C.J.R., Enkin, R.J., and Engebretson, D.C., 1996, Yellowstone in Yukon: The Late <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1440\">Cretaceous<\/a> Carmacks Group: Geology, v. 24, no. 11, p. 997\u20131000.<\/li>\n<li class=\"csl-entry\">Kearey, P., Klepeis, K.A., and Vine, F.J., 2009, Global <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Tectonics<\/a>: Oxford\u202f; Chichester, West Sussex\u202f; Hoboken, NJ, Wiley-Blackwell, 496 p.<\/li>\n<li class=\"csl-entry\">Le Pichon, X., 1968, Sea-floor spreading and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2575\">continental<\/a> drift: J. Geophys. Res., v. 73, no. 12, p. 3661\u20133697.<\/li>\n<li class=\"csl-entry\">Lehmann, I., 1936, P\u2019, Publ: Bur. Centr. Seism. Internat. Serie A, v. 14, p. 87\u2013115.<\/li>\n<li class=\"csl-entry\">Mantovani, R., 1889, Les <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1934\">fractures<\/a> de l\u2019\u00e9corce terrestre et la th\u00e9orie de Laplace: Bull. Soc. Sc. et Arts R\u00e9union, p. 41\u201353.<\/li>\n<li class=\"csl-entry\">Mason, R.G., 1958, A magnetic survey off the west <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2890\">coast<\/a> of the United-States between latitudes 32-degrees-N and 36-degrees-N longitudes 121-degrees-W and 128-degrees-W: Geophysical Journal of the Royal Astronomical Society, v. 1, no. 4, p. 320.<\/li>\n<li class=\"csl-entry\">Mason, R.G., and Raff, A.D., 1961, Magnetic Survey Off the West <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2890\">Coast<\/a> of North America, 32\u00b0 N. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3372\">Latitude<\/a> to 42\u00b0 N. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3372\">Latitude<\/a>: Geological Society of America Bulletin, v. 72, no. 8, p. 1259\u20131265., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2\">10.1130\/0016-7606(1961)72[1259:MSOTWC]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">McCollom, T.M., 1999, Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2921\">hydrothermal<\/a> systems on Europa: J. Geophys. Res., v. 104, no. E12, p. 30729\u201330742., doi: <a href=\"https:\/\/doi.org\/10.1029\/1999JE001126\">10.1029\/1999JE001126<\/a>.<\/li>\n<li class=\"csl-entry\">McKenzie, D.P., and Parker, R.L., 1967, The North Pacific: an Example of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2576\">Tectonics<\/a> on a Sphere: Nature, v. 216, p. 1276\u20131280., doi: <a href=\"https:\/\/doi.org\/10.1038\/2161276a0\">10.1038\/2161276a0<\/a>.<\/li>\n<li class=\"csl-entry\">Miller, A.R., Densmore, C.D., Degens, E.T., Hathaway, J.C., Manheim, F.T., McFarlin, P.F., Pocklington, R., and Jokela, A., 1966, Hot brines and recent iron deposits in deeps of the Red Sea: Geochimica et Cosmochimica Acta, v. 30, no. 3, p. 341\u2013359., doi: <a href=\"https:\/\/doi.org\/10.1016\/0016-7037(66)90007-X\">10.1016\/0016-7037(66)90007-X<\/a>.<\/li>\n<li class=\"csl-entry\">Morgan, W.J., 1968, Rises, trenches, great <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">faults<\/a>, and crustal blocks: J. Geophys. Res., v. 73, no. 6, p. 1959\u20131982., doi: <a href=\"https:\/\/doi.org\/10.1029\/JB073i006p01959\">10.1029\/JB073i006p01959<\/a>.<\/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_138_2602\">subduction<\/a>: J. Geophys. Res. [Solid Earth], v. 96, no. B1, p. 651\u2013665.<\/li>\n<li class=\"csl-entry\">Oldham, R.D., 1906, The constitution of the interior of the Earth, as revealed by earthquakes: Q. J. Geol. Soc. London, v. 62, no. 1\u20134, p. 456\u2013475.<\/li>\n<li class=\"csl-entry\">Pasyanos, M.E., 2010, Lithospheric thickness modeled from long-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2192\">period<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3089\">surface wave<\/a> dispersion: Tectonophysics, v. 481, no. 1\u20134, p. 38\u201350.<\/li>\n<li class=\"csl-entry\">Powell, R.E., and Weldon, R.J., 1992, Evolution of the San Andreas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3065\">fault<\/a>: Annu. Rev. Earth Planet. Sci., v. 20, p. 431.<\/li>\n<li class=\"csl-entry\">Raff, A.D., and Mason, R.G., 1961, Magnetic Survey Off the West <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2890\">Coast<\/a> of North America, 40 N. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3372\">Latitude<\/a> to 52 N. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3372\">Latitude<\/a>: Geological Society of America Bulletin, v. 72, no. 8, p. 1267\u20131270., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2\">10.1130\/0016-7606(1961)72[1267:MSOTWC]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Runcorn, S.K., 1965, Palaeomagnetic comparisons between Europe and North America: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 258, no. 1088, p. 1\u201311.<\/li>\n<li class=\"csl-entry\">Saito, T., Ewing, M., and Burckle, L.H., 1966, Tertiary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2678\">sediment<\/a> from the mid-atlantic ridge: Science, v. 151, no. 3714, p. 1075\u20131079., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.151.3714.1075\">10.1126\/science.151.3714.1075<\/a>.<\/li>\n<li class=\"csl-entry\">Satake, K., and Atwater, B.F., 2007, Long-term perspectives on giant earthquakes and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3194\">tsunamis<\/a> at <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> zones*: Annu. Rev. Earth Planet. Sci., v. 35, p. 349\u2013374.<\/li>\n<li class=\"csl-entry\">Scheidegger, A.E., 1953, Examination of the physics of theories of orogenesis: Geol. Soc. Am. Bull., v. 64, no. 2, p. 127\u2013150., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1953)64[127:EOTPOT]2.0.CO;2\">10.1130\/0016-7606(1953)64[127:EOTPOT]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Simpson, G.G., 1943, Mammals and the nature of continents: Am. J. Sci., v. 241, no. 1, p. 1\u201331.<\/li>\n<li class=\"csl-entry\">Starr, A.M., 2015, Ambient <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3103\">resonance<\/a> of rock arches: Salt Lake City, Utah, University of Utah, 134 p.<\/li>\n<li class=\"csl-entry\">Stern, R.J., 1998, A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a> primer for instructors of introductory geology courses and authors of introductory-geology textbooks: J. Geosci. Educ., v. 46, p. 221.<\/li>\n<li class=\"csl-entry\">Stern, R.J., 2004, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">Subduction<\/a> initiation: spontaneous and induced: Earth Planet. Sci. Lett., v. 226, no. 3\u20134, p. 275\u2013292.<\/li>\n<li class=\"csl-entry\">Stich, D., Mancilla, F. de L., Pondrelli, S., and Morales, J., 2007, Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake: Geophys. Res. Lett., v. 34, no. 12, p. L12308.<\/li>\n<li class=\"csl-entry\">Tatsumi, Y., 2005, The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2602\">subduction<\/a>\u00a0factory: how it operates in the evolving Earth: GSA Today, v. 15, no. 7, p. 4.<\/li>\n<li class=\"csl-entry\">Todo, Y., Kitazato, H., Hashimoto, J., and Gooday, A.J., 2005, Simple foraminifera flourish at the ocean\u2019s deepest point: Science, v. 307, no. 5710, p. 689., doi: <a href=\"https:\/\/doi.org\/10.1126\/science.1105407\">10.1126\/science.1105407<\/a>.<\/li>\n<li class=\"csl-entry\">Tolstoy, I., and Ewing, M., 1949, North Atlantic hydrography and the Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 60, no. 10, p. 1527\u20131540., doi: <a href=\"https:\/\/doi.org\/10.1130\/0016-7606(1949)60[1527:NAHATM]2.0.CO;2\">10.1130\/0016-7606(1949)60[1527:NAHATM]2.0.CO;2<\/a>.<\/li>\n<li class=\"csl-entry\">Vine, F.J., and Matthews, D.H., 1963, Magnetic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1719\">anomalies<\/a> over <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2581\">oceanic<\/a> ridges: Nature, v. 199, no. 4897, p. 947\u2013949.<\/li>\n<li class=\"csl-entry\">W\u00e4chtersh\u00e4user, G., 1990, Evolution of the first metabolic cycles: Proc. Natl. Acad. Sci. U. S. A., v. 87, no. 1, p. 200\u2013204.<\/li>\n<li class=\"csl-entry\">Wadati, K., 1935, On the activity of deep-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3080\">focus<\/a> earthquakes in the Japan Islands and neighbourhoods: Geophys. Mag., v. 8, no. 3\u20134, p. 305\u2013325.<\/li>\n<li class=\"csl-entry\">Waszek, L., Irving, J., and Deuss, A., 2011, Reconciling the hemispherical structure of Earth\/\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2596\">inner core<\/a> with its super-rotation: Nat. Geosci., v. 4, no. 4, p. 264\u2013267., doi: <a href=\"https:\/\/doi.org\/10.1038\/ngeo1083\">10.1038\/ngeo1083<\/a>.<\/li>\n<li class=\"csl-entry\">Wegener, A., 1912, Die Entstehung der Kontinente: Geol. Rundsch., v. 3, no. 4, p. 276\u2013292., doi: <a href=\"https:\/\/doi.org\/10.1007\/BF02202896\">10.1007\/BF02202896<\/a>.<\/li>\n<li class=\"csl-entry\">Wegener, A., 1920, Die entstehung der kontinente und ozeane: \u0420\u0438\u043f\u043e\u043b \u041a\u043b\u0430\u0441\u0441\u0438\u043a.<\/li>\n<li class=\"csl-entry\">Wells, H.G., Huxley, J., and Wells, G.P., 1931, The Science of Life: Philosophy, v. 6, no. 24, p. 506\u2013507.<\/li>\n<li class=\"csl-entry\">White, I.C., and Moreira, C., 1908, Commiss\u00e3o de estudos das minas de Carv\u00e3o de Pedra do Brazil:<\/li>\n<li class=\"csl-entry\">de Wijs, G.A., Kresse, G., Vo\u010dadlo, L., Dobson, D., Alf\u00e8, D., Gillan, M.J., and Price, G.D., 1998, The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_3368\">viscosity<\/a> of liquid iron at the physical conditions of the Earth\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2589\">core<\/a>: Nature, v. 392, no. 6678, p. 805\u2013807., doi: <a href=\"https:\/\/doi.org\/10.1038\/33905\">10.1038\/33905<\/a>.<\/li>\n<li class=\"csl-entry\">Wilson, J.T., 1966, Did the Atlantic close and then re-open? Nature.<\/li>\n<li class=\"csl-entry\">Wilson, M., 1993, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2591\">Plate<\/a>-moving mechanisms: constraints and controversies: Journal of the Geological Society, v. 150, no. 5, p. 923\u2013926., doi: <a href=\"https:\/\/doi.org\/10.1144\/gsjgs.150.5.0923\">10.1144\/gsjgs.150.5.0923<\/a>.<\/li>\n<li class=\"csl-entry\">Wyllie, P.J., 1970, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1957\">Ultramafic<\/a> rocks and the upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">mantle<\/a>, <i>in<\/i> Morgan, B.A., editor, Fiftieth anniversary symposia: Mineralogy and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1173\">petrology<\/a> of the Upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2586\">Mantle<\/a>; <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_1921\">Sulfides<\/a>; Mineralogy and geochemistry of non-<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2883\">marine<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2842\">evaporites<\/a>: Washington, DC, Mineralogical Society of America, p. 3\u201332.<\/li>\n<li class=\"csl-entry\">Zhou, Z., 2004, The origin and early evolution of birds: discoveries, disputes, and perspectives from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_138_2176\">fossil<\/a> evidence: Naturwissenschaften, v. 91, no. 10, p. 455\u2013471.<\/li>\n<\/ol>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_138_2576\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2652\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2575\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2591\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1181\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2600\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2602\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2599\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2624\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2630\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2601\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2601\"><div tabindex=\"-1\"><p>Place where two plates slide past each other, creating strike slip 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_138_1447\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1447\"><div tabindex=\"-1\"><p>Stress within an object that causes a side-to-side movement within an internal fabric or weakness.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2637\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2637\"><div tabindex=\"-1\"><p>The cycle of opening ocean basins with rifting and seafloor spreading, then closing the basin via subduction and collision, creating a supercontinent.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1461\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2638\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2672\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2655\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1933\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1706\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1706\"><div tabindex=\"-1\"><p>Relative balance of an object based on how it floats.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2176\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2176\"><div tabindex=\"-1\"><p>Any evidence of ancient life.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2891\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2891\"><div tabindex=\"-1\"><p>Submerged part of the continental mass, with a gentle slope.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2910\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2910\"><div tabindex=\"-1\"><p>Deposition and erosion tied to glacier 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_138_2960\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2581\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2464\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1710\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1710\"><div tabindex=\"-1\"><p>Long term averages and variations within the conditions of the atmosphere.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1719\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1719\"><div tabindex=\"-1\"><p>Data which is out of the ordinary and does not fit previous trends.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1702\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1702\"><div tabindex=\"-1\"><p>Wobbles in the Earth's axis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2577\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3364\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3364\"><div tabindex=\"-1\"><p>An acronym for SOund Navigation And Ranging, sonar uses sound waves to navigate and map surfaces.&nbsp; Sound waves created by an observer reflect off of surfaces and return to the observer.&nbsp; The amount of time it takes for the sound to return is a function of the distance the surface is from the observer.&nbsp; Bats use sonar to navigate through the dark.&nbsp; Ships use sonar to map the ocean floor.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2885\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3916\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3916\"><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_138_3085\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3085\"><div tabindex=\"-1\"><p>The location at the surface directly above the focus of an earthquake, typically associated with strong damage.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2678\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2579\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2579\"><div tabindex=\"-1\"><p>As a rock cools, the iron minerals within the rock align with the current magnetic field. Since the magnetic field changes (by where you are on Earth, by flips where \"north\"and \"south\" switch, and by migration of the magnetic north pole), scientists use the magnetic alignment within rocks to determine past movement or the magnetic field itself, along with the movement of rocks and plates via 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_138_2675\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2687\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2673\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3372\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3372\"><div tabindex=\"-1\"><p>The measure of degrees north or south from the equator, which has a latitude of 0 degrees.&nbsp; The Earth's north and south poles have latitudes of 90 degrees north and south, respectively.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_3087\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2586\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2202\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2202\"><div tabindex=\"-1\"><p>A stoney and\/or metallic object from our solar system which was never incorporated into a planet and has fallen onto Earth. Meteorite is used for the rock on Earth, meteoroid for the object in space, and meteor as the object travels in Earth's atmosphere.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2831\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2700\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2689\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2580\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1962\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1961\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2583\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2583\"><div tabindex=\"-1\"><p>A property of solids in which a force applied to an object causes the object to fracture, break, or snap. Most rocks, at low temperatures, are brittle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2582\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2582\"><div tabindex=\"-1\"><p>A property of a solid, such that when a force is applied, the solid flows, stretches, or bends along with the force, instead of cracking or breaking. For example, many plastics are ductile.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2584\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2584\"><div tabindex=\"-1\"><p>Short for&nbsp;Mohorovi\u010di\u0107 Discontinuity, it is the seismically-recognized layer within the Earth in which the crust ends and the mantle begins. Because the crust is very different in composition to the mantle, the moho is easy to find, since seismic waves travel differently through the two materials.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1924\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2589\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2622\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2622\"><div tabindex=\"-1\"><p>Rocks of the ocean floor, such as mid-ocean ridge rocks, which are brought to 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_138_2587\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2588\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1957\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3345\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3345\"><div tabindex=\"-1\"><p>Minerals with a luster similar to metal and contain metals, including valuable elements like lead, zinc, copper, tin, 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_138_2915\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2915\"><div tabindex=\"-1\"><p>Force applied to an object, typically dealing with forces 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_138_2590\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2592\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2593\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2594\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2594\"><div tabindex=\"-1\"><p>Also called lower mantle, a solid, more brittle physical layer&nbsp;of the Earth, below the asthenosphere.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2595\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2596\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2596\"><div tabindex=\"-1\"><p>The innermost physical layer of the Earth, which is solid.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2667\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2598\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2598\"><div tabindex=\"-1\"><p>A boundary between continental and oceanic plates that has no relative movement, making it a place where an oceanic plate is connected to a continental plate, but it is not a plate boundary.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2620\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2603\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2603\"><div tabindex=\"-1\"><p>Deepest part of the ocean where a subducting plate dives below the overriding plate.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2604\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2604\"><div tabindex=\"-1\"><p>Mix of sediments that form as a subducting plate descends and the overriding plate scrapes material and material is added.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2686\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2686\"><div tabindex=\"-1\"><p>A geological province which is added (accreted) to a continental mass via subduction and 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_138_2605\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2607\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2607\"><div tabindex=\"-1\"><p>The area of the mantle where volatiles rise from the slab, causing flux melting and 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_138_2606\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2608\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2890\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2890\"><div tabindex=\"-1\"><p>The entire area which is related to land-sea interactions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2703\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3194\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3065\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3065\"><div tabindex=\"-1\"><p>Planer feature where two blocks of bedrock move past each other 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_138_3098\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3098\"><div tabindex=\"-1\"><p>A measure of earthquake strength. Scales include Richter and Moment.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2614\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2614\"><div tabindex=\"-1\"><p>Any depression formed between the arc and the trench, commonly between the arc and the accretionary wedge.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2617\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1448\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1448\"><div tabindex=\"-1\"><p>A strain that occurs in a substance in which the item changes shape due to a stress.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1445\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2610\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2610\"><div tabindex=\"-1\"><p>Area behind the arc, which can be subject to compressional (causing thrusted mountain belts) or extensional (causing back-arc basins) 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_138_1446\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3074\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3074\"><div tabindex=\"-1\"><p>A low-angle reverse fault, common in mountain building.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2611\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2611\"><div tabindex=\"-1\"><p>Faulting that is not deep into the crust, and typically only involves sedimentary cover, not basement 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_138_2612\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2612\"><div tabindex=\"-1\"><p>Faulting that is deep into the crust, and typically involves crystalline basement 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_138_2585\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2585\"><div tabindex=\"-1\"><p>The process of uplifting mountain within mountain belts, primarily via tectonic movement. <strong>Orogenic belts<\/strong> are the mountain belts that result from these movements, and <strong>orogenesis<\/strong> is the name for the process of forming mountain belts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1440\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1440\"><div tabindex=\"-1\"><p>The last period of the Mesozoic, 145-66 million years ago.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2192\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2616\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2616\"><div tabindex=\"-1\"><p>Where an ocean plate subducts beneath a continental plate, causing a volcanic arc to form.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2618\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2618\"><div tabindex=\"-1\"><p>Where a dense ocean plate subducts beneath a less dense oceanic&nbsp;plate, causing an island arc to form.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2619\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2619\"><div tabindex=\"-1\"><p>Place where oceanic-oceanic subduction causes volcanoes to form on an overriding oceanic plate, making a chain of active 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_138_2623\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2623\"><div tabindex=\"-1\"><p>An arrangement of many continental masses collided together into one larger mass. According to the Wilson Cycle, this occurs every half billion years or so.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_3366\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3366\"><div tabindex=\"-1\"><p>The most recent supercontinent, which formed over 300 million years ago and started breaking apart less than 200 million years ago. Africa and South America, as well as Europe and North America, bordered 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_138_2211\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2211\"><div tabindex=\"-1\"><p>The supercontinent that existed before Pangea, about 1 billion years ago. North America was positioned in the center of the land 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_138_2621\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2621\"><div tabindex=\"-1\"><p>Process which allows a continental plate to bring up oceanic plate, frequently occurring in 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_138_2626\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2626\"><div tabindex=\"-1\"><p>A valley formed by normal faulting.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2625\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2625\"><div tabindex=\"-1\"><p>Uplifted mountain block caused by normal faulting.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_3071\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3071\"><div tabindex=\"-1\"><p>A valley formed by normal faulting on just one side.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2640\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2640\"><div tabindex=\"-1\"><p>The stable interior part of a continent, typically more than a billion years old, and sometimes as old as &nbsp;2.5-3 billion years. When exposed on the surface, a craton is called a shield.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1934\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2627\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2627\"><div tabindex=\"-1\"><p>A section of a rift that starts but does not complete. This typically occurs at 120\u00b0 angles to the active rift.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2628\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2628\"><div tabindex=\"-1\"><p>A depression that occurs in an area that was subject to earlier 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_138_3134\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2629\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2629\"><div tabindex=\"-1\"><p>Place where three plate boundaries (typically divergent) extend&nbsp;from a single point at 120\u00b0 angles.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2479\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2479\"><div tabindex=\"-1\"><p>Steep spire carved by several glaciers.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2462\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1176\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3365\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1917\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1956\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2884\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2884\"><div tabindex=\"-1\"><p>The deep, flat part of the ocean. Also known as the ocean floor.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2921\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2815\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1184\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3076\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3076\"><div tabindex=\"-1\"><p>Faulting that occurs with shear forces, typically on vertical fault plaines as two fault blocks slide past 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_138_3077\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3077\"><div tabindex=\"-1\"><p>Movement in a transform or strike-slip setting which it toward the right across the fault.&nbsp;As viewed across the fault, objects will move to the right.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2632\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2632\"><div tabindex=\"-1\"><p>A strike-slip or transform motion in which the relative motion is to the left. As viewed across the fault, objects will move to the 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_138_3367\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3367\"><div tabindex=\"-1\"><p>Fault, or movement along a fault, that does not have earthquake activity.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2634\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2634\"><div tabindex=\"-1\"><p>A segment along a transform or strike-slip fault which has a compressional component, sometimes creating related thrust faulting and mountains.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2635\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2635\"><div tabindex=\"-1\"><p>A place along a transform or strike-slip fault with an extensional component, sometimes including normal faulting, basin formation, and 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_138_2636\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2636\"><div tabindex=\"-1\"><p>An object that is cut by a fault which allows the amount of movement to be determined. This is useful for all faults, but more commonly used in strike-slip 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_138_2664\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1708\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2639\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1187\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2899\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3096\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3096\"><div tabindex=\"-1\"><p>A process of using 3D seismic arrays to get subsurface images.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1197\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1188\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1953\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1949\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1441\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_1441\"><div tabindex=\"-1\"><p>The last (and current) era of the Phanerozoic eon, starting 66 million years ago and spanning through the present.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2609\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2609\"><div tabindex=\"-1\"><p>A chain of volcanic activity, typically in a curved pattern, rising from a subduction zone. The arc is on the overriding plate, typically a few hundred kilometers from the trench, but parallel to the trench.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_1173\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3089\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3089\"><div tabindex=\"-1\"><p>Seismic waves that only move along the surface, mainly R waves and L waves.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_3103\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_3103\"><div tabindex=\"-1\"><p>An amplification of earthquake waves due to a structure of buildings or structures.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_3080\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_3368\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_1921\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_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_138_2883\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2883\"><div tabindex=\"-1\"><p>Places that are under ocean water at all times.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_138_2842\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_138_2842\"><div tabindex=\"-1\"><p>A chemical sedimentary rock that forms as water evaporates.<\/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":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-138","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":19,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/138","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\/138\/revisions"}],"predecessor-version":[{"id":3403,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapters\/138\/revisions\/3403"}],"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\/138\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/media?parent=138"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/pressbooks\/v2\/chapter-type?post=138"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/contributor?post=138"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accintrogeology\/wp-json\/wp\/v2\/license?post=138"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}