{"id":103,"date":"2024-03-08T22:50:41","date_gmt":"2024-03-08T22:50:41","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/ppscgey1108geologyofnationalparks\/chapter\/welcome-to-geology-of-national-parks\/"},"modified":"2024-03-28T22:19:26","modified_gmt":"2024-03-28T22:19:26","slug":"welcome-to-geology-of-national-parks","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/ppscgey1108geologyofnationalparks\/chapter\/welcome-to-geology-of-national-parks\/","title":{"raw":"Introduction to Geology","rendered":"Introduction to Geology"},"content":{"raw":"
\r\n

What is Geology?<\/h2>\r\n

In its broadest sense, geology is the study of Earth\u2014its interior and its exterior surface, the minerals, rocks and other materials that are around us, the processes that have resulted in the formation of those materials, the water that flows over the surface and through the ground, the changes that have taken place over the vastness of geological time, and the changes that we can anticipate will take place in the near future. Geology is a science, meaning that we use deductive reasoning and scientific methods to understand geological problems. It is, arguably, the most integrated of all of the sciences because it involves the understanding and application of all of the other sciences: physics, chemistry, biology, mathematics, astronomy, and others. But unlike most of the other sciences, geology has an extra dimension, that of time\u2014deep time\u2014billions of years of it. Geologists study the evidence that they see around them, but in most cases, they are observing the results of processes that happened thousands, millions, and even billions of years in the past. Those were processes that took place at incredibly slow rates\u2014millimetres per year to centimetres per year\u2014but because of the amount of time available, they produced massive results.<\/p>\r\n

Geology is also about understanding the evolution of life on Earth; about discovering resources such as water, metals and energy; about recognizing and minimizing the environmental implications of our use of those resources; and about learning how to mitigate the hazards related to earthquakes, volcanic eruptions, and slope failures.<\/p>\r\n\r\n

What are scientific methods?<\/h2>\r\n

There is no single method of inquiry that is specifically the \u201cscientific method\u201d; furthermore, scientific inquiry is not necessarily different from serious research in other disciplines. The most important thing that those involved in any type of inquiry must do is to be skeptical. As the physicist Richard Feynman once said: the first principle of science is that \u201cyou must not fool yourself\u2014and you are the easiest person to fool.\u201d A key feature of serious inquiry is the creation of a hypothesis (a tentative explanation) that could explain the observations that have been made, and then the formulation and testing (by experimentation) of one or more predictions that follow from that hypothesis.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"321\"]\"Figure Figure 1.1.2: A group of people peer down at rocks in a stream[\/caption]\r\n

For example, we might observe that most of the cobbles in a stream bed are well rounded (see Figure 1.1.2<\/strong>), and then derive the hypothesis that the rocks are rounded by transportation along the stream bed. A prediction that follows from this hypothesis is that cobbles present in a stream will become increasingly rounded as they are transported downstream. An experiment to test this prediction would be to place some angular cobbles in a stream, label them so that we can be sure to find them again later, and then return at various time intervals (over a period of years) to carefully measure their locations and roundness.<\/p>\r\n

A critical feature of a good hypothesis and any resulting predictions is that they must be testable. For example, an alternative hypothesis to the one above is that an extraterrestrial organization creates rounded cobbles and places them in streams when nobody is looking. This may indeed be the case, but there is no practical way to test this hypothesis. Most importantly, there is no way to prove that it is false, because if we aren\u2019t able to catch the aliens at work, we still won\u2019t know if they did it!<\/p>\r\n\r\n

Minerals and Rocks<\/h2>\r\n

The Earth is made up of varying proportions of the 90 naturally occurring elements\u2014hydrogen, carbon, oxygen, magnesium, silicon, iron, and so on. In most geological materials, these combine in various ways to make minerals.<\/p>\r\n

A mineral is a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice. There are thousands of minerals. In nature, minerals are found in rocks, and the vast majority of rocks are composed of at least a few different minerals. A close-up view of granite, a common rock, is shown in Figure 1.4.2<\/strong>. Although a hand-sized piece of granite may have thousands of individual mineral crystals in it, there are typically only a few different minerals, as shown here.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"565\"]\"A Figure 1.4.2 A close-up view of the rock granite and some of the minerals that it typically contains (H = hornblende (amphibole), Q = quartz and F = feldspar). The crystals range from about 0.1 to 3 millimetres (mm) in diameter. Most are irregular in outline, but some are rectangular.[\/caption]\r\n

Rocks can form in a variety of ways. Igneous rocks form from magma (molten rock) that has either cooled slowly underground (e.g., to produce granite) or cooled quickly at the surface after a volcanic eruption (e.g. basalt). Sedimentary rocks, such as sandstone, form when the weathered products of other rocks accumulate at the surface and are then buried by other sediments. Metamorphic rocks form when either igneous or sedimentary rocks are heated and squeezed to the point where some of their minerals are unstable and new minerals form to create a different type of rock. An example is schist.<\/p>\r\n

A critical point to remember is the difference between a mineral and a rock. A mineral is a pure substance with a specific composition and structure, while a rock is typically a mixture of several different minerals (although a few types of rock may include only one type of mineral). Examples of minerals are feldspar, quartz, mica, halite, calcite, and amphibole. Examples of rocks are granite, basalt, sandstone, limestone, and schist.<\/p>\r\n\r\n<\/div>\r\n

\r\n

The Rock Cycle<\/h2>\r\n

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle (Figure 3.1.1<\/strong>). The rock cycle is driven by two forces: (1) Earth\u2019s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.<\/p>\r\n

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.<\/p>\r\n\r\n\r\n[caption id=\"attachment_219\" align=\"aligncenter\" width=\"501\"]\"Illustration Figure 3.1.1 A schematic view of the rock cycle.[\/caption]\r\n\r\n

Figure 3.1.1<\/strong> image description: The rock cycle takes place both above and below the Earth\u2019s surface. The rock deepest beneath the earth\u2019s surface, and under extreme heat and pressure, is metamorphic rock. This metamorphic rock can melt and become magma. When magma cools below the earth\u2019s surface, it becomes \u201cintrusive igneous rock.\u201d If magma cools above the earth\u2019s surface, it is \u201cextrusive igneous rock\u201d and becomes part of the outcrop. The outcrop is subject to weathering and erosion, and can be moved and redeposited around the earth by forces such as water and wind. As the outcrop is eroded, it becomes sediment which can be buried, compacted, and cemented beneath the Earth\u2019s surface to become sedimentary rock. As sedimentary rock gets buried deeper and comes under increased heat and pressure, it returns to its original state as metamorphic rock. Rocks in the rock cycle do not always make a complete loop. It is possible for sedimentary rock to be uplifted back above the Earth\u2019s surface and for intrusive and extrusive igneous rock to be reburied and become metamorphic rock.<\/div>\r\n

In describing the rock cycle, we can start anywhere we like, although it\u2019s convenient to start with magma. As we\u2019ll see in more detail below, magma is rock that is hot to the point of being entirely molten, with a temperature of between about 800\u00b0 and 1300\u00b0C, depending on the composition and the pressure.<\/p>\r\n\r\n\r\n[caption id=\"attachment_228\" align=\"alignleft\" width=\"300\"]\"Red Figure 3.1.2 Magma forming pahoehoe basalt at Kilauea Volcano, Hawaii.[\/caption]\r\n

Magma can either cool slowly within the crust (over centuries to millions of years)\u2014forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years)\u2014forming extrusive igneous rock (volcanic rock) (Figure 3.1.2<\/strong>). Intrusive igneous rock typically crystallizes at depths of hundreds of metres to tens of kilometres below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and then exposed by the erosion of the overlying rocks.<\/p>\r\n\r\n\r\n[caption id=\"attachment_229\" align=\"alignright\" width=\"347\"]\"Marine Figure 3.1.3 Cretaceous-aged marine sandstone overlying marine mudstone, Gabriola Island, B.C.[\/caption]\r\n

Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products\u2014mostly small rock and mineral fragments\u2014are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.<\/p>\r\n

Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of metres or more, they become compressed and cemented into sedimentary rock (See Figure 3.1.3<\/strong> for example). Again through various means, largely resulting from plate-tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock (Figure 3.1.4<\/strong>)<\/span><\/p>\r\n\r\n\r\n[caption id=\"attachment_230\" align=\"aligncenter\" width=\"483\"]\"Grey Figure 3.1.4 Metamorphosed and folded Triassic-aged limestone, Quadra Island, B.C.[\/caption]\r\n\r\n<\/div>\r\n \r\n

\r\n\r\n \r\n\r\nFundamentals of Plate Tectonics<\/span>\r\n\r\n<\/div>\r\n
\r\n

Plate Tectonics is the model or theory that has been used for the past 60 years to understand and explain how the Earth works\u2014more specifically the origins of continents and oceans, of folded rocks and mountain ranges, of earthquakes and volcanoes, and of continental drift.<\/p>\r\n\r\n\r\n[caption id=\"attachment_275\" align=\"alignright\" width=\"159\"]\"Illustration Figure 1.5.1 The components of the interior of the Earth[\/caption]\r\n

Key to understanding plate tectonics is an understanding of Earth\u2019s internal structure, which is illustrated in Figure 1.5.1<\/strong>. Earth\u2019s core consists mostly of iron. The outer core is hot enough for the iron to be liquid. The inner core\u2014although even hotter\u2014is under so much pressure that it is solid. The mantle is made up of iron and magnesium minerals. The bulk of the mantle surrounding the outer core is solid rock, but is plastic enough to be able to flow slowly. The outermost part of the mantle is rigid. The crust \u2014composed mostly of granite on the continents and mostly of basalt beneath the oceans\u2014is also rigid. The crust and outermost rigid mantle together make up the lithosphere. The lithosphere is divided into about 20 tectonic plates that move in different directions on Earth\u2019s surface.<\/p>\r\n

An important property of Earth (and other planets) is that the temperature increases with depth, from close to 0\u00b0C at the surface to about 7000\u00b0C at the centre of the core. In the crust, the rate of temperature increase is about 30\u00b0C every kilometre. This is known as the geothermal gradient.<\/p>\r\n

Heat is continuously flowing outward from Earth\u2019s interior, and the transfer of heat from the core to the mantle causes convection in the mantle (Figure 1.5.2<\/strong>). This convection is the primary driving force for the movement of tectonic plates. At places where convection currents in the mantle are moving upward, new lithosphere forms (at ocean ridges), and the plates move apart (diverge). Where two plates are converging (and the convective flow is downward), one plate will be subducted (pushed down) into the mantle beneath the other. Many of Earth\u2019s major earthquakes and volcanoes are associated with convergent boundaries.<\/p>\r\n\r\n\r\n[caption id=\"attachment_237\" align=\"aligncenter\" width=\"300\"]\"Illustration Figure 1.5.2 Depiction of the convection in the mantle and its relationship to plate motion[\/caption]\r\n

Plates, Plate Motions, and Plate Boundary Processes<\/h2>\r\n

Alfred Wegener (1880-1930) earned a PhD in astronomy at the University of Berlin in 1904, but he had always been interested in geophysics and meteorology and spent most of his academic career working in meteorology. In 1911 he happened on a scientific publication that included a description of the existence of matching Permian-aged terrestrial fossils in various parts of South America, Africa, India, Antarctica, and Australia (Figure 10.1.2<\/strong>).<\/p>\r\n\r\n\r\n[caption id=\"attachment_286\" align=\"aligncenter\" width=\"514\"]\"Map Figure 10.1.2 The distribution of several Permian terrestrial fossils that are present in various parts of continents that are now separated by oceans.[\/caption]\r\n\r\n

Figure 10.1.2 image description:<\/strong> Fossils found across different continents suggest that these continents were once joined as a super-continent. Fossil remains of Cynognathus (a terrestrial reptile) and Mesosaurus (a freshwater reptile) have been found in South America and Africa. Fossil evidence of the Lystrosaurus, a land reptile from the Triassic period, has been found in India, Africa, and Antarctica. Fossils of the fern Glossopteris have been found in Australia, Antarctica, India, Africa, and South America. When you position these continents so they fit together, the areas where these fossils were found line up.<\/div>\r\n

Wegener concluded that this distribution of terrestrial organisms could only exist if these continents were joined together during the Permian, and he coined the term Pangea (\u201call land\u201d) for the supercontinent that he thought included all of the present-day continents.<\/p>\r\n

In 1960, Harold Hess, a widely respected geologist from Princeton University, advanced a theory with many of the elements that we now accept as plate tectonics. Hess proposed that new sea floor was generated from mantle material at the ocean ridges, and that old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle. He suggested that the process was driven by mantle convection currents, rising at the ridges and descending at the trenches (Figure 10.3.8<\/strong>). He also suggested that the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding land masses were thrust up to form mountains. Hess\u2019s theory formed the basis for our ideas on sea-floor spreading and continental drift, but it did not deal with the concept that the crust is made up of specific plates. Although the Hess model was not roundly criticized, it was not widely accepted (especially in the U.S.), partly because it was not well supported by hard evidence.<\/p>\r\n\r\n\r\n[caption id=\"attachment_204\" align=\"aligncenter\" width=\"687\"]\"Illustration Figure 10.3.8 A representation of Harold Hess\u2019s model for sea-floor spreading and subduction[\/caption]\r\n

Continental drift and seafloor spreading became widely accepted around 1965 as more and more data became available and geologists started thinking in these terms. By the end of 1967 the Earth\u2019s surface had been mapped into a series of plates (Figure 10.4.1<\/strong>). The major plates are Eurasia, Pacific, India, Australia, North America, South America, Africa, and Antarctic. There are also numerous small plates (e.g., Juan de Fuca, Cocos, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. For example, the Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer) that all move in the same general direction but at slightly different rates.<\/p>\r\n\r\n[caption id=\"attachment_262\" align=\"aligncenter\" width=\"1104\"]\"A Figure 10.4.1 A map showing 15 of the Earth\u2019s tectonic plates and the approximate rates and directions of plate motions. [Image Description below.][\/caption]Figure 10.4.1 image description: Descriptions of 15 different plates and their movements<\/strong>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
\r\n

Plate name<\/strong><\/p>\r\n<\/th>\r\n

\r\n

Description of plate<\/strong><\/p>\r\n<\/th>\r\n

\r\n

Bordering plates (ordered from longest border to shortest)<\/strong><\/p>\r\n<\/th>\r\n

\r\n

Description of movement<\/strong><\/p>\r\n<\/th>\r\n<\/tr>\r\n

\r\n

Africa plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes all of Africa and the surrounding ocean, including the eastern Atlantic Ocean, the surrounding Antarctic Ocean, and the western Indian ocean.<\/p>\r\n<\/td>\r\n

\r\n

South America plate, Antarctic plate, Eurasia plate, North America plate, Arabia plate, India plate, Australia plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north east towards the Arabia and Eurasia plates.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Antarctic plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate makes up all of Antarctica and much of the surrounding ocean.<\/p>\r\n<\/td>\r\n

\r\n

Pacific plate, Australia plate, Africa plate, Scotia plate, Nazca plate, South America plate.<\/p>\r\n<\/td>\r\n

\r\n

The part of the plate around the South America plate is moving northwards and a little east. The part of the plate around the Australia plate is moving southwards.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Arabia plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes all of Saudi Arabia, and much of the Levant (up to Iraq and Syria).<\/p>\r\n<\/td>\r\n

\r\n

Eurasia plate, Africa plate, India plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north east towards the Eurasia plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Australia plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes Australia and much of the surrounding ocean. New Guinea and the northern parts of New Zealand are part of the Australia plate. The ocean area along southern Asia up to the India plate is also a part of the Australia plate.<\/p>\r\n<\/td>\r\n

\r\n

Antarctic plate, Pacific plate, Eurasia plate, India Plate, Africa plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north east towards the Eurasia plate and the Pacific plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Caribbean plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is small. It includes the central Caribbean countries and runs along the northern edge of South America.<\/p>\r\n<\/td>\r\n

\r\n

North America plate, South America plate, Cocos plate.<\/p>\r\n<\/td>\r\n

\r\n

N\/A<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Cocos plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is small. It runs along the west coast of Mexico and western Caribbean countries.<\/p>\r\n<\/td>\r\n

\r\n

Nazca plate, Pacific plate, North America plate, Caribbean plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north east towards the Caribbean and North America plates.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Eurasia plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes the northeastern part of the Atlantic Ocean, all of Europe, all of Russia (except its most eastern part), and down through southeast Asia, including China and Indonesia.<\/p>\r\n<\/td>\r\n

\r\n

North America plate, Africa plate, Australia plate, Arabia plate, India plate, Filipino plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate is rotating in a clockwise direction towards the Pacific plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Filipino plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes the islands that make up the Philippines and north to include parts of southern Japan.<\/p>\r\n<\/td>\r\n

\r\n

Eurasia plate, Pacific plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north west towards the Eurasia plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

India plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes India and the surrounding India Ocean.<\/p>\r\n<\/td>\r\n

\r\n

Australia plate, Eurasia plate, Africa plate, Arabia plate.<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving north-north east towards the Eurasia plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Juan de Fuca plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is small. It runs along the north western coast of the United States and the southern British Columbia coast.<\/p>\r\n<\/td>\r\n

\r\n

Pacific plate, North America plate.<\/p>\r\n<\/td>\r\n

\r\n

N\/A<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Nazca plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is in the Pacific Ocean between the Pacific plate and the South America plate.<\/p>\r\n<\/td>\r\n

\r\n

South America plate, Pacific plate, Antarctic plate, Cocos plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving directly east towards the South America plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

North America Plate<\/p>\r\n<\/td>\r\n

\r\n

This plate includes all of North America, Greenland, the eastern most part of Russia, northern Japan, and the northwestern part of the Atlantic Ocean.<\/p>\r\n<\/td>\r\n

\r\n

Eurasia plate, Pacific plate, Africa plate, Caribbean plate, South America plate, Cocos plate, Juan de Fuca plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is rotating counter clockwise in towards the Pacific plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Pacific plate<\/p>\r\n<\/td>\r\n

\r\n

This plate makes up most of the Pacific Ocean.<\/p>\r\n<\/td>\r\n

\r\n

North America plate, Australia plate, Antarctic plate, Nazca plate, Filipino plate, Cocos plate, Juan de Fuca plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is moving northwest towards the Australia, Filipino, and Eurasia plates.<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

Scotia plate<\/p>\r\n<\/td>\r\n

\r\n

This plate is small. It runs from the tip of South America eastwards to form a barrier between the Antarctic plate and the South America plate.<\/p>\r\n<\/td>\r\n

\r\n

Antarctic plate, South America plate.<\/p>\r\n<\/td>\r\n

\r\n

N\/A<\/p>\r\n<\/td>\r\n<\/tr>\r\n

\r\n

South America plate<\/p>\r\n<\/td>\r\n

\r\n

This plate starts at the western edge of South America and stretches east into the southwestern parst of the Atlantic Ocean.<\/p>\r\n<\/td>\r\n

\r\n

Africa plate, Nazca plate, Scotia plate, Caribbean plate, Antarctic plate, North America plate<\/p>\r\n<\/td>\r\n

\r\n

This plate moves north and slightly west towards the Caribbean plate and the North America plate.<\/p>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Check out how Earth\u2019s tectonic plates have moved over the past 200 million years: Continents collide and break apart over time<\/span><\/a> (2:09).<\/p>\r\n[embed]https:\/\/www.youtube.com\/watch?v=ADsjdu27WaM[\/embed]\r\n\r\nWant to see how things have changed closer to home? Check out How North American got its Shape<\/span><\/a> (4:58).\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=jzqnUvE66HA[\/embed]\r\n

Rates of motions of the major plates range from less than 1 cm\/y to over 10 cm\/y. The Pacific Plate is the fastest, followed by the Australian and Nazca Plates. The North American Plate is one of the slowest, averaging around 1 cm\/y in the south up to almost 4 cm\/y in the north.<\/p>\r\n

Plates move as rigid bodies, so it may seem surprising that the North American Plate can be moving at different rates in different places. The explanation is that plates move in a rotational manner. The North American Plate, for example, rotates counter-clockwise; the Eurasian Plate rotates clockwise.<\/p>\r\n

Boundaries between the plates are of three types: divergent (i.e., moving apart), <\/em>convergent <\/em>(i.e., moving together), and transform (moving side by side). The plates are made up of crust and the lithospheric part of the mantle (Figure 10.4.2)<\/strong>, and even though they are moving all the time, and in different directions, there is never a significant amount of space between them. Plates are thought to move along the lithosphere-asthenosphere boundary, as the asthenosphere is the zone of partial melting. It is assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates.<\/p>\r\n\r\n\r\n[caption id=\"attachment_263\" align=\"aligncenter\" width=\"780\"]\"Illustration Figure 10.4.2 The crust and upper mantle. Tectonic plates consist of lithosphere, which includes the crust and the lithospheric (rigid) part of the mantle.[\/caption]\r\n

It is possible for a single plate to be made up of both oceanic and continental crust. For example, the North American Plate includes most of North America, plus half of the northern Atlantic Ocean. Similarly, the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean. The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault.<\/p>\r\n\r\n

Divergent Boundaries<\/h2>\r\n

Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure 10.4.3<\/strong>). Most divergent boundaries are located at the oceanic ridges (although some are on land), and the crustal material created at a spreading boundary is always oceanic in character. Spreading rates vary considerably, from 2 cm\/y to 6 cm\/y in the Atlantic, to between 12 cm\/y and 20 cm\/y in the Pacific.<\/p>\r\n

Some of the processes taking place in this setting include:<\/p>\r\n\r\n