The shape and position of the continents have changed dramatically over the last billion years, and geologic processes continue these changes today. The Earth's outer layer—the crust—is dynamic, consisting of constantly moving plates that are made of a rigid continental and oceanic lithosphere overlying a churning, plastically flowing asthenosphere (the outer part of the Earth's mantle). These plates are slowly pulling apart, colliding, or sliding past one another with great force, creating strings of volcanic islands, new ocean floor, earthquakes, and mountains. The continents are likewise continuously shifting position relative to each other. These movements not only shape the land, but also affect the distribution of rocks and minerals, natural resources, climate, and life on Earth. Active plate margins are the boundaries between two plates of the Earth's crust that are colliding, pulling apart, or sliding past each other as they move over the mantle. The three major types of plate boundaries are:
World map showing the boundaries of the tectonic plates. The North American Continent takes up only part of the North American plate (brown). The Mid-Atlantic Ridge, a spreading center (in other words, a place where plates are moving away from one another) is also labelled. The arrows indicate the directions of plate movement. Convergent boundaries are indicated by arrows pointing toward each other, divergent boundaries are indicated by arrows pointing away from each other, and transform boundaries are indicated by arrows pointed in opposite directions parallel to the plate margin. Map modified from USGS tectonic plates map, via Wikimedia Commons (public domain).
The lithosphere is the outermost layer of the Earth. It is rigid layer of crust and upper mantle that is broken up into large fragments called plates. The asthenosphere is the layer of the Earth below the lithosphere. Although the rock of the asthenosphere would seem very solid if you could look at it, under long-term stress it slowly bends and flows, like very thick syrup. The boundary between the rigid lithosphere and flowing asthenosphere is found in the mantle, the region between the Earth's crust and its core. This transition from lithosphere to asthenosphere is caused by increasing temperature, which is correlated with increasing depth beneath the Earth's surface. In tectonically active regions of extension (essentially, regions where crust is stretching and new crust is formed), such as mid-ocean ridges, temperature rises rapidly with depth. Thus, the asthenosphere begins nearly at the base of the crust. In more tectonically stable regions, temperature increases more slowly with depth. In these regions, the asthenosphere may begin deeper within the mantle. A cross-section of the Earth showing the layers from the center (inner core) to the surface (lithosphere). Note that the solid crust that makes up the lithosphere sits on the outer part of the mantle, which is called the asthenosphere. The asthenosphere exhibits plastic flow, like a very thick liquid. Over time, this allows the plates of the lithosphere to shift and move on the asthenosphere. Image modified from original by Jim Houghton, first published in The Teacher-Friendly Guide to Earth Science series published by the Paleontological Research Institution (CC BY-NC-SA 4.0 license). The difference between crust and mantle is mainly chemical. The lithosphere's composition typically varies from mafic basalt in oceanic crust to felsic granite in continental crust. The mantle is composed of homogenous ultramafic material. The differences among felsic, mafic, and ultramafic rocks are as follows:
For further discussion of igneous rock classification, see Here on Earth: Introduction to Rocks. Felsic minerals and rocks. Left: A herkimer diamond, not a true diamond but a type of quartz found in the region of Herkimer, New York. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized). Right: Salisbury pink granite from North Carolina. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped). Recently-erupted basalt lava (a mafic igneous rock) on the Big Island of Hawai'i. Photograph by Jonathan R. Hendricks ([email protected]).
Subduction is the process by which one plate moves under another (is subducted), sinking into the mantle. The process of subduction often triggers volcanism and mountain-building on the overriding plate. Collisions between oceanic and continental platesContinental crust, made mostly of rocks such as granite, is less dense but much thicker than oceanic crust. Oceanic crust is made mostly of dense rocks such as basalt. The higher density of oceanic crust means that when continental crust collides with oceanic crust, the denser oceanic crust will usually be dragged (or subducted) under the buoyant continental crust. The subduction of an oceanic plate beneath a continental plate as they collide produces an accretionary wedge. These rocks, which include deep-sea sediments overlying the crust, the oceanic crust itself, and rock from the upper mantle, are scraped off the descending plate and attached to the continental crust. If the rocks are forced up onto the continental crust and exposed at the surface, the are called ophiolites. On the oceanic side of the accretionary wedge is a trench where the subducting plate is descending. Downwarping of the crust on the continental side of the accretionary wedge produces a forearc basin. Simplified diagram of a convergent boundary at which continental crust and oceanic crust are colliding. The denser oceanic crust is subducted, or forced under, the more buoyant continental crust. The oceanic crust melts as it is subducted, and an accretionary wedge forms where the oceanic plate is scraped against the continental crust; in front of the wedge is a trench. Subduction of the oceanic crust produces magma, which wells up, causing volcanic activity. Image modified for [email protected] from original by Jim Houghton, first published in The Teacher-Friendly Guide series (CC BY-NC-SA 4.0 license). As the subducting plate is carried deep into the mantle, seawater and sediment carried down with the plate lower the melting point of the surrounding mantle rock. The melted mantle rock, or magma, is hot and buoyant, so it rises to the surface and creates volcanoes. The Cascade volcanoes that occur in the northwestern United States (Oregon, Washington, and northern California) are the result of subduction at a convergent boundary between an oceanic plate (the Juan de Fuca plate) and a continental plate (the North American plate). These volcanoes are some of the most famous in the U.S., including Mt. Hood, Mt. St. Helens, Mt. Rainier, Mt. Shasta, and Crater Lake. The volcanoes that form due to subduction are stratovolcanoes. A stratovolcano is a conical volcano made up of many lava flows as well as layers of ash and breccia from explosive eruptions. In fact, stratovolcanoes are often characterized by their periodic violent eruptions. While young stratovolcanoes tend to have steep cone shapes, the symmetrical shape is easily changed by massive eruptions, such as at Mt. St. Helens in 1980. Perhaps one of the most dramatic subduction zones formed by the convergence of a continental and an oceanic plate is found along the western coast of South America. Here, the Nazca Plate is being subducted beneath the South American Plate. As a result, the Andes Mountains have formed. The highest peak in the Andes and in South America is Aconcagua in Argentina; it is more than 6,900 meters (over 22,500 feet) tall. Mount Hood in Oregon is a stratovolcano in the Cascades. Source: U.S. Forest Service-Pacific Region (flickr, public domain). Explosive eruptions of Mt. St. Helens, a stratovolcano in the Cascades, Washington, on May 18, 1980. This eruption, possibly the most well-know in U.S. history, blew apart a large portion of the volcano's summit and killed 57 people. Source: Washington DNR (flickr, Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license). Left: Relief map of South America showing the Peru-Chile Trench off the west coast of South America and the Andes Mountains that stretch the length of the western side of the continent. Source: NOAA ETOPO1 Global Relief Model (doi: 10.7289/V5C8276M). Right: Aconcagua (foreground). Photo by Bjørn Christian Tørrissen (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized).
Islands sometimes collide with continents. Such islands occur as fragments of land on oceanic plates. As an oceanic plate is subducted beneath a continental plate, any islands on the oceanic plate will slowly creep towards the subduction zone at the edge of the continental crust. When the island gets close enough, it collides with and sticks to the edge of the continent in a process known as docking. Thus, continents can grow by accretion, meaning that they grow in area by adding small bits of land to their edges over time. Islands may be volcanic or continental in origin. Volcanic islands may form over hot spots or regions where oceanic plates collide, causing subduction and volcanism (both scenarios are discussed further below). Some islands are exotic terranes. An exotic terrane is a piece of crustal material that has broken off from its parent continent and has become attached to another plate. Because an exotic terrane originated on another continent, it will have distinctly different geologic characteristics than the surrounding rocks. For example, in the Ordovician, the Carolina Terrane collided with and attached to what is now part of the southeastern U.S., forming the Carolina Slate Belt and the Talladega Slate Belt. Florida also began as an exotic terrane, although it was not an island. It became attached to North America when North America collided with the large southern continent of Gondwana in the late Paleozoic. Idealized diagram showing the docking and accretion of a volcanic island at the edge of a continent. Upper diagram (1): A volcanic island (center) on oceanic crust moves toward the edge of a large continent (right). As the oceanic crust is subducted, an accretionary wedge forms at the edge of the continent and the melting oceanic crust in the subduction zone produces magma, which rises through the continental crust and forms a volcanic mountain range. Lower diagram (2): The volcanic island has docked on the edge of the continent. The active subduction zone has moved seaward, meaning that the original continental volcanic mountain range has gone extinct and an active continental volcanic mountain range has formed near the new subduction zone. The old accretionary wedge is sandwich between the extinct volcanic mountain range and the new active continental volcanic mountain range. A new accretionary wedge has formed at the active subduction zone. Image modified for [email protected] from original by Frank Granshaw, first published in The Teacher-Friendly Guide to the Geology of the Western US (CC BY-NC-SA 4.0 license).
Volcanic islands are common at subduction zones between colliding oceanic plates, where one oceanic plate is subducted beneath the other. As the plates press together, friction between them generates enough heat and pressure to melt some of the crust. The molten rock rises through the crust and creates volcanoes along the edge of the overriding plate. Because island arc volcanoes mix the more mafic composition of the ocean floor with the more felsic composition of overlying sediment from continents, they are usually of "intermediate" composition along the mafic-to-felsic spectrum. Volcanic islands association with collisions between oceanic plates frequently form in curved lines, and are therefore called island arcs. The Aleutian Islands, Philippine Islands, and Lesser Antilles are all modern examples of volcanic island arcs associated with subduction zones. The Mariana Volcanic Arc in the southwestern Pacific is on a subduction zone between the ocean crusts of the smaller Mariana Plate or Phillipine Plate (the overriding plate) and the giant Pacific Plate (the subducting plate). This arc is made up of undersea volcanoes, with the tops of only a few, like Asuncion and Pagan, emerging above sea level. (The better-known islands in the Marianas like Guam and Saipan are not part of the volcanic arc, but were created by deposits from the forearc.) The Mariana Trench that is part of this subduction zone is the deepest part of the any of the world's oceans, reaching more than 10,900 meters (more than 35,800 feet) in depth. Coastal relief map of southern Alaska. The subduction trench on the border of the Aleutian volcanic island arc is clearly visible as a curving, dark blue line. The trench parallels the southern edge of the Aleutian Island chain and diverges slightly seaward as it approaches mainland Alaska (top center of the image). Source: NOAA (Lim, E., B.W. Eakins, and R. Wigley, Coastal Relief Model of Southern Alaska: Procedures, Data Sources and Analysis, NOAA Technical Memorandum NESDIS NGDC-43, 22 pp., August 2011). The Mariana Trench is a trench associated with a convergent boundary between the ocean crusts of the overriding Mariana Plate and the subducting Pacific Plate. The island of Guam in the Pacific is part of a volcanic island arc associated with this subduction zone. The photo above right shows the underwater eruption of NW Rota-1, an underwater volcano, at a depth of 560 meters (nearly 1840 feet). Source for map: NOAA/PMEL, NSF, "Submarine Ring of Fire 2014-Ironman (used following NOAA Ocean Exploration Editorial Policy). Source for photo: NOAA Photo Library (flickr, Creative Commons Attribution 2.0 Generic license, image slightly cropped). Photos of North Pagan, Mariana Volcanic Arc, 1981. Pagan includes two volcanoes (North Pagan and South Pagan) connected by an isthmus of land. Left: View of North Pagan. Photo by Norm Banks, USGS, Global Volcanism Program Image GVP-02127 (Smithsonian Institution National Museum of Natural History, public domain). Right: Lava flow (at the base of the crater where smoke is rising) near a lagoon on North Pagan. The surface of the water is covered with black volcanic material from an eruption. Photo by Norm Banks, USGS, Global Volcanism Program Image GVP-02111 (Smithsonian Institution National Museum of Natural History, public domain).
Although mountains are created when oceanic and continental crust collide due to the compression of the two plates in the collision zone, taller ranges are usually produced by continent-to-continent collisions. This is because when two buoyant continental crusts collide, there is nowhere for the crust to go but up. The modern Himalayas, at the collision site of the Eurasian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts. The central and southern Applachian Mountains were also formed by a continent-to-continent collision. Although these mountains may have once been as tall as the Himalayas, millennia of weathering and erosion have worn them down. A convergent boundary at which two continents are colliding (the two continents are colored two shades of brown in the diagram). Image modified from original by Jim Houghton, first published in The Teacher-Friendly Guide to the Geology of the Southeastern U.S., 2nd ed., edited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published by the Paleontological Research Institution) (CC BY-NC-SA 4.0 license). Left: The changing positions of the Indian subcontinent in the Late Cretaceous (70 million years ago), the Paleogene (50 million years ago) and today. Illustration from Kious and Tilling (1996) This Dynamic Earth: the Story of Plate Tectonics (USGS). Right: Map of the modern Himalayan Mountains in Asia. These mountains formed when the Indian subcontinent collided with Asia. Figure 1 from Manish and Pandit (2018) PeerJ 6:e5919 (Creative Commons Attribution 4.0 International license, image resized). The Himalayan Mountains are at a convergent boundary were the Eurasian Plate is colliding with the Indian Plate. This satellite image shows the two plates and the mountains forming as they collide. Image modified from original by Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC (Nasa Visible Earth, used following image use policy).
Divergent plate boundaries exist where tectonic plates are under tension; essentially, when plates are being pulled apart. This tension leads to the upwelling of magma, which forces its way through the separating crust onto the surface. In continental settings, the tension causes faulting and shallow earthquakes. As the faults pull apart and thin the crust, the decrease in overlying pressure can result in the formation of small volcanoes called cinder cones. In oceanic settings, new oceanic crust is created, and shallow, minor earthquakes occur. Continental riftsOn a continent, a divergent boundary appears as a rift. A rift forms when a single plate begins to break apart, or when previously fused plates begin to separate again. When a rift occurs in continental crust, elongated fissures (faults) begin to form in the crust as it thins and stretches. Blocks of crust drop down between the faults, forming basins that fill with sediment. Magma rises up through the margins of the basins and may be extruded at the surface as lava flows. A rifting continental crust will have characteristic long, roughly parallel valleys. Some of these may fill with water to form lakes. One of the most famous rift areas today occurs in East Africa. There, a large rift system formed by the stretching of the African plate has created a landscape with rift valleys and deep rift lakes, like Lake Tanganyika and Lake Malawi of the African Great Lakes region. A typical rift basin. The fault is the break in the rock along which the downdropped block has slid, forming a depression known as a rift basin. Sediment begins to fill the basin as it erodes from the surrounding landscape. Magma in the form of lava flows may well up through cracks in the basin, creating layers of igneous rock (basalt). Image modified from original by Jim Houghton, first published in The Teacher-Friendly Guide to the Geology of the Southeastern U.S., 2nd ed., edited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published by the Paleontological Research Institution) (CC BY-NC-SA 4.0 license). East African Rift Zone. Left: Map showing the East African Rift Zone (dotted lines), a rift zone formed by the slow breaking apart of the African plate. Map by Sémhur, modified by Furado (Wikimedia Commons, Creative Commons Attribution-Share Alike 3.0 Unported license, image modified). Right: Satellite view of the area to the south of Lake Bogoria (the bottom of the lake can be seen in the center top of the image), Rift Valley Province, Kenya. Note the parallel ridges and valleys formed by the rifting process. Source: NASA simulated nature color image created by Jesse Allen (NASA Earth Observatory, used following NASA's image use policy, image cropped and resized).
As rifting continues, the continental crust stretches and thins further and further until it eventually ruptures. The region where the crust breaks floods with water, forming a narrow sea. As the edges of the broken crust continue to separate, magma rises up through the margins of the rift, cooling to produce new dense, mafic oceanic crust. The submerged rift thus transforms into a spreading center on the seafloor. The continuing separation between the edges of the broken plate and formation of new oceanic crust continue to widen the sea slowly over time, with the spreading center forming a mid-ocean ridge on the seafloor. Mid-ocean ridges are the most common type of divergent boundary and are characterized by the eruption of bulbous pillow-shaped basalt lavas and hydrothermal fluids. Because ocean crust is constantly being created at spreading centers and recycled (melted) at subduction zones, most of the world's ocean crust is Mesozoic or younger in age. Although spreading centers are typically submerged, Iceland is part of the Mid-Atlantic Ridge, a long spreading center running roughly north-south in the middle of the Atlantic Ocean between the Americas on one side and Europe and Africa on the other. Formation of a rift valley. As the continent begins to separate, faults and basins form. The basins may fill with water to form rift lakes. Modified from a diagram by Hannes Grobe (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.5 Generic license). Formation of a new ocean basin. The continental crust has ruptured and the rift is now flooded with seawater. Oceanic crust begins to form in the new oceanic basin. Modified from a diagram by Hannes Grobe (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.5 Generic license). Mature ocean basin. As the pieces of the continent continue to separate, new oceanic crust is formed at a mid-ocean ridge. Modified from a diagram by Hannes Grobe (Wikimedia Commons, Creative Commons Attribution-ShareAlike 2.5 Generic license). Left: The Red Sea between Africa (left) and the Arabian Peninsula (right) formed in a rifted basin. Photo by Jeff Schmaltz, MODIS Rapid Reponse Team, NASA/GSFC (used under NASA image use policy, image cropped and resized). Right: The Mid-Atlantic Ridge is a spreading center along which the continents on the western and eastern sides of the Atlantic are separating. The ridge is the light blue line running more or less vertically through the Atlantic. Photo by NOAA (Wikimedia Commons, public domain). The East Pacific Rise is a spreading center in the Pacific off the west coast of South America. Left: Map showing the location of the East Pacific Rise. Source: NOAA (via Wikimedia Commons, public domain). Right: Hydrothermal vent in basalt, southern East Pacific Rise, in 1994. The animals in the photo are shrimp. Source: Global Volcanism Program Image GVP-11882 (Smithsonian Institution National Museum of Natural History, public domain). Rock from a "black smoker," a type of hydrothermal vent on the East Pacific Rise, a spreading center off the west coast of South America. This rock is high in sulfur. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license). Map of the world showing plate boundaries with the ocean crust shaded to indicate its age. The youngest crust (red) occurs around active spreading centers. Source: NOAA National Geophysical Data Center, Marine Geology and Geophysics Division. Almannagja, Thingvellir National Park, Iceland. This rift valley has formed as the North American and Eurasian plates have moved away from one another at the Mid-Atlantic Ridge. Photo by Diego Delso, delso.photo (via Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).
In some cases, a rift fails. A failed rift occurs when the existing crust is stretched thin and magma begins to well up, but the plate is never completely broken. An example of a failed rift is the Midcontinent Rift System (Keweenawan Rift), an ancient rift in the North American continent that extends in an arc from eastern Kansas to Minnesota, through lake Superior, and down into Michigan. This rift originated about 1.1 billion years ago and is thought to have formed over a hot spot in the Earth's mantle (read more below). Rifting may have stopped when the hot spot disappeared. Ancient igneous and sedimentary rocks are found in this failed rift zone. The Midcontinent Rift System. Left: Map showing the boundary of the Midcontinent Rift (black line) and the rocks types in it. Green = sedimentary rock; blue = metamorphic rock; purple = volcanic rock; pink = intrusive igneous rock. Map from USGS "Mineral Deposits of the Midcontinent Rift System" (U.S. government work/no other use policy found). Right: Palisade rhyolite (an extrusive igneous or volcanic rock), about 1.1 billion years old, shore of Lake Superior, northeastern Minnesota. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).
Transform boundaries exist where adjacent tectonic plates are moving alongside each other in opposite directions. Most transform faults or boundaries can be found in the ocean, where they connect segments of divergent boundaries. The transform faults connecting divergent boundary segments tend to give spreading centers a stair-step or jagged appearance. The San Andreas Fault in California is a rare transform boundary mainly located on land. The San Andreas Fault creates ridges that run parallel to the fault. Mountains and valleys occur where the fault changes direction. Image of the Pacific Ocean showing the East Pacific Rise and Pacific-Antarctic Ridge, spreading centers on the eastern and southern margins of the Pacific plate. Many transform faults or boundaries occur between segments of divergent boundaries; two large transform faults are indicated in the image. Source: NOAA (via Wikimedia Commons, public domain).
A hot spot is a volcanic region thought to be fed by a spot in the underlying mantle that is anomalously hot. In a hot spot, plumes of magma rise from the layer where the Earth's mantle and core meet. The hotter material rises because it is less dense and therefore more buoyant than its surroundings. The magma from the hot spot pushes its way up through the overlying crust, creating igneous intrusions (subsurface igneous rocks, like granite) and sometimes erupting from volcanoes on the surface as lava. Hot spots can occur under both continental and oceanic crust, and they provide evidence that the Earth’s tectonic plates move. Since a hot spot is nearly stationary in the mantle, it remains in place as the plate slowly moves over it, forming a chain of volcanic features that increase in age as one moves away from the hot spot. Hot spots are sometimes associated with rifting because a hot spot may cause expansion and thinning of the overlying crust, making it susceptible to rupturing. Hot spots in the U.S.Two well-known hot spots occur in the United States: the Hawaiian Islands and Yellowstone National Park. The Hawaiian Islands have formed over a hot spot underneath the oceanic crust of the Pacific Plate. As the plate moves over the hot spot, new volcanoes are created, eventually erupting above the water's surface. Older islands that have moved away from the hot spot erode, eventually disappearing beneath the sea as they weather away. In fact, the Hawaiian Islands are part of a chain of seamounts (undersea mountains) that extends from the middle of the Pacific Ocean to the edge of the Aleutian Islands. The geysers and other geothermal features at Yellowstone National Park are the result of a hot spot under the North American plate. The hot spot formed in the Miocene Epoch, about 16.5 million years ago. Volcanic rocks marking the location of the hot spot have been moving northeast as the North American plate gradually drifts over it. Diagram showing the hot spot beneath Hawaii. Magma rises up in a plume from the Earth's core, forming a magma in the upper mantle. The magma rises, causing volcanic activity on the surface. Hawaii is the youngest of the Hawaiian islands and active volcanism still occurs there. As the Pacific Plate continues to move, it will eventually be supplanted by another island. Modified from a diagram by Wade Greenberg-Brand (originally published in The Teacher-Friendly Guide to the Earth Sciences of the Western US, after a diagram by Tasa Graphic Arts, Inc.). False-color image of the Pacific showing bathymetry and elevation. The Hawaiian Seamounts and the Emperor Seamounts are a continuous chain of islands and seamounts (volcanic mountains that are submerged) produced by a hot spot under the Pacific Plate. The seamount chain extends for 5800 kilometers (3600 miles), ending near the eastern end of the Aleutian Island chain, which was formed by subduction of the Pacific Plate. Image by National Geophysical Data Center/USGS (Wikimedia Commons, public domain). Original caption: "Map of the northwestern U.S., showing the approximate locations of Yellowstone hotspot volcanic fields (orange) and Columbia River Basalts (gray). Boundary of Yellowstone National Park is shown in yellow. Modified from Barry et al. (GSA Special Paper 497, p. 45-66, 2013), Smith and Siegel (Windows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks: Oxford University Press, 2000), and Christiansen (USGS Professional Paper 729-G, 2001)." Source: USGS (public domain). Back to Introductions to Earth Science Topics main page |
An island arc is located near the boundary between which two tectonic plates
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