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PLATE TECTONIC BOUNDARIES
Places, where oceanic and continental lithospheric tectonic plates meet and move relative to each other, are called active margins (e.g., the western coasts of North and South America). A location where continental lithosphere transitions into oceanic lithosphere without movement is known as a passive margin (e.g., the eastern coasts of North and South America). This is why tectonic plates may be made of both oceanic and continental lithosphere. In the process of plate tectonics, the lithospheric plates movement is the primary force that causes the majority of features and activity on the Earth’s surface that can be attributed to plate tectonics. This movement occurs (at least partially) via the drag of motion within the asthenosphere and because of density.
As they move, the tectonic plates interact with each other at the boundaries between the tectonic plates. These interactions are the primary drivers of mountain building, earthquakes, and volcanism on the planet. In a simplified plate tectonic model, plate interaction can be placed in one of three categories. In places where plates move toward each other, the boundary is known as convergent. In places where plates move apart, the boundary is known as divergent. In places where the plates slide past each other, the boundary is known as a transform boundary. The next three subchapters will explain the details of the movement at each type of boundary.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
Convergent boundaries, sometimes called destructive boundaries, are places where two or more tectonic plates have a net movement toward each other. Convergent boundaries, more than any other, are known for orogenesis, the process of building mountains and mountain chains. The key to convergent boundaries is understanding the density of each plate involved in the movement. Continental lithosphere is always lower in density and is buoyant when compared to the asthenosphere. Oceanic lithosphere, on the other hand, is denser than continental lithosphere and, when old and cold, may even be denser than the asthenosphere. When plates of different density converge, the more dense plate sinks beneath, the less dense plate, a process called subduction.
Subduction is when oceanic lithosphere descends into the mantle due to its density. The average rate of subduction of oceanic crust worldwide is 25 miles per million years, about a half inch per year. Continental lithosphere can partially subduct if attached to sinking oceanic lithosphere, but its buoyancy does not allow it to subduct fully. As the tectonic plate descends, it also pulls the ocean floor down in a feature known as a trench. On average, the ocean floor is around 3-4 km deep. In trenches, the ocean can be more than twice as deep, with the Mariana Trench approaching a staggering 11 km.
Within the trench is a feature called the accretionary wedge, sometimes known as melange or accretionary prism, which is a mix of ocean floor sediments that are scraped and compressed at the boundary between the subducting plate and the overriding plate. Sometimes pieces of continental material, like microcontinents, riding with the subducting plate will become sutured to the accretionary wedge, forming a terrane. In fact, large portions of California are comprised of accreted terranes.
When the subducting plate, known as a slab, submerges into the depths of the mantle, the heat and pressure are so immense that lighter materials, known as volatiles, like water and carbon dioxide are pushed out of the subducting plate into an area called the mantle wedge above. The volatiles are released mostly via hydrated minerals that revert to non-hydrated forms in these conditions. These volatiles, when mixed with asthenospheric material above the tectonic plate, lower the melting point of the material. At the temperature of that depth, the material melts to form magma. This process of magma generation is called flux melting. Magma, because of its lower density, migrates toward the surface, creating volcanism. This forms a curved chain of volcanoes, due to many boundaries being curved on a spherical Earth, a feature called an arc. The overriding plate which contains the arc can be either oceanic or continental, where some features are different, but the general architecture remains the same.
How subduction initiates is still a matter of some debate. Presumably, this would start at passive margins where oceanic and continental crust meet. At the current time, there is oceanic lithosphere that is denser than the underlying asthenosphere on either side of the Atlantic Ocean that is not currently subducting. Why has it not turned into an active margin? Firstly, there is strength in the connection between the dense oceanic lithosphere and the less dense continental lithosphere it is connected to, which needs to be overcome. Gravity could cause the denser oceanic plate to force itself down, or the plate can start to flow ductility at a low angle. There is evidence that new subduction is starting off the coast of Portugal. Large earthquakes, like the 1755 Lisbon Earthquake, may even have something to do with this process of creating a subduction zone, though it is not definitive. Transform boundaries that have brought areas of different densities together are also thought to start subduction possibly.
Besides volcanism, subduction zones are also known for the largest earthquakes in the world. In places, the entire subducting slab can become stuck, and when the energy has built up too high, the entire subduction zone can slide at once along a zone extending for hundreds of kilometers along the trench, creating enormous earthquakes and tsunamis. The earthquakes can not only be large, but they can be deep, outlining the subducting slab as it descends. Subduction zones are the only places on Earth with fault surfaces large enough to create magnitude nine earthquakes. Also, because the faulting occurs beneath seawater, subduction also can create giant tsunamis, such as the 2004 Indian Ocean Earthquake and the 2011 Tōhoku Earthquake in Japan.
Subduction, which is a convergent motion, can have varying degrees of convergence. In places that have a high rate of convergence, mostly due to young, buoyant oceanic crust subducting, the subduction zone can create faulting behind the arc area itself, known as back-arc faulting. This faulting can be tensional, or this area is subject to compressional forces. A modern example of this occurs in the two ‘spines’ of the Andes Mountains. In the west, the mountains are formed from the volcanic arc itself; in the east, thrust faults have pushed up another, non-volcanic mountain range still part of the Andes. This type of thrusting can typically occur in two styles: thin-skinned, which only faults surficial rocks, and thick-skinned, which thrusts deeper crustal rocks. Thin-skinned deformation notably occurred in the western U.S. during the Cretaceous Sevier Orogeny. Near the end of the Sevier Orogeny, thick-skinned deformation also occurred in the Laramide orogeny.
The Laramide Orogeny is also known for another subduction feature: flat slab subduction. When the slab subducts at such a low angle, there is an interaction between the slab and the overlying continental plate. Magmatic activity can give rise to mineral deposits, and deformation can occur well into the interior of the overriding plate. All subduction zones have a forearc basin, which is an area between the arc and the trench. This is an area of a high degree of thrust faulting and deformation, seen mostly within the accretionary wedge. There are also places where the convergence shows the results of tensional forces. A variety of causes have been proposed for this, including slab roll-back due to density or ridge migration. This causes extension behind the volcanic or island arc, known as a back-arc basin. These can have so much extension that rifting and divergence can develop, though they can be more asymmetric than their mid-ocean ridge counterparts.
Oceanic-continental subduction occurs when an oceanic plate dives below continental plates. This boundary has a trench and mantle wedge, but the volcanoes are expressed in a feature known as a volcanic arc. A volcanic arc is a chain of mountain volcanoes, with famous examples including the Cascades of the Pacific Northwest (map) and the Andes of South America (map).
Oceanic-oceanic subduction zones have two significant differences from boundaries that have continental lithosphere. Firstly, each plate in an ocean-ocean plate boundary is capable of subduction. Therefore, it is typical that the denser, older, and colder of the two plates is the one that subducts. Secondly, since both plates are oceanic, volcanism creates volcanic islands instead of continental volcanic mountain ranges. This chain of active volcanoes is known as an island arc. There are many examples of this on Earth, including the Aleutian Islands off of Alaska (map), the Lesser Antilles in the Caribbean (map), and several island arcs in the western Pacific.
In places where two continental plates converge toward each other, subduction is not possible. This occurs where an ocean basin closes, and a passive margin is attempted to be driven down with the subducting slab. Instead of subducting beneath the continent, the two masses of continental lithosphere slam into each other in a process known as a collision. Collision zones are known for tall mountains and frequent, massive earthquakes, with little to no volcanism. With subduction ceasing with the collision, there is not a process to create the magma for volcanism.
Continental plates are too low density to subduct, which is why the process of collision occurs instead of subduction. Unlike the dense subducting slabs that form from oceanic plates, any attempt to subduct continental plates is short lived. An occasional exception to this is obduction, in which a part of a continental plate is caught beneath an oceanic plate, formed in collision zones or with small plates caught in subduction zones. This imbalance in density is solved by the continental material buoying upward, bringing oceanic floor and mantle material to the surface, and is the primary source of ophiolites. An ophiolite consists of rocks of the ocean floor that are moved onto the continent, which can also expose parts of the mantle on the surface.
Foreland basins can also develop near the mountain belt, as the lithosphere is depressed due to the mass of the mountains themselves. While subduction mountain ranges can cause this, collisions have many examples, with possibly the best modern example being the Persian Gulf, a feature only there due to the weight of the nearby Zagros Mountains. Collisions are powered by the subducting oceanic lithosphere, and eventually stop as the continental plates combine into a larger mass. In truth, a small portion of the continental crust can be driven down into the subduction zone, though due to its buoyancy, it returns to the surface over time. Because of the relative plastic nature of continental lithosphere, the zone of deformation is much broader. Instead of earthquakes located along a narrow boundary, collision earthquakes can be found hundreds of miles from the suture between the land masses.
The best modern example of this process occurs concurrently in many locations across the Eurasian continent and includes mountain building in the Pyrenees (the Iberian Peninsula converging with France, map), Alps (Italy converging into central Europe, map), Zagros (Arabia converging into Iran, map), and Himalayan (India converging into Asia, map) ranges. Eventually, as ocean basins close, continents join together to form a massive accumulation of continents called a supercontinent, a process that has taken place in hundreds of million-year cycles over earth’s history.
Divergent boundaries, sometimes called constructive boundaries, are places where two or more plates have a net movement away from each other. They can occur within a continental plate or an oceanic plate, though the typical pattern is for divergence to begin within continental lithosphere in a process known as “rift to drift,” described below.
Because of the thickness of continental plates, heat flow from the interior is suppressed. The shielding that supercontinents provide is even stronger, eventually causing upwelling of hot mantle material. This material uplift weakens overlying continental crust, and as convection beneath naturally starts pulling the material away from the area, the area starts to be deformed by tensional stress forming a valley feature known as a rift valley. These features are bounded by normal faults and include tall shoulders called horsts, and deep basins called grabens. When rifts form, they can eventually cause linear lakes, linear seas, and even oceans to form as divergent forces continue.
This breakup via rifting, while initially seeming random, actually has two influences that dictate the shape and location of rifting. First of all, the stable interiors of some continents, called a craton, are seemingly too strong to be broken apart by rifting. Where cratons are not a factor, rifting typically occurs along the patterns of a truncated icosahedron, or “soccer ball” pattern. This is the geometric pattern of fractures that requires the least amount of energy when expanding a sphere equally in all directions. Taking into account the radius of the Earth, this includes ~110 km segments of deformation and volcanism which have 120 degree turns, forming something known as failed rift arms. Even if the motion stops, a minor basin can develop in this weak spot called an aulacogen, which can form long-lived basins well after tectonic processes stop. These are places where extension started but did not continue. One famous example is the Mississippi Valley Embayment, which forms a depression through which the upper end of the Mississippi River flows. In places where the rift arms do not fail, for example, the Afar Triangle, three divergent boundaries can develop near each other forming a triple junction.
Rifts come in two types: narrow and broad. Narrow rifts contain concentrated stress or divergent action. The best active example is the East African Rift Zone, where the horn of Africa near Somalia is breaking away from mainland Africa (map). Lake Baikal in Russia is also an active rift (map). Broad rifts distribute the deformation over a wide area of many fault-bounded locations, like in the western United States in a region known as the Basin and Range (map). The Wasatch Fault, which created the Wasatch Range in Utah, marks the eastern edge of the Basin and Range (map).
Earthquakes, of course, do occur at rifts, though not at the severity and frequency of some other boundaries. Volcanism is also frequent in the extended, faulted, and thin lithosphere found at rift zones due to decompressional melting and faults acting as conduits for the lava reaching the surface. Many relatively young volcanoes dot the Basin and Range, and very strange volcanoes occur in East Africa like Ol Doinyo Lengai in Tanzania, which erupts carbonatite lavas, relatively cold liquid carbonate.
As rifting and volcanic activity progress, the continental lithosphere becomes more mafic and thinner, with the eventual result transforming the plate under the rifting area into the oceanic lithosphere. This is the process that gives birth to a new ocean, much like the narrow Red Sea (map) emerged with the movement of Arabia away from Africa. As the oceanic lithosphere continues to diverge, a mid-ocean ridge is formed.
A mid-ocean ridge, also known as a spreading center, has many distinctive features (map). They are the only places on Earth where the new oceanic lithosphere is being created, via slow oozing volcanism. As the oceanic lithosphere spreads apart, rising asthenosphere melts due to decreasing pressure and fills in the void, making the new lithosphere and crust. These volcanoes produce more lava than all the other volcanoes on Earth combined, and yet are not usually listed on maps of volcanoes due to the vast majority of mid-ocean ridges being underwater. Only rare locations, such as Iceland, are the volcanism and divergent characteristics seen on land. Technically, these places are not mid-ocean ridges, because they are above the surface of the seafloor. The video below is drone imagery of the Icelandic Lava River.
Alfred Wegener even hypothesized this concept of mid-ocean ridges. Because the lithosphere is very hot at the ridge, it has a lower density. This lower density allows it to isostatically ‘float’ higher on the asthenosphere. As the lithosphere moves away from the ridge by continued spreading, the plate cools and starts to sink isostatically lower, creating the surrounding abyssal plains with lower topography. Age patterns also match this idea, with younger rocks near the ridge and older rocks away from the ridge. Sediment patterns also thin toward the ridge, since the steady accumulation of dust and biologic material takes time to accumulate.
Another distinctive feature around mid-ocean ridges is magnetic striping. Called the Vine-Matthews-Morley Hypothesis, it states that as the material moves away from the ridge, it cools below the Curie Point, which is the temperature at which the magnetic field is imprinted on the rock as the rock freezes. Over time, the Earth’s magnetic field has flipped back and forth, and it is this change in the field that causes the stripes. This pattern is an excellent record of past ocean-floor movements and can be used to reconstruct past tectonics and determine rates of spreading at the ridges.
Mid-ocean ridges also are home to some of the unique ecosystems ever discovered, found around hydrothermal vents that circulate ocean water through the shallow oceanic crust and send it back out rich with chemical compounds and heat. While it was known for some time that hot fluids could be found on the ocean floor, it was only in 1977 when a team of scientists using the Diving Support Vehicle Alvin discovered a thriving community of organisms, including tube worms bigger than people. This group of organisms is not at all dependent on the sun and photosynthesis but instead relies on chemical reactions with sulfur compounds and heat from within the Earth, a process known as chemosynthesis. Before this discovery, the thought in biology was that the sun was the ultimate source of energy in ecosystems; now we know this to be false. Not only that, some have suggested it is from this that life could have started on Earth, and it now has become a target for extraterrestrial life (e.g., Jupiter’s moon Europa).
A transform boundary, sometimes called a strike-slip or conservative boundary, is a place where the motion is of the plates sliding past each other. They can move in either dextral fashion with the side opposite moving toward the right or a sinistral fashion with the side opposite moving toward the left. Most transform boundaries can be viewed as a single fault or as a series of faults. As stress builds on adjacent plates attempting to slide them past each other, eventually a fault occurs and releases stress with an earthquake. Transform faults have a shearing motion and are common in places where tectonic stresses are transferred. In general, transform boundaries are known for only earthquakes, with little to no mountain building and volcanism.
The majority of transform boundaries are associated with mid-ocean ridges. As spreading centers progress, these aseismic fracture zone transform faults accommodate different amounts of spreading due to Eulerian geometry that a sphere rotates faster in the middle (Equator) than at the top (Poles) than along the ridge. However, the more significant transform faults, in the eyes of humanity, are the places where the motion occurs within continental plates with a shearing motion. These transform faults produce frequent moderate to large earthquakes. Famous examples include California’s San Andreas Fault (map), both the Northern and Eastern Anatolian Faults in Turkey (map), the Altyn Tagh Fault in central Asia (map), and the Alpine Fault in New Zealand (map).
Transpression and Transtension
In places where transform faults are not straight, they can create secondary faulting. Transpression is defined as places where there is an extra component of compression with shearing. In these restraining bends, mountains can be built up along the fault. The southern part of the San Andreas Fault has a large area of transpression known as the “big bend” and has built, moved, and even rotated many mountain ranges in southern California.
Transtension is defined as places where there is an extra component of extension with shearing. In these releasing bends, depressions and sometimes volcanism are formed along the fault. The Dead Sea and California’s the Salton Sea are examples of basins formed by transtensional forces.
A piercing point is a feature that is cut by a fault, and thus can be used to recreate past movements along the fault. While this can be used on all faults, transform faults are most adapted for this technique. Normal and reverse faulting and divergent and convergent boundaries tend to obscure, bury, or destroy these features; transform faults generally do not. Piercing points usually consist of unique lithologic, structural, or geographic patterns that can be matched by removing the movement along the fault. Detailed studies of piercing points along the San Andreas Fault has shown over 225 km of movement in the last 20 million years along three different active traces of the fault.
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