Landforms of Plate Boundaries | Sciencing
This section will introduce you to the concept of plate tectonics, how it works, why it . This animation shows the relationship between subduction of the lithosphere and The Cascade Mountains of the Pacific Northwest are a continental arc. Mountains, hills, plateaus, and plains are the four major types of Tectonic plate movement under the Earth can create landforms by pushing. Plate tectonics, theory dealing with the dynamics of Earth's outer shell—the associated with plate motion is provided in the articles tectonic landform and ocean. . This density difference creates two principal levels of Earth's surface. .. Adapted from: C.R. Scotese, The University of Texas at Arlington.
At greater depths the subducted plate is partially recycled into the mantle. The site of subduction is marked by a deep trench, between 5 and 11 km 3 and 7 miles deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts. The overriding plate scrapes sediments and elevated portions of ocean floor off the upper crust of the lower plate, creating a zone of highly deformed rocks within the trench that becomes attached, or accreted, to the overriding plate.
This chaotic mixture is known as an accretionary wedge. The rocks in the subduction zone experience high pressures but relatively low temperatures, an effect of the descent of the cold oceanic slab.
Under these conditions the rocks recrystallize, or metamorphose, to form a suite of rocks known as blueschists, named for the diagnostic blue mineral called glaucophanewhich is stable only at the high pressures and low temperatures found in subduction zones. See also metamorphic rock. At deeper levels in the subduction zone that is, greater than 30—35 km [about 19—22 miles]eclogiteswhich consist of high-pressure minerals such as red garnet pyrope and omphacite pyroxeneform.
The formation of eclogite from blueschist is accompanied by a significant increase in density and has been recognized as an important additional factor that facilitates the subduction process. Island arcs When the downward-moving slab reaches a depth of about km 60 milesit gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone known as the mantle wedge.
Melting in the mantle wedge produces magmawhich is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate, known as a volcanic arctypically a few hundred kilometres behind the oceanic trench.
The distance between the trench and the arc, known as the arc-trench gap, depends on the angle of subduction. Steeper subduction zones have relatively narrow arc-trench gaps.
A basin may form within this region, known as a fore-arc basin, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust.
If both plates are oceanic, as in the western Pacific Ocean, the volcanoes form a curved line of islandsknown as an island arcthat is parallel to the trench, as in the case of the Mariana Islands and the adjacent Mariana Trench. If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America. Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust.
In either case, the composition of the volcanic mountains formed tends to be more silicon -rich and iron - and magnesium -poor relative to the volcanic rocks produced by ocean-ocean convergence.
Back-arc basins Where both converging plates are oceanic, the margin of the older oceanic crust will be subducted because older oceanic crust is colder and therefore more dense. This results in a process known as back-arc spreading, in which a basin opens up behind the island arc.
The crust behind the arc becomes progressively thinner, and the decompression of the underlying mantle causes the crust to melt, initiating seafloor-spreading processessuch as melting and the production of basalt; these processes are similar to those that occur at ocean ridges.
The geochemistry of the basalts produced at back-arc basins superficially resembles that of basalts produced at ocean ridgesbut subtle trace element analyses can detect the influence of a nearby subducted slab.
This style of subduction predominates in the western Pacific Oceanin which a number of back-arc basins separate several island arcs from Asia. However, if the rate of convergence increases or if anomalously thick oceanic crust possibly caused by rising mantle plume activity is conveyed into the subduction zone, the slab may flatten. Such flattening causes the back-arc basin to close, resulting in deformationmetamorphismand even melting of the strata deposited in the basin. Mountain building If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, a convergent margin forms as the ocean initially contracts.
This process can lead to collision between the approaching continentswhich eventually terminates subduction. Mountain building can occur in a number of ways at a convergent margin: Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America —which includes the Rocky Mountains as well as the Cascadesthe Sierra Nevadaand other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion.
As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes.
Over the past 70 million years the subduction of the Neo-Tethys Seaa wedge-shaped body of water that was located between Gondwana and Laurasialed to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia.
These collisions culminated in the formation of the Alps and the Himalayas. Jurassic paleogeographyDistribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period.
Included in the paleogeographic reconstruction are the locations of the interval's subduction zones. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zoneand, therefore, the rocks in this region are warm and weak.
Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward.
For example, rapid uplift of the Andes approximately 25 million years ago is evidenced by a reversal in the flow of the Amazon River from its ancestral path toward the Pacific Ocean to its modern path, which empties into the Atlantic Ocean.
In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which have influenced the plate-tectonic evolution of the western Pacific for at least the past million years. Mountains by terrane accretion As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin.
Since the late Devonian and early Carboniferous periods, some million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes.
The piecemeal addition of these accreted terranes has added an average of km miles in width along the western margin of the North American continentand the collisions have resulted in important pulses of mountain building. The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise.
The continental shelf, slope, and rise are collectively called the continental margin. During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted. Where this occurs, rare slices of ocean crust, known as ophiolitesare preserved on land.
They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land.
A classic example is the Coast Range ophiolite of Californiawhich is one of the most extensive ophiolite terranes in North America. These ophiolite deposits run from the Klamath Mountains in northern California southward to the Diablo Range in central California.
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- Tectonic Landforms and Mountain Building
This oceanic crust likely formed during the middle of the Jurassic Periodroughly million years ago, in an extensional regime within either a back-arc or a forearc basin. In the late Mesozoicit was accreted to the western North American continental margin.
Because preservation of oceanic crust is rare, the recognition of ophiolite complexes is very important in tectonic analyses. Until the mids, ophiolites were thought to represent vestiges of the main oceanic tract, but geochemical analyses have clearly indicated that most ophiolites form near volcanic arcs, such as in back-arc basins characterized by subduction roll-back the collapse of the subducting plate that causes the extension of the overlying plate.
The recognition of ophiolite complexes is very important in tectonic analysis, because they provide insights into the generation of magmatism in oceanic domains, as well as their complex relationships with subduction processes. See above back-arc basins. Mountains by continental collision Continental collision involves the forced convergence of two buoyant plate margins that results in neither continent being subducted to any appreciable extent.
A complex sequence of events ensues that compels one continent to override the other. The subducted slab still has a tendency to sink and may become detached and founder submerge into the mantle.
Plate Tectonics: Earth in Motion
The crustal root undergoes metamorphic reactions that result in a significant increase in density and may cause the root to also founder into the mantle. Both processes result in a significant injection of heat from the compensatory upwelling of asthenosphere, which is an important contribution to the rise of the mountains.
Continental collisions produce lofty landlocked mountain ranges such as the Himalayas. Much later, after these ranges have been largely leveled by erosionit is possible that the original contact, or suture, may be exposed. The balance between creation and destruction on a global scale is demonstrated by the expansion of the Atlantic Ocean by seafloor spreading over the past million years, compensated by the contraction of the Pacific Oceanand the consumption of an entire ocean between India and Asia the Tethys Sea.
The northward migration of India led to collision with Asia some 40 million years ago. Since that time India has advanced a further 2, km 1, miles beneath Asia, pushing up the Himalayas and forming the Plateau of Tibet. Pinned against stable SiberiaChina and Indochina were pushed sideways, resulting in strong seismic activity thousands of kilometres from the site of the continental collision.
Plate Tectonics: Earth in Motion
Transform faults are so named because they are linked to other types of plate boundaries. The majority of transform faults link the offset segments of oceanic ridges. However, transform faults also occur between plate margins with continental crust—for example, the San Andreas Fault in California and the North Anatolian fault system in Turkey. These boundaries are conservative because plate interaction occurs without creating or destroying crust.
Because the only motion along these faults is the sliding of plates past each other, the horizontal direction along the fault surface must parallel the direction of plate motion.
The fault surfaces are rarely smooth, and pressure may build up when the plates on either side temporarily lock. This buildup of stress may be suddenly released in the form of an earthquake.
Geological Survey Many transform faults in the Atlantic Ocean are the continuation of major faults in adjacent continents, which suggests that the orientation of these faults might be inherited from preexisting weaknesses in continental crust during the earliest stages of the development of oceanic crust.
On the other hand, transform faults may themselves be reactivated, and recent geodynamic models suggest that they are favourable environments for the initiation of subduction zones.
Linear chains of islandsthousands of kilometres in length, that occur far from plate boundaries are the most notable examples.
These island chains record a typical sequence of decreasing elevation along the chain, from volcanic island to fringing reef to atoll and finally to submerged seamount. An active volcano usually exists at one end of an island chain, with progressively older extinct volcanoes occurring along the rest of the chain.
Tuzo Wilson and American geophysicist W. Jason Morgan explained such topographic features as the result of hotspots. The principal tectonic plates that make up Earth's lithosphere.
Also located are several dozen hot spots where plumes of hot mantle material are upwelling beneath the plates. Black dots indicate active volcanoes, whereas open dots indicate inactive ones. The number of these hotspots is uncertain estimates range from 20 tobut most occur within a plate rather than at a plate boundary. Hotspots are thought to be the surface expression of giant plumes of heat, termed mantle plumesthat ascend from deep within the mantle, possibly from the core-mantle boundary, some 2, km 1, miles below the surface.
These plumes are thought to be stationary relative to the lithospheric plates that move over them. A volcano builds upon the surface of a plate directly above the plume.
As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct. Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atollsand eventually they sink below the surface of the sea to form a seamount.
At the same time, a new active volcano forms directly above the mantle plume. Diagram depicting the process of atoll formation. Atolls are formed from the remnant parts of sinking volcanic islands. The best example of this process is preserved in the Hawaiian-Emperor seamount chain. The plume is presently situated beneath Hawaii, and a linear chain of islandsatollsand seamounts extends 3, km 2, miles northwest to Midway and a further 2, km 1, miles north-northwest to the Aleutian Trench.
The age at which volcanism became extinct along this chain gets progressively older with increasing distance from Hawaii —critical evidence that supports this theory. Hotspot volcanism is not restricted to the ocean basins ; it also occurs within continents, as in the case of Yellowstone National Park in western North America.
Measurements suggest that hotspots may move relative to one another, a situation not predicted by the classical model, which describes the movement of lithospheric plates over stationary mantle plumes. This has led to challenges to this classic model. Furthermore, the relationship between hotspots and plumes is hotly debated.
Proponents of the classical model maintain that these discrepancies are due to the effects of mantle circulation as the plumes ascend, a process called the mantle wind. Data from alternative models suggest that many plumes are not deep-rooted.
Instead, they provide evidence that many mantle plumes occur as linear chains that inject magma into fractures, result from relatively shallow processes such as the localized presence of water-rich mantle, stem from the insulating properties of continental crust which leads to the buildup of trapped mantle heat and decompression of the crustor are due to instabilities in the interface between continental and oceanic crust.
In addition, some geologists note that many geologic processes that others attribute to the behaviour of mantle plumes may be explained by other forces. The point of emergence of the axis through the surface of the sphere is known as the pole of rotation.
Therefore, the relative motion of two rigid plates may be described as rotations around a common axis, known as the axis of spreading. Application of the theorem requires that the plates not be internally deformed—a requirement not absolutely adhered to but one that appears to be a reasonable approximation of what actually happens.
Application of this theorem permits the mathematical reconstruction of past plate configurations. Theoretical depiction of the movement of tectonic plates across Earth's surface.Landforms at Plate Boundaries
Movement on a sphere of two plates, A and B, can be described as a rotation around a common pole. Circles around that pole correspond to the orientation of transform faults that is, single lines in the horizontal that connect to divergent plate boundaries, marked by double lines, in the vertical. Several different kinds of interactions exist. In some places the edges come together, in other places they pull apart, and in still others, the plates slide past each other.
All this interaction creates many different landforms. Trenches The deepest landforms on Earth are the trenches in the ocean. These landforms are created when one plate slides underneath another.
This action is known as subduction. Some tectonic plates are much heavier than others. The heavy plate slides under the lighter plate. The edge between the two plates formed by this interaction is a deep trench. One of the most famous trenches is called the Marianas Trench. As the Philippine plate slides underneath the Pacific plate, the deepest trench known on Earth is continually formed. Volcanoes and Ridges Volcanoes and ridges are landforms that are created by the movement of tectonic plates.
Some volcanoes are formed when the plates pull apart under the ocean. A crack in Earth's crust forms. Magma rises through the crack, forming ridges. One example is the San Juan Ridge, a wide area of young volcanoes.