Subduction
Overview
 
In geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, subduction is the process that takes place at convergent boundaries
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...

 by which one tectonic plate
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 moves under another tectonic plate, sinking into the Earth's mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones". A subduction zone is an area on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 where two tectonic plates move towards one another and one slides under the other. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows).

Plates can be formed from either oceanic lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 or continental lithosphere.
Encyclopedia
In geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, subduction is the process that takes place at convergent boundaries
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...

 by which one tectonic plate
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 moves under another tectonic plate, sinking into the Earth's mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones". A subduction zone is an area on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 where two tectonic plates move towards one another and one slides under the other. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows).

Plates can be formed from either oceanic lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 or continental lithosphere. Stable subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate (that is, the subducted plate is always oceanic while the subducting plate may or may not be oceanic). Subduction zones are often noted for their high rates of volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

, earthquakes, and mountain building
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

. Continental collision
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...

 results where a continental plate is subducted.

Orogenesis, or mountain-building, occurs when large pieces of material on the subducting plate (such as island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....

s) are pressed into the overriding plate. These areas are subject to many earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

s, which are caused by the interactions between the subducting slab
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...

 and the mantle, the volcanoes, and (when applicable) the mountain-building related to island arc collisions.

General description

Subduction zones mark sites of convective downwelling
Downwelling
Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together...

 of the Earth's lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 (the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 plus the top brittle portion of the upper mantle). Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 converges with another plate. The down-going slab
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...

 -- the subducting plate—is overridden by the leading edge of the other plate. The slab sinks at an angle of approximately 25 to 45 degrees to the surface of the Earth. At a depth of approximately 80–120 km, the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 of the oceanic slab is converted to a metamorphic rock called eclogite
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...

. At this point, the density of the oceanic lithosphere increases and it is carried into the mantle by the downwelling convective currents. It is at subduction zones that the Earth's lithosphere, oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

, sedimentary layers, and some trapped water are recycled into the deep mantle. Earth is the only planet where subduction is known to occur. Without subduction, plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 could not exist.
Subduction zones dive down into the mantle beneath 55,000 km of convergent plate margins (Lallemand, 1999), almost equal to the cumulative 60,000 km of mid-ocean ridges. Subduction zones burrow deeply but are imperfectly camouflaged, and we can use geophysics
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...

 and geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

 to study them. Not surprisingly, the shallowest portions of subduction zones are known best. Subduction zones are strongly asymmetric for the first several hundred kilometers of their descent. They start to go down at oceanic trench
Oceanic trench
The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....

es. Their descents are marked by inclined zones of earthquakes
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

 that dip away from the trench beneath the volcanoes and extend down to the 660 km discontinuity. Subduction zones are defined by the inclined array of earthquakes known as the “Wadati-Benioff Zone” after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at enormously greater depths than elsewhere on Earth, where seismicity is limited to the outermost 20 km of the solid Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.

The subducting basalt and sediment are normally rich in hydrous minerals and clays. During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...

. The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle where it lowers the pressure in (and thus the melting temperature of) the mantle rock to the point of actual melting, generating magma. These magmas, in turn, rise, because they are less dense than the rocks of the mantle. These mantle-derived magmas (which are basaltic in composition) can continue to rise, ultimately to the Earth's surface, resulting in a volcanic eruption. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt (a) interacts with (melts) the Earth's crust and/or (b) undergoes fractional crystallization
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...

.

Above subduction zones, volcanoes exist in long chains called volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...

s. Volcanoes that exist along arcs tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive. Krakatoa, Nevado del Ruiz, and Mount Vesuvius are all examples of arc volcanoes. Arcs are also known to be associated with precious metals such as gold, silver and copper - again believed to be carried by water and concentrated in and around their host volcanoes in rock termed "ore".

Subduction results from convection in the asthenosphere. The heat from the core of the earth that is imparted to the mantle causes the mantle to convect much the way boiling water convects in a pan on the stove. Hot mantle at the core-mantle boundary rises while cool mantle sinks, causing convection cells to form. At points where two downward moving convecting cells meet (cold mantle sinking), convection can occur, forcing the oceanic crust below either continents or other oceanic crust. Continental crust tends to override oceanic crust because it consists of less dense granite compared to the basalt of the oceanic crust.

Theory on origin

Although the process of subduction as it occurs today is fairly well understood, its origin remains a matter of discussion and ongoing study. A recent paper by V.L. Hansen in Geology
Geology (journal)
Geology is a publication of the Geological Society of America . The GSA claims that it is the most widely read scientific journal in the field of earth science...

 presented a hypothesis that mantle upwelling and similar thermal processes, combined with an impact from an extraterrestrial source, would give the early earth the discontinuities in the crust for the subduction of the denser material underneath lighter material.

A model of the initiation of subduction, based on analytic and analog modeling, presumes that the difference of density between two adjacent lithopsheric slabs is sufficient to lead to the initiation of subduction. The analytic part of the model shows that where two lithospheric slabs of different densities are positioned one next to the other, maximum differential lithostatic pressure would occur at the base of the denser slab directed towards the lighter one. The resulting strain would lead to the rotation of the contact zone between the slabs to dip towards the lighter slab, and the dip would be reduced until offset along the contact zone would be enabled. The parameters that constrain the rotation of the contact zone are known as "Argand Numbers". Analog experiments based on this concept were carried out using a centrifuge, comprising lighter and denser brittle and ductile "lithosphere" floating on still denser "asthenosphere". The analog experiments suggested that the initiation of subduction started with the penetration of the denser ductile "lithosphere" below its lighter counterpart. Consequently, the lighter "lithosphere" was uplifted, then collapsed on the denser slab, increasing the load on its edge and driving the denser sequence further under the lighter slab. It was presumed further that once the denser "lithosphere" was set below the lighter one, it underwent conversion to eclogite which increased its density and drove it to subduct into the "asthenosphere". The rate of this part of the subduction process was determined by friction. Reduction of slab friction in nature could result from serpentinization and other water-related processes.

According to the geophysicist Don L. Anderson
Don L. Anderson
Don Lynn Anderson is an American geophysicist who has made important contributions to the determination of the large-scale structure of the Earth's interior, especially using seismological methods. He is Eleanor and John R. McMillan Professor emeritus of geophysics at the California Institute of...

, it is conceivable that plate tectonics could not happen without the calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 laid down by living beings at the edges of subduction zones. The massive weight of these sediments could be softening the underlying rocks, making them pliable enough to plunge.

Volcanic activity

Volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es that occur above subduction zones, such as Mount St. Helens
Mount St. Helens
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is south of Seattle, Washington and northeast of Portland, Oregon. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a...

 and Mount Fuji
Mount Fuji
is the highest mountain in Japan at . An active stratovolcano that last erupted in 1707–08, Mount Fuji lies about south-west of Tokyo, and can be seen from there on a clear day. Mount Fuji's exceptionally symmetrical cone is a well-known symbol of Japan and it is frequently depicted in art and...

, at ~ 100 km from the trench in arcuate chains, hence the term volcanic arc
Volcanic arc
A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...

. Two kinds of arcs are generally observed on Earth: island arcs that formed on oceanic lithosphere, such as the Mariana
Mariana Islands
The Mariana Islands are an arc-shaped archipelago made up by the summits of 15 volcanic mountains in the north-western Pacific Ocean between the 12th and 21st parallels north and along the 145th meridian east...

 or Tonga
Tonga
Tonga, officially the Kingdom of Tonga , is a state and an archipelago in the South Pacific Ocean, comprising 176 islands scattered over of ocean in the South Pacific...

 island arcs, or continental arc that formed on the continent, such as the Cascade Volcanic Arc. Island arcs are produced by the subduction of oceanic lithosphere beneath another oceanic lithosphere (oceanic subduction), while continental arcs formed during subduction of oceanic lithosphere beneath a continental lithosphere.

The arc magmatism occurs 100–200 km away from the trench and ~ 100 km from the subducting slab. This depth of arc magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 generation is the consequence of the interaction between fluids, released from the subducting slab, and the arc mantle wedge that is hot enough to generate hydrous melting. Arcs produce about 25% of the total volume of magma produced each year on Earth (~30–35 km³), much less than the volume produced at mid-ocean ridges, and they contribute to the formation of new continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

. Arc volcanism has the greatest impact on humans, because many arc volcanoes lie above sea level and erupt violently. Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of the Earth's climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 and affect air travel.

Earthquakes and tsunamis

The strains caused by plate convergence in subduction zones cause at least three different types of earthquakes. Earthquakes mainly propagate in the cold subducting slab and define the Wadati-Benioff zone. Seismicity shows that the slab can be tracked down to the upper mantle - lower mantle boundary (~ 600 km depth).

Nine out of the ten largest earthquakes to occur in the last 100 years were subduction zone events. This includes the 1960 Great Chilean Earthquake, which at M 9.5 was the largest earthquake ever recorded, the 2004 Indian Ocean earthquake and tsunami, and the 2011 Tōhoku earthquake and tsunami
2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...

. The subduction of cold oceanic crust into the mantle depresses the local geothermal gradient
Geothermal gradient
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied...

 and causes a larger portion of the earth to deform in a more brittle fashion than it would in a normal geothermal gradient setting. Because earthquakes can only occur when a rock is deforming in a brittle fashion, subduction zones can create large earthquakes. If such an earthquake causes rapid deformation of the sea floor, there is potential for tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate
Indo-Australian Plate
The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and surrounding ocean, and extends northwest to include the Indian subcontinent and adjacent waters...

 under the Eurasian Plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...

 on December 26, 2004 that devastated the areas around the Indian Ocean
2004 Indian Ocean earthquake
The 2004 Indian Ocean earthquake was an undersea megathrust earthquake that occurred at 00:58:53 UTC on Sunday, December 26, 2004, with an epicentre off the west coast of Sumatra, Indonesia. The quake itself is known by the scientific community as the Sumatra-Andaman earthquake...

. Small tremors that create small, non-damaging tsunamis occur frequently.

Outer rise
Outer trench swell
The outer trench swell, outer trench high, or outer rise is a subtle ridge on the seafloor near an oceanic trench, where a descending plate begins to flex and fault in preparation for its descent into the mantle at a subduction zone...

 earthquakes occur when normal faults oceanward of the subduction zone are activated by flexture of the plate as it bends into the subduction zone. The Samoa earthquake of 2009 is an example of this type of event. Displacement of the sea floor caused by this event generated a 6m tsunami in nearby Samoa.

Anomalously deep events are a characteristic of subduction zones which produce the deepest earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

s on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than 20 km. However, in subduction zones, earthquakes occur at depths as great as 700 km. These earthquakes define inclined zones of seismicity known as Wadati-Benioff zones
Benioff zone
A Wadati–Benioff zone is a deep active seismic area in a subduction zone. Differential motion along the zone produces deep-seated earthquakes, the foci of which may be as deep as about . They develop beneath volcanic island arcs and continental margins above active subduction zones...

, after the scientists who discovered them, which trace the descending lithosphere. Seismic tomography
Seismic tomography
Seismic tomography is a methodology for estimating the Earth's properties. In the seismology community, seismic tomography is just a part of seismic imaging, and usually has a more specific purpose to estimate properties such as propagating velocities of compressional waves and shear waves . It...

 has helped detect subducted lithosphere in regions where there are no earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about 670 km, whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary
Core-mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

. The great seismic discontinuities in the mantle - at 410 and 670 km depth - are disrupted by the descent of cold slabs in deep subduction zones.

Orogeny

Subducting plates can bring island arcs and sediments to convergent margins. This material often does not subduct with the rest of the plate, but instead is accreted to the continent in the form of exotic terranes. These cause crustal thickening and mountain-building.

Subduction angle

Subduction typically occurs at a moderately steep angle right at the point of the convergent plate boundary. However, anomalous shallower angles of subduction are known to exist as well some extremely steep.
  • Flat-slab subduction (<30°): occurs when subducting lithosphere, called a slab, subducts horizontally or nearly-horizontally. The flat slab can extend for hundreds of to over a thousand kilometers. This is abnormal, as the dense slab typically sinks at a much steeper angle directly at the subduction zone. Because subduction of slabs to depth is necessary to drive subduction zone volcanism (through the destabilization and dewatering of minerals and the resultant flux melting
    Flux melting
    In igneous petrology flux melting occurs when water and other volatile components are added to a hot solid rock. The ultramafic rock of the upper mantle can be melted by the addition of volatiles from below. The source of the water and volatiles is the subducting slab of oceanic crust which...

     of the mantle wedge
    Mantle wedge
    A mantle wedge is a zone of mantle, triangular in cross–section, that lies between subducting and overriding tectonic plates. Flow inside the wedge is aligned with plate motions. Flux melting of material within wedge due to the release of volatiles from the underlying subducting slab results in...

    ), flat-slab subduction can be invoked to explain volcanic gaps. Flat-slab subduction is ongoing beneath part of the Andes
    Andes
    The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...

     causing segmentation of the Andean Volcanic Belt
    Andean Volcanic Belt
    The Andean Volcanic Belt is a major volcanic belt along the Andean cordillera in Colombia, Bolivia, Peru, Ecuador, Chile and Argentina. It formed as a result of subduction of the Nazca Plate and Antarctic Plate underneath the South American Plate. The belt is subdivided into four main volcanic...

     into four zones. The flat-slab subdution in northern Peru and Norte Chico
    Norte Chico
    Norte Chico or Near North Coast ranges over five river valleys north of present-day Lima: the Chancay River, the Huaura River, Supe River, Fortaleza River and Pativilca River....

     region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge
    Nazca Ridge
    The Nazca Ridge is an ocean ridge in the southern Pacific Ocean. It lies on the Nazca Plate and is being subducted in the Peru-Chile Trench under the South American Plate by ongoing plate motion....

     and the Juan Fernández Ridge
    Juan Fernández Ridge
    The Juan Fernández Ridge is a volcanic island and seamount chain on the Nazca Plate. It runs in a west-east direction from the Juan Fernández hotspot to the Peru-Chile Trench at a latitude of 33° S near Valparaíso. The Juan Fernández Islands are the only seamounts that reach the surface....

     respectively. Around Taitao Peninsula
    Taitao Peninsula
    The Taitao Peninsula is a westward projection of the mainland of Chile, with which it is connected by the narrow Isthmus of Ofqui, over which the natives and early missionaries were accustomed to carry their boats between the Moraleda Channel and Gulf of Penas...

     flat-slab dubduction is attributed to the subduction of the Chile Rise
    Chile Rise
    The Chile Rise or Chile Ridge is an oceanic ridge, a tectonic divergent plate boundary between the Nazca and Antarctic Plates. Its eastern end is the Chile Triple Junction where the Chile Rise is being subducted below the South American Plate in the Peru-Chile Trench...

    , a spreading ridge
    Mid-ocean ridge
    A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...

    . The Laramide Orogeny
    Laramide orogeny
    The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...

     in the Rocky Mountains
    Rocky Mountains
    The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

     of United States
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

     is attributed to flat-slab subduction. During this time, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the basement
    Basement (geology)
    In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...

    -cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being.

  • Steep-angle subduction (>70°): occurs in subduction zones where earth's oceanic crust
    Oceanic crust
    Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

     and lithosphere are old and thick and have therefore lost buoyancy. At present the steepest dipping subduction zone lies the Marianas Trench where also the earths oldest oceanic crust lies (Jurassic
    Jurassic
    The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

     age), if exempting ophiolites. Steep-angle subduction is in contrast to flat-slab subduction associated with back-arc extension of crust making volcanic arcs and fragments of continental crust wander away from continents over geological times leaving behind a marginal sea
    Marginal sea
    The term marginal sea has differing meanings. In one sense the term is equivalent to territorial waters. In another sense the term indicates a partially enclosed sea adjacent to or widely open to the open ocean, but bounded by submarine ridges...

    .

Importance

Subduction zones are important for several reasons:
  1. Subduction Zone Physics: Sinking of the oceanic lithosphere
    Lithosphere
    The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

     (sediments + crust + mantle), by contrast of density
    Density
    The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

     between the cold and old lithosphere and the hot asthenospheric mantle wedge, is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection
    Mantle convection
    Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...

    .
  2. Subduction Zone Chemistry: The subducted sediments and crust dehydrate and release water-rich (aqueous) fluids into the overlying mantle, causing mantle melting and fractionation
    Fractionation
    See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...

     of elements between surface and deep mantle reservoirs, producing island arcs and continental crust
    Continental crust
    The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

    .
  3. Subduction zones drag down subducted oceanic sediments, oceanic crust, and mantle lithosphere that interact with the hot asthenospheric mantle
    Mantle
    A mantle is an ecclesiastical garment in the form of a very full cape which extends to the floor, joined at the neck, that is worn over the outer garments....

     from the overriding plate to produce calc-alkaline
    Calc-alkaline
    The calc-alkaline magma series is one of two main magma series in igneous rocks, the other magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...

     series melts, ore deposits, and continental crust.


Subduction zones have also been considered as possible disposal sites for nuclear waste, where the action would carry the material into the planetary mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

, safely away from any possible influence on humanity or the surface environment, but this method of disposal is currently banned by international agreement.

See also

  • List of tectonic plate interactions
  • Back-arc basin
    Back-arc basin
    Back-arc basins are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the...

  • Oceanic trench
    Oceanic trench
    The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....

  • Obduction
    Obduction
    Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode....

  • Slab window
    Slab window
    In geology, a slab window is a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone. They are commonly associated with the formation of convergent plate boundaries and they have had significant effects on the formation of the North American Cordillera.A...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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