Geometry of a subduction zone - insets to show accretionary prism and partial melting of hydrated asthenosphere. An accretionary wedge is Sedimentary rock material that is accreted onto the non- subducting Tectonic plate at a convergent plate boundary The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the

In geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another, with one sliding underneath the other and moving down into the mantle, at rates typically measured in centimeters per year. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided An oceanic plate ordinarily slides underneath a continental plate or another oceanic plate; this often creates an orogenic zone or volcanic arc subject to many earthquakes. Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer In a sense, subduction zones are the opposite of divergent boundaries, areas where material rises up from the mantle and plates are moving apart. In Plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a

General description

Subduction zones mark sites of convective downwelling of the Earth's lithosphere (the crust plus the strong portion of the upper mantle). 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 The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. In Geology, a crust is the outermost solid shell of a planet or moon Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate and sinks below it to depth of approximately 100 km. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. At that depth the peridotite of the oceanic slab is converted to eclogite, the density of the edge of the oceanic lithosphere increases and it sinks into the mantle. The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided It is at subduction zones that the Earth's lithosphere, oceanic crust, sedimentary layers, and trapped water are recycled into the deep mantle. Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) Earth is the only planet where subduction is known to occur. Without subduction, plate tectonics could not exist and Earth would be a very different planet: Earth's crust would not have differentiated into continents and oceans and all of the solid Earth would lie beneath a global ocean. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere

Subduction results from the difference in density between lithosphere and underlying asthenosphere. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the Where, very rarely, lithosphere is denser than asthenospheric mantle, it can easily sink back into the mantle at a subduction zone; however, subduction is resisted where lithosphere is less dense than underlying asthenosphere. Whether or not lithosphere is denser than underlying asthenosphere depends on the nature of the associated crust. Crust is always less dense than asthenosphere or lithospheric mantle and continental lithosphere is always less dense than oceanic lithosphere. Exceptionally, the presence of the large areas of flood basalt that are called large igneous provinces (LIPs), which result in extreme thickening of the oceanic crust, can cause some sections of older oceanic lithosphere to be too buoyant to subduct. A flood basalt or trap basalt is the result of a giant Volcanic eruption or series of Eruptions that coats large stretches of land or the Ocean floor Large Igneous provinces (LIPS were originally defined by Coffin and Eldholm (1992 as areas of Earth's surface that contain very large volumes of magmatic rocks (typically Where lithosphere on the downgoing plate is too buoyant to subduct, a collision occurs, hence the adage "Subduction leads to orogeny". Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and

Subduction zones are arc-shaped, with the concave side oriented away from the direction of subduction because of the curvature of the Earth. This can easily be seen by making a cut into an orange, with the knife blade representing a subducting slab.

Theory on origin

There have been some recent theories on the beginnings of subduction and Plate tectonics generally. A recent paper by V. L. Hansen in Geology 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. Geology is a publication of the Geological Society of America. ^[1]

Associated volcanic activity

Oceanic plates are subducted creating oceanic trenches. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor

Subduction causes oceanic trenches, such as the Mariana trench. The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor The Mariana Trench (or Mariana's Trench) is the deepest part of the world's Oceans and the deepest location on the surface of the Earth 's Trenches occur where one plate begins its descent beneath another. Volcanoes that occur above subduction zones, such as Mount St. Helens and Mount Fuji, often occur in arcuate chains, hence the term volcanic arc or island arc. Plate tectonics and hotspots Divergent plate boundaries At the Mount St Helens is an active Stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States is the highest Mountain in Japan at.An Active volcano that last erupted in 1707–08 it straddles the boundary of Shizuoka and A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under Not all "volcanic arcs" are arced: trenches and arcs are often linear.

The magmatism associated with the volcanic arc occurs 100-300 km away from the trench. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet However, a relationship has been found, which relates volcanic arc location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many volcanic arcs around the world have revealed that volcanic arcs tend to form at a location where the subducted slab has reached a depth of about 100 km. This has interesting implications for the mechanism that causes the magmatism at these arcs. 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. Nevertheless, arc volcanism has the greatest impact on humans, because many arc volcanoes lie above sealevel and erupt violently. Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of the Earth's climate. Particulates, alternatively referred to as particulate matter (PM or fine particles, are tiny particles of solid or liquid suspended in a gas Climate encompasses the temperatures humidity rainfall atmospheric particle count and numerous other meteorogical factors in a given region over long periods of

Earthquakes and tsunamis

Subduction zones are also notorious for producing devastating earthquakes because of the intense geological activity. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer The introduction of cold oceanic crust into the mantle depresses the local geothermal gradient 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 have the potential to create very large earthquakes. If this earthquake occurs under the ocean it has the potential to create tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate under the Eurasian Plate on December 26, 2004, that devastated the areas around the Indian Ocean. A tsunami ((tsuːˈnɑːmi is a series of waves created when The Indo-Australian Plate is a major Tectonic plate that includes the continent of Australia and surrounding Ocean, and extends northwest to include the The Eurasian Plate is a Tectonic plate which includes most of the continent of Eurasia (a landmass consisting of the traditional Continents of Europe Events 1481 - Battle of Westbrook - Holland defeats troops of Utrecht. "MMIV" redirects here For the Modest Mouse album see " Baron von Bullshit Rides Again " The 2004 Indian Ocean earthquake was an undersea Earthquake that occurred at 005853 UTC on December 26 2004 with an Epicentre off the west coast of Small tremors that create tiny, unnoticeable tsunamis happen all the time because of the dynamics of the earth.

Subduction zones are associated with the deepest earthquakes on the planet. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer 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 (after the scientists who discovered them), which outline the descending lithosphere. A Wadati-Benioff zone (also Benioff-Wadati zone or Benioff zone) is a deep active seismic area in a Subduction zone Seismic tomography has helped outline subducted lithosphere in regions where there are no earthquakes. Seismic Tomography is a methodology for estimating the earth's properties 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. The core-mantle boundary (or CMB in the parlance of solid earth geophysicists lies between the Earth's Silicate mantle and its liquid Iron - Nickel 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.

Importance

Cartoon representation of the Subduction Factory, from Y. Tatsumi JAMSTEC.

Subduction zones are important for several reasons:

1. Subduction Zone Physics: Sinking of mantle lithosphere is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection. Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density
2. Subduction Zone Chemistry: The cold material sinking in subduction zones releases water into the overlying mantle, causing mantle melting and fractionating elements (buffering) between surface and deep mantle reservoirs, producing island arcs and continental crust. Fractionation is a Separation process in which a certain quantity of a Mixture (solid liquid solute or suspension is divided up in a number of smaller quantities The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed
3. Subduction Zone Biology: Because subduction zones are the coldest parts of the Earth's interior and life cannot exist at temperatures >150°C, subduction zones are almost certainly associated with the deepest (highest pressure) biosphere. The biosphere is the broadest level of ecological study the global sum of all Ecosystems.
4. Subduction zones mix subducted sediments, oceanic crust, and mantle lithosphere and mix this with mantle from the overriding plate to produce fluids, calc-alkaline series melts, ore deposits, and continental crust. Calc-alkaline and calc-alkalic are equivalent terms applied to groups of Igneous rocks that commonly occur together and that have compositions related by the characteristic

Subduction zones have also being considered as possible disposal sites for nuclear waste, where the action would carry the material into the planetary mantle, safely away from any possible influence on humanity or the surface environment, but this method of disposal is currently banned by international agreement^[2]. Radioactive wastes are Waste types containing radioactive Chemical elements that do not have a practical purpose The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided

See also

• Plate tectonics
• List of tectonic plate interactions
• Back-arc basin
• Oceanic rift
• Oceanic trench
• Obduction
• Orogeny

References

External links

• Animation of a subduction zone.