Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches.

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.

Trenches define one of the most important natural boundaries on the Earth’s solid surface, that between two lithospheric plates. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. There are three types of lithospheric plate boundaries: divergent (where lithosphere and oceanic crust is created at mid-ocean ridges), convergent (where one lithospheric plate sinks beneath another and returns to the mantle), and transform (where two lithospheric plates slide past each other). In Plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region A transform fault is a fault which runs along the boundary of a Tectonic plate. Trenches are the spectacular and distinctive morphological features of plate boundaries. Plates move together along convergent plate boundaries at convergence rates that vary from a few millimeters to ten or more centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. 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 Trenches are generally parallel to a volcanic island arc, and about 200 km from a volcanic arc. A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under Oceanic trenches typically extend 3 to 4 km (1. 9 to 2. 5 mi) below the level of the surrounding oceanic floor. The deepest ocean depth to be sounded is in the Challenger Deep of the Mariana Trench at a depth of 10,911 m (35,798 ft) below sea level. The Challenger Deep is the deepest surveyed point in the oceans with a depth of about 11000 metres (about 36000 feet 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 Oceanic lithosphere disappears into trenches at a global rate of about a tenth of a square meter per second.

Geographic distribution

There are about 50,000 km of convergent plate margins, mostly around the Pacific Ocean – the reason for the reference “Pacific-type” margin - but they are also in the eastern Indian Ocean, with relatively short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea. In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Indian Ocean is the third largest of the world's Oceanic divisions covering about 20% of the water on the Earth 's surface Trenches are sometimes buried and lack bathymetric expression, but the fundamental structures that these represent mean that the great name should also be applied here. This applies to Cascadia, Makran, southern Lesser Antilles, and Calabrian trenches. The Cascadia Subduction zone is a Subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to Makran ( Urdu / Persian: مکران) is a semi-desert coastal strip in the south of Balochistan, in Iran and Pakistan, along The Lesser Antilles, also known as the Caribbees, are part of the Antilles, which together with the Bahamas and Greater Antilles form the Calabria ( Latin: Brutium) is a region in southern Italy, south of Naples, located at the "toe" of Trenches along with volcanic arcs and zones of earthquakes that dip under the volcanic arc as deeply as 700 km are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. 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 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 Trenches are related to but distinguished from continental collision zones (like that between India and Asia to form the Himalaya), where continental crust enters the subduction zone. The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed When buoyant continental crust enters a trench, subduction eventually stops and the convergent plate margin becomes a collision zone. Features analogous to trenches are associated with collisions zones; these are sediment-filled foredeeps referred to as peripheral foreland basins, such as that which the Ganges River and Tigris-Euphrates rivers flow along. The Ganges (ˈgænʤiːz also Ganga, Devanāgarī: hi गंगा in most Indian languages) is the major river in the Indian subcontinent

History of the term "Trench"

Trenches are the most spectacular morphologic feature on Earth’s solid surface but they were not clearly defined until the late 1940’s and 1950’s. The bathymetry of the ocean was of no real interest until the late 19th and early 20th centuries, with the initial laying of Transatlantic telegraph cables on the seafloor between the continents. Bathymetry is the underwater equivalent to Hypsometry. The name comes from Greek βαθυς deep, and μετρον measure. The first transatlantic Telegraph Cable crossed the Atlantic Ocean from Foilhommerum Valentia Island, in western Ireland to Even then the elongated bathymetric expression of trenches was not recognized until well into the 20th century. The term “trench” does not appear in Murray and Hjort’s (1912) classic oceanography book. Sir John Murray KCB (3 March 1841 – 16 March 1914 was a pioneering Scots-Canadian Oceanographer and Marine biologist. Oceanography (from the greek words Ωκεανός meaning Ocean and γράφω meaning to write also called oceanology or Instead they applied the term “deep’ for the deepest parts of the ocean, such as Challenger Deep. The Challenger Deep is the deepest surveyed point in the oceans with a depth of about 11000 metres (about 36000 feet Experiences from World War I battlefields emblazoned the concept of the trench warfare as an elongate depression defining an important boundary, so it was no surprise that the term “trench” was used to describe natural features in the early 1920’s. World War I (abbreviated WWI; also known as the First World War, the Great War, and the War to End All Trench warfare is a form of warfare where both combatants have fortified positions and fighting lines are static The term was first used in a geologic context by Scofield two years after the war ended to describe a structurally-controlled depression in the Rocky Mountains. Mountain peaks of the Rocky Mountains The Rocky Mountains, often called the Rockies, are a Mountain range in western North America. Johnstone, in his 1923 textbook An Introduction to Oceanography first used the term in its modern sense for any marked, elongate depression of the sea bottom.

During the 1920’s and 1930’s, Felix Andries Vening Meinesz developed a unique gravimeter that could measure gravity in the stable environment of a submarine and used it to measure gravity over trenches. Felix Andries Vening Meinesz ( The Hague July 30 1887 - Amersfoort August 10 1966) was a Dutch geophysicist and Gravimetry#How_gravity_is_measured A gravimeter or Gravitometer, is an instrument used in Gravimetry for measuring the local Gravitational field. Gravitation is a natural Phenomenon by which objects with Mass attract one another His measurements revealed that trenches are sites of downwelling in the solid Earth. 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 concept of downwelling at trenches was characterized by Griggs in 1939 as the tectogene hypothesis, for which he developed an analogue model using a pair of rotating drums. World War II in the Pacific led to great improvements of bathymetry in especially the western and northern Pacific, and the linear nature of these deeps became clear. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including The rapid growth of deep sea research efforts, especially the widespread use of echosounders in the 1950’s and 1960’s confirmed the morphological utility of the term. The important trenches were identified, sampled, and their greatest depths sonically plumbed. The heroic phase of trench exploration culminated in the 1960 descent of the Bathyscaphe "Trieste", which set an unbeatable world record by diving to the bottom of the Challenger Deep. Design Trieste was designed by the Swiss scientist Auguste Piccard and built in Italy Following Robert S. Dietz’ and Harry Hess’ articulation of the seafloor spreading hypothesis in the early 1960’s and the plate tectonic revolution in the late 1960’s the term ‘trench’ has been redefined with plate tectonic as well as bathymetric connotations!

Modern understanding of trenches

Trenches define one of the most important natural boundaries on the Earth’s solid surface, that between two lithospheric plates. Robert Sinclair Dietz ( September 14, 1914 &ndash May 19, 1995) was Professor of Geology at Arizona State University. Harry Hess (Born July 5, 1968 in Oshawa Ontario, Canada) is a Canadian Record producer, Singer and Guitarist Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere There are three types of lithospheric plate boundaries: divergent (where lithosphere and oceanic crust is created at mid-ocean ridges), convergent (where one lithospheric plate sinks beneath another and returns to the mantle), and transform (where two lithospheric plates slide past each other). Trenches are the spectacular and distinctive morphological features of convergent plate boundaries. Plates move together along convergent plate boundaries at convergence rates that vary from a few millimetres to ten or more centimetres per year. Trenches form where oceanic lithosphere is subducted at a convergent plate margin, presently at a global rate of about a tenth of a square metre per second.

Trench rollback

Although trenches would seem to be positionally stable over time, it is hypothesized that some trenches, particularly those associated with subduction zones where two oceanic plates converge, retrograde, that is, they move backward into the plate which is subducting, akin to a backward-moving wave. This has been termed trench rollback (also hinge rollback). This is one explanation for the existence of back-arc basins. Back-arc basins (or retro-arc basins are geologic features submarine basins associated with Island arcs and Subduction zones.

Morphologic expression

     The Peru-Chile Trench

Trenches are centerpieces of the distinctive physiography of a convergent plate margin. The Peru-Chile Trench, also known as the Atacama Trench, is an Oceanic trench in the eastern Pacific Ocean, about 160 kilometers (100 mi off the coast Transects across trenches yield asymmetric profiles, with relatively gentle (~5°) outer (seaward) slope and a steeper (~10-16°) inner (landward) slope. This asymmetry is due to the fact that the outer slope is defined by the top of the downgoing plate, which must bend as it starts its descent. The great thickness of the lithosphere requires that this bending be gentle. As the subducting plate approaches the trench, it is first bent upwards to form the outer trench swell, then descends to form the outer trench slope. The outer trench swell 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 The outer trench slope is disrupted by a set of subparallel normal faults which staircase the seafloor down to the trench. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement The plate boundary is defined by the trench axis itself. Beneath the inner trench wall, the two plates slide past each other along the subduction decollement, the seafloor intersection of which defines the trench location. A décollement horizon is a Tectonic surface that acts as a gliding plane between two masses in a Thrust fault relationship The overriding plate contains volcanic arc (generally) and a forearc. A volcanic arc is a chain of volcanic islands or Mountains formed by Plate tectonics as an oceanic Tectonic plate subducts under A forearc is a depression in the Sea floor located between a Subduction zone and an associated Volcanic arc. The volcanic arc is caused by physical and chemical interactions between the subducted plate at depth and asthenospheric mantle associated with the overriding plate. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the The forearc lies between the trench and the volcanic arc. Forearcs have the lowest heatflow from the interior Earth because there is no asthenosphere (convecting mantle) between the forearc lithosphere and the cold subducting plate. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the

The inner trench wall marks the edge of the overriding plate and the outermost forearc. The forearc consists of igneous and metamorphic crust, and this crust acts as buttress to a growing accretionary prism (sediments scraped off the downgoing plate onto the inner trench wall, depending on how much sediment is supplied to the trench). Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change If the flux of sediments is high, material will be transferred from the subducting plate to the overriding plate. In this case an accretionary prism grows and the location of the trench migrates progressively away from the volcanic arc over the life of the convergent margin. Convergent margins with growing accretionary prisms are called accretionary convergent margins and make up nearly half of all convergent margins. If the sediment flux is low, material will be transferred from the overriding plate to the subducting plate by a process of tectonic ablation known as subduction erosion and carried down the subduction zone. Forearcs undergoing subduction erosion typically expose igneous rocks. In this case, the location of the trench will migrate towards the magmatic arc over the life of the convergent margin. Convergent margins experiencing subduction erosion are called nonaccretionary convergent margins and comprise more than half of convergent plate boundaries. This is an oversimplification, because different parts of a convergent margin can experience sediment accretion and subduction erosion over its life.

The asymmetric profile across a trench reflects fundamental differences in materials and tectonic evolution. The outer trench wall and outer swell comprise seafloor that takes a few million years to move from where subduction-related deformation begins near the outer trench swell until sinking beneath the trench. In contrast, the inner trench wall is deformed by plate interactions for the entire life of the convergent margin. The forearc is continuously subjected to subduction-related earthquakes. 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 This protracted deformation and shaking ensures that the inner trench slope is controlled by the angle of repose of whatever material it is composed of. Because they are composed of igneous rocks instead of deformed sediments, non-accretionary trenches have steeper inner walls than accretionary trenches.

Filled trenches

The composition of the inner trench slope and a first-order control on trench morphology is determined by sediment supply. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of Active accretionary prisms are common for trenches near continents where large rivers or glaciers reach the sea and supply great volumes of sediment which naturally flow to the trench. A continent is one of several large Landmasses on Earth. They are generally identified by Convention rather than any strict criteria with seven regions "Riverine" redirects here For the use of that term in Maritime geography, see there "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period. These filled trenches are confusing because in a plate tectonic sense they are indistinguishable from other convergent margins but lack the bathymetric expression of a trench. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Bathymetry is the underwater equivalent to Hypsometry. The name comes from Greek βαθυς deep, and μετρον measure. The Cascadia margin of the northwest USA is a filled trench, the result of sediments delivered by the rivers of the NW USA and SW Canada. The Cascadia Subduction zone is a Subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to The Lesser Antilles convergent margin shows the importance of proximity to sediment sources for trench morphology. The Lesser Antilles, also known as the Caribbees, are part of the Antilles, which together with the Bahamas and Greater Antilles form the In the south, near the mouth of the Orinoco River, there is no morphological trench and the forearc plus accretionary prism is almost 500 km wide. ORiNOCO is the brand name that was used for a family of wireless networking solutions by Proxim (previously Lucent) The accretionary prism is so large that it forms the islands of Barbados and Trinidad. Barbados ( Portuguese word for bearded-ones, bɑrˈbeɪdoʊz -dɒs situated just east of the Caribbean Sea, is an independent Island nation Trinidad ( Spanish: " Trinity " is the largest and most populous of the two major islands and Northward the forearc narrows, the accretionary prism disappears, and only north of 17°N the morphology of a trench is seen. In the extreme north, far away from sediment sources, the Puerto Rico Trench is over 8600 m deep and there is no active accretionary prism. The Puerto Rico Trench is an Oceanic trench located on the boundary between the Caribbean Sea and the Atlantic Ocean. A similar relationship between proximity to rivers, forearc width, and trench morphology can be observed from east to west along the Alaskan-Aleutian convergent margin. Alaska ( Аляска Alyaska) is a state in the United States of America, in the northwest of the North American continent The convergent plate boundary offshore Alaska changes along its strike from a filled trench with broad forearc in the east (near the coastal rivers of Alaska) to a deep trench with narrow forearc in the west (offshore the Aleutian islands). Another example is the Makran convergent margin offshore Pakistan and Iran, which is a trench filled by sediments from the Tigris-Euphrates and Indus rivers. Makran ( Urdu / Persian: مکران) is a semi-desert coastal strip in the south of Balochistan, in Iran and Pakistan, along The Tigris is the eastern member of the two great Rivers that define Mesopotamia, along with the Euphrates, which flows from the mountains of southeastern The Euphrates ( ( Arabic: ar نهر الفرات; Turkish: tr Fırat Syriac: syr ܦܪܬ; Hebrew: he פרת The Indus River { Sanskrit: सिन्धु Sindhu; Urdu: urd {{Nastaliq سندھ}} Sindh; Sindhi: snd Thick accumulations of turbidites along a trench can be supplied by down-axis transport of sediments that enter the trench 1000-2000 km away, as is found for the Peru-Chile Trench south of Valparaíso and for the Aleutian Trench. Turbidite Geological formations have their origins in Turbidity current deposits, which are deposits from a form of underwater Avalanche that The Peru-Chile Trench, also known as the Atacama Trench, is an Oceanic trench in the eastern Pacific Ocean, about 160 kilometers (100 mi off the coast Valparaíso (literally in Spanish: Valle Paraíso (Paradise Valley and also called "Valpo" locally is a major city in Chile Convergence rate can also be important for controlling trench depth, especially for trenches near continents, because slow convergence causes the capacity of the convergent margin to dispose of sediment to be exceeded.

There an evolution in trench morphology can be expected as oceans close and continents converge. While the ocean is wide, the trench may be far away from continental sources of sediment and so may be deep. As the continents approach each other, the trench may become filled with continental sediments and become shallower. A simple way to approximate when the transition from subduction to collision has occurred is when the plate boundary previously marked by a trench is filled enough to rise above sealevel.

Accretionary prisms and sediment transport

Accretionary prisms grow by frontal accretion, whereby sediments are scraped off, bulldozer-fashion, near the trench, or by underplating of subducted sediments and perhaps oceanic crust along the shallow parts of the subduction decollement. ----A bulldozer is a crawler ( Caterpillar tracked Tractor) equipped with a substantial metal plate (known as a blade) used to push large quantities Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Frontal accretion over the life of a convergent margin results in younger sediments defining the outermost part of the accretionary prism and the oldest sediments defining the innermost portion. Older (inner) parts of the accretionary prism are much more lithified and have steeper structures than the younger (outer) parts. Underplating is difficult to detect in modern subduction zones but may be recorded in ancient accretionary prisms such as the Franciscan Group of California in the form of tectonic mélanges and duplex structures. Different modes of accretion are reflected in morphology of the inner slope of the trench, which generally shows three morphological provinces. The lower slope comprises imbricate thrust slices that form ridges. The mid slope may comprise a bench or terraces. The upper slope is smoother but may be cut by submarine canyons. A submarine canyon is a steep-sided Valley on the Sea floor of the Continental slope. Because accretionary convergent margins have high relief, are continuously deformed, and accommodate a large flux of sediments, they are vigorous systems of sediment dispersal and accumulation. Sediment transport is controlled by submarine landslides, debris flows, turbidity currents, and contourites. A landslide is a geological phenomenon which includes a wide range of ground movement such as rock falls deep failure of slopes and shallow debris flows which can occur A turbidity current or density current is a current of rapidly moving sediment-laden water moving down a slope through air water or another fluid Submarine canyons transport sediment from beaches and rivers down the upper slope. These canyons form by channelized turbidites and generally lose definition with depth because continuous faulting disrupts the submarine channels. Sediments move down the inner trench wall via channels and a series of fault-controlled basins. The trench itself serves as an axis of sediment transport. If enough sediment moves to the trench, it may be completely filled so that turbidity currents are able to carry sediments well beyond the trench and may even surmount the outer swell. Sediments from the rivers of SW Canada and NW USA spill over where the Cascadia trench would be and cross the Juan de Fuca plate to reach the spreading ridge several hundred kilometres to the west. The Juan de Fuca Plate, named after the explorer, is a Tectonic plate arising from the Juan de Fuca Ridge, and subducting under the northerly

The slope of the inner trench slope of an accretionary convergent margin reflects continuous adjustments to the thickness and width of the accretionary prism. The prism maintains a ‘critical taper’, established in conformance with Mohr-Coulomb Theory for the pertinent materials. Mohr-Coulomb theory is a Mathematical model (see Yield surface) describing the response of a material such as rubble piles or Concrete to shear stress A package of sediments scraped off the downgoing lithospheric plate will deform until it and the accretionary prism that it has been added to attain a critical taper (constant slope) geometry. Once critical taper is attained, the wedge slides stably along its basal decollement. Strain rate and hydrologic properties strongly influence the strength of the accretionary prism and thus the angle of critical taper. Fluid pore pressures modify rock strength and are important controls of critical taper angle. Low permeability and rapid convergence may result in pore pressures that exceed lithostatic pressure and a relatively weak accretionary prism with a shallowly tapered geometry, whereas high permeability and slow convergence result in lower pore pressure, stronger prisms, and steeper geometry.

The Hellenic trench system is unusual because this convergent margin subducts evaporites. Evaporites (iˈvæpəraɪt are water-soluble Mineral sediments that result from the Evaporation of bodies of surficial Water. The slope of the surface of the southern flank of the Mediterranean Ridge (its accretionary prism) is low, about 1°, which indicates very low shear stress on the decollement at the base of the wedge. Evaporites influence the critical taper of the accretionary complex, as their mechanical properties differ from those of siliciclastic sediments, and because of their effect upon fluid flow and fluid pressure, which control effective stress. In the 1970s, the linear deeps of the Hellenic trench south of Crete were interpreted to be similar to trenches at other subduction zones, but with the realization that the Mediterranean Ridge is an accretionary complex, it became apparent that the Hellenic trench is actually a starved forearc basin, and that the plate boundary lies south of the Mediterranean Ridge. Crete ( Greek: Κρήτη transliteration: Krētē, modern transliteration Kriti) is the largest of the Greek islands and the

Water and biosphere

The volume of water escaping from within and beneath the forearc results in some of Earth’s most dynamic and complex interactions between aqueous fluids and rocks. A forearc is a depression in the Sea floor located between a Subduction zone and an associated Volcanic arc. Most of this water is trapped in pores and fractures in the upper lithosphere and sediments of the subducting plate. The average forearc is underrun by a solid volume of oceanic sediment that is 400 m thick. This sediment enters the trench with 50-60% porosity. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% These sediments are progressively squeezed as they are subducted, reducing void space and forcing fluids out along the decollement and up into the overlying forearc, which may or may not have an accretionary prism. Sediments accreted to the forearc are another source of fluids. Water is also bound in hydrous minerals, especially clays and opal. Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and OPAL ( Open Pool Australian Lightwater reactor) is a 20 megawatt pool-type nuclear research reactor that was officially opened in April 2007 Increasing pressure and temperature experienced by subducted materials converts the hydrous minerals to denser phases that contain progressively less structurally-bound water. Water released by dehydration accompanying phase transitions is another source of fluids introduced to the base of the overriding plate. These fluids may travel through the accretionary prism diffusely, via interconnected pore spaces in sediments, or may follow discrete channels along faults. Sites of venting may take the form of mud volcanoes or seeps and are often associated with chemosynthetic communities. Fluids escaping from the shallowest parts of a subduction zone may also escape along the plate boundary but have rarely been observed draining along the trench axis. All of these fluids are dominated by water but also contain dissolved ions and organic molecules, especially methane. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Methane is often sequestered in an ice-like form (methane clathrate, also called gas hydrate) in the forearc. Methane clathrate, also called methane hydrate or methane ice, is a solid form of water that contains a large amount of Methane within its Crystal These are a potential energy source and can rapidly break down. Destabilization of gas hydrates has contributed to global warming in the past and will likely do so in the future.

Chemosynthetic communities thrive where cold fluids seep out of the forearc. Chemosynthesis is the biological conversion of one or more carbon molecules (usually Carbon dioxide or Methane) and nutrients into organic matter using the Oxidation Cold seep communities have been discovered in inner trench slopes down to depths of 6000 m in the western Pacific, especially around Japan, in the Eastern Pacific along North, Central and South America coasts from the Aleutian to the Peru-Chile trenches, on the Barbados prism, in the Mediterranean, and in the Indian Ocean along the Makran and Sunda convergent margins. These communities receive much less attention than the chemosynthetic communities associated with hydrothermal vents. A hydrothermal vent is a Fissure in a planet's surface from which geothermally heated Water issues Chemosynthetic communities are located in a variety of geological settings: above over-pressured sediments in accretionary prisms where fluids are expelled through mud volcanoes or ridges (Barbados, Nankai and Cascadia); along active erosive margins with faults; and along escarpments caused by debris slides (Japan trench, Peruvian margin). Surface seeps may be linked to massive hydrate deposits and destabilization (e. g. Cascadia margin). High concentrations of methane and sulfide in the fluids escaping from the seafloor are the principal energy sources for chemosynthesis. The term sulfide ( sulphide in British English) refers to several types of Chemical compounds containing Sulfur in its lowest Oxidation

Empty trenches and subduction erosion

Trenches distant from an influx of continental sediments lack an accretionary prism, and the inner slope of such trenches is commonly composed of igneous or metamorphic rocks. Non-accretionary convergent margins are characteristic of (but not limited to) primitive arc systems. Primitive arc systems are those built on oceanic lithosphere, such as the Izu-Bonin-Mariana, Tonga-Kermadec, and Scotia (South Sandwich) arc systems. The inner trench slope of these convergent margins exposes the crust of the forearc, including basalt, gabbro, and serpentinized mantle peridotite. These exposures allow easy access to study the lower oceanic crust and upper mantle in place and provide a unique opportunity to study the magmatic products associated with the initiation of subduction zones. Most ophiolites probably originate in a forearc environment during the initiation of subduction, and this setting favors ophiolite emplacement during collision with blocks of thickened crust. Not all non-accretionary convergent margins are associated with primitive arcs. Trenches adjacent to continents where there is little influx of sediments carried by rivers, such as the central part of the Peru-Chile Trench, may also lack an accretionary prism.

Igneous basement of a nonaccretionary forearc may be continuously exposed by subduction erosion. This transfers material from the forearc to the subducting plate and can be accomplished by frontal erosion or basal erosion. Frontal erosion is most active in the wake of seamounts being subducted beneath the forearc. Subduction of large edifices (seamount tunneling) oversteepens the forearc, causing mass failures that carry debris towards and ultimately into the trench. This debris may be deposited in graben of the downgoing plate and subducted with it. In contrast, structures resulting from subduction erosion of the base of the forearc are difficult to recognize from seismic reflection profiles, so the possibility of basal erosion is difficult to confirm. Subduction erosion may also diminish a once-robust accretionary prism if the flux of sediments to the trench diminishes.

Nonaccretionary forearcs may also be the site of serpentine mud volcanoes. The serpentine group describes a group of common rock-forming hydrous Magnesium Iron phyllosilicate (()3 Minerals they Note See the Volcano article for information on Magmatic volcanoes such as Mount St These form where fluids released from the downgoing plate percolate upwards and interact with cold mantle lithosphere of the forearc. Mantle peridotite is hydrated into serpentinite, which is much less dense than peridotite and so will rise diapirically when there is an opportunity to do so. A peridotite is a dense coarse-grained Igneous rock, consisting mostly of the minerals Olivine and Pyroxene. Some nonaccretionary forearcs are subjected to strong extensional stresses, for example the Marianas, and this allows buoyant serpentinite to rise to the seafloor where they form serpentinite mud volcanoes. Chemosynthetic communities are also found on non-accretionary margins such as the Marianas, where they thrive on vents associated with serpentinite mud volcanoes.

Factors affecting trench depth

The Puerto Rico Trench

There are several factors that control the depth of trenches. The Puerto Rico Trench is an Oceanic trench located on the boundary between the Caribbean Sea and the Atlantic Ocean. The most important control is the supply of sediment, which fills the trench so that there is no bathymetric expression. Bathymetry is the underwater equivalent to Hypsometry. The name comes from Greek βαθυς deep, and μετρον measure. It is therefore not surprising that the deepest trenches (deeper than 8,000 m) are all nonaccretionary. In contrast, all trenches with growing accretionary prisms are shallower than 8000 m. A second order control on trench depth is the age of the lithosphere at the time of subduction. Because oceanic lithosphere cools and thickens as it ages, it subsides. Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins The older the seafloor, the deeper it lies and this determines a minimum depth from which seafloor begins its descent. This obvious correlation can be removed by looking at the relative depth, the difference between regional seafloor depth and maximum trench depth. Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths. Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlying asthenosphere to flow around the edges of the sinking plate. The asthenosphere (from an invented Greek a + ' sthenos "without strength" and Greek word σφαίρα (sphera meaning globe is the Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as the Challenger Deep. The Challenger Deep is the deepest surveyed point in the oceans with a depth of about 11000 metres (about 36000 feet

Major oceanic trenches

Ancient oceanic trenches

References

• R. 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 The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Tonga Trench is located in the Pacific Ocean and is 10882 meters (35702 ft deep at its deepest point known as the Horizon Deep. The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Kuril-Kamchatka Trench or Kuril Trench ( Russian Курило-Камчатский жёлоб) is an Oceanic trench with a maximum depth The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Philippine Trench or Philippine Deep or Mindanao Trench or Mindanao Deep is a Submarine trench to the east of the Philippine Islands The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Kermadec trench is one of Earth's deepest Oceanic trenches reaching a depth of 10047 m The Pacific Ocean is the largest of the Earth 's Oceanic divisions The, also known as Izu-Bonin Trench is an Oceanic trench in the western Pacific Ocean. The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Japan Trench is an Oceanic trench, a part of the Pacific Ring of Fire, in the floor of the northern Pacific Ocean off northeast Japan The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Puerto Rico Trench is an Oceanic trench located on the boundary between the Caribbean Sea and the Atlantic Ocean. The Peru-Chile Trench, also known as the Atacama Trench, is an Oceanic trench in the eastern Pacific Ocean, about 160 kilometers (100 mi off the coast The Peru-Chile Trench, also known as the Atacama Trench, is an Oceanic trench in the eastern Pacific Ocean, about 160 kilometers (100 mi off the coast The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Aleutian Trench (or Aleutian Trough) is a Subduction zone and Oceanic trench which runs along the southern coastline of Alaska and the Alaska ( Аляска Alyaska) is a state in the United States of America, in the northwest of the North American continent New Guinea, located just north of Australia, is the world's second largest island, having become separated from the Australian mainland when the area now known The Cayman Trough, or Cayman Trench, also called Bartlett Deep, or Bartlett Trough, is a complex Transform fault zone which contains a small For the region see Caribbean. The Caribbean Sea (kəˈrɪbiən or /ˌkærɨˈbiːən/ is a tropical Sea in the Western Hemisphere Baja California (pronounced ˈbɑːhɑː kælɨˈfɔrnjə in English is the northernmost state of Mexico. The Hikurangi Trench (also called the Hikurangi Trough) is a linear deep in the Pacific Ocean off the east coast of the North Island of New Zealand New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island The Japan Trench is an Oceanic trench, a part of the Pacific Ring of Fire, in the floor of the northern Pacific Ocean off northeast Japan For a topic outline on this subject see List of basic Japan topics. The Kuril-Kamchatka Trench or Kuril Trench ( Russian Курило-Камчатский жёлоб) is an Oceanic trench with a maximum depth The Kuril Islands (ˈkʊrɪl or /ˈkjuˈriˈl/ Кури́льские острова́ əstrʌˈva ru-Latn ''Kuril'skie ostrova'' or Kurile Islands in Russia 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 An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. The Pacific Ocean is the largest of the Earth 's Oceanic divisions The Mariana Islands (also the Marianas; up to the early 20th century sometimes called Ladrones Islands, from Spanish Islas de los Ladrones meaning The Middle America Trench is a major Subduction zone, an Oceanic trench in the eastern Pacific Ocean off the southwestern coast of Middle America For the former North American fur-trading district see New Caledonia (Canada, and for the Scottish colony in Panama see Darien scheme. The Puerto Rico Trench is an Oceanic trench located on the boundary between the Caribbean Sea and the Atlantic Ocean. For the region see Caribbean. The Caribbean Sea (kəˈrɪbiən or /ˌkærɨˈbiːən/ is a tropical Sea in the Western Hemisphere The Peru-Chile Trench, also known as the Atacama Trench, is an Oceanic trench in the eastern Pacific Ocean, about 160 kilometers (100 mi off the coast Peru (Perú Piruw Piruw officially the Republic of Peru ( reˈpuβlika del peˈɾu is a country in western South America. Chile, officially the Republic of Chile ( Spanish:) is a country in South America occupying a long and narrow Coastal strip wedged between the The Philippine Trench or Philippine Deep or Mindanao Trench or Mindanao Deep is a Submarine trench to the east of the Philippine Islands The Philippines ( Filipino: Pilipinas, officially known as the Republic of the Philippines (fil ''Republika ng Pilipinas'' RP The Ryukyu Trench is an 2250 kilometer (1398 mi long ocean trench running north along the eastern edge of Japan 's Ryukyu Islands in the Philippine Sea The Ryukyu Islands, in Japanese called the (literally Southwest Islands are a chain of Japanese islands in the western Pacific Ocean at the eastern limit The South Sandwich Trench is a deep arcuate trench in the South Atlantic Ocean lying 100 km to the east of the South Sandwich Islands. The Sunda Arc is a Volcanic arc that has produced the islands of Sumatra and Java and the Sunda Strait and the Lesser Sunda Islands The Java Trench, also called Sunda Trench, located in the northeastern Indian Ocean, with a length of 2600 km and a maximum depth of 7725 meters (at 10°19'S 109°58'E The Tonga Trench is located in the Pacific Ocean and is 10882 meters (35702 ft deep at its deepest point known as the Horizon Deep. For a topic outline on this subject see List of basic Australia topics. The Yap Trench is an Oceanic trench in the western Pacific Ocean. Palau, officially the Republic of Palau (Beluu er a Belau is an island nation in the Pacific Ocean, some 500 miles (800  km) east The Intermontane Trench was an ancient Oceanic trench during the Triassic time The Intermontane Islands were a giant chain of active Volcanic islands somewhere out in the Pacific Ocean during the Triassic time beginning around 245 million The Insular Islands were a giant chain of active Volcanic islands somewhere in the Pacific Ocean during the Cretaceous time that rode on top a microplate The Farallon Trench was an ancient Oceanic trench on the west coast of North America during the late Cretaceous period The Tethyan Trench was an ancient Oceanic trench that existed in the northern part of the Tethys Ocean during the middle Mesozoic to early Cenozoic Turkey (Türkiye known officially as the Republic of Turkey ( is a Eurasian Country that stretches For a topic outline on this subject see List of basic Iran topics. Definitions of Tibet See also Definitions of Tibet Name In English The English word Tibet, like the word for Tibet in most European J. Stern 2002. "Subduction Zones". Reviews of Geophysics. 10. 1029/2001RG000108
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See also

• Oceanic ridge
• Plate tectonics
• Physical Oceanography
• List of Landforms
• List of Deepest Ocean Trenches