Editing Subduction (section)

===Arc-trench complex===
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The surface expressions of subduction zones are arc-trench complexes. On the ocean side of the complex, where the subducting plate first approaches the subduction zone, there is often an ''[[outer trench high]]'' or ''outer trench swell''. Here the plate shallows slightly before plunging downwards, as a consequence of the rigidity of the plate.<ref>{{cite journal |last1=Whitman |first1=Dean |title=The Isostatic Residual Gravity Anomaly of the Central Andes, 12° to 29° S: A Guide to Interpreting Crustal Structure and Deeper Lithospheric Processes |journal=International Geology Review |date=May 1999 |volume=41 |issue=5 |pages=457–475 |doi=10.1080/00206819909465152|bibcode=1999IGRv...41..457W |s2cid=129797807 }}</ref> The point where the slab begins to plunge downwards is marked by an ''[[oceanic trench]]''. Oceanic trenches are the deepest parts of the ocean floor.

Beyond the trench is the ''[[forearc]]'' portion of the overriding plate. Depending on sedimentation rates, the forearc may include an [[accretionary wedge]] of sediments scraped off the subducting slab and accreted to the overriding plate. However, not all arc-trench complexes have an accretionary wedge. Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc basin is poorly developed in non-accretionary arcs.{{sfn|Stern|2002|pp=25-26}}

Beyond the forearc basin, volcanoes are found in long chains called ''[[volcanic arc]]s''. The subducting basalt and sediment are normally rich in [[hydrous]] minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward.<ref>{{cite journal |last1=Fujie |first1=Gou |display-authors=etal |year=2013 |title=Systematic changes in the incoming plate structure at the Kuril trench |journal=Geophysical Research Letters |volume=40 |issue=1 |pages=88–93 |doi=10.1029/2012GL054340 |bibcode=2013GeoRL..40...88F |doi-access=free }}</ref> 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]].{{sfn|Stern|2002|pp=6-10}} The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating [[magma]] via [[flux melting]].{{sfn|Schmincke|2003|pp=18,113-126}} The magmas, in turn, rise as [[diapir]]s because they are less dense than the rocks of the mantle.{{sfn|Stern|2002|pp=19-22}} The mantle-derived magmas (which are initially basaltic in composition) can ultimately reach the Earth's surface, resulting in volcanic eruptions. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Earth's crust or undergoes [[fractional crystallization (geology)|fractional crystallization]]. Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive.{{sfn|Stern|2002|p=27–28}} [[Krakatoa]], [[Nevado del Ruiz]], and [[Mount Vesuvius]] are all examples of arc volcanoes. Arcs are also associated with most [[ore]] deposits.{{sfn|Stern|2002|pp=19-22}}

Beyond the volcanic arc is a [[back-arc region]] whose character depends strongly on the angle of subduction of the subducting slab. Where this angle is shallow, the subducting slab drags the overlying continental crust partially with it, which produces a zone of shortening and crustal thickening in which there may be extensive [[Folding (geology)|folding]] and [[thrust fault]]ing. If the angle of subduction steepens or rolls back, the upper plate lithosphere will be put in [[Tension (geology)|tension]] instead, often producing a [[back-arc basin]].{{sfn|Stern|2002|p=31}}