Submarine trench
Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr.
There are about 50,000 km (31,000 mi) 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. Globally, there are over 50 major ocean trenches covering an area of 1.9 million km2 or about 0.5% of the oceans. Trenches that are partially infilled are known as "troughs" and sometimes they are completely buried and lack bathymetric expression, but the fundamental plate tectonics structures that these represent mean that the great name should also be applied here. This applies to the Cascadia, Makran, southern Lesser Antilles, and Calabrian trenches. Trenches along with volcanic arcs and zones of earthquakes that dip under the volcanic arc as deeply as 700 km (430 mi) are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. Trenches are related to but distinguished from continental collision zones (such as that between India and Asia forming the Himalaya), where continental crust enters a subduction zone. When buoyant continental crust enters a trench, subduction eventually stops and the area becomes a zone of continental collision. Features analogous to trenches are associated with collisions zones, including sediment-filled foredeeps, such as those the Ganges River and Tigris-Euphrates rivers flow along.
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