Instead they applied the term “deep“ for the deepest parts of the ocean, such as Challenger Deep. The term “trench” does not appear in Murray and Hjort’s (1912) classic oceanography book. Even then the elongated bathymetric expression of trenches was not recognized until well into the 20th century. 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. Trenches were not clearly defined until the late 1940s and 1950s. 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. When buoyant continental crust enters a trench, subduction eventually stops and the convergent plate margin becomes a collision zone. 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. 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. This applies to Cascadia, Makran, southern Lesser Antilles, and Calabrian trenches. 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. 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. Major Pacific trenches (1-10) and fracture zones (11-20): 1. Oceanic lithosphere moves into trenches at a global rate of about a tenth of a square metre per second. The greatest ocean depth to be sounded is in the Challenger Deep of the Mariana Trench, at a depth of 10,911 m (35,797 ft) below sea level. Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Along convergent plate boundaries, plates move together at rates that vary from a few mm to over ten cm per year. Trenches are a distinctive morphological feature of plate boundaries. 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 define one of the most important natural boundaries on the Earth’s solid surface: the one between two lithospheric plates. They are also the deepest parts of the ocean floor. The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches.
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