Trench

A trench is a long, narrow, deep depression in the seafloor that marks where one tectonic plate subducts beneath another. It is the surface trace of the subduction zone and holds the ocean’s greatest depths: the Mariana Trench reaches about 10,920 meters in the Challenger Deep. The trench is asymmetric, with a steep landward inner wall rising to the accretionary prism and arc and a gentler outer wall on the bending subducting plate. Sediment fill varies from starved in the western Pacific to thick where a large fan enters. The trench axis defines the hadal zone and the deepest part of an active margin.