A deep-sea trench is a narrow, elongate, v-shaped depression in the ocean floor. Trenches are the deepest parts of the ocean, and the lowest points on Earth, reaching depths of nearly 7 mi (10 km) below sea level. These long, narrow, curving depressions can be thousands of miles in length, yet as little as 5 mi (8 km) in width. Deep-sea trenches are part of a system of tectonic processes termed subduction. Subduction zones are one type of convergent plate boundary where either an oceanic or a continental plate overrides an oceanic plate. A trench is formed where the oceanic plate dives below (is subducted by) the (less dense) overriding plate. They are associated with a certain type of volcanic chain called an island arc and with zones of high earthquake activity. The trenches can extend for thousands of kilometers parallel to the volcanoes of the island arcs located on...