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 the overriding plate. Examples include the Aleutian islands, an arc bordered to the south by the Aleutian trench, and the Marianas, bordered by the Mariana trench, the deepest in the world. Along the western coast of South America, the Peru-Chile trench marks where the Nazca plate is being subducted beneath the South American plate. The volcanic activity and uplift of the Andes mountains are a result of the subduction process.
Trenches and active subduction zones are found along much of the Ring of Fire, a zone of volcanism and earthquake activity that borders the Pacific ocean. The tectonic processes of subduction form the trenches and island arcs and are also responsible for the earthquake activity. Major earthquakes occur along the plunging boundary between the subducting and overriding plates. This boundary is called a Benioff zone. Many scientists believe that the volcanic island arcs are formed from magmas produced by the partial melting of the descending and/or the overriding plate. Considerable volcanic activity worldwide is the result of subduction.
Most geologists argue that the size of the earth has not changed significantly in the past several hundred million years. According to tectonic theory, new crust is generated along the divergent plate boundaries such as the mid-Atlantic ridge at rates on the order of a few centimeters a year. As the new crust is created, an equal amount must be destroyed at roughly the same rate for the size of Earth to remain unchanged. Subduction along the trenches of the convergent plate boundaries appears to maintain that balance.
