Source Encyclopædia Britannica, Inc.

This term describes any seafloor feature that is a long, narrow depression in the seafloor in maximal depths from 7,300 to 11,000 metres depth. These trenches form in locations where tectonic plates are in contact and one plate slides under another. The trench forms an arc usually and can be dotted with undersea volcanoes. The are also the centre of big earthquakes and their resultant tsunamis.The deepest is the Mariana Trench in the western North. The longest trench is the Peru-Chile Trench, which extends some 5,900 km along the west coast of South America. Trenches are relatively narrow, usually less than 100 km wide. Of the Earth’s 20 major trenches, 17 are found in the Pacific basin. The Java Trench extends from northern Australia to the northwestern end of Sumatra. The New Britain and Solomon trenches, the New Hebrides Trench, the Tonga and Kermadec trenches are all found in our region. Deep-sea trenches are V-shaped with steep sides. Narrow, flat abyssal plains of ponded sediment generally occupy the bottom of the trench. The bending forces will usually create a ridge of up to 1000 metres height close to the trench. This sometimes breaks the surface to create islands, as in the Java Trench. A line of volcanoes often forms about 100 km from the trench. They can form active volcanic island chains, such as the Mariana Islands. Some seafloor features are called trenches but they are not true ocean trenches caused by overlapping plates. The Diamantina trench (Diamantina Fracture Zone) extends westward from the southwest coast of Australia. It is a rift valley that was formed when Australia separated from Antarctica between 60 and 50 million years ago. Trenches aren’t usually filled with as much sediment as the moving plates draws old sediment into the Earth’s crust. They tend to be filled with soils and ash from landslides along the trench walls. In the immense pressures and total darkness of a trench it could be assumed that there is little life. Instead there are a vast array of very highly specialised creatures. We rarely visit trenches, but when we do we always find something new.