trench

trench, a linear feature in the ocean where the depth exceeds 6,000 metres (19,700 ft). Trenches occur along the active margins of oceans where the oceanic crust is sliding under continental land masses or island arcs. They are highly active seismically; one of the most violent earthquakes ever recorded was associated with a trench off Chile. The active margin of the Pacific Ocean is known as the ‘Ring of Fire’; ocean crust is sliding under the continents at a rate of 6 centimetres (2.4 in.) a year. Onshore a great chain of volcanoes stretches from the Philippines, along Japan, Kamchatka, the Aleutian Islands, and then down the full length of the west coast of the Americas. Offshore lies a series of trenches. The older the ocean crust, the deeper the trench, because the crust is colder and stiffer. The deepest of all is the Marianas Trench. Scientists aboard HMS Challenger were the first to try and plumb it, and in 1960 the bathyscaphe Trieste made the deepest manned descent of it. It was remeasured in 2001 and was found to be 10,926 metres (35,838 ft) deep. The seabed of each trench is inhabited by a unique community of animals, but their remoteness means that their faunas remain poorly known. See also geological oceanography.

www.geocities.com/thesciencefiles/marianas/trench.html

M. V. Angel