Oceanic Trenches

Oceanic trenches are narrow (up to 100 km wide), elongate depressions on the seafloor that are adjacent to active continent margins and island arc systems. Although most of them are found in the Pacific Ocean, there are a few in the Caribbean Sea and the Atlantic and Indian oceans. The Challenger Deep in the Marianas Trench adjacent to the Philippines is the deepest place on earth, reaching 10.7 km below the surface, a vertical relief greater than that of Mount Everest. The Puerto Rican Trench reaches a depth of 8.4 km. The 5,900km Peru-Chile Trench is the longest; it is bordered by volcanoes on South America.

It was not until the modern concept of plate tectonics was developed that an understanding of how oceanic trenches form was established. These trenches are associated with both earthquakes and volcanic activity and are the result of the interaction of two crustal plates, one sliding under the other and descending. As the underlying plate moves downward at a steep angle into hotter regions of the earth (a process known as subduction), pulled by convection currents and gravity, a trench is formed where one plate slides below the other. Subduction produces additional heat by friction. Heat causes the plate to melt, and the resulting molten material (magma) rises to the surface, erupts, and builds volcanoes. The downward movement also causes earthquakes. Where two oceanic plates converge, a volcanic island arc system of basaltic rock develops in the direction of the downward-dipping plate, such as occurs in the Aleutian Islands. An oceanic-continental interaction creates mountains as sea sediments are squeezed and raised upward, and volcanoes as a result of the melting of the inclined plate. In this situation the molten material rises through the mountains, is contaminated by the surrounding rocks, and erupts high on the mountains to form andesite volcanoes.

Trenches are relatively steeper on the landward side and gentler on the ocean basin side. Sediments accumulating in the trench derived from the erosion of the adjacent continent tend to be thicker on the landward side.

The oceanic trenches are home to famous hydrothermal vent faunas.

—Sidney Horenstein

See also: Deep-Sea Hydrothermal Vent Faunas;

Oceans; Plate Tectonics

References

Gurnis, Michael. 2001. "Sculpting the Earth from

Inside Out." Scientific American 284 (3): 40-47; Ham-

blin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Kennish, Michael J., ed. 2001. Practical Handbook of Marine Science, 3rd ed. Boca Raton, FL: CRC Press; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill Pub. Co.; Siebold, E. and W. H. Beyer. 1993. The Sea Floor: An Introduction to Marine Geology. Berlin: Springer Verlag.

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