What Is An Ocean Trench

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deep-sea trench, besides called oceanic trench, any long, narrow, steep-sided depression in the ocean bottom in which occur the maximum oceanic depths, approximately 7,300 to more eleven,000 metres (24,000 to 36,000 feet). They typically form in locations where i tectonic plate subducts under another. The deepest known depression of this kind is the Mariana Trench, which lies east of the Mariana Islands in the western Northward Pacific Ocean; it reaches 11,034 metres (36,200 anxiety) at its deepest indicate.

Types

Deep-sea trenches generally prevarication seaward of and parallel to adjacent island arcs or mountain ranges of the continental margins. They are closely associated with and found in subduction zones—that is, locations where a lithospheric plate bearing oceanic crust slides down into the upper mantle nether the force of gravity. The upshot is a topographic depression where the oceanic plate comes in contact with the overriding plate, which may be either oceanic or continental. If the overriding plate is oceanic, an island arc develops. The trench forms an arc in plan view, and islands with explosive volcanoes develop on the overriding plate. If the overriding plate is continental, a marginal trench forms where the topographic depression appears to follow the outline of the continental margin. Explosive volcanoes are plant there too.

Indian Ocean

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Indian Bounding main: Trenches

The Indian Sea has the fewest trenches of whatever of the globe's oceans. The narrow (50 miles [80 km]), volcanic, and seismically active Java...

Both types of subduction zones are associated with big earthquakes that originate at a depth of equally much as 700 km (435 miles). The deep earthquakes below subduction zones occur in a plane that dips 30° or more nether the overriding plate. Typical trench depths are 8 to 10 km (5 to 6 miles). The longest trench is the Peru-Chile Trench, which extends some five,900 km (nigh three,700 miles) along the west coast of South America. Trenches are relatively narrow, usually less than 100 km (almost threescore miles) broad.

Of the Earth's 20 major trenches, 17 are found in the Pacific basin, a vast area rimmed by trenches of both marginal and island arc varieties. Marginal trenches bound the w coast of Central and South America from the Gulf of California to southern Republic of chile. Although they are deeply buried in sediment, trenches are found forth the western North American continental margin from Greatcoat Mendocino (in northern California) to the Canadian border. The Aleutian Trench extends from the northernmost point in the Gulf of Alaska west to the Kamchatka Peninsula in far eastern Russia. It tin can be classified as a marginal trench in the east only is more than properly termed an island arc westward of Alaska.

In the western Pacific the trenches are associated with island arcs. These include the Kuril, Japan, Bonin, Mariana, Ryukyu, and Philippine trenches that extend from Kamchatka to almost the Equator. A complex pattern of island arcs is institute in Republic of indonesia. The major island arc here is the Java Trench extending from northern Australia to the northwestern stop of Sumatra in the northeast Indian Sea. The region of New Guinea and the Solomon Islands includes the New United kingdom and Solomon trenches, the latter of which joins the New Hebrides Trench directly to the due south. East of this surface area the Tonga and Kermadec trenches extend south from the Fiji Islands to New Zealand.

Two island arcs occur in the Atlantic Ocean. The South Sandwich Trench is located west of the Mid-Atlantic Ridge between Due south America and Antarctica. The Puerto Rico Trench joins the Lesser Antilles Isle arc in the eastern Caribbean.

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A few trenches are partially filled with sediments derived from the bordering continents. The Aleutian Trench is finer buried east of Kodiak Isle in the Gulf of Alaska. There the ocean floor is smooth and flat. To the west, farther from the sediment supply of Alaska, the trench reaches depths of more than 7 km (near iv miles). The Lesser Antilles trench in the eastern Caribbean area is buried by sediments originating from South America.

Structure

Deep-sea trenches and their approaches are striking features on the ocean floor. In general, the cantankerous sections of deep-sea trenches are Five-shaped with steeper landward sides. Typical slopes range between four° and sixteen°, although slopes as steep as 45° have been measured in the Tonga Trench of the equatorial S Pacific. Narrow, flat abyssal plains of ponded sediment generally occupy trench axes; nevertheless, in about deep-bounding main trenches the accumulated material is relatively shallow since the bottom of the trench subducts into Earth's interior.

Oceanward of trenches the seafloor is usually bulged upward in an outer ridge or ascension of up to ane,000 metres (near 3,300 feet) in relief. This condition is thought to be the rubberband response of the oceanic plate bending down into a subduction zone. The landward or island-arc slope of the trench is often interrupted by a submarine ridge, which sometimes breaks the sea surface, every bit in the instance of the Coffee Trench. Such a ridge is constructed from deformed sediments scraped off the superlative of the descending oceanic plate and is termed an accretionary prism. A line of explosive volcanoes, extruding (erupting) a lava that forms the volcanic rock andesite, is found on the overriding plate normally 100 km (well-nigh 60 miles) or so from the trench. In marginal trenches these volcanoes form mountain chains, such equally the Cascades in the Pacific Northwest or the not bad volcanoes of the Andes. In isle arcs they form active volcanic isle bondage, such as the Mariana Islands.

Backside the volcanic line of isle arcs are sometimes found immature, narrow ocean basins. These basins are bounded on the opposite side past submarine ridges. Such interarc, or backarc, basins are sites of seafloor spreading directly caused past the dynamics of subduction. They originate at the volcanic line, so that the outer bounding submarine ridge, or third arc, represents an older portion of the volcanic line that has spread abroad. These backarc basins acquit many of the features feature of oceanic spreading centres. Well-studied examples of these features are found in the Lau Basin of the Tonga arc and also west of the Mariana Islands. The Bounding main of Nihon originated from backarc spreading backside the Japanese arc that began some 30 1000000 years agone. At least two backarc basins accept opened behind the Mariana arc, creating seafloor in 2 phases from about 30 to 17 million years agone in the western Parece Vela Basin and from v million years ago in the Mariana Trough adjacent to the islands. The Mariana Trough is a dorsum-arc basin occurring due eastward of the Mariana arc.

It is important to note that some seafloor features bear the name trench and are deep linear troughs; notwithstanding, they do not occur in subduction zones. The Vema Trench on the Mid-Indian Ridge is a fracture zone. The Vityaz Trench northwest of Fiji is an aseismic (inactive) feature of unknown origin. The Diamantina trench (Diamantina Fracture Zone) extends w from the southwest coast of Australia. Information technology is a rift valley that was formed when Australia separated from Antarctica between 60 and l 1000000 years ago.

Origin of deep-body of water trenches

Geophysical data provide important clues apropos the origin of trenches. No abnormalities in the menstruum of internal Globe heat or variations in the Earth's magnetic field occur at trenches. Precision measurements reveal that the force of gravity more often than not is lower than normal, withal. These negative gravity anomalies are interpreted to mean that the segments of the lithosphere (that is, the crust and upper mantle comprising the rigid, outermost shell of the World) that underlie trenches are being forced downwards confronting buoyant isostatic forces.

This estimation of gravity information is substantiated past seismological studies. All trenches are associated with zones of earthquake foci. Along the periphery of the Pacific Ocean, earthquakes occur close to and landward of the trenches, at depths inside the Earth of 55 km (34 miles) or less. With increased landward altitude abroad from the trenches, earthquakes occur at greater and greater depths—500 km (310 miles) or more. Seismic foci thus ascertain tabular zones approximately 20 km (12 miles) thick that dip landward at about 45° beneath the continents. Analyses of these seismic zones and of individual earthquakes suggest that the seismicity results from the descent of a lithospheric plate with its associated chaff into the asthenosphere (that is, the partially molten layer beneath the lithosphere); oceanic trenches are topographic expressions of this movement.

The sinking of oceanic lithosphere helps to explain the relative scarcity of sediment that has accumulated within the trenches. Small quantities of brown or ruddy clay, siliceous organic remains, volcanic ash and lapilli, and coarse, graded layers that result from turbidity currents and from the slumping of the trench walls occur at trench axes. Sediments on trench walls shallower than 4,500 metres (about 14,800 feet) are predominantly calcareous foraminiferal oozes. Large quantities of sediment cannot accumulate because they either are dragged into the Earth'due south interior by the plunging oceanic lithosphere or are distorted into folded masses and molded into new material of the continental periphery.

This article was most recently revised and updated by John P. Rafferty.