Nov 09, · 3. Adjacent to an arc-shapped chain of volcanic islands or adjacent to a chain of active volcanoes on larger land mass (example Japan Trench next to Japanese islands with active volcanoes).. 4. Deep focus earthquakes. 5. Thrust faulting on hanging wall of (upper plate that is . Deep-sea trenches and their approaches are striking features on the ocean floor. In general, the cross sections of deep-sea trenches are V-shaped with steeper landward sides. Typical slopes range between 4° and 16°, although slopes as steep as 45° have been .
There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page. Adjacent to an arc-shapped chain of volcanic islands or adjacent to a chain of active volcanoes on larger land mass example Japan Trench next to Japanese islands with active volcanoes.
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Black man shot by deputy after calling for help. Answer Save. Turbidite Lv 6. They are deeper than the typical abyssal plane, usually greater than 20, feet. Deep focus earthquakes.
Thrust faulting on hanging wall of upper plate that is not being subducted. Oldest sediments on the subducting plate are closer to the trench. Still have questions? Get your answers by asking now.
The Challenger Deep, at the bottom of the Mariana Trench, lies deep in the Pacific Ocean near the island of Guam. Only three people have visited the Challenger Deep, the deepest ocean trench in the world: a joint French-American crew (Jacques Piccard and Don Walsh) in and National Geographic Explorer-in-Residence James Cameron in The following features are shown at example depths to scale, though each feature has a considerable range at which it may occur: continental shelf ( feet), continental slope (, feet), abyssal plain (>10, feet), abyssal hill (3, feet up from the abyssal plain), seamount (6, feet up from the abyssal plain), ocean trench (36, feet), and volcanic island (above sea level).
Deep-sea trench , also called oceanic trench , any long, narrow, steep-sided depression in the ocean bottom in which occur the maximum oceanic depths, approximately 7, to more than 11, metres 24, to 36, feet. They typically form in locations where one 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 North Pacific Ocean ; it reaches 11, metres 36, feet at its deepest point.
Deep-sea trenches generally lie 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 under the force of gravity.
The result 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 found there too. Both types of subduction zones are associated with large earthquakes that originate at a depth of as much as km miles. Typical trench depths are 8 to 10 km 5 to 6 miles.
The longest trench is the Peru-Chile Trench , which extends some 5, km about 3, miles along the west coast of South America.
Trenches are relatively narrow, usually less than km about 60 miles wide. Although they are deeply buried in sediment, trenches are found along the western North American continental margin from Cape Mendocino in northern California to the Canadian border.
It can be classified as a marginal trench in the east but is more properly termed an island arc west of Alaska. In the western Pacific the trenches are associated with island arcs. A complex pattern of island arcs is found in Indonesia.
The major island arc here is the Java Trench extending from northern Australia to the northwestern end of Sumatra in the northeast Indian Ocean.
Two island arcs occur in the Atlantic Ocean. A few trenches are partially filled with sediments derived from the bordering continents. 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 about 4 miles. The Lesser Antilles trench in the eastern Caribbean is buried by sediments originating from South America. Deep-sea trenches and their approaches are striking features on the ocean floor. In general, the cross sections of deep-sea trenches are V-shaped with steeper landward sides. Oceanward of trenches the seafloor is usually bulged upward in an outer ridge or rise of up to 1, metres about 3, feet in relief.
This condition is thought to be the elastic 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 ocean surface, as in the case of the Java Trench. Such a ridge is constructed from deformed sediments scraped off the top 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 usually km about 60 miles or so from the trench. In marginal trenches these volcanoes form mountain chains, such as the Cascades in the Pacific Northwest or the great volcanoes of the Andes. In island arcs they form active volcanic island chains, such as the Mariana Islands.
Behind the volcanic line of island arcs are sometimes found young, narrow ocean basins. These basins are bounded on the opposite side by submarine ridges. Such interarc , or backarc, basins are sites of seafloor spreading directly caused by 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 away.
These backarc basins bear many of the features characteristic 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 Sea of Japan originated from backarc spreading behind the Japanese arc that began some 30 million years ago. At least two backarc basins have opened behind the Mariana arc, creating seafloor in two phases from about 30 to 17 million years ago in the western Parece Vela Basin and from 5 million years ago in the Mariana Trough next to the islands.
The Mariana Trough is a back-arc basin occurring due east of the Mariana arc. It is important to note that some seafloor features bear the name trench and are deep linear troughs; however, they do not occur in subduction zones.
The Vityaz Trench northwest of Fiji is an aseismic inactive feature of unknown origin. 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. Geophysical data provide important clues concerning the origin of trenches.
Precision measurements reveal that the force of gravity generally is lower than normal, however. 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 Earth that underlie trenches are being forced down against buoyant isostatic forces.
This interpretation of gravity data is substantiated by 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 within the Earth of 55 km 34 miles or less. With increased landward distance away from the trenches, earthquakes occur at greater and greater depths— km miles or more.
Analyses of these seismic zones and of individual earthquakes suggest that the seismicity results from the descent of a lithospheric plate with its associated crust 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 red 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, metres about 14, feet are predominantly calcareous foraminiferal oozes.
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The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree See Article History. Three-dimensional diagram showing crustal generation and destruction according to the theory of plate tectonics; included are the three kinds of plate boundaries—divergent, convergent or collision , and strike-slip or transform.
Read More on This Topic. The narrow 50 miles [80 km] , volcanic, and seismically active Java Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Learn More in these related Britannica articles:. Elsewhere they are absent. In contrast to ocean ridges, trenches…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox!
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