deep-sea trench

deep-sea trench, any long, narrow, steep-sided depression in the ocean bottom in which occur the maximum oceanic depths, approximately 7,300 to more than 11,000 metres (24,000 to 36,000 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 11,034 metres (36,200 feet) at its deepest point.

Types

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.

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Indian Ocean: Trenches

Both types of subduction zones are associated with large earthquakes that originate at a depth of as much as 700 km (435 miles). The deep earthquakes below subduction zones occur in a plane that dips 30° or more under 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 5,900 km (about 3,700 miles) along the west coast of South America. Trenches are relatively narrow, usually less than 100 km (about 60 miles) wide.

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 west coast of Central and South America from the Gulf of California to southern Chile. 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. The Aleutian Trench extends from the northernmost point in the Gulf of Alaska west to the Kamchatka Peninsula in far eastern Russia. 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. These include the Kuril, Japan, Bonin, Mariana, Ryukyu, and Philippine trenches that extend from Kamchatka to near the Equator. 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. The region of New Guinea and the Solomon Islands includes the New Britain and Solomon trenches, the latter of which joins the New Hebrides Trench directly to the south. East of this 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 South America and Antarctica. The Puerto Rico Trench joins the Lesser Antilles Island arc in the eastern Caribbean.

A few trenches are partially filled with sediments derived from the bordering continents. The Aleutian Trench is effectively buried east of Kodiak Island 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 (about 4 miles). The Lesser Antilles trench in the eastern Caribbean 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 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 measured in the Tonga Trench of the equatorial South Pacific. Narrow, flat abyssal plains of ponded sediment generally occupy trench axes; however, in most deep-sea 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 rise of up to 1,000 metres (about 3,300 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 100 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 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 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.

Origin of deep-sea trenches

Geophysical data provide important clues concerning the origin of trenches. No abnormalities in the flow of internal Earth heat or variations in the Earth’s magnetic field occur at 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—500 km (310 miles) or more. Seismic foci thus define 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 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 4,500 metres (about 14,800 feet) are predominantly calcareous foraminiferal oozes. Large quantities of sediment cannot accumulate because they either are dragged into the Earth’s interior by the plunging oceanic lithosphere or are distorted into folded masses and molded into new material of the continental periphery.