OCEANIC TRENCH

OCEANIC TRENCHES

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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 7,300 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.

Trench, Ocean, Maximum Depth

1. Mariana, Trench Pacific Ocean, 11,034 m (36,201 ft)

2. Tonga Trench, Pacific Ocean, 10,882 m (35,702 ft)

3. Philippine Trench, Pacific Ocean, 10,545 m (34,596 ft)

4. Kuril–Kamchatka Trench, Pacific Ocean, 10,542 m (34,587 ft)

5. Kermadec Trench, Pacific Ocean, 10,047 m (32,963 ft)

6. Izu-Bonin Trench (Izu-Ogasawara Trench), Pacific Ocean, 9,810 m (32,190 ft)

7. Japan Trench, Pacific Ocean, 10,375 m (34,039 ft)

8. Puerto Rico Trench, Atlantic Ocean, 8,800 m (28,900 ft)

9. South Sandwich Trench, Atlantic Ocean, 8,428 m (27,651 ft)

10. Peru–Chile Trench or Atacama Trench, Pacific Ocean, 8,065 m (26,460 ft)

How Ocean Trenches Form

Subduction Zones

When the leading edge of a dense tectonic plate meets the leading edge of a less-dense plate, the denser plate bends downward. This place where the denser plate subducts is called a subduction zone.

Oceanic subduction zones almost always feature a small hill preceding the ocean trench itself. This hill, called the outer trench swell, marks the region where the subducting plate begins to buckle and fall beneath the more buoyant plate.

Some ocean trenches are formed by subduction between a plate carrying continental crust and a plate carrying oceanic crust. Continental crust is always much more buoyant than oceanic crust, and oceanic crust will always subduct.

Ocean trenches formed by this continental-oceanic boundary are asymmetrical. On a trench’s outer slope (the oceanic side), the slope is gentle as the plate gradually bends into the trench. On the inner slope (continental side), the trench walls are much more steep. The types of rocks found in these ocean trenches are also asymmetrical. The oceanic side is dominated by thick sedimentary rocks, while the continental side generally has a more igneous and metamorphic composition.

Some of the most familiar ocean trenches are the result of this type of convergent plate boundary. The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the Nazca plate subducting beneath the continental crust of the South American plate. The Ryukyu Trench, stretching out from southern Japan, is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the Eurasian plate.

More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Mariana Trench, in the South Pacific Ocean, is formed as the mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate.

In a subduction zone, some of the molten material—the former seafloor—can rise through volcanoes located near the trench. The volcanoes often build volcanic arcs—island mountain ranges that lie parallel to the trench. The Aleutian Trench is formed where the Pacific plate subducts beneath the North American plate in the Arctic region between the U.S. state of Alaska and the Russian region of Siberia. The Aleutian Islands form a volcanic arc that swings out from the Alaskan Peninsula and just north of the Aleutian Trench.

Not all ocean trenches are in the Pacific, of course. The Puerto Rico Trench is a tectonically complex depression in part formed by the Lesser Antilles subduction zone. Here, the oceanic crust of the enormous North American plate (carrying the western Atlantic Ocean) is being subducted beneath the oceanic crust of the smaller Caribbean plate.

Accretionary Wedges

Accretionary wedges form at the bottom of ocean trenches created at some convergent plate boundaries. The rocks of an accretionary wedge are so deformed and fragmented they are known as melange—French for “mixture.”

Accretionary wedges form as sediments from the dense, subducting tectonic plate are scraped off onto the less-dense plate. Sediments often found in accretionary wedges include basalts from the deep oceanic lithosphere, sedimentary rocks from the seafloor, and even traces of continental crust drawn into the wedge. The most common type of continental crust found in accretionary wedges is volcanic material from islands on the overriding plate.

Accretionary wedges are roughly shaped like a triangle with one angle pointing downward toward the trench. Because sediments are mostly scraped off from the subducting plate as it falls into the mantle, the youngest sediments are at the bottom of this triangle and the oldest are at the more flattened area above. This is the opposite of most rock formations, where geologists must dig deep to find older rocks.

Active accretionary wedges, such as those located near the mouths of rivers or glaciers, can actually fill the ocean trench on which they form. (Rivers and glaciers transport and deposit tons of sediment into the ocean.) This accreted material can not only fill trenches, but rise above sea level to create islands that “hide” the ocean trenches beneath. The Caribbean island of Barbados, for example, sits atop the ocean trench created as the South American plate subducts beneath the Caribbean plate.

What is it like in a trench?

The great depth of ocean trenches creates an environment with water pressures more than 1,000 times greater than the surface, constant temperatures just above freezing, and no light to sustain photosynthesis. While this may not seem like conditions suitable to life, the combination of extremely high pressure, the gradual accumulation of food along trench axes, and the geographical isolation of hadal systems are believed to have created habitats with an extraordinarily high abundance of a few highly specialized organisms.



How does life survive there?

Many of the organisms living in trenches have evolved surprising ways to survive in these unique environments. Recent discoveries in the hadal zone have revealed organisms with proteins and biomolecules suited to resisting the crushing hydrostatic pressure and others able to harness energy from the chemicals that leak out of hydrocarbon seeps and mud volcanoes on the seafloor. Other hadal species thrive on the organic material that that drifts down from the sea surface and is funneled to the axis of the V-shaped trenches.