What are Ocean Trenches and How are They Formed?

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What are Ocean Trenches and How are They Formed?

Around 72 percent of the Earth’s surface is found under the oceans. It has always been contributing to the dynamically changing face of the Earth through the tectonic phenomenon occurring at its depths. Such a naturally occurring phenomenon is referred to as the oceanic trench. ScienceStruck takes a plunge into the process of formation of these deep sea valleys.

The Pacific ‘Ring of Fire’ is a horseshoe-shaped, 25,000 mile belt surrounding the Pacific stretch that encompasses about three-fourth of the volcanoes on the globe. It is known to be surrounded by a relentless array of oceanic trenches, which includes 17 out of the 20 major trenches known so far.

The Earth consists of many layers or spheres concentric to a molten core. The lithosphere is the uppermost layer or crust of the Earth. It is rigid and floats above the asthenosphere. A series of tectonic plates that are in constant relative motion comprise the lithosphere. This motion of plates result in different topological features on the Earth. One of these features found under the oceans is the ocean trench.

Ocean trenches may be defined as narrow V-shaped depressions stretched over long distances at the active areas of the ocean basins. They are also sometimes called submarine valleys. They are very similar to the deep ravines found on the surface of the Earth.

The 50,000 km (31,000 mi) of convergent plate margins found on the Earth’s oceanic crust constitute the ‘hadal zone’, named after the Greek underworld god ‘Hades’. Most of them are found in the Pacific Ocean, but some of them are located in and around the Indian Ocean, and margins of the Atlantic Ocean and Mediterranean Sea.

The environmental conditions at such a depth as that of ocean trenches is quite adverse. The pressure at the trenches goes as high as 1,000 times the pressure at the surface. There is almost no light reaching up to that depth, and it is pitch black down in the chasms of the trenches. The temperature in the trenches is known to be slightly greater than the freezing temperature of water.

Subduction Zone

Subduction Zone

Subduction accounts for the geophysical process where one of the tectonic plates on the Earth’s surface slides beneath the other. A subduction zone and its occurrence can be traced from the formation of mid-ocean ridges. These mid-ocean ridges are a range of volcanic hills found underwater. The flowing mantle material out of these mid-ocean ridges forms the new oceanic crust. As these newly formed crusts move away from the ridges, they start spreading over the ocean bed and cool down. The cooled oceanic crust gets denser and heavier as it starts contracting. When this crust converges with another oceanic or continental crust, the heavier crust melts and slides (subducts) under the lighter crust due to gravitational action. The heavier plate sinks into or converges into the mantle. A depression is formed along the converging ridge of the plates. These depressions are called oceanic trenches.

Formation of the Oceanic Trench

Oceanic Trench

The subduction zones and the activities resulting from it are the cause of formation of oceanic trenches. The movement of the Earth’s crust at the convergent boundaries of tectonic plates, be it oceanic crust and continental crust or two oceanic crusts, result in a steep deep valley with the two crusts acting as its walls. Oceanic crust is always heavier than the continental crust. The heavier crust has a moderate slope, whereas the lighter crust forms the inner wall, and it has a steep slope. The rate of disappearance of the oceanic crust into the trenches is estimated to be about the tenth part of a square meter per second every year.

When trenches occur, then they give rise to various natural phenomenon such as mountain ranges, earthquakes, and volcanic arcs. The reason behind these subsequent occurrences is increase in the mantle material due to the melting of ocean floor subduction. The molten material then rises above the crust through volcanic chains parallel to the trench and around it. Continental shelves or volcanic belts are found within 200 km from an oceanic trench and are considered to be the landmark for locating it. The Aleutian Arc has been formed as the Pacific Plate subducts beneath the North American Plate.

Some Famous Trenches

‘Challenger Deep’ is the deepest portion present in the crescent-shaped Mariana Trench found in the Pacific Ocean. Mariana Trench is famed as the deepest trench in the entire stretch of oceanic crust on the Earth and is located in the east of Mariana islands near Guam. It is estimated to be 36,037 feet (10,984 m) below the ocean’s surface.

The deepest trench in the crust of Atlantic Ocean is the Puerto Rico Trench, which is 8,600 meters (28,232 feet) deep, and its length is around 280 kilometers (175 miles). It is formed at the convergence of the North Atlantic Plate sliding and the Caribbean Plate.

The longest trench known in the world is the Peru-Chile Trench, which extends to a distance of 5,900 km (about 3,700 miles) along the west coast of South America.

Due to the hostile atmospheric conditions present at these deep ocean trenches, its exploration is still a challenge to human technology. Till date, very few expeditions have ventured as far as 6,000 meters under the ocean’s surface. Out of these, most of them have been unmanned and have been carried out by automated robotic probes. Despite the life-opposing situation, researchers were surprised to find a unique adaptation by biolife present there. These findings have opened up an entire field of research regarding biochemical processes and medical science. The studies so far seem promising to deal with the cure for various diseases as well as to study the evolution of biolife on Earth.

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