Ocean trenches are chasms or depressions in the ocean floor. They are usually narrow but extremely deep, reaching some of the lowest below sea level points ever discovered. These trenches are formed at subduction zones, which mark the boundaries of lithospheric plates.
The Earth’s lithospheric layer includes the upper mantle and the crust, or surface. All of the planet’s water and land is found on the lithosphere. This layer is not one entire mass, but a congregation of different slabs, called plates. These plates are huge and can be the bedrock of entire oceans and continents. The study of these lithospheric plates and their movement is called plate tectonics, a main theoretical field of geology.
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There are three main types of boundaries where these plates meet. Transform boundaries are where plates move past each other, divergent boundaries are where plates move away from each other, and convergent boundaries are where one plate slides beneath another. Ocean trenches are formed at convergent boundaries, called subduction zones, which are usually the center of a lot of tectonic activity.
Trenches in the ocean are commonly found parallel to volcanic islands. This is often due to dehydration reactions, which occur when an underlying plate sinks beneath an upper one. This movement often causes water to make contact with hot mantle from under the lithosphere, and the resulting chemical reaction creates a land mass. Volcanoes and chains of volcanic islands, called volcanic arcs, are formed when the lava is released and builds up above the water line.
The Mariana Trench, located near Japan in the Pacific Ocean, is the deepest ocean trench. It is also the deepest point in the Earth’s surface, reaching depths of 36,201 feet (11,033 meters). This means it is bigger than Earth’s highest point, Mount Everest, which measures 29,035 feet (8,850 meters). The deepest point of the Mariana Trench, called the Challenger Deep, was named after HMS Challenger II, the British exploration vessel that first reached that depth in 1960.
Aquatic life thrives even in these trenches, which are deprived of almost any sort of light and under extreme pressure. A number of diverse species, ranging from microorganisms like bacteria to jellyfish, lobsters, and octopi, have been found in them. A sample of dirt taken from the Challenger Deep even showed signs of foraminifera, which are single-celled organisms resembling some of the earliest life forms discovered on Earth.