How Deep Ocean Trenches are Formed

Deep ocean trenches are formed when a continental plate descends underneath an oceanic plate. This is commonly a process of plate tectonics. The Earth’s lithosphere is divided into 17 large tectonic plates and about 20 smaller plates, all of which are  continuously moving with respect to each other. This creates zones where plates converge, diverge or get transformed.  While divergent plates create crust by sea floor spreading, another process, such as the convergence of plates, destroys some of the Earth’s crust. The process by which the Earth’s crust is destroyed is called subduction, and it most commonly occurs at the deep ocean regions of the world, where ocean trenches are formed.

What’s an ocean trench?

Ocean trenches are long steep-sided depressions on the ocean floor, extending for thousands of miles in length and as little as a few miles across. Ocean trenches are driven by tectonics and are most commonly found along the Pacific Ocean in a zone known as the ring of fire. They are usually formed at the boundaries of convergent plates, at a region where a continental plate submerges beneath an oceanic plate. Based on the tectonic theory, new crust is created along divergent plate boundaries, such as in a mid-ocean ridge; however, for the size of the Earth to remain unchanged, an equal amount must be destroyed somewhere; this most commonly occurs at subduction zones, where deep ocean trenches are formed.

Subduction zones

Typically, subduction zones occur around the edges of oceans, as a result of the sinking of an oceanic plate under a continental plate or another slow-moving oceanic plate. At the boundary of a lighter plate and a heavier plate, the heavier plate starts sliding down into the mantle, starting to melt and creating magma. Plumes of magma begin to erupt through the edge of the continental plate, forming volcanic islands (volcanic arcs) that lie parallel to the trench.  Some trenches are formed near the continental shelves, while others are created far away from continental shelves; however, both of them form volcanic chains, which are known as volcanic arcs.

Volcanic arcs

The point at which one tectonic plate is being subducted under another plate creates a fracture on the overriding plate. Plumes of molten material begin to erupt through the fractures. The magma erupts and forms an arc of volcanoes along the plate margin. The magmas produced by this type of eruptions are highly viscous and contain a higher concentration of dissolved gases, making them highly explosive. Volcanic arcs are associated with high earthquake activity. Some examples of volcanic arcs are the Mariana islands, the Japanese Archipelago, the Philippine Islands, and the Aleutian Islands, among others.

Deepest ocean trenches

The subduction of one plate beneath another creates the deepest regions in the ocean, known as ocean trenches. The trenches are located at the deepest regions; they are v-shaped and steep-sloped at the continental side of the plate and less steep on the oceanic plate. Seventeen out of 20 major oceanic trenches in the world lie in the Pacific ocean. The Mariana Trench, which is located off the Mariana Islands in the Pacific ocean, is the deepest ocean trench in the world, and is situated at approximately 11,000 meters (36,000 ft.) below the surface of the ocean. This trench took its name after the Challenger Deep, which made the first sounding in 1875. In 1960, the bathyscaphe Trieste descended with two men on board, making them the first men to have descended there.

Before the era of deep oceanic exploration, it was thought that the bottom of the ocean was a lifeless world; however, explorations of the deep ocean on board submersibles, including the Trieste, demonstrated that it is possible for marine creatures to survive under extreme pressures. The deepest trench in the world is found in the Pacific Ocean near the island of Guam and is known as the Mariana Trench, at 10,994 meters (36,070 ft.) depth, while the deepest trench in the Atlantic ocean is the Puerto Rico Trench, with 8,600 meters (28,232 ft.) below the surface of the ocean and about 280 km (175 miles) in length. According to National Geographic, ocean trenches are formed due to the movement of the Earth’s crust, or plate tectonics. Tectonic plates slide under one another in a process known as subduction.