It is a crescent-shaped scar in the Earth’s crust.
The trench is not the part of the seafloor closest to the center of the Earth. This is because the Earth is not a perfect sphere; its radius is about 25 kilometres (16 mi) less at the poles than at the equator.
The Mariana Trench is part of the Izu-Bonin-Mariana subduction system that forms the boundary between two tectonic plates.
In this system, the western edge of one plate, the Pacific Plate, is subducted (i.e., thrust) beneath the smaller Mariana Plate that lies to the west.
Crustal material at the western edge of the Pacific Plate is some of the oldest oceanic crust on earth (up to 170 million years old), and is therefore cooler and more dense; hence its great height difference relative to the higher-riding (and younger) Mariana Plate.
The deepest area at the plate boundary is the Mariana Trench proper.
The movement of the Pacific and Mariana plates is also indirectly responsible for the formation of the Mariana Islands.
These volcanic islands are caused by flux melting of the upper mantle due to release of water that is trapped in minerals of the subducted portion of the Pacific Plate.
Challenger Deep:
The Challenger Deep is the deepest known point in the Earth’s seabed hydrosphere.
It is in the Pacific Ocean, at the southern end of the Mariana Trench near the Mariana Islands group.
The Challenger Deep is a relatively small slot-shaped depression in the bottom of a considerably larger crescent-shaped oceanic trench, which itself is an unusually deep feature in the ocean floor.
