What causes an ocean trench to form?

The subduction of plate tectonics causes an ocean trench to form. This means one of the Earth's tectonic plate slides under another tectonic plate.

When forming an ocean trench, a heavy tectonic plate's edge presses against a lighter tectonic plate's edge. At the place where the plates meet, the heavier tectonic plate subducts below the lighter tectonic plate to form the subduction zone. The subduction zone is the site called an ocean trench. Ocean trenches most commonly form when a tectonic plate comprised of oceanic crust meets a tectonic plate comprised of continental crust, although some rarer trenches have been formed by two layers of oceanic crust meeting.