text: describe a deep-ocean trench. how does one form?   the deepest parts of the ocean are found in arc-shaped depressions in the ocean floor known as trenches. these trenches form at convergent plate boundaries when one plate is subducted beneath another. the arc shape results from the geometry of plate movement across the spherical earth, and the convex side of the arc points toward the open ocean. these trenches, because of the tectonic and volcanic behavior associated with them, are often accompanied by arc-shaped bands of islands.