Final answer:
Deep-ocean trenches are formed through the process of subduction, where one tectonic plate is forced beneath another into the mantle. This creates subduction zones marked by ocean trenches. The subducted plate is eventually melted and recycled.
Step-by-step explanation:
Deep-ocean trenches are formed through the process of subduction, which occurs when two tectonic plates converge. One plate is forced beneath the other into the mantle. This subduction creates a subduction zone marked by an ocean trench, such as the deep Japan trench along the coast of Asia. The subducted plate is eventually melted and recycled into a downward-flowing convection current.