Final answer:
When oceanic crust falls into a trench, it is forced beneath another plate in a subduction zone. It melts and is recycled into a convection current, balancing the flow of material in the Earth's crust.
Step-by-step explanation:
When oceanic crust falls into a trench, it is forced beneath another plate in what is called a subduction zone. The subducted plate is pushed down into the upper mantle, where it eventually melts and is recycled into a convection current. This process helps balance the flow of material in the Earth's crust.