Answer: Through the Process of Subduction, old dense crust is pushed toward a deep ocean trench. There, it sinks under lighter crust. It returns to molten rock, becoming a part of the mantle again.
Step-by-step explanation:
What is Subduction?
Subduction occurs when two plates converge, and one slides under the other. This is common when continental plates meet oceanic plates. Because the oceanic plate is denser, it slides under the continental plate.
What is a trench (in the ocean)?
Ocean trenches are long, narrow depressions on the seafloor. These chasms are the deepest parts of the ocean—and some of the deepest natural spots on Earth.
What is Mantle?
The mantle is the mostly-solid bulk of Earth's interior. The mantle lies between Earth's dense, super-heated core and its thin outer layer, the crust.
I hope this helps!