Final answer:
The old crust is recycled in subduction zones where it is forced under another plate, melts, and joins the mantle's convection currents.
Step-by-step explanation:
When new crust is made at rift zones, the old crust is recycled at subduction zones. A subduction zone is where one tectonic plate is forced underneath another, often forming a deep ocean trench like the Japan trench. This process balances the creation and destruction of Earth's crust. The material of the subducted plate melts several hundred kilometers below the surface and becomes part of a convection current, which is part of Earth's internal heat transfer system. Thanks to this process, the oceanic crust is relatively young, renewing entirely in just over 100 million years.