Final answer:
At some convergent plate boundaries under the ocean, old crust is recycled by being subducted into the upper mantle, where it melts and is eventually recycled into convection currents, balancing the crust formation at rift zones.
Step-by-step explanation:
Under the ocean at some convergent plate boundaries, old crust is recycled by subduction. At these plate boundaries, the thinner oceanic plate is forced beneath another plate into the upper mantle. This process involves the old crust being thrust into regions of high pressure and temperature, ultimately melting and being recycled into Earth's mantle via downward-flowing convection currents. This activity helps balance the material creation at rift zones, where new crust is formed.
As the subducted plate descends, it marks a subduction zone, often indicated by the presence of deep ocean trenches like the Japan trench. Subduction is a key part of the cycle of plate tectonics and plays a crucial role in the renewal and destruction of Earth's crust.