Describe how Hess's theory explains why there is no ocean rock older than 200 million years

Oceanic rocks are continually recycled.

Explanation:

Hess proposed that no ocean rock is older than 200 million years because it is constantly being replaced by the upwelling of magma from the mantle. The magma leaves the mantle at the divergent plate boundaries and creates underwater ridges and canyons.

At the divergent plate boundaries, the tectonic plates move away from each other, but the older rock one again enters the mantle at the subduction zones where it slides underneath less dense plates of continental crust.