12) Except for the Mediterranean Sea, the oldest oceanic crust found in the oceans is around 180 Ma. Why is there no older oceanic crust than this?

a. all older oceanic crust has been subducted into the mantle  

Explanation:

Oceanic crusts are generally not known to survive as long as that of one found in the Mediterranean sea which is said to be more than 300 million years old. This is because approaching 200 million years old, oceanic crusts go back into the mantle at subduction zones in a process that can most accurately be described as recycling.

It is said that the oceanic crust in question is so old that it may have been part of Tethys, one of Earth's earlier oceans.