Question

All rock on the ocean floor is less than 180 million years old because _____.

Answer 1

Answer:

older rock would have returned to the mantle through subduction

Explanation:

The reason why the ocean floor is only 180 million years old even though the oceans exist much longer then that on Earth, is because of the movement of the tectonic plates, a process through which the oceanic plates are getting subducted and melted into the mantle. The oceans that have existed prior to the ones that exist today haven't really left significant traces of their crust behind them. The reason for this is that the oceanic plates over which these oceans were got below the continental plates, thus got subducted. As they got subducted, they got destroyed in the mantle. The oceanic crust that we have nowadays is only 180 million years old, but as the time passes, the older oceanic crust will face the same destiny, and there will constantly be new one replacing it.