There is an upper layer to the Earth’s Mantle called the Asthenosphere. This comprises of semi-molten rock. Crustal rocks of the Earth’s tectonic plates seem to almost float on this. The heavier rocks of the ocean crust float lower than the less dense rocks of the continental crust. This suggests that the rocks of the ocean floor would be older than the rocks of the continent. However, this is not the case.

Image: Subduction-en.svg, Wikimedia Commons, author: KDS4444, license: CC BY-SA 4.0.