An oceanic plate is a tectonic plate at the bottom of the oceans. It is primarily made of mafic rocks, rich in iron and magnesium. It is thinner than the continental crust (generally less than 10 kilometers thick) and denser. It is also younger than continental crust. When they collide, the oceanic plate moves underneath the continental plate because of its density. As a result, it melts in the mantle and reforms. The oldest oceanic rocks are less than 200 million years old.