Oceanic crust is the part of Earth's lithosphere which underlies the ocean basins. It is thinner (generally less than 10 km thick) but more dense than continental crust, about 3.3 g/cc (grams per cubic centimeter). Oceanic crust is composed of mafic basaltic rocks. Most of the present day oceanic crust is less than 200 million years old because it is continuously being created at oceanic ridges and destroyed by being pulled back into the mantle in subduction zones by the processes of plate tectonics.