Oceanic crust is the part of the Earth's crust that makes up the seafloor. It's thinner, denser, and simpler in structure than the continental crust. Oceanic crust is also younger, on average; from its birth at a mid-ocean ridge to its end at a subduction zone is no more than 250 million years. term that is sometimes used for oceanic crust rocks is sima, which is short for magnesium silicate, a common component of these rocks. Another term for these undersea igneous rocks is mafic, which comes from the fact that they are high in magnesium and iron. Basalt and gabbro are examples of mafic rocks.