Oceanic Crust is made up of silicon, oxygen, and aluminum. It is denser and thinner than continental crust because it contains more iron, calcium, and magnesium. The crust underneath the ocean is about 5 km thick. Oceanic crust takes up 71% of Earth's surface. The rocks on the ocean floor consist of basalt. Basalt is an extrusive igneous rock that forms when lava solidifies.