The oceanic crust is composed primarily of basalt, a dark, fine-grained igneous rock that forms when molten rock from the Earth's mantle cools and solidifies. Basalt is the most common type of volcanic rock on Earth and is found in both oceanic and continental crust. The oceanic crust is much thinner than the continental crust, with an average thickness of only 5-10 kilometers. The continental crust, on the other hand, can be up to 70 kilometers thick. The oceanic crust is also denser than the continental crust, with a density of about 3 grams per cubic centimeter compared to 2.7 grams per cubic centimeter for the continental crust. This difference in density is due to the fact that the oceanic crust is composed primarily of basalt, while the continental crust is composed primarily of granite, a lighter-colored, coarse-grained igneous rock.