Basalt is the main type of rock in the oceanic crust. Basalt is formed when magma cools quickly at the Earth's surface, making a type of igneous rock.
Though basalt does sometimes appear on land, particularly on volcanic islands like Hawaii and Iceland, it mainly forms in the ocean. It is a dense, dark-colored rock that has small crystals, many too tiny to see. It is rich in magnesium and iron. The oceanic crust, largely made up of basalt is located under the ocean floor. Measuring about 4 miles thick, it is thinner but heavier than its counterpart, the continental crust.