90% of Earth's volcanic rock is BASALT, a dark-colored, fine-grained igneous rock composed mainly of plagioclase and pyroxene minerals. BASALT is the most common type of volcanic rock and is formed when lava cools quickly at the surface of the Earth. It is found in oceanic crust and forms most of the seafloor, as well as on land in areas such as Hawaii and Iceland. BASALT is also used in construction, for example in making cobblestones and monuments.