Basalt

Basalt ; buh-SALT) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt has the same chemical composition and mineralogy as slow-cooling, coarse-grained gabbro. The er... (Wikipedia)