Basalt is primarily formed by the extrusion of lava flows onto the surface of the earth during a volcanic eruption. The intrusion of a narrow sill or igneous dike below the crust also results in the formation of basalt.
Basalt is a type of igneous rock that is typically dark in coloration with a fine-grained texture. It is the most predominant rock type found in Earth's crust, and mainly consists of pyroxene and plagioclase minerals. Volcanoes that form basalt are commonly found near the ocean basins. Some areas where basalt is present in large quantities include oceanic divergent boundaries (such as the Juan de Fuca Ridge), mantle plumes and hotspots below Earth's surface (such as the Columbia River Flood Basalts) and oceanic hotspots (such as the Hawaiian Islands).