Basalt is an extrusive volcanic rock. Basalt is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or gray.
Basalt magmas have formed by decompression melting of the Earth's mantle and by partial melting of rock in the interiors of Mars and the Earth's moon. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.
Pliny used the word basalt and it is said to have had an Ethiopian origin, meaning a black stone.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as dolerite (also called diabase) or gabbro.
Source: Wikipedia.Org
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