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Basalt (pronounced /bəˈsɔːlt, ˈbeisɔːlt, ˈbæsɔːlt/) is a common extrusive volcanic rock. Where fresh, it is gray to black and fine-grained. It may be porphyritic containing larger crystals in a fine-grained matrix, vesicular with gas bubbles, or frothy scoria. The oceanic crust is composed predominantly of basalt and its intrusive counterpart, gabbro. Basalt is also common on the continents in areas of young volcanism (e.g., along continental rifts and above mantle plumes).

On Earth, most basalt magmas form by decompression melting of the mantle. Source rocks for these partial melts are dominated by peridotite, an olivine and pyroxene-rich rock. Basalt has also formed on Earth's Moon, on Mars (planet) and Venus, and even on the asteroid Vesta.