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Basalt is a heavy, basic, dark igneous rock with a fined grained texture, which is the most abundant rock found in cooled lava sheets. Gas escaping from the molten matter leaves vesicles (bubbles), which later can be lined or filled with mineral deposits, called amygdules. The chief mineral visible in basalt is usually olivine (MgFeSiO4). Basalt magmas are very hot and fluid, and these factors, coupled with the immense volume of lava erupted enables basaltic lavas to cover many square kilometers. Basalt is often associated with columnar jointing, which produces long upright vertical pillars.
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