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( Basalt)
Basalt (pronounced /'bæs?lt, b?'s??lt/) is a common mafic extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or gray. On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. 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. 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. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.
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