Basalt is an extrusive igneous (volcanic) rock that is low in silica content, dark in colour, and comparatively rich in iron and magnesium. The dominant igneous rock on the Earth's surface is basalt. It appears that such is also the case on Earth's close neighbours. The lunar maria are covered with basalt lava flows. These lunar basalts have a mineralogy similar to that of terrestrial basalts, but chemically they have no water, a lower amount of alkalis and alumina, and a higher iron oxide and chromium content. On the lunar highlands, plagioclase-rich rocks are most common; these include anorthosites, gabbros, troctolites (olivine-plagioclase rock), and minor basalt. It appears that basalt is common on Mars as well. The large shield volcano Olympus Mons must have been formed from eruptions of fluid basalt flows. The X-ray fluorescence analyses performed by the Vikings 1 and 2 landers showed that the rocks are basaltic. Venus apparently has volcanic features, but full chemical analysis has not yet revealed the rock compositions.
Excerpt from the Encyclopedia Britannica without permission.