Basalt
A dark, fine-grained volcanic rock made mainly of pyroxene and plagioclase feldspar, found on Earth and on other worlds.
Basalt is what you get when runny, iron-rich lava cools quickly at or near a planet's surface. Its tiny crystals are mostly two minerals: pyroxene, which is dark, and plagioclase feldspar, which is pale. Basalt makes the ocean floors on Earth and builds islands such as Hawaii and Iceland. The dark patches you can see on the Moon, called maria, are huge plains of basalt too.
Some meteorites are basalts from other worlds. Eucrites are basalts that are thought to come from the asteroid Vesta. Juvinas, a eucrite that fell in France in 1821, is one of the best known. Many martian meteorites called shergottites are basaltic as well, and some lunar meteorites are mare basalts. Studying them tells scientists that volcanoes erupted on small asteroids as well as on large planets.
Earthly basalt is also one of the commonest rocks mistaken for a meteorite, because it is dark and fairly heavy. Clues that a dark rock is ordinary basalt include gas bubbles (vesicles) all through it, no thin fusion crust, and a weathered grey or brown skin. A eucrite, by contrast, often has a glossy black crust over a pale grey interior.
Related terms
Level: Explorer Ages 10 and up