Basalt

Terrestrial Basalts

Basalt and related rock types like andesite and dacite are rocks that form when volcanoes erupt magma (lava) onto the surface of the earth and the magma cools.

Basalt flows on Hawaii (photos by Randy Korotev)

Terrestrial basalts come in a wide variety of textures and colors. Some have vesicles (gas bubbles); some don’t. Some are gray, others are reddish or even greenish. Basalt is one of the most common rock types on Earth. Except for the coral, nearly all of the rocks of Hawaii are basalts. Much of Oregon and Idaho is covered with basalt.

Basalt cobbles are commonly found on beaches, especially on volcanic islands (Hawaii, top). Most of the dark rocks in the bottom photo are basalts (Lake Superior).

Closeup of a piece of fresh basalt from Hawaii. Sometimes the glassy surface of a fresh basalt is mistaken for a meteorite fusion crust. This basalt rock does not have the rounded, aerodynamic shape of a meteorite and it contains far too many vesicles for a meteorite. (photo by Randy Korotev)

Planetary Basalts

Basaltic volcanism occurred on other terrestrial planets, too. Venus, Mars, the Moon, and some of the largest asteroids had volcanoes that produced basalt. Basaltic meteorites dominate that 6-7% of all meteorites that are called achondrites (see statistics). Eucrites are basaltic meteorites from the asteroid belt. About 17% of the Moon’s surface is covered with basalt and about 8% of the lunar meteorites are unbrecciated basalts. Most of the Martian meteorites are basalts (all of those known as shergottites). All basalts, regardless of where they were formed, are similar because the process that formed them (melting in the interior of a planet) is the same. In the absence of a fusion crust, the only sure way to distinguish a terrestrial (Earth) basalt from a basaltic meteorite (Moon, Mars, asteroid) is with expensive chemical or mineralogical tests.

This handful of rocks came out of a bag sold at a garden shop. They are intended to be used in landscaping or in barbecue grills, where they help distribute the heat evenly. All are vesicular basalts.

Bottom Line:

If you have a basalt or other volcanic rock, then it’s not a meteorite. Such rocks are very common on Earth but uncommon among meteorites.