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 in Hawaii.
Click on image for enlargement.
(photos by Randy Korotev)
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
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. Click on image for
(photo by Randy Korotev)
Basaltic volcanism occurred on other terrestrial
planets, too. Venus, Mars, the Moon, and some of the largest asteroids had
volcanoes that produced basalt. Eucrites are basaltic meteorites from the
asteroid belt. About 17% of the Moon is covered with basalt and about 14%
of the lunar meteorites are basalts. (See "How Do We Know It's a Rock From the Moon?") 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. Unfortunately, the only
way to distinguish a terrestrial (Earth) basalt from a basaltic meteorite
(Moon, Mars, asteroid) is with expensive chemical and mineralogical tests.
If you find a basalt, it's probably not a meteorite.
The handful of rocks below came out of a bag
sold at a garden shop. They are intended to be used in landscaping or in
barbeque grills, where they help distribute the heat evenly. All are vesicular basalts.