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
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 with basalt.
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 enlargement. (photo by
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.