Igneous rocks are formed from magma (molten rock; the term igneous
derives from the Latin ignis for "fire") that
cools at the earth's surface or in the upper regions of the
crust. The magma that reaches the surface, called lava, is exposed
to much cooler water or air; and because it forms outside of the
crust, it is called extrusive rock. It solidifies relatively quickly,
sometimes within minutes or hours. Since magma typically reaches
the surface through volcanoes, this extrusive rock is commonly labeled
volcanic. Since the cooling process is so short, the chemical components
don't have the time to develop into large mineral crystals,
and so tend to be very small, giving the rock a fine-grained texture.
In some cases lava cools so fast that there are no discernible grains
and the rock instead has a glassy texture. A good example of this
is obsidian, a dark rock used by some ancient cultures to make stone
cutting tools. Other common or well-known volcanic rocks include
basalt, rhyolite, and pumice.
To learn more about volcanic rocks, visit the USGS Photo
Glossary of Volcano Terms.