The igneous rocks that form on the earth’s surface are called
_extrusive_ or _volcanic_ igneous rocks. When magma flows to the
surface, it cools and hardens quickly. The mineral grains that form
during this fast cooling may be too small to be distinguished from each
other. Some lava cools too quickly for minerals to crystallize—then the
rock is volcanic glass.
[Illustration: Extrusive igneous rocks form at the earth’s surface from
lava that cools and hardens relatively quickly.]
No volcanic igneous rocks are forming in Texas now. However, during
Tertiary time, in the Big Bend area and in other parts of the
Trans-Pecos country of west Texas, lava came to the surface and
hardened. (The physiographic outline map, p. 42, shows where these areas
are located.)