Identification of volcanic rocks is dependant on the
type of eruption and the original chemistry of the lava. Lavas that are
rich in minerals like quartz and feldspar are usually
light in color. They are usually associated with volcanism on land.
Rocks that are rich in hornblende, augite, and olivine
are darker in color and associated with eruptions under the oceans.
All volcanic rocks will cool much quicker than
plutonic rocks and this is reflected in the size of the minerals. In a
hand sample, the minerals are difficult to distinguish. But, you can
look at the rock and get a basic idea. If it is a dark colored rock and
fine grained, it can be basalt. If it is lighter colored, almost a
pink-gray, then it is andesite.