Igneous rocksthose which originate from magma fall naturally
into two categories, extrusive and intrusive. Extrusive rocks
are erupted from volcanoes or seafloor fissures, or they freeze
at shallow depths. This means that they cool relatively quickly
and under low pressures, therefore they are typically fine
grained and gassy. The other category is intrusive rocks, which
solidify slowly at depth and do not release gases. Some of these
rocks are clasticcomposed of rock and mineral fragments, or
clasts, rather than solidified melt. Technically that makes them
sedimentary rocks, but these volcaniclastic rocks have many
differences from other sedimentary rocksin their chemistry and
the role of heat, especiallyand geologists tend to lump them
with the igneous rocks. I'm being a little strict, then, and
calling this whole gallery "volcanic rocks." Learn more about
igneous rocks