If magma ascends faster than it cools, it erupts and solidifies at the surface to form
a fine-grained volcanic rock. The magma erupts from volcanoes, which are
named after Vulcan, the Roman god of fire. The grains of this volcanic
rock are enclosed in a glassy matrixevidence that the magma cooled
within minutes to days, turning the liquid portion to glass.
If magma cools faster than it ascends it becomes a mush of crystals
and liquid that stops in its tracks underground to form a coarse-grained
plutonic rock. Plutonic rocks are named after Pluto, the Roman god of
the underworld. Like all plutonic rocks, this one is made completely
of crystals, with no glass or gas bubbles. The large crystals fit closely
togetherevidence that the rock cooled underground over tens to
hundreds of thousands of years.