Pumice is a rock formed from a very gas-rich magma. As this
magma rises in the vent of a volcano, the gas expands and
forces the magma out explosively. In some cases what gets
blasted out of the volcano is a froth of gas bubbles held in
a relatively small amount of liquid magma. The froth freezes
in the air, the liquid turning to glass and trapping the
bubbles, forming lumps of pumice. Pumice is usually so light
that it will float on water.