Pumice
Pumice is a light, porous volcanic rock that forms during explosive eruptions.
It resembles a sponge because it consists of a network of gas bubbles frozen
amidst fragile volcanic glass and minerals. All types of magma (basalt, andesite,
dacite, and rhyolite) will form pumice.
Pumice is similar to the liquid foam generated when a bottle of pressurized
soda is opened--the opening depressurizes the soda and enables dissolved carbon
dioxide gas to escape or erupt through the opening. During an explosive
eruption, volcanic gases dissolved in the liquid portion of magma also expand
rapidly to create a foam or froth; in the case of pumice, the liquid part of the
froth quickly solidifies to glass around the glass bubbles.
Note the
closed pores of this pumice closeup. As a result, pumice is impermeable,
and floats on water. The glassy nature of the material could potentially
cause alkali reactivity in concrete.
Click on hot link for more pictures of pumice.
