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.
Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals.
Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes.
-- Excerpt from:
Volcano World Website, August 2001; and USGS Volcano Hazards Website Photoglossary, August 2001