Pronunciation: PUM•iss
Description*: Pumice is a frothy, gas-filled volcanic rock in which the walls of the gas bubbles and gas tubes are composed primarily of volcanic glass. Pumice is typically a light-toned rock (white, gray, tan) whose very low density (due to the gas bubbles) allows it to float in water, at least until the bubble structure fills with water.
Volcanic glass forms naturally when magma is cooled so rapidly that the atoms do not have time to connect with one another in regular lattice structures. As a result of its irregular structure, glass is unstable and eventually breaks down (devitrifies) to form a variety of minerals that are stable at Earth's surface temperature and pressure. This process is usually completed within ~20 million years.
*Description from Cronin, V.S., 2001, Geology laboratory projects for group learning: Primis McGraw-Hill, 156 pp., ISBN 0-07-252348-4
Used by permission of the author.
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