pumice (pŭmˈĭs) [key], volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colorless or
pumice (pŭmˈĭs) [key], volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colorless or light gray and has the general appearance of a rock froth. The viscosity of the lava, the quantity of water vapor and gas, and the rate of cooling together determine the fineness of the vesicular substance. Large amounts of gas result in a finer-grained variety known as pumicite. The chemical composition is that of granite. Coarser-grained rock, with fewer and larger air spaces, is called scoria; it is usually associated with dark-colored igneous rocks of diorite or gabbro composition. Pumice is used chiefly as an abrasive and is inc