Pumice, igneous rock having a spongy or frothy texture, and composed largely of glass. It is frequently made up of parallel fibers or threads, with intervening spaces to form a delicate structure. Pumice is produced by the expansion of the occluded, or internal, gases of lavas when they reach the surface of the earth (see Lava). It is most abundantly developed in lavas of rhyolitic composition, because these are usually very viscous (see Rhyolite).