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*A light-colored, frothy volcanic rock, usually of [[dacite ]] or [[rhyolite ]] composition, formed by the expansion of gas in erupting [[lava]]. Commonly perceived as lumps or fragments of pea size and larger but can also occur abundantly as ash-size particles. <ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology (Foxworthy and Hill, 1982) ]</ref>  *Because of its numerous gas bubbles, pumice commonly floats on water. <ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology]</ref>