Turbulent, low-density cloud of hot rock debris and gases that moves over the ground surface at high speed.<ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology]</ref> | Turbulent, low-density cloud of hot rock debris and gases that moves over the ground surface at high speed.<ref>[http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html Glossary of Volcano and Related Terminology]</ref> |