A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity from the base of a vertical eruption column. Can accompany phreatomagmatic eruptions
U.S. Geological Survey, U.S. Department of the Interior, Volcano Hazards Program, Glossary, Washington, DC, USA 2011. http://volcanoes.usgs.gov/vsc/glossary/alluvium.ht ml, (ND/ Volc Emr - Amer) {795}