Natural Disaster. Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian forms in lava flows where the lava cools so fast that crystals do not have time to grow
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}