Volcanic rock (or lava) that characteristically is dark in color (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium. Basalticlavas are more fluid than andesites or dacites, which contain more silica
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}