A region of the earth's crust or lithosphere that is spreading apart due to tectonic forces acting on the surrounding areas. Back-arc basins are regions of extension behind subduction zones and are often locations where volcanoes form
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}