In volcanology: rocks that are not from the primary eruptive magma, but are instead pulled from the walls of the magma storage zone or conduit during an eruption. Previously formed rocks not from the primary magma source
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}