A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward. The surfaces of pahoehoe flows on flat or gentle slopes often exhibit elliptical, domed structures called tumuli
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}