A line on a map that connects geologic units of equal rock thickness. In volcanology, this typically relates to the thickness of an ash or tephradeposit from an explosive eruption
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}