Post‐eruption erosion and deposition in the 1980 crater of Mount St Helens, Washington, determined from digital maps

Abstract

AbstractThe crater of Mount St Helens shows one of the world's highest known rates of mass wasting. On many summer days, rockfall is almost continuous, and many large rock and dirty‐snow avalanches have travelled several kilometres from their sources on the crater walls. Since formation of the crater on 18 May 1980, talus cones exceeding 100 m in thickness have formed at the base of the unstable 600 m high crater walls. To estimate rates of erosion and deposition, a series of digitized topographic maps made from aerial photographs taken of the crater in 1980, 1981, 1983, 1986 and 1988 were analysed using a geographic information system. Between 1980 and 1988, 30 × 106 m3 of rock were eroded from the crater wall, representing a mean retreat rate of 2.1 m yr−1. To account for the volume increase that occurs when bedrock is transformed into scree, this volume is multiplied by 4/3; this provides an estimate of the rock‐debris volume supplied to the crater floor of 40 × 106 m3. The actual volume of deposits that accumulated during this 8 year period, however, is 68 × 106 m3. The difference of 28 × 106 m3 is presumably the volume of snow intercalated between insulating layers of rock debris. Similar calculations for each of four time intervals between 1980 and 1988 suggest that wall erosion and thus talus accumulation rates are declining, but that rates will probably remain high for decades to come.