Joumalof volcanology and geothennal research ELSEVIER Journal of Volcanology and Geothennal Research 66 ( 1995) 185-202 The intensity and magnitude of Holocene plinian eruptions from Mount St. Helens volcano S. Carey, J. Gardner, H. Sigurdsson Graduate School of Oceanography. University of Rhode Island. Narragansett. RI02882. USA Received 16 October 1991; accepted 7 July 1994 Abstract Dispersal characteristics of the T. We, Wn, Pu, Ps, Ye, Yn and Yb plinian fall deposits of Mount St. Helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport. Isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities (magma discharge) based on a theoretical model of tephra dispersal. New proximal thickness measurements are combined with an empirical distal extrapolation, based on studies of 53 plinian deposits, to calculate the magnitude (erupted mass) of each eruption. Layer Yn (3510 yr B.P.) represents the highest intensity and largest magnitude eruption at Mount St. Helens in post-glacial times. Modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase ( - 26 hours). Several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of Mount St. Helens. Peak intensity of the plinian phase was 10 8 kg/s and the total erupted volume was 4 km 3 (DRE of magma). Small plinian-style eruptions are represented by layers such as Ps and Pu of the Pine Creek eruptive period (3000-2500 yr B.P.) and have intensities of only _10 6 kg/so When compared with plinian eruptions from other volcanoes, the Holocene eruptions of Mount St. Helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate- sized stratovolcanoes. There is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle. This relationship may be related to a time-dependent process for the accumulation of differentiated and volatile-rich magma within the chamber beneath Mount St. Helens. 1. Introduction Mount St. Helens has erupted explosively numerous times during the past 40,000 years (Crandell et aI., 1975; Mullineaux et aI., 1975). More than 100 tephra fall layers have been identified on the flanks of the volcano and, in some cases, over large areas of the northwestern United States and Canada (e.g., Crandell et aI., 1962; Moody, 1977; Mullineaux, 1986). This record of frequent and violent eruptions led Crandell et aI. (1975) to suggest that Mount St. Helens was the most active volcano of the Cascade Range and there 0377-0273/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0377-0273(94)00059-X was a high probability that another eruption would take place during this century. In 1980 their forecast was realized as the volcano entered into a new period of explosive activity and dome building (Lipman and Mullineaux, 1981). The 1980 eruptions were intensely studied and sig- nificant progress was made in relating the physical characteristics of pyroclastic deposits, in particular the tephra falls, to the dynamics of the eruptions and atmos- pheric transport of ash (Carey and Sigurdsson, 1982, 1985; Hopkins and Bridgman, 1985; Armienti et aI., 1988; Carey et aI., 1990). Recent developments in the-