Revised tephra volumes for Mount St. Helens and Glacier Peak volcanoes

Abstract:

Isopach maps from 8 tephra eruptions from Mount St. Helens were reported in Carey et al. (1995) and for 3 eruptions from Glacier Peak in Gardner et al. (1998). These isopach data only define single slopes on a thickness versus square root of area plot (Fierstein and Nathenson, 1992) whereas one expects a second slope in the medial to distal region for larger eruptions. A model was proposed by Carey et al. (1995) for estimating the second slope to calculate volumes. A more recent study by Sulpizio (2005) for estimating the second slope involves a systematic analysis of many eruptions to provide correlation equations. The purpose of this study is to recalculate the volumes of Mount St. Helens and Glacier Peak eruptions and compare results from the two methods for estimating second slopes.

In order to gain some perspective on the methods for estimating the second slope, we use data for thickness versus distance beyond the last isopach that is available for some of the eruptions. The thickness versus square root of area method is extended to thickness versus distance by developing an approximate relation between the two, assuming elliptical isopachs. Thickness versus distance data tend to support the Sulpizio method.

The volumes derived using the Sulpizio method are 20 % or less of the values for the Mount St. Helens layers given in Carey et al. (1995) and about 50 % of the values for the Glacier Peak layers given in Gardner et al (1998). For example, for Mount St. Helens layer Wn, the volume calculated from the isopachs is 0.55 km³, using the Carey et al. (1995) method it is 7.7 km³, and using the Sulpizio (2005) method it is 1.4 km³.

Carey, S., Gardner, J., and Sigurdsson, H., 1995, J. Volc. and Geoth. Res. 66, 185–202.

Fierstein, J., and Nathenson, M., 1992, Bull. Volc. 54, 156–167.

Gardner, J.E., Carey, S., and Sigurdsson, H., 1998, Geol. Soc. of Am. Bull. 110, 173–187.

Sulpizio, R., 2005, J. Volc. Geoth. Res. 145, 315–336.