Blast ashfall deposit of May 18, 1980 at Mount St. Helens, Washington

Abstract

The ashfall deposit produced by the major directed blast at Mount St. Helens on the morning of May 18, 1980 is a thin (≤, 65 mm), small-volume (0.03 km 3 dense rock equivalent), fine-grained crystal-lithic ash with a large dispersal area (> 10 4 km 2). The blast ashfall deposit is characterized by accretionary lapilli and smaller ash aggregates within 50 km of the volcano. The maximum deposit thickness, finest median grain size, most efficient sorting and largest and most abundant accretionary lapilli are approximately coincident and located 17 km north of the volcano, adjacent to a major reentrant in the northern margin of the devastated zone. It is suggested that processes of flow-convergence and arrested dispersal in the blast due to topographic barriers led to higher suspended ash concentrations in that area, resulting in increased aggregation and fallout of fine-grained ash. The most distinctive characteristic of the blast ashfall that indicates an origin from a directed blast is its combined features of chiefly dense lithic and crystal pyroclasts in a widely dispersed, fine-grained deposit. The common presence of shredded vegetation is a further indication of a directed blast.