BLAST ASHFALL DEPOSIT OF MAY 18, 1980 AT MOUNT ST-HELENS, WASHINGTON

The ashfall deposit produced by the major directed blast at Mount St. Helens on the morning of May 18, 1980 is a thin (less than or equal to 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 bl ast ashfall deposit is characterized by accretionary lapilli and small er ash aggregates within 50 km of the volcano. The maximum deposit thi ckness, finest median grain size, most efficient sorting and largest a nd 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 proce sses of flow-convergence and arrested dispersal in the blast due to to pographic barriers led to higher suspended ash concentrations in that area, resulting in increased aggregation and fallout of fine-grained a sh. The most distinctive characteristic of the blast ashfall that indi cates an origin from a directed blast is its combined features of chie fly dense lithic and crystal pyroclasts in a widely dispersed, fine-gr ained deposit. The common presence of shredded vegetation is a further indication of a directed blast.