Abstract

A new, statistical sampling strategy for pyroclastic deposits has yielded data that reveal the depositional characteristics of the May 18, 1980, blast flow of Mount St. Helens. Samples were collected from each layer along a profile parallel to flow lines, as interpreted from tree blow-down directions. The decrease in the mass of sediment of different grain sizes with distance supports a turbulent-gravitational mechanism of suspended sedimentation, and transport within a dilute blast cloud having a density only slightly greater than that of the atmosphere. Traditional sampling strategies would have been unable to yield data suitable to test this settling mechanism hypothesis.

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