TRANSPORT AND DEPOSITION IN THE MAY 18, 1980, MOUNT-ST-HELENS BLAST FLOW

A new statistical sampling strategy for pyroclastic deposits has yield ed data that reveal the depositional characteristics of the May 18, 19 80, blast flow of Mount St. Helens. Samples were collected from each l ayer along a profile parallel to flow lines, as interpreted from tree blow-down directions. The decrease in the mass of sediment of differen t grain sizes with distance supports a turbulent-gravitational mechani sm of suspended sedimentation, and transport within a dilute blast clo ud 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.