Abstract
Mount St. Helens ash is a pumaceous, pyroclastic material which has been dispersed in varying amounts over most of the Pacific Northwest on a number of occasions during 1980, including the catastrophic eruption of May 18 1980. Samples have been collected and examined from several sites between the crater itself and northwestern Oregon from March 20, 1980 through October 17, 1980. Ejected material consists of both older mountain and new particles. Tephra composition varies in proportions of glass and glass-coated minerals with length of plume travel time. Portland area fallout has ranged from coarse, steel gray, low in glass on June 12, 1980 (travel time two hours) to fine, talcy, brownish gray, high in glass on May 25, 1980 (travel time six to eight hours).Crystalline, sub-micron sized particles of the ash are being studied by the selected-area electron diffraction technique with the TEM in an effort to determine the nature of the mineral content of the ash. The particles are deposited on carbon-coated grids containing small pieces of evaporated gold film for determination of accurate camera constants. Several plagioclase feldspar minerals (70-75% of the mineral fraction for Cascade volcanoes) have been identified from random samples of the atmospheric fallout.
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