During each of the magmatic eruptions of Mount St. Helens on May 25, June 12, and August 7, a vertical eruptive column rose intermittently to altitudes of 12-15 km, from which pumice, lithic fragments, and crystals settled downwind in lobes that generally become thinner and finer away from the volcano. Each ejecta lobe is asymmetric according to several criteria, including (1) the axes of maximum thickness and of maximum pumice size are not midway between the two margins of the lobe, (2) the axis of maximum pumice size does not correspond to the axis of thickness, and (3) the median size of particles grades through several grain-size intervals from one lateral margin to the other. The fining in grain size across the lobe is due to the rotation of wind directions with altitude, so material falling from a high-level airborne plume is winnowed as it falls through transverse low-level winds. Wind directions that rotate clockwise with increasing altitude effect an air-fall lobe whose axis of maximum coarseness is clockwise of the axis of maximum thickness; wind directions that rotate counterclockwise with increasing altitude effect an air-fall lobe whose trend of maximum coarseness is counterclockwise of the axis of maximum thickness.
The thickness of air-fall deposits from eruptions on May 25 through August 7 range variously from one-third to one-fortieth that of the May 18 air-fall deposit at a given distance from the volcano. The post-May 18 deposits are an order of magnitude thinner than Mount St. Helens pumice layer T (A.D. 1800) and two orders of magnitude thinner than Mount St. Helens pumice layer Yn (3400 yr B.P.), which is similar in thickness to the most voluminous air-fall deposits of other Cascade Range volcanoes. The maximum size of pumice within the May 18 air-fall lobe is 5-10 times that of the post-May 18 lobes. The overlapping air-fall lobes of May 25, June 12, July 22, and August 7 form a stratigraphic layer that in most places is indivisible into deposits of the separate eruptions.
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