Areal Distribution, Thickness, Mass, Volume, and Grain Size of Air-Fall Ash from the Six Major Eruptions of 1980

The airborne-ash plume front from the Mount St. Helens eruption of May 18 advanced rapidly to the northeast at an average velocity of about 250 km/hr during the first 13 min after eruption. It then traveled to the east-northeast within a high-velocity wind layer at altitudes of 10-13 km at an average velocity of about 100 km/hr over the first 1,000 km. Beyond about 60 km, the thickest ash fall was east of the volcano in Washington, northern Idaho, and western Montana. A distal thickness maximum near Ritzville, Wash., is due to a combination of factors: (1) crude sorting within the vertical eruptive column, (2) eruption of finer ash above the high-velocity wind layer at altitudes of 10-13 km, and (3) settling of ash through and below that layer. Isopach maps for the May 25, June 12, August 7, and October 16-18 eruptions show distal thickness maximums similar to that of May 18. A four-unit tephra stratigraphy formed by the May 18 air fall within proximal areas east of the volcano changes to three units, two units, and one unit at progressively greater distances downwind. Much of the deposits beyond 200 km from the volcano has two units. A lower thin dark lithic ash is inferred to represent products that disintegrated from the volcano\u27s summit in the initial part of the eruption and early juvenile pumice and glass. An upper, thicker, light-gray ash rich in pumice and volcanic-glass shards represents the later voluminous eruption of juvenile magma. The axis of the dark-ash lobe in eastern Washington and norther Idaho is south of the axis of the light-gray ash lobe because the high-velocity wind layer shifted northward during the eruption. The areal distribution of ash on the ground is offset to the north relative to the mapped position of the airborne-ash plume, because the winds below the high-velocity wind layer were more northward. Except for the distal thickness near Ritzville, Wash., mass per area, thickness, and bulk density of the May 18 ash decrease downwind, because larger grains and heavier lithic and crystal grains settled out closer to the volcano than did the lighter pumice and glass shards. A minimum volume of 1.1 km3 of uncompacted tephra is estimated for the May 18 eruption; this volume is equivalent to about 0.20-0.25 km3 of solid rock, assuming an average density of between 2.0 and 2.6 g/cm3 for magma and summit rocks. The estimated total mass from the May 18 eruption is 4.9 x 1014 g, and the average uncompacted bulk density for downwind ash is 0.45 g/cm3. Masses and volumes for the May 24 and June 12 eruptions are an order of magnitude smaller than those of May 18, but average bulk densities are higher (about 1.00 and 1.25), owing to compaction by rain that fell during or shortly after the two eruptions. Volume and mass of the July 22 eruption are two orders of magnitude smaller than those of May 18, and those of the August 7 and October 16-18 eruptions are three orders of magnitude smaller. The eruption of May 18, however, is smaller than five of the last major eruptions of Mount St. Helens in terms of volume of air-fall tephra produced, but probably is intermediate if the directed-blast deposit is included with the air-fall tephra