The large amount of scientific data collected on the Mount St. Helens eruption has resulted in significant changes in thinking about the atmospheric hazards caused by explosive volcanic activity. The hazard posed by fine silicate ash with long residence time in the atmosphere is probably much less serious than previously thought. The Mount St. Helens eruption released much fine ash in the upper atmosphere. These silicates were removed very rapidly due to a process of particle aggregation (Sorem, 1982; Carey and Sigurdsson, 1982; Rose and Hoffman, 1982). There is some evidence to suggest that particle aggregation is particularly successful in removing glass shards with high surface areas/mass ratios. The primary atmospheric hazard of explosive eruptions is volcanic sulfur, which is converted to sulfuric acid and sulfate crystals. Although the Mount St. Helens dacite magma had a very low sulfur content before eruption, the eruptions did make a significant contribution to the stratospheric sulfate layer (Newell, 1982). Evidence based on measurements of S and Cl in erupted rocks, glass inclusions, gas samples, and atmospheric samples collected for both Mount St. Helens and Fuego volcanoes, suggests that both volcanoes released substantial contributions of S from intrusive (non-eruptive) magma. The amount of sulfur contributed to the atmosphere by an explosive eruption thus depends not only on the volume of magma erupted and its sulfur content, but also on the degree of near-surface non-eruptive magma.
The data collected to assess atmospheric hazard and to evaluate the processes and mechanisms of explosive volcanic eruptions have helped illuminate our understanding of: (1) the dispersion and atmospheric fractionation of volcanic ash and (2) the determination of the size and degassing energetics of shallow magma bodies beneath volcanoes.
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