Bimodal grain size distribution and secondary thickening in air-fall ash layers

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Investigation of the ashfall deposit from the 1980 Mount St Helens eruption reveals that the deposit is bimodal, the finer population maintaining a uniform grain size distribution away from source, and that the deposit exhibits secondary thickening beyond the 1-cm isopach. Grain size distribution shows that at least 64% of the deposit is finer than 63 µm, far more than is generally supposed for plinian eruptions. An investigation of other ashfall deposits reveals similar grain size bimodality features, which are interpreted as evidence of aggregation of fine particles in the eruption cloud. Our data suggest that aggregation may be a common process during ash fallout from explosive eruptions and that secondary thickening may often occur, providing that other conditions are met.