GEOLOGICAL NOTE Sizes and Shapes of 10-Ma Distal Fall Pyroclasts in the Ogallala Group, Nebraska

Size distributions of distal ashfall particles from correlated 10-Ma layers in Nebraska, measured using laser diffraction methods, are lognormal with mode diameters of ∼90 mm. This ashfall is ∼100% bubble-wall shards of rhyolite glass and apparently represents a distal ashfall from an eruption 1400 km away. Measured terminal velocities of these ash particles are 0.2–18 cm/s, consistent with Stokes Law settling of spherical particles with diameters of 9–50 mm. Surface area of the ash particles, measured with gas adsorption, is 20–30 times the surface area of equivalent Stokes spheres. These results highlight the effects of shape and atmospheric drag in distal ashfalls. They also highlight atmospheric transport and fallout of distal ashfall particles, because these deposits resemble many other ashfalls preserved in the Great Plains of North America throughout the Tertiary and Quaternary. Because the ashfalls preserve major mammalian death assemblages, they demonstrate that deposits with modes of optical diameters 1100 mm are still hazardous by aerodynamic definitions of lung disease risk and include particles substantially within hazardous PM10 ranges. The aerodynamically fine particle size may lead to substantial aeolian redistribution, causing local thicknesses of 12 m. Overall, the ashfall thicknesses observed are at least several times larger than would be expected based on exponential thinning from the volcano. Shape measurements of distal ash particles may be necessary to assess risk. The possible health risks in the central United States from a future rhyolitic eruption in the western United States may be significant.