El Chichón volcano , April 4 , 1982 : volcanic cloud history and fine ash fallout

This retrospective study focuses on the fine silicate particles (\62 lm in diameter) produced in a large eruption that was otherwise well studied. Fine particles represent a potential hazard to aircraft, because as simple particles they have very low terminal velocities and could potentially stay aloft for weeks. New data were collected to describe the fine particle size distributions of distal fallout samples collected soon after eruption. Although, about half of the mass of silicate particles produced in this eruption of *1 km dense rock equivalent magma were finer than 62 lm in diameter, and although these particles were in a stratospheric cloud after eruption, almost all of these fine particles fell to the ground near (\300 km) the volcano in a day or two. Particles falling out from 70 to 300 km from the volcano are mostly \62 lm in diameter. The most plausible explanation for rapid fallout is that the fine ash nucleates ice in the convective cloud and initiates a process of meteorological precipitation that efficiently removes fine silicates. These observations are similar to other eruptions and we conclude that ice formation in convective volcanic clouds is part of an effective fine ash removal process that affects all or most volcanic clouds. The existence of pyroclastic flows and surges in the El Chichón eruption increased the overall proportion of fine silicates, probably by milling larger glassy pyroclasts.