Fall-out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of St. Vincent

Abstract The 1979 explosive activity of Soufriere Volcano, St. Vincent, commenced on April 13th and continued for two weeks, producing eleven eruption columns. The activity produced air-fall ash with abundant accretionary lapilli, plus minor pyroclastic flows and base surge deposits. The dispersal of the columns was observed by an SMS-1 geostationary satellite. All but two of the plumes were dispersed entirely to the east. The last event on April 26th formed a minor lobe to the east and a major lobe to the south-southeast. The April 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour. The magma discharge rate was greater than 3000 m3/s. The plume expanded at a rate of 4000 km2/hr and was dispersed at a mean velocity of 15 m/s. The ash deposits show bimodal grain-size distribution, with 95% of the ash collected being sub 1 mm diameter. Modelling of the ash fall-out has allowed interpretation of grainsize parameters in a quantitative manner. The fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density. The coarse population of the April 26th deposit displayed a systematic change in Mdθand σθ away from source. The fine mode displays uniform Mdθ and σθ everywhere and is interpreted as having been deposited in the form of accretionary lapilli.