1. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Abstract

Measurements of pore size distribution in five samples of pumice, formed during explosive eruptions of plinian type, show three distinct sizes of vesicle. The characteristics of the three pore size populations are similar in the samples although they were collected from plinian deposits of five different volcanoes. We interpret the pore size populations as representing the degassing of silicic magma on three very different time scales. The coarse population (»60 m) represents slow vesiculation within the magma chamber over periods of weeks, months or years at small levels of gas supersaturation. The medium-sized population (50–5 m) represents rapid vesiculation of magma on ascent to the surface during the plinian eruption over periods of minutes to hours. The fine population (5–0.5 m) represents bubbles formed during explosive disruption of magma and rapid ejection into the atmosphere, involving short time scales (a few seconds), but large decompression rates.