Explosive volcanic eruptions emit large proportions of very fine ash (<30 μm) into the atmosphere, posing hazards to aviation, infrastructure, and human health. Here we present an analysis of bubble size distributions at the point of fragmentation during the 18 May 1980 eruption of MSH through the examination of simple ash particles in distally deposited fall samples. The external surfaces of individual fine ash grains preserve the morphology of the bubbles that burst to form the ash, so bubble sizes can be measured using stereo‐scanning electron microscopy. Simple ash particles are those that allow the measurement of a single vesicle imprint per individual grain. These simple ash particles are the finest component of the tephra, and can thus travel great distances from the source volcano. Analyses of samples provided bubble volume distributions with a dominant peak between 560 and 5600μm3, corresponding to equivalent vesicle diameter modes of 10–22 μm, and these values were consistent for all examined sample locations. Determination of syn‐eruptive bubble sizes thus makes it possible to glean information regarding conduit dynamics at the point of magma fragmentation from observed ash deposits, to parameterize numerical eruption models in ways not previously possible, and to quantify the size of bubbles that burst to create the ash component most hazardous to the aviation industry and human health.
