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Abstract

Systematic measurements of the spatial variations of pyroclast grain size distributions in fall deposits are used to infer the mass eruption rates, eruption cloud heights, gas eruption speeds and released magma volatile contents during 31 explosive eruptions in regions where such events are common. The compilation of eruption cloud heights in this way is an important input to statistical models predicting volcanic hazard due to pyroclast dispersal. The estimates of mass eruption rate are found to be generally quite reliable, with estimated errors of no more than a factor of 2, corresponding to implied errors of c. 20% in eruption cloud height and clast dispersal. The estimates of magma volatile content and eruption speed, however, are very much less reliable. This highlights the extreme difficulty, despite meticulous fieldwork, of obtaining enough measurements of pyroclast size variations very near to a vent to allow good estimates of this parameter to be made. This is unfortunate, because improved values for released magma gas contents would allow quantification of the likely stability of eruption columns against collapse to form pyroclastic density currents, which would add an extra dimension to hazard modelling.

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