The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions

In an explosive eruption, the atmospheric column dynamics depend strongly o n the mass fraction of gas in the erupting mixture, which is fixed by fragm entation in the volcanic conduit. At fragmentation, gas present in vesicula r magmatic liquid gets partitioned between a continuous phase separating ma gma clasts and a dispersed phase in individual bubbles within the clasts, A s regards flow behavior, it is the former, continuous, gas phase which matt ers, and we show that its amount is determined by the fragment size. Analys is of 25 fall deposits and 37 flow deposits demonstrates that ash and pumic e populations follow a power law size distribution such that N, the number of fragments with radii larger than r, is given by N;proportional to r(-D). D values range from 2.9 to 3.9 and are always larger than 3.0 in fall depo sits. D values for pyroclastic flow deposits are systematically smaller tha n those of fall deposits. We show that at fragmentation the amount of conti nuous gas phase is an increasing function of the D value. Large D values ca nnot be attributed to a single fragmentation event and are due to secondary fragmentation processes. Laboratory experiments on bubbly magma and on sol id pumice samples demonstrate that primary breakup leads to D values of 2.5 +/-0.1 and that repeated fragment collisions act to increase the D value,. A model for size-dependent refragmentation accounts for the observations. W e propose that in a volcanic conduit, fragmentation proceeds as a sequence of events. Primary breakup releases a small amount of gas and is followed b y fragment collisions. Due to refragmentation and decompression, both the m ass and volume fractions of continuous gas increase. The final D value, and hence the mass fraction of continuous gas at the vent: depends on the time spent between primary fragmentation and eruption out of the vent.