The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions (original) (raw)

1998, Journal of Geophysical Research: Solid Earth

In an explosive eruption, the atmospheric column dynamics depend strongly on the mass fraction of gas in the erupting mixture, which is fixed by fragmentation in the volcanic conduit. At fragmentation, gas present in vesicular magmatic liquid gets partitioned between a continuous phase separating magma clasts and a dispersed phase in individual bubbles within the clasts. As regards flow behavior, it is the former, continuous, gas phase which matters, and we show that its amount is determined by the fragment size. Analysis of 25 f•ll deposits and 37 flow deposits demonstrates that ash and pumice populations follow a power law size distribution such that N, the number of fragments with radii larger than r, is given by N • r-D. D values range from 2.9 to 3.9 and are always larger than 3.0 in fall deposits. D values for pyroclastic flow deposits are systematically smaller than those of f•ll deposits. We show that at fragmentation the amount of continuous gas phase is an increasing function of the D value. Large D values cannot be attributed to a single fragmentation event and are due to secondary fragmentation processes. Laboratory experiments on bubbly magma and on solid pumice samples demonstrate that primary breakup leads to D va, lues of 2.54-0.1 and that repeated fragment collisions act to increase the D value. A model for size-dependent refragmentation accounts for the observations. We propose that in a volcanic conduit, fragmentation proceeds as a sequence of events. Primary breakup releases a small amount of gas and is followed by fragment collisions. Due to refragmentation and decompression, both the mass 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. 29,759 29,760 KAMINSKI AND JAUPART: FRAGMENTATION IN EXPLOSIVE VOLCANIC ERUPTIONS Exsolution Figure 1. Schematic representation of the main processes occurring in an eruptive conduit. At fragmentation, bubbly magma breaks up into a number of fragments. A common assumption ß •s that all the gas present collects into a continuous phase carrying the fragments. This implies that only ash particles are generated by fragmentation, which is not consistent with the presence of pumice samples in pyroclastic deposits. every gas bubble gets disrupted and all magma fragments are vesicle free. We shall call this the "complete atomization limit," such that all the exsolved gas collects into a continuous phase (Figure 1). In the other limit, the magma fragments are large and retain a large number of bubbles inside. In this case, the continuous gas phase is a tortuous network separating the fragments, and the mixture of gas and fragments does not behave as a suspension. Pumice samples from pyroclastic deposits are highly vesicular and provide evidence for gas kept within fragments [Gardner et al., 1996; Kaminski and Jaupart, 1997] (Appendix A), suggesting that the "complete atomization" limit may not be a valid approximation. The above argument implies that the mass fraction of continuous gas in the volcanic mixture depends on fragmentation. Thus fragmentation not only separates between "explosive" and "effusive" eruption regimes but may also determine which explosive regime ensues. Consider, for example, the Plinian and pyroclastic flow regimes. In the Plinian case, the erupted material becomes lighter than surrounding air and a buoyant column develops to high altitudes in the atmosphere. In the pyroclastic flow regime, the eruption column collapses at some height above the vent [Sparks and Wil-son, 1976; Woods, 1995]. Specifying which regime prevails requires knowledge of the mass fraction of gas at the vent, which involves three steps. The first step is to estimate the amount of volatiles dissolved in the melt at depth [Rutherford et al., 1985; Anderson et al., 1989]. The second step is to predict how bubbles nucleate and expand due to pressure release and to specify the mechanism of fragmentation. This has been the focus of much recent research, involving field studies [