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

Abstract

Abstract. 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.

Percolating magmas in three dimensions

“…Our simulations confirmed that the power law bubble distributions did not alter this value (Fig. 4a) which is very close to the empirical value B 3,f ≈1.1±0.1 from Plinian fragments (Kaminski and Jaupart, 1998). Note that in some of the literature (e.g.…”

“…Figure 4b shows that -at least in the case of power law bubble distributions -that the primary fragments have size distributions with long tails not too far from the theoretical power law with exponent B 3f (hence close to the observations of Kaminski and Jaupart, 1998). The only direct way to test this model would be either empirically by testing the yield strengths of volcanic products as functions of their vesicularities (something which to our knowledge has not yet been done systematically), or to perform (difficult) numerical or theoretical calculations on percolating systems (see however below).…”

“…This value B 3f >1 contrasts with the much smaller value obtained in 2-D percolation (≈B 2 /2; see Gaonac'h et al, 2003, for a discussion). In addition, as noted by Kaminski and Jaupart (1998) Fig. 4b.…”

“…Note that in some of the literature (e.g. Kaminski and Jaupart, 1998;Kueppers et al, 2006) the exponent 3B 3f is used instead. It is denoted by the symbol D and is incorrectly referred to as a "fractal dimension" even though it is not the fractal dimension of any set of points; indeed, since 3B 3,f >3, it could not possibly represent a set embedded in three-dimensional space.…”

Erupción del volcán de fuego (Guatemala, 1974): ¿Evidencia de una fragmentación terciaria?

“…En este contexto se entenderá por fragmentación secundaria aquella en la cual la partícula, como consecuencia de colisiones se fragmenta, dejando expuesta una porción de las vesículas que contiene en su interior, favoreciendo una subsecuente fracturación al explotar las vesículas debido a que su presión interna es mucho mayor que la externa (Kaminski & Jaupart, 1997, 1998.…”