The evolution of bubble size distributions in volcanic eruptions (original) (raw)
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Scaling vesicle distributions and volcanic eruptions
Bulletin of Volcanology, 2004
Models of coalescence-decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities (n(V)) are of the scaling form n V ð Þ / V ÀB 3 À1 where V is the volume of the vesicles and B 3 the 3dimensional scaling (power law) exponent. We analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from %10 mm to 3 cm, they are well fit with B 3 %0.85. We show that to within experimental error, this exponent is the same as that reported in the literature for basaltic lavas, and other volcanic products. The importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at "packing" bubbles allowing very high vesicularities to be reached before the critical percolation threshold, a process which-for highly stressed magmas-would trigger explosion. In this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics.
Relating vesicle shapes in pyroclasts to eruption styles
Bulletin of Volcanology, 2013
Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that is smaller for eruptions with Ca 1 than for eruptions with Ca 1. Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have Ca 1, in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has Editorial responsibility: been controversial, are also characterized by Ca 1 and larger values of than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.
Enhancement of eruption explosivity by heterogeneous bubble nucleation triggered by magma mingling
Scientific Reports
We present new evidence that shows magma mingling can be a key process during highly explosive eruptions. Using fractal analysis of the size distribution of trachybasaltic fragments found on the inner walls of bubbles in trachytic pumices, we show that the more mafic component underwent fracturing during quenching against the trachyte. We propose a new mechanism for how this magmatic interaction at depth triggered rapid heterogeneous bubble nucleation and growth and could have enhanced eruption explosivity. We argue that the data support a further, and hitherto unreported contribution of magma mingling to highly explosive eruptions. This has implications for hazard assessment for those volcanoes in which evidence of magma mingling exists.
The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions
Journal of Geophysical Research: Solid Earth, 1998
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 [
Journal of Volcanology and Geothermal Research, 2010
Soufrière Hills Volcano had two periods of repetitive Vulcanian activity in 1997. Each explosion discharged the contents of the upper 0.5-2 km of the conduit as pyroclastic flows and fallout: frothy pumices from a deep, gas-rich zone, lava and breadcrust bombs from a degassed lava plug, and dense pumices from a transition zone. Vesicles constitute 1-66 vol.% of breadcrust bombs and 24-79% of pumices, all those larger than a few tens of µm being interconnected. Small vesicles (< few tens of µm) in all pyroclasts are interpreted as having formed syn-explosively, as shown by their presence in breadcrust bombs formed from originally non-vesicular magma. Most large vesicles (> few hundreds of µm) in pumices are interpreted as pre-dating explosion, implying pre-explosive conduit porosities up to 55%. About a sixth of large vesicles in pumices, and all those in breadcrust bombs, are angular voids formed by syn-explosive fracturing of amphibole phenocrysts. An intermediate-sized vesicle population formed by coalescence of the small syn-explosive bubbles. Bubble nucleation took place heterogeneously on titanomagnetite, number densities of which greatly exceed those of vesicles, and growth took place mainly by decompression.
Bulletin of Volcanology, 2006
Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 10 7-10 9 cm-3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h.