The evolution of bubble size distributions in volcanic eruptions (original) (raw)
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.
Thermal vesiculation during volcanic eruptions
Nature, 2015
Terrestrial volcanic eruptions are the consequence of magmas ascending to the surface of the Earth. This ascent is driven by buoyancy forces, which are enhanced by bubble nucleation and growth (vesiculation) that reduce the density of magma. The development of vesicularity also greatly reduces the 'strength' of magma, a material parameter controlling fragmentation and thus the explosive potential of the liquid rock. The development of vesicularity in magmas has until now been viewed (both thermodynamically and kinetically) in terms of the pressure dependence of the solubility of water in the magma, and its role in driving gas saturation, exsolution and expansion during decompression. In contrast, the possible effects of the well documented negative temperature dependence of solubility of water in magma has largely been ignored. Recently, petrological constraints have demonstrated that considerable heating of magma may indeed be a common result of the latent heat of crystalli...
Bubble coalescence in basaltic lava: Its impact on the evolution of bubble populations
Journal of Volcanology and Geothermal Research, 1997
. Morphological properties bubble number density, porosity, mean radius, specific surface area of vesicular rocks provide quantitative information on the rates of bubble nucleation, growth and coalescence in magmas when measured as a function of time. Such data are useful in constraining the timing and style of gas release during volcanic eruptions. Volcanic rocks commonly show strong zonation with respect to bubble size and porosity, indicating a variation in the amount of bubble Ž . growth and coalescence preserved within a single sample. Morphological properties and bubble size distributions BSD's were measured in a suite of zoned alkali basalts using image analysis and the data were compared to theoretical predictions. Our data indicate that at porosities greater than 35%, extensive coalescence occurred during the growth of bubbles with restricted nucleation; at lower porosities, vesiculation is dominated by nucleation and diffusion with no coalescence. The interiors of many of our samples have undergone 4-7 binary coalescence events after eruption in a time of around 15 min. The Ostwald ripening effect has not significantly modified the BSD's.
2005
We conducted rapid decompression experiments using bubble-bearing viscoelastic fluid in a vertical shock tube. We varied vesicularity / and pressure difference between the inside P g and the outside P o of the bubbles, DP = P g À P o , to understand the behavior of bubbly-magmas under rapid decompression. We find that the potential energy, which depends on the initial vesicularity /, P g , and P o , determines the expansion velocity of the bubbly fluid during rapid decompression. Higher potential energy, caused by a higher / and a larger DP, leads to faster expansion. The expansion style also depends on the vesicularity / and on the pressures P g and P o . We observe five different styles of expansion during the rapid decompression that depend on / and DP. When both / and DP are small, bnothingQ occurs. As / and DP increase, the bubbly fluid reacts more violently. First, the surface of the bubbly fluid bdeformsQ and the fluid elongates in the vertical direction. For sufficient elongation the fluid can bdetachQ from the tube wall. As / and DP continue to increase, bubble walls can break, a process we refer to as bpartial ruptureQ. Finally, for still larger DP and /, both bubble walls and plateau borders break allowing the fluid to bfragmentQ into discrete pieces and erupt explosively. Our experiments show that a larger potential energy, which results from higher / and larger DP, causes a faster expansion of magma which in turn promotes fragmentation and thus explosive eruption. If we assume that the pressure inside bubbles P g scales with the depth of bubbly magma, measuring the magma vesicularity in conduits or domes as a function of the depth before eruption would help assess volcanic hazard. D
Coupling of viscous and diffusive controls on bubble growth during explosive volcanic eruptions
Earth and Planetary Science Letters, 2001
The coupling of viscosity and diffusivity during explosive volcanic degassing is investigated using a numerical model of bubble growth in rhyolitic melts. The model allows melt viscosity and water diffusivity to vary spatially and temporally with water content. We find that the system is highly sensitive to the distribution of volatiles around the bubble, primarily as a consequence of the