The dynamics of slug trains in volcanic conduits: Evidence for expansion driven slug coalescence

Strombolian volcanism is a ubiquitous form of activity, driven by the ascent and bursting of bubbles of slug morphology. Whilst considerable attention has been devoted to understanding the behaviour of individual slugs in this regime, relatively little is known about how inter-slug interactions modify flow conditions. Recently, we reported on high temporal frequency strombolian activity on Etna, in which the larger erupted slug masses were followed by longer intervals before the following explosion than the smaller bursts (Pering et al., 2015). We hypothesised that this behaviour arose from the coalescence of ascending slugs causing a prolonged lag before arrival of the next distinct bubble. Here we consider the potential importance of inter-slug interactions for the dynamics of strombolian volcanism, by reporting on the first study into the behaviour of trains of ascending gas slugs, scaled to the expansion rates in volcanic conduits. This laboratory analogue study illustrates that slugs in trains rise faster than individual slugs, and can be associated with aspects of co-current flow. The work also highlights that coalescence and inter-slug interactions play an important role in modulating slug train behaviour. We also report, for the first time, on slug coalescence driven by vertical expansion of the trailing slug, a process which can occur, even where the leading slug base ascent velocity is greater than that of the trailing slug.

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title: Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions
abstract: Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid-dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvos, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic-flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built-in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time-dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi-steady lava fountains (oscillating at the frequency of annular-flow turbulence).

title: Modelling the rapid near-surface expansion of gas slugs in low-viscosity magmas
abstract: The ascent of large gas bubbles (slugs) in vertical cylindrical conduits and low-viscosity magmas is simulated using 1D mathematical and 3D computational fluid dynamic (CFD) models. Following laboratory evidence, the 1D model defines a constant rise velocity for the slug base and allows gas expansion to accelerate the slug nose through the overlying fluid during ascent. The evolution of rapidly expanding gas slugs observed in laboratory experiments is reproduced well and, at volcano scales, predicts at-surface overpressures of several atmospheres without requiring any initial overpressure at depth. The near-surface dynamics increase slug nose velocities through the overlying magma by a factor of c. 2.5 and the gas expansion results in pre-burst magma surface velocities of c. 35 m s−1. To examine pressure distributions and the forces exerted on a conduit, 3D CFD simulations were carried out. At volcano scales, the vertical single forces during final slug ascent to the surface are c. 106 N, two orders of magnitude smaller than those associated with very-long-period seismic events at Stromboli. This supports a previous interpretation of these events in which they are generated by gas slugs flowing through changes in conduit geometry, rather than being the direct result of slug eruption processes.

title: Degassing at low magma-viscosity volcanoes: Quantifying the transition between passive bubble-burst and Strombolian eruption
abstract: At many volcanoes, low magma-viscosities allow persistent degassing over a range of styles from explosive Strombolian activity to gas ‘puffing’ and passive degassing. It is generally accepted that Strombolian eruptions reflect the bursting of large bubbles (gas slugs) at the magma surface and that relatively quiescent ‘puffing’ indicates the presence of significantly smaller bubbles or slugs. Here, we address this qualitative range and derive a dimensionless parameter, Pslim⁎, to quantify and distinguish different regimes of ‘burst vigour’. For Pslim⁎ ≤ 1, ‘passive’ activity is anticipated and measurable parameters such as infrasonic amplitude will be small. For Pslim⁎ > 1, bursting slugs will be energetic, and surface effects are anticipated to increase with increasing Pslim⁎. Various physical parameters recorded during laboratory experiments and simulated through numerical models are shown to demonstrate the same trends when parameterised by Pslim⁎. Hence, Pslim⁎ provides a straightforward relationship between changes in measurable surface effects and the system's subsurface physical parameters and, for energetic activity of a particular system, burst effects are shown to be proportional to the square root of the gas mass involved. Effect magnitudes are also shown to be a function of atmospheric pressure and application of Pslim⁎ parameterisation to planetary scenarios produces results in line with our current understanding of planetary volcanism.

title: The mechanism of basaltic explosions: Experimental modeling
abstract: An instrument package for simulating basaltic eruptions (IPSBE) with a height of 18 m has been developed for investigating the processes that occur during Strombolian eruptions. The device follows the geometrical ratio between the actual plumbing system of a volcano, with the ratio of conduit diameter to conduit height being 1 to 1000. For the first time in physical modeling studies, we created conditions in which a moving gas-saturated model liquid enters the conduit; this enabled us to study bubble nucleation, expansion, and coalescence, the generation and transformation of gas structures, and the kinetic features shown by the evolution of the gas phase. These experiments revealed a novel (previously unknown) flow pattern of two-phase mixtures in a vertical column, viz., a cluster flow that involves the regular alternation of compact clusters of gas bubbles that are separated by a fluid that does not involve a free gas phase. It is shown that the liquid, bubble, cluster, and slug flow patterns are mutually transformed under certain conditions; they are polymorphous modifications of a gas-saturated liquid moving in a vertical pipe. The data thus acquired suggested a new model for the gas-liquid movement of a magma melt in a conduit: depending on the type of gas-liquid flow behavior at the vent, the crater will exhibit different types of explosive activity, including actual explosions.

title: It takes three to tango: 2. Bubble dynamics in basaltic volcanoes and ramifications for modeling normal Strombolian activity
abstract: This is the second paper in a two-part series examining numerical simula- tions of buoyancy-driven ow in the presence of large viscosity contrasts. In the rst pa- per, we demonstrated that a combination of three numerical tools, an extended ghost- uid-type method, the level set approach, and the extension velocity technique accurately simulate complex interface dynamics in the presence of large viscosity contrasts. In this paper, we use this three-fold numerical method to investigate bubble dynamics in the conduits of basaltic volcanos with a focus on normal Strombolian eruptions. Strombolian- type activity, named after the famously episodic eruptions at Stromboli volcano, is char- acterized by temporally discrete fountains of incandescent clasts. The mildly explosive nature of normal Strombolian activity, as compared to more eusive variants of basaltic volcanism, is related to the presence of dissolved gas in the magma, yielding a complex two-phase-ow problem. We present a detailed scaling analysis allowing identication of the pertinent regime for a given ow problem. The dynamic interactions between gas and magma can be clas- sied into three non-dimensional regimes based on bubble sizes and magma viscosity. Re- solving the uid dynamics at the scale of individual bubbles is not equally important in all three regimes: As long as bubbles remain small enough to be spherical, their dy- namic interactions are limited compared to the rich spectrum of coalescence and breakup processes observed for deformable bubbles, in particular once inertia ceases to be neg- ligible. One key nding in our simulations is that both large gas bubbles and large conduit- lling gas pockets ('slugs') are prone to dynamic instabilities that lead to their rapid breakup during buoyancy-driven ascent. We provide upper-bound estimates for the maximum sta- ble bubble size in a given magmatic system and discuss the ramications of our results for two commonly used models of normal Strombolian-type activity, the Rise-Speed-Dependent (RSD) model and the Collapsing-Foam (CF) model.

title: It takes three to tango: 2. Bubble dynamics in basaltic volcanoes and ramifications for modeling normal Strombolian activity
abstract: This is the second paper in a two-part series examining numerical simula- tions of buoyancy-driven ow in the presence of large viscosity contrasts. In the rst pa- per, we demonstrated that a combination of three numerical tools, an extended ghost- uid-type method, the level set approach, and the extension velocity technique accurately simulate complex interface dynamics in the presence of large viscosity contrasts. In this paper, we use this three-fold numerical method to investigate bubble dynamics in the conduits of basaltic volcanos with a focus on normal Strombolian eruptions. Strombolian- type activity, named after the famously episodic eruptions at Stromboli volcano, is char- acterized by temporally discrete fountains of incandescent clasts. The mildly explosive nature of normal Strombolian activity, as compared to more eusive variants of basaltic volcanism, is related to the presence of dissolved gas in the magma, yielding a complex two-phase-ow problem. We present a detailed scaling analysis allowing identication of the pertinent regime for a given ow problem. The dynamic interactions between gas and magma can be clas- sied into three non-dimensional regimes based on bubble sizes and magma viscosity. Re- solving the uid dynamics at the scale of individual bubbles is not equally important in all three regimes: As long as bubbles remain small enough to be spherical, their dy- namic interactions are limited compared to the rich spectrum of coalescence and breakup processes observed for deformable bubbles, in particular once inertia ceases to be neg- ligible. One key nding in our simulations is that both large gas bubbles and large conduit- lling gas pockets ('slugs') are prone to dynamic instabilities that lead to their rapid breakup during buoyancy-driven ascent. We provide upper-bound estimates for the maximum sta- ble bubble size in a given magmatic system and discuss the ramications of our results for two commonly used models of normal Strombolian-type activity, the Rise-Speed-Dependent (RSD) model and the Collapsing-Foam (CF) model.

title: Volcano infrasonic signals and magma degassing:first-order experimental insights and application to Stromboli
abstract: We demonstrate the rise and expansion of a gas slug as a fluid dynamic source mechanism for infrasonic signals generated by gas puffing and impulsive explosions at Stromboli. The fluid dynamics behind the rise, expansion and burst of gas slugs in the confines of an experimental tube can be characterised into different regimes. Passive expansion occurs for small gas masses, where negligible dynamic gas over-pressure develops during bubble ascent and, prior to burst, meniscus oscillation forms an important infrasonic source. With increasing gas mass, a transition regime emerges where dynamic gas over-pressure is significant. For larger gas masses, this regime transforms to fully explosive behaviour, where gas over-pressure dominates as an infrasonic source and bubble bursting is not a critical factor. The rate of change of excess pressure in the experimental tube was used to generate synthetic infrasonic waveforms. Qualitatively, the waveforms compare well to infrasonic waveforms measured from a range of eruptions at Stromboli. Assuming pressure continuity during flow through the vent, and applying dimensionless arguments from the first-order experiments, allows estimation of eruption metrics from infrasonic signals measured at Stromboli. Values of bubble length, gas mass and over-pressure calculated from infrasonic signals are in excellent agreement with those derived by independent means for eruptions at Stromboli, therefore providing a method of estimating eruption metrics from infrasonic measurement.

title: Gas slug ascent in a stratified magma:implications of flow organisation and instability for Strombolian eruption dynamics
abstract: The canonical Strombolian paradigm of a gas slug ascending and bursting in a homogeneous low-viscosity magma cannot explain the complex details in eruptive dynamics recently revealed by field measurements and textural and geochemical analyses. Evidence points to the existence of high-viscosity magma at the top of the conduit of Strombolian-type volcanoes, acting as a plug. Here, new experiments detail the range of flow configurations that develop during the ascent and burst of a slug through rheologically stratified magma within a conduit. End-member scenarios of a tube fully filled with either high- or low-viscosity liquid bracket three main flow configurations: (1) a plug sufficiently large to fully accommodate an ascending gas slug; (2) A plug that can accommodate the intrusion of low-viscosity liquid driven by the gas expansion, but not all the slug volume, so the slug bursts with the nose in the plug whilst the base is still in the low-viscosity liquid; (3) Gas expansion is sufficient to drive the intrusion of low-viscosity liquid through the plug, with the slug bursting in the low-viscosity layer emplaced dynamically above the plug. We show that the same flow configurations are viable at volcanic-scale through a new experimentally-validated 1D model and 3D computational fluid dynamic simulations. Applied to Stromboli, our results demonstrate that the key parameters controlling the transition between each configuration are gas volume, plug thickness and plug viscosity. The flow processes identified include effective dynamic narrowing and widening of the conduit, instabilities within the falling magma film, transient partial and complete blockage of the conduit, and slug disruption. These complexities influence eruption dynamics and vigour, promoting magma mingling and resulting in pulsatory release of gas.

title: Pressure changes associated with the ascent and bursting of gas slugs in liquid-filled vertical and inclined conduits.
abstract: At basaltic volcanoes, the sources of long-period and very-long-period seismicity and acoustic signals are frequently described in terms of fluid dynamic processes, in particular the formation and ascent of gas slugs within the magma column and their bursting at the surface. To investigate pressure changes associated with these processes, two-phase flow experiments have been carried out in vertical and inclined pipes with both single gas slugs and a continuously supplied gas phase. The ascent of individual gas slugs is accompanied by strong dynamic pressure variations resulting from the flow of liquid around the slug. These dynamic transients generate sub-static pressures below the ascending slug in viscosity-controlled systems, and produce super-static pressures when the slug reaches the surface and motion ceases in inertia-dominated systems. Conduit inclination promotes a change of regime from bubbly to slug flow and favours an increase in size and velocity of the slugs at the expense of their frequency of occurrence during continuously supplied two-phase flow. The experimental pressure data support previous theoretical analyses of oscillatory sources in ascending slugs as the slugs approach the surface and burst. Pressure oscillations are also observed during the release of gas slugs and in their wake region.

title: Cyclicity in slug-driven basaltic eruptions: insights from large-scale analogue experiments
abstract: Basaltic eruptions commonly exhibit cyclical or pulsatory behaviour. Strombolian eruptions are typically discrete and rhythmic, with return interval of minutes-to-hours; lava fountains may fluctuate over similar, or shorter, timescales. In both cases the cyclicity results from the separation of gas from the magma, and its localization into discrete gas slugs or gas-rich packets. We report analogue experiments which model the fluid dynamics of slug-driven basaltic eruptions. Experiments were conducted in liquid-filled vertical pipes at a range of scales, from 0.02 to 0.2 m in diameter, and 2 to 13 m in height, allowing us to investigate Reynolds numbers 10􀀀3 < Re <105, encompassing the natural range for volcanoes. The dynamics of both discrete gas slugs (Taylor bubbles) and continuous sluggy flow were quantified. A significant novelty of this study is that we explore the role played by the boundary conditions at the top and bottom of the conduit, which may be either closed (zero flux) or held at constant pressure. This setup allows us to mimic plugged or open vent, and the influence of a magma chamber. Our study combines the direct observation of in-conduit fluid dynamic processes with measured pressure variations inside the conduit. Our results demonstrate that, when discrete gas slugs are injected, plugging the vent has a strong influence on the development of overpressure in the system, and on the potential for the system to manifest cyclic behaviour. When gas is injected continuously, with constant pressure boundaries at the top and bottom of the conduit, the system spontaneously self-organizes into rhythmic sluggy flow when the injection rate exceeds a critical value. In both cases we find that the capacity of the system to sustain well-formed, discrete slugs depends strongly on the Reynolds number of the flow. Well-formed Taylor bubbles – which have a smooth hemispherical cap, occupy the width of the conduit, and ascend relatively slowly – only form when the liquid column is stagnant, or is flowing in the laminar regime. Slugs that rise through turbulent liquid are poorly-formed and, at the extreme, ascend as highly-turbulent gas-rich packets; these occupy less of the conduit width and therefore ascend more rapidly than true Taylor bubbles. A major factor influencing the turbulence in the liquid is the separation between slugs; when slugs are closely spaced, the wake of one slug tends to disrupt the next. At the onset of eruption, therefore, the tendency is for poorly-formed, fast-moving slugs to catch up with the well-formed, slow-moving lead slug, increasing the potential for an impulsive, explosive onset. During sustained eruption, lower gas fluxes lead to lower-frequency, higher amplitude fluctuations in eruption rates, whilst higher gas fluxes lead to higher-frequency, lower amplitude fluctuations. The spectrum of natural activity, from discrete Strombolian eruptions to sustained, pulsatory lava fountaining can, therefore, be characterized as a change in the behaviour of the separated gas phase in the conduit.

title: Inverting the source mechanism of Strombolian explosions at Mt. Yasur, Vanuatu, using a multi-parameter dataset
abstract: The source mechanism of Strombolian explosions at Mt. Yasur, Vanuatu, is analyzed using a unique data set which enables us to combine different methodologies. The novelty of the approach presented here lies in the combination of seismic, acoustic and Doppler-radar data. Thus, using infrasound and Doppler radar observations we are able to provide independent estimates of the volume of the eruptive source. In the following, we show that a correlation exists between source volume and the maximum particle velocity of the expelled volcanic products. In order to determine the source location and the associated mechanism and to compare those two methods, we use time reversal imaging and moment tensor inversion. While the first method also applies to extended sources, the latter might be biased by its intrinsic point source approximation. We show that time reversal imaging can deliver an estimate of the source depth without any a priori assumptions even in an unfavorable scenario in terms of topography and receiver setup. The inverted source mechanism retrieved from moment tensor inversion is found to be largely isotropic, with minor deviatoric components. The inverted source location points towards an area to the north-west of the active craters, indicative of a common feeder system of the activity at Mt. Yasur. However, it is also evident, that to fully locate the source of Strombolian activity at Yasur location precision must be enhanced.

title: Blast waves from violent explosive activity at Yasur Volcano, Vanuatu
abstract: Infrasonic and seismic waveforms were collected during violent strombolian activity at Yasur Volcano (Vanuatu). Averaging ~3000 seismic events showed stable waveforms, evidencing a low-frequency (0.1–0.3 Hz) signal preceding ~5–6 s the explosion. Infrasonic waveforms were mostly asymmetric with a sharp compressive (5–106 Pa) onset, followed by a small long-lasting rarefaction phase. Regardless of the pressure amplitude, the ratio between the positive and negative phases was constant. These waveform characteristics closely resembled blast waves. Infrared imagery showed an apparent cold spherical front ~20 m thick, which moved between 342 and 405 m/s before the explosive hot gas/fragments cloud. We interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front. We suggest that violent strombolian activity at Yasur was driven by supersonic dynamics with gas expanding at 1.1 Mach number inside the conduit.

title: Array tracking of infrasonic sources at Stromboli volcano
abstract: Infrasonic arrays are a powerful tool for volcanic monitoring and hazard assessment. Explosions were recorded at Stromboli using a small aperture array of 4 infrasonic stations, allowing precise vent location. The acoustic signals were delayed-and-summed, revealing the existence of two main groups of infrasonic waves. The NE crater produces short (<3 s) high amplitude (20–80 Pa) pressure waves while the SW crater produces small acoustic pressure (10–30 Pa) with long (5–15 s) coda. The two groups reflect different in explosive styles and similar spectral content, centered on 5–6 Hz. When stacked together, acoustic waveforms for each crater reveal the same pressure pulse, which indicates a common source process. We infer that the acoustic onset at both craters is generated by the burst of a large gas bubble while the acoustic coda is controlled by a sustained pressure release.

title: Acoustic measurements of the 1999 basaltic eruption of Shishaldin volcano, Alaska 1. Origin of Strombolian activity
abstract: The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) displayed both classical Strombolian activity and an explosive Subplinian plume. Strombolian activity at Shishaldin occurred in two major phases following the Subplinian activity. In this paper, we use acoustic measurements to interpret the Strombolian activity. Acoustic measurements of the two Strombolian phases show a series of explosions that are modeled by the vibration of a large overpressurised cylindrical bubble at the top of the magma column. Results show that the bubble does not burst at its maximum radius, as expected if the liquid film is stretched beyond its elasticity. But bursting occurs after one cycle of vibration, as a consequence of an instability of the air–magma interface close to the bubble minimum radius. During each Strombolian period, estimates of bubble length and overpressure are calculated. Using an alternate method based on acoustic power, we estimate gas velocity to be 30–60 m/s, in very good agreement with synthetic waveforms. Although there is some variation within these parameters, bubble length and overpressure for the first Strombolian phase are found to be c82F11 m and 0.083 MPa. For the second Strombolian phase, bubble length and overpressure are estimated at 24F12 m and 0.15 MPa for the first 17 h after which bubble overpressure shows a constant increase, reaching a peak of 1.4 MPa, just prior to the end of the second Strombolian phase. This peak suggests that, at the time, the magma in the conduit may contain a relatively large concentration of small bubbles. Maximum total gas volume and gas fluxes at the surface are estimated to be 3.3 � 10 7 and 2.9 � 10 3 m 3 /s for the first phase and 1.0 � 10 8 and 2.2 � 10 3 m 3 /s for the second phase. This gives a mass flux of 1.2 � 10 3 and 8.7 � 10 2 kg/s, respectively, for the first and the second Strombolian phases.

title: From Strombolian explosions to fire fountains at Etna Volcano (Italy): what do we learn from acoustic measurements?
abstract: The 2001 eruption of Etna volcano, prior to the flank eruption, was marked by 16 episodes separated from one another by few days of quiescence. Insights into fire fountain formation are provided by a close comparison of the sound produced by an episode solely involving a series of Strombolian explosions (4 July) and one also showing a transition to a fire fountain (12 July). The best fit between measured and synthetic waveforms gives the temporal evolution of the bubble length, 8–100 m, and overpressure, 0.2 MPa. Both episodes result from the coalescence of a foam layer trapped at the top of the reservoir. At the transition towards a fire fountain, the number of explosions and the bubble length increase simultaneously, suggesting that the foam destabilization is more efficient when a fire fountain is produced. Acoustic records give access to the gas volume trapped within the foam, called ‘active’ degassing, while the height of fire fountains also includes the gas from passive degassing. The small bubbles from passive degassing are carried to the surface via the wake of the slugs, coming from the depth of the reservoir. The proportion between active and total gas volume represents 38–48%.

title: Characteristics of puffing activity revealed by ground-based, thermal infrared imaging: the example of Stromboli Volcano (Italy)
abstract: Puffing, i.e., the frequent (1 s ca.) release of small (0.1–10 m3), over-pressurized pockets of magmatic gases, is a typical feature of open-conduit basaltic volcanoes worldwide. Despite its non-trivial contribution to the degassing budget of these volcanoes and its recognized role in volcano monitoring, detection and metering tools for puffing are still limited. Taking advantage of the recent developments in high-speed thermal infrared imaging, we developed a specific processing algorithm to detect the emission of individual puffs and measure their duration, size, volume, and apparent temperature at the vent. As a test case, we applied our method at Stromboli Volcano (Italy), studying “snapshots” of 1 min collected in the years 2012, 2013, and 2014 at several vents. In all 3 years, puffing occurred simultaneously at three or more vents with variable features. At the scale of the single vent, a direct relationship links puff temperature and radius, suggesting that the apparent temperature is mostly a function of puff thickness, while the real gas temperature is constant for all puffs. Once released in the atmosphere, puffs dissipate in less than 20 m. On a broader scale, puffing activity is highly variable from vent to vent and year to year, with a link between average frequency, temperature, and volume from 136 puffs per minute, 600 K above ambient temperature, 0.1 m3, and the occasional ejection of pyroclasts to 20 puffs per minute, 3 K above ambient, 20 m3, and no pyroclasts. Frequent, small, hot puffs occur at random intervals, while as the frequency decreases and size increases, an increasingly longer minimum interval between puffs, up to 0.5 s, appears. These less frequent and smaller puffs also display a positive correlation between puff volume and the delay from the previous puff. Our results suggest an important role of shallow bubble coalescence in controlling puffing activity. The smaller and more frequent puffing at “hotter” vents is in agreement with the rapid rise of small gas pockets with only limited coalescence, while slower rise, allowing more time for coalescence, leads to larger but less frequent puffing at “colder” vents. This link between puffing style and vent thermal state points to a feedback between gas flux and magma temperature (and viscosity), where higher gas flux stirs and heats the magma, which, by getting less viscous, becomes a preferential way for bubble rise. Such a link has implications for the monitoring of the state of the shallow conduit at open-vent volcanoes as well as for determination of their total gas budget, relevant for hazard forecast and environmental studies.

title: The role of large bubbles detected from acoustic measurements on the dynamics of Erta 'Ale lava lake (Ethiopia)
abstract: The activity at the surface of the lava lake on Erta 'Ale volcano (Ethiopia) shows that large bubbles are regularly breaking at a fixed position on the lava lake. This is also where the small lava fountains are sometimes produced. Since this location is likely to be directly above the volcanic conduit feeding the lava lake, we have done continuous measurements between March 22 and 26, 2003 to understand the degassing of a volcano in permanent activity. The bubble size has been first estimated from videos, which once combined with the acoustic pressure, can constrain the source of the sound. The gas volume and overpressure stayed roughly constant, between 36–700 m3 and 4 × 103–1.8 × 104 Pa, respectively. Simultaneous thermal measurements showed regular peaks, which occurred when the crust was broken by a large bubble, hence gave a direct indication on the typical return time between the bubbles (1 h). These spherical cap bubbles had a high Reynolds number, 4600–20000, therefore a wake, periodically unstable, formed and detached from the bubble bottom. The bubbly wake, if the detachment occurs close to the surface, can explain the duration of lava fountains, measured on the videos. The periodic arrival of bubbly wakes, which mostly detach from the driving spherical cap within the lava lake, could explain the absence of cooling at Erta 'Ale, Erebus (Antartica), Villarica (Chile) and Nyiragongo (Democratic Republic of Congo) without invoking a convective downflow of magma in the conduit, as previously done.

title: Dynamics of mild strombolian activity on Mt. Etna
abstract: Here we report the first measurements of gas masses released during a rare period of strombolian activity at the Bocca Nuova crater, Mt. Etna, Sicily. UV camera data acquired for 195 events over an ≈ 27 minute period (27th July 2012) indicate erupted SO2 masses ranging from ≈ 0.1 to ≈ 14 kg per event, with corresponding total gas masses of ≈ 0.1 to 74 kg. Thus, the activity was characterised by more frequent and smaller events than typically associated with strombolian activity on volcanoes such as Stromboli. Events releasing larger measured gas masses were followed by relatively long repose periods before the following burst, a feature not previously reported on from gas measurement data. If we assume that gas transport within the magma can be represented by a train of rising gas pockets or slugs, then the high frequency of events indicates that these slugs must have been in close proximity. In this case the longer repose durations associated with the larger slugs would be consistent with interactions between adjacent slugs leading to coalescence, a process expedited close to the surface by rapid slug expansion. We apply basic modelling considerations to the measured gas masses in order to investigate potential slug characteristics governing the observed activity. We also cross correlated the acquired gas fluxes with contemporaneously obtained seismic data but found no relationship between the series in line with the mild form of manifest explosivity.

title: On the gas expansion and gas hold-up in vertical slugging columns—A simulation study
abstract: A study on the gas phase expansion and gas hold-up occurring in free bubbling vertical slug flow is reported. A slug flow simulator (SFS) supported by air–water experimental data was used for this purpose. The study was accomplished by implementing in the simulator the gas phase expansion along the column. Effects over bubble lengths and bubble velocities were considered. The flow in 6.5 and 20 m long columns with internal diameter of 0.032 m was simulated. Expansion of gas phase along the column is shown to slightly decrease the occurrence of bubble coalescence. Liquid free surface oscillations (due to bubble burst events and continuous inlet of liquid and gas in the column) were found to affect the expansion of the gas phase, especially for high gas flow rates. The gas phase expansion for different outlet column configurations was studied. The use of a high and large cross sectional tank (to dampen free surface oscillations) is shown not to assure a permanent expansion rate of the gas phase. Simulations with and without gas expansion along the column were compared for the computation of average flow parameters. Approximate approaches (with constant UG, corrected for the mid-column pressure) were found suitable for the prediction of the average slug length and gas hold-up. Those approaches are, however, inadequate for the computation of the average bubble length and velocity along the vertical coordinate of the column.

title: Flow around individual Taylor bubbles rising in a vertical column with water: Effect of gas expansion
abstract: This paper presents an experimental study to evaluate the effect of bubble expansion on the dynamics of an isolated Taylor bubble rising in a vertical column of water by inspection of the liquid flow around the bubble front. An increase in bubble volume results from gas decompression due to hydrostatic pressure drop as the bubble ascends through the liquid. Two reduced pressures (33.3 and 20.0 kN/m 2 ) and atmospheric pressure were maintained at the free liquid surface to provide a wide range of expansion rates. PIV coupled with a Shadowgraphy Technique was used to simultaneously obtain the flow field ahead the bubble and a well-defined gas–liquid interface. A very good correlation was found between predicted bubble volume expansion rates considering ideal gas behavior and the upward net volumetric liquid flow rate obtained by integration of the velocity profiles. Above the bubble nose, the liquid velocity at the centerline decreases rapidly from the value of the Taylor bubble velocity at the tip of the bubble. For distances greater than 0.5 D , the liquid flow is undisturbed, with a significant axial velocity component for high bubble volume expansion rates due to the displacement of the liquid promoted by gas expansion. The Reynolds number calculated for the liquid flow was less than 2100 for all conditions used. While this suggests a laminar flow regime in the liquid, coefficient C from Nicklin’s equation obtained from data was equal to 1.4. The shapes of the velocity profiles obtained at the undisturbed liquid flow show that for high expansion rates they resemble the power-law velocity profile for turbulent flow. The bubble volume expansion rate modifies continuously during the Taylor bubble rise and a well-defined liquid velocity profile is never attained.

title: Coalescence of two gas slugs rising in a co-current flowing liquid in vertical tubes
abstract: An experimental study is presented about the interaction between two slugs rising through a co-current flowing liquid in vertical tubes with 22, 32 and 52 mm internal diameters. For the conditions studied the liquid flow pattern in the wake of the slugs was turbulent and the main liquid flow regime was either laminar or turbulent. When the liquid flow regime is turbulent the minimum distance between slugs above which there is no interaction is about 5 D . For lower distances the slugs coalesce during their rise. When the liquid flow regime is laminar two different situations are observed: —if the ratio between the average velocity in the full developed film around the slug and the average velocity in the main liquid is greater than 25, coalescence between slugs is observed. The minimum distance between slugs above which there is no interaction is about 10 D ; —if the value of that ratio is lower than 25 and the initial distance between slugs is greater than the wake length of the leading slug, the distance between slugs increases during their rising in the column and slugs coalescence is not observed. This behaviour has never been referred in the literature. The wake length is found to be 5 D whatever is the main liquid flow regime.

title: On the interaction of Taylor bubbles rising in two-phase co-current slug flow in vertical columns: turbulent wakes
abstract: An experimental study on the interaction between Taylor bubbles rising through a co-current flowing liquid in a vertical tube with 32 mm of internal diameter is reported. The flow pattern in the bubble's wake was turbulent and the flow regime in the liquid slug was either turbulent or laminar. When the flow regime in the liquid slug is turbulent (i) the minimum distance between bubbles above which there is no interaction is 5D-6D; (ii) the bubble's rising velocity is in excellent agreement with the Nicklin relation; (iii) the experimental values of the bubble length compare well with theoretical predictions (Barnea 1990); (iv) the distance between consecutive bubbles varied from 13D to 16D and is insensitive to the liquid Reynolds number. When the flow regime in the liquid slug is laminar (i) the wake length is about 5D-6D; (ii) the minimum distance between bubbles above which there is no interaction is higher than 25D; (iii) the bubble's rising velocity is significantly smaller than theoretical predictions. These results were explained in the light of the findings of Pinto et al. (1998) on coalescence of two Taylor bubbles rising through a co-current liquid.

title: Coalescence of two gas slugs rising in a vertical column of liquid
abstract: This work describes an experimental investigation about the coalescence of pairs of gas slugs rising in vertical columns of liquids covering a wide range of liquid viscosities. The experiments were performed in columns with 19, 32 and 52 mm of internal diameter and slug coalescence was followed by means of a new experimental technique, based on the signals of differential pressure transducers. Important data reported in this work are: • the minimum distance between slugs above which there is no interaction, l min ; • the velocity of approach of the trailing slug as a function of its distance to the leading one. Values of l min are shown to be related to the flow pattern in the wake of liquid behind gas slugs and correlations are provided for the determination of l min ; in all cases l min is about four times the wake length, l w .

title: Flow of two consecutive Taylor bubbles through a vertical column of stagnant liquid-A CFD study about the influence of the leading bubble on the hydrodynamics of the trailing one
abstract: A detailed numerical study on the interaction between two consecutive Taylor bubbles rising through vertical columns of stagnant Newtonian liquids is reported in this work. The CFD method used is based on the VOF technique implemented in the commercial package ANSYS FLUENT. Simulations were made for a large set of flow conditions, within the laminar regime, covering a range of Morton number of 4.72×10 −5 –104 and an interval of Eotvos number between 15 and 575. The changes in the shape of the bubbles interface were followed throughout the approach process, and the main hydrodynamic features characterizing the liquid film and the wake region of the trailing bubble were determined as the separation distance became smaller. Numerical data on the velocity ratio between the trailing and the leading bubble are presented, and two distinct regions are identified (acceleration and deceleration) in the corresponding evolution curves. The velocity ratio curves produced for several flow conditions were fitted to equations describing the acceleration and the deceleration behaviour of the trailing bubble. These equations together with the fitting parameters obtained here can be very useful to improve simulators of continuous slug flow.

title: CFD Study of the Hydrodynamics of Slug Flow Systems: Interaction between Consecutive Taylor Bubbles
abstract: Slug flow is one of the most frequently occurring multiphase flow patterns in industrial processes. A deep knowledge of its fundamentals is necessary to accurately model not only the fluid flow but also reaction and heat and mass transfer in several operation units. A numerical study is reported on the dynamics of slug flow, under laminar regime, in vertical columns of stagnant and co-current Newtonian and non-Newtonian liquids (shear-thickening and shear-thinning). A CFD package (Ansys FLUENT) with the VOF methodology was applied to simulate the flow of individual and pairs of consecutive Taylor bubbles. The behaviour of the most relevant hydrodynamic features with the approach of the trailing bubble towards the leading one is addressed, with particular emphasis to the role of the liquid rheology and flow configuration. The main results presented are the velocity ratio curves between consecutive bubbles, the variation of the bubbles shape, and the axial velocity and viscosity fields in the surrounding liquid. This bubble-bubble interaction data can be a keystone to improve and complement continuous slug flow simulators used for very long columns.

title: Modelling flow pattern transitions for steady upward gas‐liquid flow in vertical tubes
abstract: Models for predicting flow pattern transitions during steady gas-liquid flow in vertical tubes are developed, based on physical mechanisms suggested for each transition. These models incorporate the effect of fluid properties and pipe size and thus are largely free of the limitations of empirically based transition maps or correlations.

title: Stability of annular flow and slugging
abstract: In this work we propose an effective viscosity criterion for the stabilization of annular gas-liquid and liquid-particle flows and an inertial mechanism which drives waves into slugs in slugging gas-liquid flows. Annular flow is stable when the fluid having the higher effective viscosity occupies the core region and the lower viscosity fluid is in the annulus. The eddy viscosity criterion is shown to be very consistent with published work on annular flow transitions in horizontal and vertical gas-liquid flows. It also applies to a variety of liquid-solid and gas-solid flows. In the second part of the paper we propose a mechanism for explaining the growth of initially small waves and initiation of slugs in gas-liquid flow.

title: Vertical slug flow in laminar regime in the liquid and turbulent regime in the bubble wake—Comparison with fully turbulent and fully laminar regimes
abstract: An experimental and simulation study on free bubbling vertical slug flow in laminar regime in the main liquid and turbulent regime in the near-wake bubble region is reported. A non-intrusive image analysis technique and a previously developed slug flow simulator (SFS) were used. Two aqueous glycerol solutions (0.012–0.013 and 0.022 Pa s) were studied. A single bubble-to-bubble interaction curve was obtained. Strong interaction was found for bubbles flowing less than 3–4D apart, with slight interaction persisting for longer distances. The shape of the interaction curve bridges those for fully turbulent and fully laminar regimes. The experimental average bubble velocity in undisturbed conditions was shown not to follow the correlation-based predictions for laminar regime in the liquid. Alternative fitting coefficients are proposed. An entrance length of 50–80D (or 90–170D) was obtained for normal inlet slug length distributions centred on 2–5D (or 2–6 D ), for superficial gas and liquid velocities up to 0.40 and 0.30 m/s, respectively. More contrasting inlet slug length distributions were found not to converge within the length of the column (6.5 m). An overall comparison between the three regimes is presented.

title: Gas holdup behavior in a tall bubble column with perforated plate distributors
abstract: The gas holdup in an 8-m high bubble column with a perforated plate distributor is measured over a superficial gas velocity range of 0.0075 to 0.54 m/s for an air-tap water system. Various combinations of the distributor design parameters, viz. the number, diameter and pitch of the holes, are tested for elucidating their effects on the gas holdup. The gas holdup behavior is described in relation to the bubble flow regime, which can be divided, in order of increasing gas velocity, into the dispersed bubble, the clustered/coalesced bubble and the slugging regimes. A correlation scheme is developed for predicting the gas holdup-superficial gas velocity relationship. Among correlation parameters, the critical gas holdup for the transition from the dispersed to the clustered bubble regime is predicted based on the bubble-wake concept; the onset of bubble clustering is induced via the bubble-wake interaction, which restricts the maximum gas holdup allowable for a homogeneous bubble dispersion. The proposed correlation scheme is capable of reproducing not only the “standard” curve for single hole gas spargers but also the complex behavior observed for typical multi-hole distributors.

title: Universal correlation for the rise velocity of long gas bubbles in round pipes
abstract: We collected all of the published data we could find on the rise velocity of long gas bubbles in stagnant fluids contained in circular tubes. Data from 255 experiments from the literature and seven new experiments at PDVSA Intevep for fluids with viscosities ranging from 1 mPa s up to 3900 mPa s were assembled on spread sheets and processed in log–log plots of the normalized rise velocity, $\hbox{\it Fr} \,{=}\,U/(gD)^{1/2}$ Froude velocity vs. buoyancy Reynolds number, $R\,{=}\,(D^{3}g (\rho_{l}-\rho_{g}) \rho_{l})^{1/2}/\mu $ for fixed ranges of the Eötvös number, $\hbox{\it Eo}\,{=}\,g\rho_{l}D^{2}/\sigma $ where $D$ is the pipe diameter, $\rho_{l}$, $\rho_{g}$ and $\sigma$ are densities and surface tension. The plots give rise to power laws in $Eo$; the composition of these separate power laws emerge as bi-power laws for two separate flow regions for large and small buoyancy Reynolds. For large $R$ ($>200$) we find \[\hbox{\it Fr} = {0.34}/(1+3805/\hbox{\it Eo}^{3.06})^{0.58}.\] For small $R$ ($<10$) we find \[ \hbox{\it Fr} = \frac{9.494\times 10^{-3}}{({1+{6197}/\hbox{\it Eo}^{2.561}})^{0.5793}}R^{1.026}.\] The flat region for high buoyancy Reynolds number and sloped region for low buoyancy Reynolds number is separated by a transition region ($10\,{<}\,R\,{<}\, 200$) which we describe by fitting the data to a logistic dose curve. Repeated application of logistic dose curves leads to a composition of rational fractions of rational fractions of power laws. This leads to the following universal correlation: \[ \hbox{\it Fr} = L[{R;A,B,C,G}] \equiv \frac{A}{({1+({{R}/{B}})^C})^G} \] where \[ A = L[\hbox{\it Eo};a,b,c,d],\quad B = L[\hbox{\it Eo};e,f,g,h],\quad C = L[\hbox{\it Eo};i,j,k,l],\quad G = m/C \] and the parameters ($a, b,\ldots,l$) are \begin{eqnarray*} &&\hspace*{-5pt}a \hspace*{-0.8pt}\,{=}\,\hspace*{-0.8pt} 0.34;\quad b\hspace*{-0.8pt} \,{=}\,\hspace*{-0.8pt} 14.793;\quad c\hspace*{-0.8pt} \,{=}\,\hspace*{-0.6pt}{-}3.06;\quad d\hspace*{-0.6pt} \,{=}\, \hspace*{-0.6pt}0.58;\quad e\hspace*{-0.6pt} \,{=}\,\hspace*{-0.6pt} 31.08;\quad f\hspace*{-0.6pt} \,{=}\, \hspace*{-0.6pt}29.868;\quad g\hspace*{-0.6pt}\,{ =}\,\hspace*{-0.6pt}{ -}1.96;\\ &&\hspace*{-5pt}h = -0.49;\quad i = -1.45;\quad j = 24.867;\quad k = -9.93;\quad l = -0.094;\quad m = -1.0295.\end{eqnarray*} The literature on this subject is reviewed together with a summary of previous methods of prediction. New data and photographs collected at PDVSA-Intevep on the rise of Taylor bubbles is presented.

title: The thickness of the falling film of liquid around a Taylor bubble
abstract: We present the results of laboratory experiments that quantify the physical controls on the thickness of the falling film of liquid around a Taylor bubble, when liquid–gas interfacial tension can be neglected. We find that the dimensionless film thickness λ′ (the ratio of the film thickness to the pipe radius) is a function only of the dimensionless parameter , where ρ is the liquid density, g the gravitational acceleration, D the pipe diameter and μ the dynamic viscosity of the liquid. For , the dimensionless film thickness is independent of Nf with value λ′≈0.33; in the interval , λ′ decreases with increasing Nf; for film thickness is, again, independent of Nf with value λ′≈0.08. We synthesize existing models for films falling down a plane surface and around a Taylor bubble, and develop a theoretical model for film thickness that encompasses the viscous, inertial and turbulent regimes. Based on our data, we also propose a single empirical correlation for λ′(Nf), which is valid in the range 10−1<Nf<105. Finally, we consider the thickness of the falling film when interfacial tension cannot be neglected, and find that film thickness decreases as interfacial tension becomes more important.

title: Flow patterns in the wake of a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids: An experimental study
abstract: The flow in the wake and near-wake regions of individual Taylor bubbles rising through stagnant and co-current vertical columns of Newtonian liquids was studied, employing simultaneously particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST). Experiments were made with water and aqueous glycerol solutions covering a wide range of viscosities (1 × 10 −3 Pa s < < 1. 5P a s), in an acrylic column of 32 mm ID. Different wake structures (laminar, transitional and turbulent) are identified, in both stagnant and co-current flow conditions. In stagnant liquids, the wake flow pattern is only dependent on the dimensionless group Nf . The different types of wakes obtained are in accordance with the critical Nf numbers proposed in previous works. For co-current flow conditions, the flow patterns in the wake depend on the Reynolds number based on the relative (to the bubble) average velocity of the upward liquid flow, the laminar-transitional and transitional-turbulent limits being for the first time experimentally determined. The wake flow patterns are quantified by means of instantaneous and average flow fields. Values for the wake length and wake volume are also presented and compare well with correlations found in literature. Study of the flow in the near-wake zone enabled determination of the distance needed to recover the undisturbed liquid velocity profile. The detailed study of the flow in the wake and near-wake regions is an important contribution to better understanding the interaction and coalescence mechanisms between Taylor bubbles. The data reported are relevant to the validation of numerical simulation codes in the vertical slug flow regime.

title: Viscous plugging can enhance and modulate explosivity of strombolian eruptions
abstract: Strombolian activity is common in low-viscosity volcanism. It is characterised by quasi-periodic, short-lived explosions, which, whilst typically weak, may vary greatly in magnitude. The current paradigm for a strombolian volcanic eruption postulates a large gas bubble (slug) bursting explosively after ascending a conduit filled with low-viscosity magma. However, recent studies of pyroclast textures suggest the formation of a region of cooler, degassed, more-viscous magma at the top of the conduit is a common feature of strombolian eruptions. Following the hypothesis that such a rheological impedance could act as a ‘viscous plug’, which modifies and complicates gas escape processes, we conduct the first experimental investigation of this scenario. We find that: 1) the presence of a viscous plug enhances slug burst vigour; 2) experiments that include a viscous plug reproduce, and offer an explanation for, key phenomena observed in natural strombolian eruptions; 3) the presence and extent of the plug must be considered for the interpretation of infrasonic measurements of strombolian eruptions. Our scaled analogue experiments show that, as the gas slug expands on ascent, it forces the underlying low-viscosity liquid into the plug, creating a low-viscosity channel within a high-viscosity annulus. The slug's diameter and ascent rate change as it enters the channel, generating instabilities and increasing slug overpressure. When the slug reaches the surface, a more energetic burst process is observed than would be the case for a slug rising through the low-viscosity liquid alone. Fluid-dynamic instabilities cause low and high viscosity magma analogues to intermingle, and cause the burst to become pulsatory. The observed phenomena are reproduced by numerical fluid dynamic simulations at the volcanic scale, and provide a plausible explanation for pulsations, and the ejection of mingled pyroclasts, observed at Stromboli and elsewhere.

title: Flow in the nose region and annular film around a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids
abstract: The flow in the nose region and in the annular film around individual Taylor bubbles rising through stagnant and co-current vertical columns of liquid were studied, employing particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST) at the same time. The combined techniques enabled simultaneous determination of the bubble shape and the velocity profiles in the liquid film. Experiments were performed with water and aqueous glycerol solutions in a wide range of viscosities (1 × 10−3 Pa s < < 1.5 Pa s), in an acrylic column of 32 mm ID. Values for the distance ahead of the nose in which the flow is disturbed by the presence of the bubble are presented for the conditions studied. The bubble shapes in the nose region are compared with Dumitrescu’s shape for potential flow. The velocity profiles show that after the nose region the liquid begins to accelerate downwards, and at a certain distance from the bubble nose the velocity profile and the liquid film thickness stabilise. The liquid film acquires characteristics of a free-falling film. Values of the developing length and film thickness are reported for the experimental conditions studied. Average velocity profiles in the fully developed film are also presented. A critical Reynolds number of around 80 (based on the mean absolute velocity in the liquid film and on the film thickness) is reported for the transition from laminar to turbulent regime. Shear stress profiles (in the fully developed film) are also provided. The data reported are relevant for the validation of numerical codes in slug flow.

title: Rise velocity of a swarm of large gas bubbles in liquids
abstract: This paper develops a procedure for estimation of the rise velocity of a swarm of large gas bubbles in a bubble column operating in the churn-turbulent flow regime. The large bubble swarm velocity is estimated by introducing two correction factors into the classical Davies–Taylor (1950) relation for rise of a single spherical cap bubble in a liquid 
Vb=0.71gdb(SF)(AF). 
The scale correction factor (SF) accounts for the influence of the column diameter. This correction is given by the Collins relation ( 1967) and is a function of the ratio of the bubble diameter db to the column diameter DT. Volume-of-fluid simulations confirm the validity of the Davies–Taylor–Collins relations for a variety of liquid properties. The acceleration factor (AF) accounts for the increase in the rise velocity of a bubble because of its interaction with the wake of a bubble preceding it. By analysis of video recordings of the interactions between two bubbles, both in-line and off-line, it is found that the acceleration factor AF increases linearly as the vertical distance of separation between the two bubbles decreases. Increasing liquid viscosity reduces this wake acceleration effect. With the aid of an extensive data set on the large bubble swarm velocity in columns of 0.051, 0.1, 0.174, 0.19, 0.38 and 0.63 m in diameter a correlation is developed for the acceleration factor. The large bubble swarm velocity is found to be three to six times higher than that of a single isolated bubble.

title: Strombolian explosions 1. A large bubble breaking at the surface of a lava
abstract: Strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column. Acoustic pressure has been measured for 36 explosions at Stromboli. We propose that sound is generated by the vibration of the bubble before it bursts. Oscillations are driven by an initial overpressure inside the bubble, assumed to be initially at rest, just below the magma-air interface. Inertia effects cause the bubble to overshoot its equilibrium radius. Then the bubble becomes underpressurized and contracts because of gas compressibility. These oscillations are only slightly damped by viscous effects in the magma layer above the bubble. The bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking, near the minimum radius. Assuming a simple geometry, we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure. The fit between synthetic and observed waveforms is very good, both for frequency, m 60 rad s -1, and amplitude. The initial bubble radius is m 1 m, and the length varies between several and a few tens of meters. From the initial overpressure, approximately 10  Pa, we calculate the maximum radial velocity of ejecta, m 30 m s -1. The generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics.

title: The first second of volcanic eruptions from the Erebus volcano lava lake, Antarctica—Energies, pressures, seismology, and infrasound
abstract: We describe a multiparameter experiment at Erebus volcano, Antarctica, employing Doppler radar, video, acoustic, and seismic observations to estimate the detailed energy budget of large (up to 40 m-diameter) bubble bursts from a persistent phonolite lava lake. These explosions are readily studied from the crater rim at ranges of less than 500 m and present an ideal opportunity to constrain the dynamics and mechanism of magmatic bubble bursts that can drive Strombolian and Hawaiian eruptions. We estimate the energy budget of the first second of a typical Erebus explosion as a function of time and energy type. We constrain gas pressures and forces using an analytic model for the expansion of a gas bubble above a conduit that incorporates conduit geometry and magma and gas parameters. The model, consistent with video and radar observations, invokes a spherical bulging surface with a base diameter equal to that of the lava lake. The model has no ad hoc free parameters, and geometrical calculations predict zenith height, velocity, and acceleration during shell expansion. During explosions, the energy contained in hot overpressured gas bubbles is freed and partitioned into other energy types, where by far the greatest nonthermal energy component is the kinetic and gravitational potential energy of the accelerated magma shell (> 10 9 J). Seismic source energy created by explosions is estimated from radar measurements and is consistent with source energy determined from seismic observations. For the generation of the infrasonic signal, a dual mechanism incorporating a terminally disrupted slug is proposed, which clarifies previous models and provides good fits to observed infrasonic pressures. A new and straightforward method is presented for determining gas volumes from slug explosions at volcanoes from remote infrasound recordings.

title: Explosive volcanic eruptions—IX. The transition between Hawaiian-style lava fountaining and Strombolian explosive activity.
abstract: The commonest eruption styles of basaltic volcanoes involve Hawaiian lava fountaining or intermittent Strombolian explosions. We investigate the ways in which magma rise speed at depth, magma volatile content and magma viscosity control which of these eruption styles takes place. We develop a model of the degree of coalescence between gas bubbles in the magma which allows us to simulate the transition between the two extreme styles of activity. We find that magma rise speed is the most important factor causing the transition, with gas content and viscosity also influencing the rise speed at which the transition occurs. Counter to intuitive expectations, a decrease in gas content does not cause a transition from Hawaiian to Strombolian activity, but instead causes a transition to passive effusion of vesicular lava. Rather, a change from Hawaiian to Strombolian style requires a significant reduction in magma rise speed.