Response of a bubble bearing viscoelastic fluid to rapid decompression: implications for explosive volcanic eruptions (original) (raw)
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Bubble growth during decompression of magma: experimental and theoretical investigation
Journal of Volcanology and Geothermal Research, 2004
A model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt. The model accounts for the interplay of three dynamic processes: decompression rate of the magma, deformation of the viscous melt around the growing bubble, and diffusion of volatiles into the bubble. Generally, these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically. For a better understanding of the physics of the processes, we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance, diffusion or linear decompression rate. We show that the solutions are a function of time and two dimensionless numbers, which are the ratios of either the diffusive or viscous time scales over the decompression time scale. The conditions for each growth regime are provided as a function of the two governing dimensionless parameters. Analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts. The model solutions, including the division to the growth regimes as function of the two parameters, provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure, supersaturation and gas volume fraction. The model results are in agreement with the conditions of Plinian explosive eruption (e.g. Mount St. Helens, 18 May 1980), where high gas overpressure is expected. The conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions, mostly in equilibrium degassing and partly in overpressure conditions. We show that in a situation of quasi-static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble, insensitive to the diffusivity profile. ß
Explosive expansion of a slowly decompressed magma analogue: Evidence for delayed bubble nucleation
Geochemistry, Geophysics, Geosystems, 2013
While ascending in the plumbing system of volcanoes, magma undergoes decompression at rates spanning several orders of magnitude and set by a number of factors internal and external to the volcano. Slow decompression generally results in an effusive or mildly explosive expansion of the magma, but counterexamples of sudden switches from effusive to explosive eruptive behavior have been documented at various volcanoes worldwide. The mechanisms involved in this behavior are currently debated, in particular for basaltic magmas. Here, we explore the interplay between decompression rate and vesiculation vigor by decompressing a magma analogue obtained by dissolving pine resin into acetone in varying proportions. Analogue experiments allow direct observations of the processes of bubble nucleation and growth, flow dynamics, and fragmentation that is not currently possible with magmatic systems. Our mixtures contain solid particles, and upon decompression, nucleation of acetone bubbles is ob...
Earth and Planetary Science Letters, 2006
To visualize the behavior of erupting magma in volcanic conduits, we performed shock tube experiments on the ductile-brittle response of a viscoelastic medium to diffusion-driven bubble expansion. A sample of shear-thinning magma analogue is saturated by gas Ar under high pressure. On rapid decompression, Ar supersaturation causes bubbles to nucleate, grow, and coalesce in the sample, forcing it to expand, flow, and fracture. Experimental variables include saturation pressure and duration, and shape and lubrication of the flow path. Bubble growth in the experiments controls both flow and fracturing, and is consistent with physical models of magma vesiculation. Two types of fractures are observed: i) sharp fractures along the uppermost rim of the sample, and ii) fractures pervasively diffused throughout the sample. Rim fractures open when shear stress accumulates and strain rate is highest at the margin of the flow (a process already inferred from observations and models to occur in magma). Pervasive fractures originate when wall-friction retards expansion of the sample, causing pressure to build-up in the bubbles. When bubble pressure overcomes wall-friction and the tensile strength of the porous sample, fractures open with a range of morphologies. Both types of fracture open normally to flow direction, and both may heal as the flow proceeds. These experiments also illustrate how the development of pervasive fractures allows exsolving gas to escape from the sample before the generation of a permeable network via other processes, e.g., bubble coalescence. This is an observation that potentially impact the degassing of magma and the transition between explosive and effusive eruptions.
Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling
Journal of Geophysical Research, 1998
We have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity, volatile concentration, and diffusivity. On the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems. We conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined. Variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2-4 orders of magnitude. Our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions. An analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics R -t 1/2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship R -log(t) is more appropriate during active bubble growth under certain conditions. A second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression. Model results for degassing of initially water-saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent. On the basis of decompression history, explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates > 1-5 m s-1.
Dynamics of explosive degassing of magma: Observations of fragmenting two-phase flows
Journal of Geophysical Research, 1996
Liquid explosions, generated by rapid degassing of strongly supersaturated liquids, have been investigated in the laboratory with a view to understanding the basic physical laX•sses operating during bubble nucleation and growth and the subsequent behavior of the expanding two-phase flow. Experimenkq are carried out in a shock tube and ,me monitored by high-speed photography and pressure trm•sducers. Theoretical CO2 supersaturations up to 455 times the ambient saturation concentration ,are generated by a chemical reaction; K2CO3 solution is suddenly injected into an excess of HC1 solution in such a way as to mix the two solutions rapidly. Immediately after file injection event, a bubble nucleation delay of a few milliseconds is followed by rapid nucleation ,mid explosive expmlsion of CO2 bubbles forming a highly heterogeneous foam. Enhanced diffusion due to advection in the 11ow coupled with continuous mixing of tile reactants, and hence ongoing bubble nucleation after injection, generates an increasingly accelerating flow until the reactants become depleted at peak accelerations of around 150 g and velocities of about 15 m s-•. Stretching of the accelerating two-phase mixture enhances the mixing. Liberation of CO2 vapor is spatially inhomogeneous leading to ductile fragmentation occurring throughout the flow in regions of greatest gas release as the consequence of the collision and stretching of lluid streams. Tile violence of the eruptions is controlled by using different concentrations of tile HCI and K2CO 3 solutions, which alters the CO2 supersaturation and yield and also file efficiency of the mixing process. Peak acceleration is proportional to theoretical supersaturation. Pressure tneasurements at the base of the shock tube show an initial nucleation delay and a pressure pulse related to the onset of explosive bubble fortnation. These chemically induced explosions differ t¾om liquid explosions created in other experiments. In explosions caused by sudden depressurization of C02-saturated water, the bubbles nucleate uniformly fl•roughout the liquid in a single nucleation event. Subsequent bubble growth causes the two-phase mixture to be accelerated upward at nearly constant accelerations. Explosively boiling liquids, in which heterogeneous nucleation is suppressed, experience an evaporation wave which propagates down into the liquid column at constant average velocity. Fragtnentation occurs at the shin'ply dellned leading edge of the wavefront. The chemical flows effectively simulate highly explosive volcanic eruptions as they are comparable in terms of flow densities, velocities, accelerations, and in the large range of scales present. The lm'ge accelm,'ations cause su'ong extensional strain and longitudinal deformation. Comparable delbrmation rates in volcanic systems could be sufficient to approach conditions for brittle l•,tgmentation. Tube pumice is a major component of plinian deposits and ignimbrites and preserves evidence of accelerating llow conditions. mounts of dissolved gas becomes strongly supersaturated on approaching the Earth's surface. Gas bubbles nucleate and grow explosively and the magma disintegrates into a two-phase mixture of gas and pyroclasts that accelerates to velocities of order of a few hundred meters per second along volcanic conduits [Wilson et al., 1980; Dobran, !992]. The timescales for these processes Copyright !996 by fl•e American Geophysical Union. Paper number 95JB02515. 0!48.1227/96/95JB.025 [5505.00 are very short. For example, in the plinian eruption of Mount St. Helens on May 18, 1980, estimates of chamber depth, magma discharge rates, conduit dimensions and volatile contents [ Carey et al., 1990] constrain the time that it takes for an individual parcel of magma to move from the chamber to the Earth's surface as about !0 min. Due to pressure variations most bubble growth is confined to the uppermost parts of the magma column so the timescale for explosive degassing must be substantially less. Estimates from modeling studies [e.g., Kief[kr, 198!; Dobran, 1992; Proussevitch et al., !993; Sparks et aL, 1994] suggest that timescales for prefragmentation bubble growth in p!inian eruptions are of order !0 to I00 s. Explosive volcanic flows are unlikely to be observed directly. Therefore the processes involved can only be studied by theoretical modeling, simulation in ana!ogue experiments, or 5547 5548 MADER ET AL.: FRAGMENTING 'IWO-PI IASE FI.OWS
Ascent and decompression of viscous vesicular magma in a volcanic conduit
During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava-filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two-dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the ``exit'' boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug-like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth-dependent radii.
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 evolution of bubble size distributions in volcanic eruptions
2002
We review observations of bubble size distributions (BSDs) generated during explosive volcanic eruptions and laboratory explosions, as inferred from vesicle size distributions found in the end products. Unimodal, polymodal, exponential and power law BSDs are common, even in the absence of coalescence, and both power law and exponential distributions have been generated in the same eruption. To date theoretical models
Journal of Applied Mechanics and Technical Physics, 2008
Experimental data and results of numerical simulations of the magma state dynamics in explosive eruptions of volcanoes are presented. The pre-explosion state of volcanoes and the cavitation processes developed in the magma under explosive decompression are studied under the assumption that the intensity of explosive volcanoes does not exert any significant effect on the eruption mechanisms. In terms of the structural features of the pre-explosion state, a number of explosive volcanic systems are close to hydrodynamic shock-tube schemes proposed by Glass and Heuckroth. High-velocity processes initiated by shock-wave loading of the liquid may be considered as analogs of natural volcanic processes, which have common gas-dynamic features and common kinetics responsible for their mechanisms, regardless of the eruption intensity.