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Simulations of volcanic explosions reveal that fragmentation is caused by expansion and acceleration of the bubbles in the magma. The bubbles are caused by the pyroclasts and gases in the magma flowing out from the volcano. The simulation utilizes a two model system which produces flow velocities and expansion rates of the bubbles that occur in the original magma. The expansion and acceleration depend on the liberation of CO2 from aqueous phase consisting of CO2, saturated water and concentrated K2CO3.

Researchers looking at sustained volcanic eruptions have been able to simulate volcanic conduit flows by rapidly decompressing large volumes of carbon dioxide-saturated water. This gives sustained explosions in a liquid where rapid exsolution of gas is taking place. It was established that the maximum average rate of propagation of the fragmentation region can be estimated if the conduit length is known. The start-up period identifies the time when the fragmentation region is propagating down the conduit.

The evolving magma rheology of three recent Ruapehu eruptions (1969, 1977, and 1995) is estimated using a combination of thermodynamic models and rheological calculations, supported by textural observations of the erupted scoria. We use a well-established thermodynamic model to determine the composition of these representative Ruapehu magmas from 300MPa to ∼30MPa. The outputs of the model provide the changing crystal and bubble content in a closed system (assuming no gas loss), as well as the fractionating melt compositions. We calculate the melt viscosity, and the effect of bubbles and crystals, to quantify the rheology of the magma during ascent (under assumed equilibrium conditions). The moderately high phenocryst content of Ruapehu scoria (∼30%) means that only a small amount of additional microlite crystallisation (∼5%) would result in a yield strength, which may lead the magma to stall. However, if the strain rates are high enough, more crystallisation would be possible without developing a yield stress. This suggests that microlite-rich magmas are almost certain to stall unless they encounter significant fluid addition from a source such as a hydrothermal system, groundwater, or surface water (i.e., Ruapehu's Crater Lake). Ruapehu magmas are initially H 2 O-undersaturated and as a consequence, crystallisation and bubble growth were suppressed until the magma achieved saturation, at ∼100 to 50 MPa. From this analysis, we suggest that Ruapehu magmas are more likely to erupt compared to magmas of a similar composition that are H 2 O-saturated. This partly explains the regular, albeit small-volume eruptions at Ruapehu and the propensity for phreatomagmatic eruptions when the magma:water ratio is low. [ABSTRACT FROM AUTHOR]

Obsidian flow emplacement is a complex and understudied aspect of silicic volcanism. Of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow. Using an array of methods we study the extreme textural heterogeneity of the Rocche Rosse obsidian flow in Lipari, a 2 km long, 100 m thick, ~ 800 year old lava flow, with respect to outgassing and emplacement mechanisms. 2D and 3D vesicle analyses and density measurements are used to classify the lava into four textural types: ‘glassy’ obsidian (< 15% vesicles), ‘pumiceous’ lava (> 40% vesicles), high aspect ratio, ‘shear banded’ lava (20–40% vesicles) and low aspect ratio, ‘frothy’ obsidian with 30–60% vesicles. Textural heterogeneity is observed on all scales (m to μm) and occurs as the result of strongly localised strain. Magnetic fabric, described by oblate and prolate susceptibility ellipsoids, records high and variable degrees of shearing throughout the flow. Total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary (magmatic) and secondary (meteoric) water. Glass water contents are between 0.08–0.25 wt.%. Water analysis also reveals an increase in water content from glassy obsidian bands towards ‘frothy’ bands of 0.06–0.08 wt.%, reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content. We present an outgassing model that reconciles textural, volatile and magnetic data to indicate that obsidian is generated from multiple shear-induced outgassing cycles, whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian, which then re-vesiculates to produce ‘dry’ vesicular magma. Repetition of this cycle throughout magma ascent results in the low water contents of the Rocche Rosse lavas and the final stage in the degassing cycle determines final lava porosity. Heterogeneities in lava rheology (vesicularity, water content, microlite content, viscosity) play a vital role in the structural evolution of an obsidian flow and overprint flow-scale morphology. Post-emplacement hydration also depends heavily on local strain, whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water, a process previously correlated to lava vesicularity alone. [ABSTRACT FROM AUTHOR]

Abstract: We consider the current state of our understanding of the rheology of two-phase magmas, that is suspensions of either bubbles or crystals in a viscous silicate melt. The discussion is restricted to strain-rates at which the suspending melt can be considered Newtonian. We start by considering the range of textures found in magmas and the bubble deformation and particle motions caused by shearing. We then review proposed models for suspensions, focussing on those functions of the form η r = f(ϕ) or that have been most widely used to describe magmatic systems (η r is the relative apparent viscosity of the suspension, ϕ is the volume fraction of the suspended phase, τ is the driving stress, and is the strain-rate). Both theoretical and empirical methods are presented and then compared against the available analogue (i.e. non-magmatic) and magmatic data. The paper contains new data and significant re-analysis of previously published data. We present a new semi-empirical constitutive model for bubble-bearing magmas that is valid for steady and unsteady flow and large strains and strain-rates. This equation utilises a new parameter, the capillarity Cx, that encapsulates the combined effect of shearing and unsteadiness on bubble suspensions. We also present a new scheme for dealing with polydispersivity of bubble suspensions. New data on the rheology of particle suspensions undergoing forced-oscillations are presented. These data show that the Cox–Merz rule only holds for dilute particle suspensions ϕ ≲0.25. A re-analysis of all available experimental data that relate rheology to particle aspect ratio provides distinct curves of maximum packing as a function of aspect ratio for smooth and rough particles with magmatic data lying on the curve appropriate for rough particles. We analyse several rheological datasets of crystal-bearing basaltic magmas and find that they are in good agreement with the constitutive equations derived from analogue data. By contrast, the same equations do not agree well with data for high-viscosity, haplogranitic melts. This may be an effect of fracturing or viscous dissipation within these samples. The paper concludes with a practical ‘rheological recipes’ section giving a step-by-step method for calculating a constitutive equation for a two-phase magmatic suspension and assessing its likely accuracy. [Copyright &y& Elsevier]

Abstract: Using constraints from literature data on the petrology and texture of erupted material from Stromboli and geochemical measurements of gas emissions together with a model of gas solubility we construct a conceptual model of quiescent degassing for this volcano. We find that within a pressure range between 100 MPa and 50 MPa (∼3.6 km and ∼1.8 km depth respectively) vesiculating magma ascending within the conduit becomes permeable to gas flow and a transition from closed- to open-system degassing takes place. Above the transition, gas, rich in the most insoluble gases, flows up through degassing magma, and thereby becomes enriched in more soluble gases during ascent to the surface. The final gas emission is therefore a superposition of gases released from magma above the percolation transition and gas that has evolved in closed-system below the transition. Steady-state gas release from Stromboli can only be sustained via magma circulation, driven by the density variation between ascending vesiculating magma and descending degassed magma. By balancing the buoyant force of ascending vesiculating magma against the viscous resistance produced by travelling through descending, degassed magma in a simple flow model we determine that a cylindrical conduit diameter of 2.5–2.9 m produces the magma mass flow rate of 575 kg s−1, required to account for the observed quiescent SO2 gas flux on Stromboli of ∼2.3 kg s−1 (200 td−1). [Copyright &y& Elsevier]

The effect of crystals and pre-existing gas phases on the dynamic degassing and fragmentation of magmas in a volcanic conduit have been simulated by analogue experiments. A series of decompression experiments of viscous solutions of gum rosin and acetone with and without the incorporation of particles (silicon carbide, glass beads, organic seeds) or pre-existing air bubbles have been performed in a transparent shock-tube. The addition of internal bubbles or solid particles in the starting solution shifts the fragmentation conditions towards lower acetone (volatile) contents and lower decompression ratios. The effect of solid particles depends on their number density and spatial distribution in the starting solution. A small number of sinking particles has little impact on the flow dynamics. However, a floating layer of particles provides a high concentration of surface bubbles that leads to an early fragmentation pulse. The observations also imply that the presence of crystals, if able to generate a high number density of bubbles, will lead to an earlier and deeper fragmentation of the magma in the volcanic conduit. [Copyright &y& Elsevier]

Abstract: We present calculations of a simplified two-layer pyroclastic flow model, which incorporates different physics to represent the dense and dilute regions of column collapse pyroclastic density currents. The two layers evolve separately, but are coupled through mass exchange as suspended ash in the dilute cloud settles into the underlying dense basal layer. The runout distance of the upper dilute current and the associated runout time are found to increase with increasing initial column height and decreasing particle size. Likewise steeper slopes increase the propagation velocity and thus the runout distance. The independent runout distance of the basal flow exhibits opposite behaviour to that of the parent dilute current, increasing with decreasing initial column height and increasing particle size. Observed runout distances can be calculated for basal flows using a Coulomb friction law, but deposit morphology is not well reproduced and is more realistic when an empirical slope-dependent sedimentation rate is included. Dominant flow behaviour is controlled by the rate of mass transfer from the parent suspension current into the dense underflow. Tall column collapses, which contain fine-grained particles, transfer their mass slowly to the dense basal flow and are well described by dilute cloud assumptions. However, for short columns containing coarse grains, the particulate mass is rapidly transferred from the collapsing dilute current into the basal flow. The bulk of the pyroclastic current material propagates as a concentrated suspension for the majority of its travel distance and is better described by the physics of granular avalanches. [Copyright &y& Elsevier]

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 have proposed incompatible mechanisms for producing the various distributions. We here present a unifying mechanism. Data from our laboratory analogue experiments suggest that power law distributions are associated with highly non-equilibrium degassing. A numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations, where, in a non-equilibrium degassing scenario, the volatile concentration remains high. This process causes the BSD to evolve from unimodal, through exponential, into a power law. The exponent of the power law is a measure of the number of nucleation events, or the duration of the nucleation period compared with the timescale of bubble growth. The mathematical inevitability of the evolution from unimodal (Poissonian) to power law is discussed. The findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non-equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions. The process of ongoing nucleation is the mechanism whereby the volcanic system maintains near-equilibrium in the case of rapid depressurisation and slow volatile diffusion. [Copyright &y& Elsevier]

Analogue materials are commonly used in volcanology to perform scaled laboratory experiments. Analogue experiments inform on fundamental fluid dynamic, structural and mechanical processes that are typically very difficult to observe and quantify directly in the natural volcanic system. Here we investigate the suitability of an aqueous solution of hydroxyethyl cellulose polymer (HEC) for use as a lava/magma analogue, with a particular focus on its rheological behaviour. We characterize a range of physical properties as functions of the concentration and temperature of the solution: density; specific heat capacity; thermal diffusivity; thermal conductivity; surface tension; as well as rheology. HEC has a non-Newtonian, shear-thinning rheology that depends on the concentration and temperature of the solution. We show that the rheology is well described by the Cross model, which was originally developed for polymer solutions, but has also been applied to bubbly magmas. Using this similarity, an approach for scaling analogue experiments that use shear-thinning polymers, like HEC, to bubbly magma is presented. A detailed workflow and a spreadsheet are provided to allow experimentalists to investigate the effects of non-Newtonian behaviour in their existing laboratory set-ups. This contribution will allow for the more complex, but often more realistic case of bubble-bearing magmas to be rigorously studied in experimental volcanology. • Bubbly magma properties are reviewed in terms of analogue experiments and scaling. • We discuss the suitability of shear-thinning polymers for use as a magma analogue. • The physical properties of hydroxyethyl cellulose (HEC) are presented. • We present a methodology for scaling shear-thinning polymers to bubble suspensions. • HEC can be used as an analogue for vesicular (bubbly) silicate melts. [ABSTRACT FROM AUTHOR]

The dichotomy between explosive volcanic eruptions, which produce pyroclasts, and effusive eruptions, which produce lava, is defined by the presence or absence of fragmentation during magma ascent. For lava fountains the distinction is unclear, since the liquid phase in the rising magma may remain continuous to the vent, fragment in the fountain, then re-weld on deposition to feed rheomorphic lava flows. Here we use a numerical model to constrain the controls on basaltic eruption style, using Kilauea and Etna as case studies. Based on our results, we propose that lava fountaining is a distinct style, separate from effusive and explosive eruption styles, that is produced when magma ascends rapidly and fragments above the vent, rather than within the conduit. Sensitivity analyses of Kilauea and Etna case studies show that high lava fountains (>50 m high) occur when the Reynolds number of the bubbly magma is greater than ∼0.1, the bulk viscosity is less than 10 6 Pa s , and the gas is well-coupled to the melt. Explosive eruptions (Plinian and sub-Plinian) are predicted over a wide region of parameter space for higher viscosity basalts, typical of Etna, but over a much narrower region of parameter space for lower viscosity basalts, typical of Kilauea. Numerical results show also that the magma that feeds high lava fountains ascends more rapidly than the magma that feeds explosive eruptions, owing to its lower viscosity. For the Kilauea case study, waning ascent velocity is predicted to produce a progressive evolution from high to weak fountaining, to ultimate effusion; whereas for the Etna case study, small changes in parameter values lead to transitions to and from explosive activity, suggesting that eruption transitions may occur with little warning. • We use a magma ascent model to constrain the controls on basaltic eruption style. • Lava fountaining is a distinct style, separate from effusive and explosive activity. • Lava fountains are produced when magma ascends rapidly and fragments above the vent. • It is unlikely that lava fountains at Kilauea would evolve into explosive eruptions. • At Etna small changes in conditions may lead to rapid transitions in eruptive style. [ABSTRACT FROM AUTHOR]