Magma Rheology Research Articles

Magma flow and interaction with waste packages in a geologic repository at Yucca Mountain, Nevada

The likelihood that a nuclear waste repository at Yucca Mountain could be intersected by igneous activity is very low, but its potential consequences are nevertheless important to performance assessments. An ongoing critical area of concern is the nature and magnitude of the thermal interaction of magma with tunnel walls, high-level nuclear waste packages, and waste forms. Previous assessments consider a variety of dynamic scenarios, but large uncertainties remain in understanding the rheological nature of the magma likely to be involved and its behavior within a repository drift. Here we specifically address the issue of magma rheology during degassing, cooling, and solidification as basaltic magma approaches Earth's surface and enters a drift. Magma containing significant amounts (> ~ 2 wt.%) of dissolved water (Wet Magma), as is anticipated for this region, at or near its liquidus temperature and saturated with water at 200 MPa is at a temperature near or below the 1-atm solidus temperature. Isentropic ascent from this near liquidus temperature promotes extensive solidification and/or glassification. Exsolving water with approach to the surface promotes rapid vesiculation leading to fragmentation and tephra production. With continued ascent the still water-saturated magma traverses the solidification phase field and undergoes a combination of rapid crystallization and quenching, becoming a glassy highly viscous (~ 10 8 Pa s) mass of greatly reduced mobility. This immobility is reflected in the high effective viscosity regulating flows from nearby cinder cones associated with wet basalt. This also matches well with the experimentally determined rheology of dry basalt glass. This rheology greatly restricts the mobility of basalt within repository drifts, amounting to < 10 m per day. Magma in this state quenches rapidly (~ 10 cm/min) on waste packages. Wet basalt is explosive, but relatively immobile as lava. Dry Magma is not explosive, but highly mobile as lava. Previous studies have tended to use an inconsistent set of mixed magma properties involving both extremes. The net effect of our results is that the portion of a repository hypothetically affected by invading magma is likely to be minimal and the number of waste packages affected may be very small. Moreover, the waste packages and/or waste materials affected will most likely be encased in quenched magma.

Viscous heating in rhyolite: An in situ experimental determination

Viscous heating during magmatic flow may play a major role in eruption dynamics. In order to document viscous heating during deformation of magma, we have conducted a series of rheological experiments where viscous heating is directly monitored via thermocouples in high-viscosity magmas. We observe experimentally the strain rate dependence of viscous heating. Viscous heating becomes rheologically significant in the highly viscous lavas investigated at strain rates above ca. 10− 3 s− 1. A simple analysis shows that the temperature increase generated through viscous heating during deformation of melts with viscosities ranging from 108 to 1012 Pa·s can account for their apparent non-Newtonian rheology in these experiments. This thermal correction transforms apparent non-Newtonian, strain rate dependent rheology of magma to a Newtonian behavior over the range of conditions accessed in this work. In this manner, this study provides an experimental basis for separating the relative roles of structural relaxation and viscous heating in the generation of apparent non-Newtonian rheology at high strain rates. Here, viscous heating dominates and the observation of the structural onset of non-Newtonian behavior is precluded by a viscous heating-induced lowering of the Newtonian viscosity. The interplay of viscous heating and structural relaxation in melts in nature is discussed briefly.

Propagation of P and S-waves in magmas with different crystal contents: Insights into the crystallinity of magmatic reservoirs

Increasing amount of crystals tends to reduce the mobility of magmas and modifies its elastic characteristics (e.g. [Caricchi, L. et al., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth and Planetary Science Letters, 264: 402–419.; Bagdassarov, N., Dingwell, D.B. and Webb, S.L., 1994. Viscoelasticity of crystal- and bubble-bearing rhyolite melts. Physics of the Earth and Planetary Interior, 83: 83–99.]). To quantify the effect of crystals on the elastic properties of magmas the propagation speed of shear and compressional waves have been measured at pressure and temperatures relevant for natural magmatic reservoirs. The measurements have been performed in aggregates at variable particle fractions (ϕ = 0–0.7). The measurements were carried out at 200 MPa confining pressure and temperatures between 300 K and 1273 K (i.e. across the glass transition temperature (Tg) from glass to melt). The specimens were mixtures of a haplogranitic melt containing 5.25 wt.% H 2O and variable amounts of sub-spherical alumina particles. Additional experiments were carried out on a sample containing both, crystals and air bubbles. The temperature derivatives of the shear (dVs/d T) and compressional wave (dVp/d T) velocities for pure glass and samples with a crystal fraction of 0.5 are different below and above the glass transition temperature. For a crystal fraction 0.7, only dVp/d T changed above the Tg. In the presence of gas bubbles, Vp and Vs decrease constantly with increasing temperature. The bubble-bearing material yields a lower bulk modulus relative to its shear modulus. The propagation velocities of compressional and shear waves increase non-linearly with increasing crystal fraction with a prominent raise in the range 0.5 < ϕ < 0.7. The speed variations are only marginally related to the density increase due to the presence of crystals, but are dominantly related to the achievement of a continuous crystal framework. The experimental data set presented here can be utilized to estimate the relative proportions of crystals and melt present in a magmatic reservoir, which, in turn, is one of the fundamental parameters determining the mobility of magma and, consequently, exerting a prime control on the likelihood of an eruption from a sub-surficial magma reservoir.

Deformation of bubbles in a highly viscous pipe flow

The rheology of bubble-bearing magma is important in volcanic conduit flow. Its rheological properties strongly depend on the deformation of bubbles in highly viscous liquid. Here we conduct an experimental investigation on the deformation of bubbles in a Stokes pipe flow, in which the shear rate is non-uniform. The deformation of the bubbles in various shear rates is observed by high-speed photography. The deformed shape is classified using the capillary number, Ca, the ratio of viscous shear stress that promotes bubble deformation to surface tension that resist deformation. We find that the length of the elongated bubbles due to shear at Ca > 1 is smaller than that in a simple shear field whose capillary numbers are equal to each other. In such a case, the bubbles have an asymmetric shape like the section of a two-dimensional airfoil with camber. Implication to the effective viscosity of the suspensions with such a wing-shaped bubble is briefly discussed.

Fudge Factors in Lessons on Crystallization, Rheology and Morphology of Basalt Lava Flows

Fudge is an excellent analog for basaltic lava and making or tasting fudge leads to memorable lessons on the importance of crystals in controlling the morphology of basaltic lava flows. In particular, students learn why ‘a’a flows have rough broken surfaces, while pahoehoe flows are smooth or folded. Furthermore, fudge provides an interesting analog in lessons on the factors that control crystal nucleation and growth as well as how crystals affect magma rheology. Although the full process of making fudge from scratch is too long for a lecture demonstration, fudge can be incorporated into lessons on basalt flows by way of taste-tests, photographs or simplified experiments with pre-made fudge and syrup. Advanced students can run experiments during a laboratory period and examine the crystal textures under a microscope, or do their own experiments in small groups outside of the classroom. Evaluation with written quizzes shows that fudge demonstrations can be an effective aid for teaching the complex concepts of crystal nucleation and growth and their effects on basalt lava flows.

Percolating magmas in three dimensions

Abstract. The classical models of volcanic eruptions assume that they originate as a consequence of critical stresses or critical strain rates being exceeded in the magma followed by catastrophic fragmentation. In a recent paper (Gaonac'h et al., 2003) we proposed an additional mechanism based on the properties of complex networks of overlapping bubbles; that extreme multibubble coalescence could lead to catastrophic changes in the magma rheology at a critical vesicularity. This is possible because at a critical vesicularity Pc (the percolation threshold), even in the absence of external stresses the magma fragments. By considering 2-D percolation with the (observed) extreme power law bubble distributions, we showed numerically that P2c had the apparently realistic value ≈0.7. The properties of percolating systems are, however, significantly different in 2-D and 3-D. In this paper, we discuss various new features relevant to 3-D percolation and compare the model predictions with empirical data on explosive volcanism. The most important points are a) bubbles and magma have different 3-D critical percolation points; we show numerically that with power law bubble distributions that the important magma percolation threshold P3c,m has the high value ≈0.97±0.01, b) a generic result of 3-D percolation is that the resulting primary fragments will have power law distributions with exponent B3f≈1.186±0.002, near the empirical value (for pumice) ≈1.1±0.1; c) we review the relevant percolation literature and point out that the elastic properties may have lower – possibly more realistic – critical vesicularities relevant to magmas; d) we explore the implications of long range correlations (power law bubble distributions) and discuss this in combination with bubble anisotropy; e) we propose a new kind of intermediate "elliptical" dimensional percolation involving differentially elongated bubbles and show that it can lead to somewhat lower critical thresholds. These percolation mechanisms for catastrophically weakening magma would presumably operate in conjunction with the classical critical stress and critical strain mechanisms. We conclude that percolation theory provides an attractive theoretical framework for understanding highly vesicular magma.

Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics

The eruptive dynamics of volcanic systems are largely controlled by the viscosity of deforming magma. Here we report the results of a series of high-temperature, high-pressure experiments at conditions relevant for volcanic conduits (250 MPa confining pressure and temperature between 500 °C and 900 °C) that were undertaken to investigate the rheology of magma with crystal fractions varying between 0.5 and 0.8 (50 to 80 wt.%) at different strain-rate conditions. The experiments demonstrate that the presence of crystals increases the relative viscosity (ratio between the viscosity of the mixture and the viscosity of the melt phase) of magmas and additionally induces a decrease of the relative viscosity with increasing strain-rate (shear thinning, non-Newtonian behavior). The experimental results, combined with existing data at low crystal fractions (0–0.3), were used to develop a semi-empirical parameterization that describes the variations of relative viscosity for crystal fractions between 0 and 0.8 and accounts for the complex non-Newtonian rheology of crystal-bearing magmas. The new parameterization, included into numerical models simulating the magma ascent dynamics, reveals that strain-rate-dependent rheology significantly modifies the dynamic behavior inside volcanic conduits, particularly affecting the magma fragmentation conditions.

Microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure

The relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high‐temperature high‐pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0% to 76% solid fraction (alumina grains). Torsion experiments were conducted at 300 MPa confining pressure, temperatures ranging from 475°C to 1000°C and shear strain rates ranging from 2.0 × 10−5 to 2.1 × 10−3 s−1 up to total strains of 21.3. Flow is Newtonian for a solid fraction of φs = 0.0–0.16 with a log dynamic viscosity η = 10.3 Pa s (T = 500°C). A deviation from Newtonian behavior is observed for φs &gt; 0.16 with an increase in apparent viscosity of about 1 order of magnitude between φs = 0.16 and 0.54. The shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction. Both shape fabric and deviation from Newtonian behavior originate from the increase in the number of particle clusters in the suspension. The apparent viscosity increases by 1.5 orders of magnitude between φs = 0.54 and 0.65, and extrapolation of the data suggests a very sharp increase in apparent viscosity for φs ≥ 0.65. At T ≥ 550°C and φs = 0.54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains. At T ≤ 550°C and φs = 0.65, intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles. The resulting bulk extensional fabric is almost parallel to the shortening direction.

The Fluid Mechanics Inside a Volcano

The style and evolution of volcanic eruptions are dictated by the fluid mechanics governing magma ascent. Decompression during ascent causes dissolved volatile species, such as water and carbon dioxide, to exsolve from the melt to form bubbles, thus providing a driving force for the eruption. Ascent is influenced not only by the nucleation and growth of gas bubbles, but also magma rheology and brittle deformation (fragmentation). In fact, all processes and magma properties within the conduit interact and are coupled. Ultimately, it is the ability of gas trapped within growing bubbles to expand or to be lost by permeable gas flow, which determines whether ascending magmas can erupt nonexplosively. We review and integrate models of the primary conduit processes to show when each process or property dominates and how these interact within a conduit. In particular, we illustrate how and why ascent rate may control eruptive behavior: slowly ascending magmas erupt effusively and rapidly ascending magmas erupt explosively.

The combined effects of chlorine and fluorine on the viscosity of aluminosilicate melts

The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na 2O–Fe 2O 3–Al 2O 3–SiO 2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na 2O + FeO) < (Al 2O 3 + Fe 2O 3)] to peralkaline [(Na 2O + FeO) > (Al 2O 3 + Fe 2O 3)] were determined over a temperature range 560–890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 10 8.8 to 10 12.0 Pa s. The compositions are based on addition of FeF 3 and FeCl 3 to aluminosilicate melts with a fixed amount of SiO 2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na 2O–Fe 2O 3–Al 2O 3–SiO 2 melts. Am. Mineral. 91, 344–352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.

火砕物の組織から読み取る火道内での脱ガス

This paper reviews recent studies on textural analysis of pyroclasts, especially those on degassing in the volcanic conduit. Potential importance of geophysical observations on eruption events inspired by progresses in magma rheology is suggested, and links between quantitative textural analysis and forward approaches from both theory and experiments are discussed. Permeability of the fragmented materials may provide useful information to consider permeable flow degassing in the volcanic conduit. Permeability data of pyroclasts reported so far, however, show somewhat dispersed values which can not be fitted by a simple relation with porosity based on a percolation theory. This indicates that geometrical diversities originated form bubble coalescence, deformation and collapse are important to control degassing efficiency. Geological observations of “fossil” conduit have been made, in which brittle behavior of viscous magmas under high shear stresses near the volcanic conduit has been underscored.

Bubble suspension rheology and implications for conduit flow

Bubbles are ubiquitous in magma during eruption and influence the rheology of the suspension. Despite this, bubble-suspension rheology is routinely ignored in conduit-flow and eruption models, potentially impairing accuracy and resulting in the loss of important phenomenological richness. The omission is due, in part, to a historical confusion in the literature concerning the effect of bubbles on the rheology of a liquid. This confusion has now been largely resolved and recently published studies have identified two viscous regimes: in regime 1, the viscosity of the two-phase (magma–gas) suspension increases as gas volume fraction ϕ increases; in regime 2, the viscosity of the suspension decreases as ϕ increases. The viscous regime for a deforming bubble suspension can be determined by calculating two dimensionless numbers, the capillary number Ca and the dynamic capillary number Cd. We provide a didactic explanation of how to include the effect of bubble-suspension rheology in continuum, conduit-flow models. Bubble-suspension rheology is reviewed and a practical rheological model is presented, followed by an algorithmic, step-by-step guide to including the rheological model in conduit-flow models. Preliminary results from conduit-flow models which have implemented the model presented are discussed and it is concluded that the effect of bubbles on magma rheology may be important in nature and results in a decrease of at least 800 m in calculated fragmentation-depth and an increase of between 40% and 250% in calculated eruption-rate compared with the assumption of Newtonian rheology.

A transient model for explosive and phreatomagmatic eruptions

Abstract We develop a model for explosive eruptions in cylindrical conduits generated by plug disruption at the top of the conduit. The eruption is calculated for a standard set of parameters for rhyolite. The model takes into account temperature variations and non-Newtonian magma rheology. We analyzed the influence of principal parameters such as fragmentation criteria, magma rheology, and initial conditions. We conclude that the variation of magma temperature is less than 70 K during eruption, and the model with bubble concentration-dependent viscosity leads to the increase of the steady discharge rate by about 40%. The eruption model is extended to include interaction between magma in the conduit and water in a surrounding aquifer. The phreatomagmatic eruption is also initiated by plug disruption. Two cases of initial aquifer pressure are considered: magmastatic and hydrostatic. These cases are valid for confined and unconfined aquifers. For the first case (confined aquifer), water influx influences the conduit flow soon after plug disruption and leads to the additional peaks of discharge rate; for the second case (unconfined aquifer), a purely magmatic, explosive eruption of longer duration is followed by a phreatomagmatic phase. In both cases, magma discharge rate increases by 10–100% in comparison with purely magmatic eruption.

Identification of variable convective regimes at Erta Ale Lava Lake

During February 17–19, 2002, we collected a combined thermal and seismic data set for persistent lava lake activity at Erta Ale volcano, Ethiopia. These data indicate that the lake cycled between periods characterized by low (0.01–0.08 m s −1) and high (0.1–0.4 m s −1) surface velocities, typically lasting tens to hundreds of minutes. These periods of high and low velocity motion define periods of vigorous and sluggish convection, respectively. Spectral analysis revealed that vigorous convection periods were characterized by high frequencies and energies in the thermal data and an increased presence of high-frequency energy in the seismic data. The data show that vigorous periods were characterized by formation of hot, short-lived, plastic crusts, and sluggish periods by cooler, longer lived, brittle crusts. Here, the higher transit velocity across the lake surface from upwelling zones of crust formation to downwelling zones of crust destruction during the vigorous periods decreases the crust lifetime. This in turn decreases the total cooling and thickening experienced by a plate of crust moving across the lake surface. Two scenarios can be envisaged to explain such convection cycles. The first relates variable convection rates to changes in the volume flux and rheology of magma entering the lake. In the second, cyclic convection is set up by the generation of convective instabilities within the lake. In this case, cooling of a surface layer generates a slow moving, viscous, increasingly dense convection layer at the lake surface which is consumed and replaced during overturn.

Controls on conduit magma flow dynamics during lava dome building eruptions

Lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity. We develop a transient model of the magma flow in a volcanic conduit from an open‐system magma chamber with continuous replenishment. The model incorporates gas exsolution, bubble growth, gas escape through the magma, and decompression‐induced crystallization and considers variations in magma temperature, water content, conduit diameter, phenocryst size, chamber volume, and magma rheology. Calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime, when discharge rate is low and crystals grow efficiently leading to high magma viscosity, to another stable state, when discharge rate is high and crystallization is negligible. The difference in discharge rates between these regimes can be several orders of magnitude. Periods are similar to the observed timescales and mainly depend on the chamber volume. The system shows strongly nonlinear responses to the variation of governing parameters. If magma has a Bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of Newtonian rheology. Large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature, water content, phenocryst size distribution, or conduit diameter. The system can fluctuate between low and high discharge rates with transitions to explosive activity.

Phase relations in silicic systems at one-atmosphere pressure

An important control on magma rheology is the extent to which the magma crystallizes during ascent as a result of the effective undercooling created by volatile exsolution. To assess this undercooling, we need to know the final (anhydrous) one-atmosphere phase relations of silicic magmas. For this reason, we have performed one-atmosphere controlled-fO2 crystallization experiments on dacitic to rhyolitic melt compositions (67–78 wt% SiO2) and determined equilibrium phase assemblages, melt fractions, and some phase compositions over a range of temperatures. Experiments were run at oxygen fugacities between NNO+1 and NNO+2 and temperatures of 1,000 to 1,250°C. Constant phase compositions and sample crystallinities in runs longer than 3.5 days suggest that these runs closely approached compositional equilibrium. Additionally, melting experiments with similar compositions yielded results closely resembling those obtained in crystallization experiments. All samples have liquidus temperatures between 1,250 and 1,200 °C, with plagioclase the liquidus phase for the two most mafic samples and quartz for the most silicic sample. When associated glass compositions are projected into the Qz-Ab-Or system they define a revised one-atmosphere quartz-feldspar cotectic 5–10% less quartz normative than previously estimated. Glass compositions from each sample plot along this cotectic between 1,100 and 1,000 °C, consistent with the plagioclase-quartz co-crystallization textures found in runs at these temperatures. This cotectic constrains glass compositions to a maximum silica content of 76±1 wt% SiO2. Reported glass compositions in excess of 77 wt% SiO2 in volcanic samples suggest non-equilibrium crystallization, perhaps a consequence of large melt undercoolings.

Percolating magmas and explosive volcanism

Magma under pressure rises in conduits, depressurizes, forms bubbles by the exsolution of gas and – at void fractions (P) typically of the order of 0.7 – can fragment and explode. The study of overlapping geometrical units – percolation theory – predicts that at a critical volume fraction Pc the size of the largest simply connected region becomes infinite. We apply percolation theory to overlapping bubbles arguing that this geometric singularity at Pc implies a physical singularity in the magma rheology. This would imply that if the magma is under stress, ‐ whether it is ductile or brittle ‐ this rapid development of a network of infinitely long “bubbles” triggers fragmentation and explosion. Classical monodisperse (equal size) continuum percolation theory predicts Pc = 0.2985 ± 0.005 which is far from the observed values. However, it has recently been shown that the bubble distribution is a power law associated with a huge range of bubble sizes. Using Monte Carlo percolation simulations, we show that distributions exhibiting the empirical exponents are very efficient at “packing” the bubbles, drastically raising Pc to the value = 0.70 ± 0.05. Explosive volcanism is thus explained by singular rheology at Pc.

Rheology of Granitic Magmas During Ascent and Emplacement

▪ Abstract Considerable progress has been made over the past decade in understanding the static rheological properties of granitic magmas in the continental crust. Changes in H2O content, CO2 content, and oxidation state of the interstitial melt phase have been identified as important compositional factors governing the rheodynamic behavior of the solid/fluid mixture. Although the strengths of granitic magmas over the crystallization interval are still poorly constrained, theoretical investigations suggest that during magma ascent, yield strengths of the order of 9 kPa are required to completely retard the upward flow in meter-wide conduits. In low Bagnold number magma suspensions with moderate crystal contents (solidosities 0.1 ≤ φ ≤ 0.3), viscous fluctuations may lead to flow differentiation by shear-enhanced diffusion. AMS and microstructural studies support the idea that granite plutons are intruded as crystal-poor liquids (φ ≤ 50%), with fabric and foliation development restricted to the final stages of emplacement. If so, then these fabrics contain no information on the ascent (vertical transport) history of the magma. Deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt, resulting in a range of local macroscopic flow structures, including layering, crystal alignment, and other mechanical instabilities such as shear zones. As the suspension viscosity varies with stress rate, it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions applies to mixtures undergoing shear. New theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved.

SO 2 emissions from Soufrière Hills Volcano and their relationship to conduit permeability, hydrothermal interaction and degassing regime

The time series of sulphur dioxide (SO 2) emissions during the continuing eruption of Soufrière Hills Volcano, Montserrat, yields insights into conduit permeability and driving pressures, the role of the hydrothermal system and changes in magma flux both at depth and to the surface. On a time scale of years, an effectively constant supply of sulphur from a more mafic magma at depth permits evaluation of changes in the permeability of the plumbing system between 1995 and 2002 (due to magma rheology changes and hydrothermal sealing), most of which take place in the upper few hundreds of metres (dome and upper conduit). A broadly increasing SO 2 emission rate from 1995 to 1997 can be attributed to a constant or increasing supply of exsolving sulphur from depth, combined with a broadly increasing magma discharge rate at the surface. Decreases in SO 2 flux over three orders of magnitude, from July 1998 to November 1999, were due to a corresponding decrease in permeability of the upper conduit and dome due to cooling and ‘sealing’ by the precipitation of hydrothermal minerals and the closure of fracture and bubble networks. The second phase of dome growth, from November 1999 to the present, April 2002, has been associated with a similar range of SO 2 fluxes to the first phase. Large dome collapses in 1997 and during a period of zero magma flux in 1998 were associated with instantaneous SO 2 emissions of >10 kt, which indicate a capacity for significant SO 2 storage in the conduit and dome prior to the collapses. SO 2 data suggest that the second phase of dome building, despite a similar sulphur budget in terms of supply from depth and mean SO 2 emission rate at the surface (around 500 t/d), is characterised by a higher bulk permeability at shallow depths and is a more ‘open’ system with respect to fluid through-flow than the first phase of dome building from 1995 to 1998. The lack of large SO 2 emissions after large dome collapses, in 2000 and 2001, suggests limited storage of SO 2 in the conduit system. The data suggest that the likelihood of a switch to explosive activity after a large collapse is more unlikely now than during the first phase of dome building. Over shorter time scales, permeability changes may be recognised from the SO 2 flux data prior to the onset of dome growth and during cycles of small explosions in 1999. On time scales of minutes to hours, pulses of SO 2-rich gas emissions occur after rockfalls and pyroclastic flows, due to the release of a SO 2-rich fluid phase stored in closed fractures and pore spaces within the dome. Long period and hybrid seismic events may be associated with changes in SO 2 emission rate at the surface at various times of the eruption, although only when the temporal resolution of SO 2 monitoring is improved, will it be possible for these short-term changes to be correlated and evaluated effectively. Monitoring SO 2 emission rates from Soufrière Hills Volcano is, at this stage, of primary value in the long run, on the time scale of years, where the relationships between deep supply and surface emissions can be used to evaluate whether the eruption might be waning, or has merely paused, which is of considerable value for hazard assessment.

Chapter 2 The origin, nature and behavior of pyroclastic density currents

Abstract In this chapter we consider the initiation and transport behaviour of pyroclastic density currents that deposit ignimbrites. We deal with a wide range of phenomena and assess the limitations in present understanding. Some limitations considered in this chapter and the next lie in the possible differences between pyroclastic currents, which are gas-particle systems, and aqueous analogue experiments from which some understanding has been gleaned. Air has a substantially lower viscosity and density than (liquid) water, is far more compressible and shows far greater thermal expansion. Therefore flow rheologies and processes, like particle settling and sorting in pyroclastic currents, are likely to differ quantitatively from those in aqueous currents, and there may also be some more fundamental differences in behaviour, such as in fluidization, the development and propagation of shock waves and thermal effects, and in the agglomeration (clustering) behaviour of fine ash particles. Origin and development of pyroclastic density currents Eruption styles Pyroclastic density currents originate in different ways and from various sources (Fig. 2.1). They may be short-lived (highly unsteady) phenomena (Fig. 2.1 A, D and E) or relatively long-lived (sustained unsteady to quasi-steady; Fig. 2.IB and C). In currents that initiate explosively, the clast concentration of the erupting dispersion relates to the eruption style (e.g. high or low pyroclastic fountaining), which is a function of (1) the magma rheology and mass flux, (2) the volatile abundances, species and exsolution rates, (3) the size, abundance and timing of formation of vesicles and cracks at the time of fragmentation, and (4) the size and abundance of accidental