experimental constraints on the rheology eruption and emplacement dynamics of analog lavas comparable to mercury s northern volcanic plains we present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of mercury in particular we focus on the northern volcanic plains nvp which correspond to the largest lava flows on mercury and possibly in the solar system high temperature viscosity measurements were performed at both superliquidus up to 1736 k and subliquidus conditions 1569 1502 k to constrain the viscosity variations as a function of crystallinity from 0 to 28 and shear rate from 0 1 to 5 s 1 melt viscosity shows moderate variations 4 16 pa s in the temperature range 1736 1600 k experiments performed below the liquidus temperature show an increase in viscosity as shear rate increases from 0 1 to 5 s 1 resulting in a shear thinning behaviour with a decrease in viscosity of ca 1 log unit the low viscosity of the studied composition may explain the ability of nvp lavas to cover long distances on the order of hundreds of kilometres in a turbulent flow regime using our experimental data we estimate that lava flows with thickness of 1 5 and 10 m are likely to have velocities of 4 8 6 5 and 7 2 m s respectively on a 5 ground slope numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates 10000 m3 s are necessary to cover the large distances indicated by satellite data from the messenger spacecraft

in situ observation of the percolation threshold in multiphase magma analogues magmas vesiculate during ascent producing complex interconnected pore networks which can act as outgassing pathways and then deflate or compact to volcanic plugs similarly in conduit fragmentation events during dome forming eruptions create open systems transiently before welding causes pore sealing the percolation threshold is the first order transition between closed and open system degassing dynamics here we use time resolved synchrotron source x ray tomography to image synthetic magmas that go through cycles of opening and closing to constrain the percolation threshold c at a range of melt crystallinity viscosity and overpressure pertinent to shallow magma ascent during vesiculation we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure at high viscosity 106 pa s and high overpressure 1 4 mpa we found that a brittle viscous regime dominates in which brittle rupture allows system spanning coalescence at a low percolation threshold c 0 17 via the formation of fracture like bubble chains percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network resulting in a hysteresis in the evolution of connectivity with porosity at low melt viscosity and overpressure we observed a viscous regime with much higher percolation threshold c 0 37 due to spherical bubble growth and lower degree of crystal connection finally our results also show that sintering of crystal free and crystal bearing magma analogues is characterised by low percolation thresholds c 0 04 0 10 we conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow crystal rich magma at initial stages of vulcanian and strombolian eruptions

rheology of bubble bearing magmas the rheology of bubble bearing suspensions is investigated through a series of three dimensional boundary integral calculations in which the effects of bubble deformation volume fraction and shear rate are considered the behaviour of bubbles in viscous flows is characterized by the capillary number ca the ratio of viscous shear stresses that promote deformation to surface tension stresses that resist bubble deformation estimates of ca in natural lava flows are highly variable reflecting variations in shear rate and melt viscosity in the low capillary number limit e g in carbonatite flows bubbles remain spherical and may contribute greater shear stress to the suspension than in high capillary number flows in which bubble deformation is significant at higher ca deformed bubbles become aligned in the direction of flow and as a result contribute less shear stress to the suspension calculations indicate that the effective shear viscosity of bubbly suspensions at least for ca 0 5 is a weakly increasing function of volume fraction and that suspensions of bubbles are shear thinning field observations and qualitative arguments however suggest that for sufficiently large ca ca greater than about 1 the effective shear viscosity may be less than that of the suspending liquid bubbles reach their quasi steady deformed shapes after strains of order one for shorter times the continuous deformation of the bubbles results in continual changes of rheological properties in particular for small strains the effective shear viscosity of the suspension may be less than that of the liquid phase even for small ca results of this study may help explain previous experimental theoretical and field based observations regarding the effects of bubbles on flow rheology

explosive expansion of a slowly decompressed magma analogue evidence for delayed bubble nucleation 1 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 observed we find that mixtures with a high acetone content containing smaller and fewer solid particles experience strong supersaturation and fragment under very slow decompressions despite having low viscosity while mixtures with lower acetone content with more and larger solid particles degas efficiently without fragmentation we interpret our results in terms of delayed bubble nucleation due to a lack of efficient nucleation sites we discuss how a similar mechanism might induce violent explosive expansion in volatile rich and poorly crystalline low silica magmas by analogy with the behavior of rhyolitic magmas

degassing behaviour at basaltic volcanoes new insights from experimental investigations of different conduit geometry and magma viscosity abstract understanding gas magma dynamics in volcanic conduits and linking them with the associated geophysical signals at the surface is of fundamental importance in monitoring active volcanoes in the past a significant number of studies have been devoted to this topic addressing the onset the key factors governing each degassing regime and the related markers at the surface particularly at basaltic volcanoes here we first review such a broad and increasing literature focusing on the main approaches used in the attempt of deciphering conduit dynamics by indirect observations 1 analogue laboratory experiments and 2 seismo acoustic measurements then we combine the two techniques into a novel set of experiments addressing a crucial yet unexplored issue the irregularity i e the departure from an ideal smooth cylindrical shape of the conduit surface we built a set of epoxy conduits with various fractal dimensions dc i e irregularity of the internal surface using silicone oil as a proxy for magma different degassing patterns bubbly slug and churn annular flow were reproduced by changing systematically 1 injected gas flux 5 to 180 10 3 l s 2 analogue magma viscosity 10 to 1000 pas 3 fractal dimension dc of the conduit surface i e dc 2 dc 2 18 and dc 2 99 the experiments were monitored by means of a video camera and a set of sensor aimed to detect the seismic and acoustic signals results show that viscosity strongly influences the transition among degassing patterns and the recurrence rate of slug bursts at the surface moreover we observed an increase of the exponent of the power law equation linking squared seismic amplitude to gas flow rate with conduit roughness the opposite trend was noticed with increasing liquid viscosity these results have important implications for linking seismic tremor to eruption source parameters such as the volume discharge rate at different volcanoes or for investigating its temporal evolution at a single vent

experimental study of bubble growth in stromboli basalt melts at 1 atm abstract volcanic eruptions are characterized by intense degassing thus it is imperative to have high quality laboratory data to constrain degassing mechanisms in order to investigate bubble formation and growth at 1 atm degassing experiments using a stromboli basalt were performed on the gsecars x ray beamline at the advanced photon source volatile bearing glasses were synthesized at 1250 c and 1000 mpa in a piston cylinder with h 2 o or mixtures of h 2 o co 2 they were then heated in situ on the x ray beamline at 1 atm bubble growth was observed in situ using x ray radiography the 3d bubble size distributions in the quenched samples and a natural stromboli pumice were studied by synchrotron x ray microtomography the results show that bubble nucleation and growth in basaltic melts are volatile concentration dependent bubbles can easily form in melts initially containing high volatile concentrations the effect of co 2 on bubble nucleation and growth becomes significant at large co 2 concentrations of 880 to 1480 ppm but is not important at lower concentrations multiple nucleation events occur in most of these degassing experiments and they are more pronounced in more supersaturated melts bubble growth is controlled by viscosity near glass transition temperatures and by diffusion at higher temperatures bubbles begin to pop 10 to 20 s after a foam is developed at vesicularities of 65 to 83 bubble size distributions follow power law relations at vesicularities of 1 to 65 and bubble size distributions evolve from power law relations to exponential relations at vesicularities of 65 to 83 this evolution is associated with the change from far from equilibrium degassing to near equilibrium degassing during far from equilibrium degassing multiple nucleation events are pronounced and possibly account for the generation of power law relations when the system reaches near equilibrium degassing coalescence is dominant and leads to the formation of bubbles of similar size therefore bubble size distributions are described by exponential relations

relating vesicle shapes in pyroclasts to eruption styles vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions although their numbers and sizes are used routinely to infer aspects of eruption dynamics vesicle shape remains an underutilized parameter we have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles using the dimensionless shape factor omega a measure of the degree of complexity of the bounding surface of an object for each of the seven eruptions we have also estimated the capillary number ca from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling we find that omega is smaller for eruptions with rm ca ll 1 than for eruptions with ca gg 1 consistent with previous studies we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth such as coalescence and shape relaxation of bubbles by capillary stresses among the samples analyzed strombolian and hawaiian fire fountain eruptions have rm ca ll 1 in contrast to vulcanian plinian and ultraplinian eruptions interestingly the basaltic plinian eruptions of tarawera volcano new zealand in 1886 and mt etna italy in 122 bc for which the cause of intense explosive activity has been controversial are also characterized by rm ca gg 1 and larger values of omega than strombolian and hawaiian style fire fountain eruptions we interpret this to be the consequence of syn eruptive magma crystallization resulting in high magma viscosity and reduced rates of bubble growth our model results indicate that during these basaltic plinian eruptions buildup of bubble overpressure resulted in brittle magma fragmentation

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

the generation of gas overpressure in volcanic eruptions abstract observations of natural eruption products show that different parts of a single magma batch may experience different degassing histories during ascent in a volcanic conduit in non explosive eruptions lava issuing from a volcanic vent may contain overpressured gas bubbles these important features of volcanic eruptions cannot be accounted for by existing flow models which rely on simplifying hypotheses for the relationship between pressures in the gas phase and in the bulk flow volcanic flows involve highly compressible material which undergoes large viscosity variations as degassing proceeds we show that these properties may lead to large gas overpressures in erupting lava the magnitude of this overpressure depends on the initial volatile content of magma and is largest for relatively volatile poor magmas due to the extreme viscosity variations at water contents less than 1 wt we develop a simple analytical model to illustrate the main features of compressible viscous flows 1 at any level gas pressure is larger near the conduit axis than at the walls 2 gas overpressure is an increasing function of mass discharge rate

crystalmom a new model for the evolution of crystal size distributions in magmas with the quadrature based method of moments nucleation and growth of crystals and the resulting crystal size distribution play a fundamental role in controlling the physical properties of magmas and consequently the dynamics of the eruptions in the past decades laboratory experiments demonstrated that size and shape of crystals strongly control the physical properties of magma and lava additionally natural and experimental samples are usually characterized in terms of their crystal size distribution to link it with physical processes that are not directly observable such as cooling or decompression mechanisms in this paper we present crystalmom a new predictive model based on the quadrature based method of moments developed for studying the kinetic of crystallization in volcanic systems the quadrature based method of moments well established in the field of chemical engineering represents a mesoscale modelling approach that rigorously simulates the space time evolution of a distribution of particles by considering its moments the method is applied here for the first time for studying the equilibrium disequilibrium crystallization in magma modelling the temporal evolution of the moments of a crystal size distribution the model verified against numerical and experimental data represents a valuable tool to infer the cooling and decompression rates from the crystal size distribution observed in natural samples

first record of rellimia leclercq bonamo aneurophytales from gondwana with comments on the earliest lignophytes

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

interactions between bubbles in magmas and lavas effects of bubble deformation the interactions between two deformable bubbles are studied in order to determine the effects of deformation on bubble dynamics and to determine the limits in which the effects of deformation can be ignored deformation due to hydrodynamic interactions with other bubbles leads to alignment of horizontally offset bubbles and thus an enhanced rate of coalescence bubble alignment may produce spatial inhomogeneities in bubble concentrations in magmas which implies temporal variations in the concentration and size of bubbles in erupting magmas and spatial variations in solidified lavas an approximate quantitative model for the rate of coalescence of deformable bubbles is developed based on a series of experiments the effects of deformation can be ignored if the largest bubbles have radii less than about 5 mm in silicate magmas

vesiculation and quenching during surtseyan eruptions at hunga tonga hunga ha apai volcano tonga surtseyan eruptions are shallow to emergent subaqueous explosive eruptions that owe much of their characteristic behavior to the interaction of magma with water the difference in thermal properties between water and air affects the cooling and postfragmentation vesiculation processes in magma erupted into the water column here we study the vesiculation and cooling processes during the 2009 and 2014 2015 surtseyan eruptions of hunga tonga hunga ha apai volcano by combining 2 d and 3 d vesicle scale analyses of lapilli and bombs and numerical thermal modeling most of the lapilli and bombs show gradual textural variations from rim to core the vesicle connectivity in the lapilli and bombs increases with vesicularity from fully isolated to completely connected and also increases from rim to core in transitional clasts we interpret the gradual textural variations and the connectivity vesicularity relationships as the result of postfragmentation bubble growth and coalescence interrupted at different stages by quenching in water the measured vesicle size distributions are bimodal with a population of small and large vesicles we interpret this bimodality as the result of two nucleation events one prefragmentation with the nucleation and growth of large bubbles and one postfragmentation with nucleation of small vesicles we link the thermal model with the textural variations in the clasts showing a dependence on particle size leidenfrost effect and initial melt temperature in particular the cooling profiles in the bombs are consistent with the gradual textural variations from rim to core in the clasts likely caused by variations in time available for vesiculation before quenching

shallow magma mingling driven strombolian eruptions at mt yasur volcano vanuatu 1 mt yasur volcano vanuatu has been increasingly recognized for its high frequency strombolian eruptions strombolian activity is often regarded as a product of the rapid ascent of gas slugs originating from a deep magma which mingle with a batch of shallow magma upon eruption heterogeneous crystal distribution as well as bimodal bubble size distributions in the eruptive products generally supports this view here the strombolian activity at mt yasur is analyzed a rheological investigation indicates that the basaltic andesitic eruptive products contain an apparently homogeneous glass phase and yet exhibit evidence of a distinct range of glass transition temperatures with multiple peaks occurring in individual samples such anomalous behavior is proposed to result from the mingling of magmas with contrasting oxidation states the unstable nature of the measured glass transition behavior leads us to the inference that mingling is located in the shallow parts of the eruptive conduits partly driven by rejuvenation of material slumped from the crater walls into an open conduit system the dynamics of this process may expose the periodicity of the eruptions themselves

the role of porosity on the engineering geological properties of 1669 lavas from mount etna abstract a comprehensive laboratory characterization of volcanic rocks belonging to the 1669 lava flow of mount etna recognized as one of the largest and most catastrophic event ever occurred is presented herein with the aim of investigating on their peculiar mechanical attitude the multi disciplinary approach proposed herein pointed out that although originated by the same eruptive event 1669 trachybasaltic lavas are characterized by relevant differences in porosity and rock fabric from both the qualitative and quantitative points of view this is an interesting aspect because such rocks are quarried for the production of high quality material to be employed for engineering geological purposes even in the val di noto unesco world heritage site in this perspective their detailed characterization is an essential practice even preparatory for the engineering geological use to this purpose this paper provides a focus on the laboratory investigation of these rocks which according to three different porosity classes underwent petrographic physical mechanical and seismic analyses interesting statistical correlations allowed understanding the mutual dependence of the main engineering geological parameters highlighting how porosity through both vesicles and microcracks plays a leading role from the engineering geological point of view achieved results provide a scientific casuistry on this topic and are a reliable reference for studies on similar rock types worldwide since lavas are among the most used rock materials in the engineering geological practice

rheological tests of polyurethane foam undergoing vesiculation deformation solidification as a magma analogue abstract in this study we examined the rheology of polyurethane foam puf as an analogue system of magma undergoing vesiculation deformation and solidification solid puf was formed by mixing two polymeric liquids followed by chemical reactions of foaming and curing the transient rheology during the processes was investigated using an instrument based on a stress controlled rheometer first the oscillatory rheology was measured applying small amplitude oscillations with cyclic frequency sweeps in the foaming stage the frequency dependence of the rheology depends on the dynamic capillary number in a similar way to the existing bubbly flow model in the curing stage the viscoelasticity of the liquid becomes dominant and the rheology of puf shows maxwell type viscoelasticity with relaxation time exponentially increasing with time when the reaction is rapid because of high temperature the viscoelasticity during curing is not scaled by the single relaxation time instead puf passes through a condition called gelation which is characterized by a power law relaxation spectrum next we examined the relationship between the dynamic viscosity measured by the small strain oscillation and the shear viscosity measured with a fixed shear rate at a large strain we found that puf obeys the cox merz rule which states that the two viscosities are equivalent when the shear rate and the angular frequency are equal specifically the shear viscosity tends to be smaller than the dynamic viscosity by a few tens of percent the difference was shown to be in the range expected for bubbly fluid in the foaming stage while it went beyond what can be explained by the bubbly fluid model when the viscoelasticity of the liquid becomes significant x ray computed tomography analyses revealed that samples in these cases contained large bubbles indicating bubble coalescence we infer that the difference is caused by bubble coalescence as well as non newtonian rheology during solidification including gelation all these processes might occur in actual magma during eruptions we show that the ranges of the capillary number comparing the deformation time and the bubble shape relaxation time and deborah number comparing the deformation time and the viscoelastic relaxation time in our experiments are comparable with their ranges expected for magma ascending in an eruptive conduit the characteristic time of viscosity increase of puf was also shown to be realizable for magma in natural volcanoes we conclude that puf is useful for simulating magma processes in eruptive conduits

a model for the rheology of particle bearing suspensions and partially molten rocks an edited version of this paper was published by agu copyright 2009 american geophysical union

gum rosin acetone system as an analogue to the degassing behaviour of hydrated magmas solutions of gum rosin and acetone reproduce the volatile and temperature dependent viscosity together with the phase behaviour of hydrated magmas a range of experimental exsolution conditions was investigated including the variation of supersaturation rate of decompression solution temperature and volatile content degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid the end products ranged from a mildly degassed liquid to a solid foam which preserved strained vesicles solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes

the rheology of two phase magmas a review and analysis 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 r f or f that have been most widely used to describe magmatic systems r is the relative apparent viscosity of the suspension is the volume fraction of the suspended phase 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 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

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

coupled degassing and crystallization experimental study at continuous pressure drop with application to volcanic bombs experiments on degassing of water saturated granite melts with a pressure drop from 100 and 450 mpa to 40 and 120 mpa respectively at temperatures close to feldspar liquidus 750 700 c were carried out to determine the modality of water exsolution and vesicle formation at the liquidus temperature pressure drop rates as small as approximately 100 bar day were used uniform space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction 0 5 vol of trapped air bubbles volume crystallization of feldspar was observed in degassed melts supplied with seeds bubble size distributions bsd measured in granite glasses after degassing are presented data on vesicle characteristics number radius area elongation were acquired on images digitized with standard software while the reconstruction of size distributions was performed with the schwartz saltikov unfolding procedure bubble size distributions of size classes in the range 5 1000 m were acquired with proper magnification and satisfactory statistical reliability of determined number densities the bsds of the experimental samples are compared with the results of measurements of rapidly degassed products of mt etna and vulcano island many particular features of the bubble nucleation and growth can be distinguished in an individual bsd however the general bsd of the whole data set including natural ones can be relatively well described with linear regression in bilogarithmic coordinates the slope of this regression is approximately 2 8 0 1 this dependence is in striking contrast with distributions theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles the theoretical distribution requires the occurrence of strong maxima that are not observed in our experimental and natural samples thus arguing for heterogeneous nucleation mechanisms

fragmentation of a porous viscoelastic material implications to magma fragmentation fragmentation of vesicular magma by rapid decompression is one of the most likely triggers for explosive eruptions in this phenomenon the decompression rate and the viscoelastic nature of magma are considered to be key factors in order to obtain a clear idea on the effects of these two factors controlled fragmentation experiments have been conducted these experiments have three advantages first the specimen is made of a viscoelastic material with controlled porosity and geometry second the fragmentation process is directly monitored finally both the magnitude and rate of decompression are controlled brittle fragmentation and ductile expansion were both observed in the same porous material at different timescales the various mechanical responses of the specimen elastic flow and fragmentation were correlated with the pressure profile measured at the base of the specimen fragmentation was noted to occur when the decompression exceeded a critical value within a critical time two relevant timescales are discussed in terms of physical mechanisms of relaxation the first is the measured glass transition time the second is the estimated timescale for the onset of viscous bubble expansion the observed phenomena bear several similarities with natural magma fragmentation it is thus considered that the present results are a useful step toward constructing a model for magma fragmentation

crustal thickening under the palaeo meso proterozoic delhi fold belt in northwestern india evidence from deep reflection profiling summary the results of deep reflection profiling studies carried out across the palaeo meso proterozoic delhi fold belt dfb and the archaean bhilwara gneissic complex bgc in the northwest indian platform are discussed in this paper this region is a zone of proterozoic collision the collision appears to be responsible for listric faults in the upper crust which represent the boundaries of the delhi exposures in these blocks the lower crust appears to lie nw of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks a fairly reflective lower crust northwest of the dfb exposures appears to be the downward continuation of the dfb upper crust the poorly reflective lower crust under the exposed dfb may be the westward extension of the bgc upper crust at depth thus the lower crust in this region can be divided into the fairly reflective marwar basin mb dfb crust and a poorly reflective bgc crust vertically oriented igneous intrusions may have disturbed the lamellar lower crustal structure of the bgc resulting in a dome shaped poorly reflective lower crust whose base not traceable in the reflection data may have a maximum depth of about 50 km as indicated by the gravity modelling the dfb appears to be a zone of thick 45 50 km crust where the lower crust has doubled in width this has resulted in three moho reflection bands two of which are dipping se from 12 5 to 15 0 s two way time twt and from 14 5 to 16 0 s twt another band of subhorizontal moho reflections at 12 5 s twt may have developed during the crustal perturbations related to a post delhi tectonic orogeny the signatures of the proterozoic collision in the form of strong se dipping reflections in the lower crust and moho have been preserved in the dfb indicating that the crust here has not undergone any significant ductile deformation since at least after the delhi rifting event

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

deformation patterns in a high viscosity lava flow inferred from the crystal preferred orientation and imbrication structures an example from salina aeolian islands southern tyrrhenian sea italy shape preferred orientation and imbrication structures of crystals have been measured on samples representative of the base centre and top of a highly viscous lava flow on salina aeolian islands southern tyrrhenian sea the data allow zones with different deformation patterns to be identified in the base and top of the flow deformation leads to the development of discrete preferred orientation and imbrication of the elongate crystals the sense of shear is right lateral at the base and left lateral at the top of the flow shear strain can be estimated by the analysis of crystal preferred orientation deformation increases from the flow centre to the outer more viscous boundary layers random orientation of crystals in the inner zone supports the presence of plug flow in a pseudoplastic lava the textural features of the studied lava may be related to different mechanisms i e lateral expansion we conclude that the observed crystal alignments and imbrication structures may be related to a plug flow moving between two non deforming walls the walls are represented by the solidified broken upper and basal crust of the flow the low shear strain values calculated in the outer margins of the flow are indicative of the last deformation event crystal preferred orientation and imbrication structures may be related to the occurrence of velocity gradients existing between the inner zone of the flow and its solidus or near solidus outer margins

coupling of viscous and diffusive controls on bubble growth during explosive volcanic eruptions abstract the coupling of viscosity and diffusivity during explosive volcanic degassing is investigated using a numerical model of bubble growth in rhyolitic melts the model allows melt viscosity and water diffusivity to vary spatially and temporally with water content we find that the system is highly sensitive to the distribution of volatiles around the bubble primarily as a consequence of the great sensitivity of melt viscosity to water content at low water concentrations the dehydrated portion of the magma near the bubble exerts the major control on the effective viscosity of the melt however the values of effective viscosity are lower than previously reported values calculated using an approximated concentration profile degassing is found to be highly sensitive to the choice of the solubility law which controls the volatile concentration near the bubble but insensitive to the equation of state the form of the concentration profile in the case of concentration dependent diffusivity is such that the effective viscosity is substantially lower by as much as an order of magnitude than for constant diffusivities of similar magnitude this leads to unexpectedly high bubble growth rates particularly for cases of highly non equilibrium degassing this is because low values of diffusivity are more than compensated by low melt viscosity due to higher volatile concentrations this complex interplay between viscosity and diffusivity means that models of bubble growth must take into account the concentration dependent nature of both these parameters approximating either of these with a constant value will lead to significant errors in estimations of bubble growth rates

an analytical model for gas overpressure in slug driven explosions insights into strombolian volcanic eruptions strombolian eruptions common at basaltic volcanoes are mildly explosive events that are driven by a large bubble of magmatic gas a slug rising up the conduit and bursting at the surface gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals we present a theoretical investigation of slug overpressure based on magma static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts slug length at burst and the depth at which the burst process begins we find that burst overpressure is controlled by two dimensionless parameters v which represents the amount of gas in the slug and a which represents the thickness of the film of magma that falls around the rising slug burst overpressure increases nonlinearly as v and a increase we consider two eruptive scenarios 1 the standard model in which magma remains confined to the vent during slug expansion and 2 the overflow model in which slug expansion is associated with lava effusion as occasionally observed in the field we find that slug overpressure is higher for the overflow model by a factor of 1 2 2 4 applying our model to typical strombolian eruptions at stromboli we find that the transition from passive degassing to explosive bursting occurs for slugs with volume 24 230 m 3 depending on magma viscosity and conduit diameter and that at burst a typical strombolian slug with a volume of 100 1000 m 3 has an internal gas pressure of 1 5 bars and a length of 13 120 m we compare model predictions with field data from stromboli for low energy puffers mildly explosive strombolian eruptions and the violently explosive 5 april 2003 paroxysm we find that model predictions are consistent with field observations across this broad spectrum of eruptive styles suggesting a common slug driven mechanism we propose that paroxysms are driven by unusually large slugs large v

orientation distribution of microlites in obsidian abstract the shape and three dimensional orientation distribution of microlites are measured in obsidian from little glass mountain ca measurements are made from thin sections using an image series of high magnification digital micrographs taken serially through different focal depths these measurements agree well with the theoretically predicted and experimentally measured distribution of long slender rods in a newtonian fluid undergoing simple shear flow in this type of flow rods in a dilute suspension rotate periodically spending most of the time aligned with the flow measurements of the detailed orientation distribution integrated with theoretical models provide a tool for inferring flow dynamics and the timing of magmatic processes

analytical models for bubble growth during decompression of high viscosity magmas analytical models for decompressional bubble growth in a viscous magma are developed to establish the influence of high magma viscosity on vesiculation and to assess the time scales on which bubbles respond to decompression instantaneous decompression of individual bubbles analogous to a sudden release of pressure e g sector collapse is considered for two end member cases the infinite melt model considers the growth of an isolated bubble before significant bubble interaction occurs the shell model considers the growth of a bubble surrounded by a thin shell and is analogous to bubble growth in a highly vesicular magmatic foam results from the shell model show that magmas less viscous than 109 pa s can freely expand without developing strong overpressures the timescales for pressure re equilibration are shortened by increased ratios of bubble radius to shell thickness and by larger decompression time scales for isolated bubbles in rhyolitic melts infinite melt model are significantly longer implying that such bubbles could experience internal pressures greater than the ambient pressure for at least a few hours following a sudden release of pressure the shell model is developed to assess bubble growth during the linear decompression of a magma body of constant viscosity for the range of decompression rates and viscosities associated with actual volcanic eruptions bubble growth continues at approximately the equilibrium rate with no attendant excess of internal pressure the results imply that viscosity does not have any significant role in preventing the explosive expansion of high viscosity foams however for viscosities of 109 pa s there is the potential for a viscosity quench under the extreme decompression rates of an explosive eruption it is proposed that the typical vesicularities of pumice of 0 7 0 8 are a consequence of the viscosity of the degassing magmas becoming sufficiently high to inhibit bubble expansion over the characteristic time scale of eruption for fully degassed silicic lavas with viscosities in the range 1010 to 1012 pa s time scales for decompression of isolated bubbles can be hours to many months

on the slow decompressive response of volatile and crystal bearing magmas an analogue experimental investigation abstract the degassing kinetics of ascending magma strongly affect eruption dynamics the kinetics are in turn influenced by magma properties the investigation of the relationship between magma properties and eruption dynamics is a key element in revealing the processes characterizing magmatic flows within the shallow conduit to explore the effects of physical properties on degassing in basaltic eruptive systems we have designed and carried out experiments on the slow decompressive response of analogue magmas composed of silicone oil based suspensions using a shock tube apparatus four series of experiments were performed 1 particle free silicone oils with viscosity ranging from 1 to 1000 pa s were used to constrain the liquid response 2 silicone oils with variable proportion of suspended micrometric spherical particles were employed to assess the effect of different crystal fractions 3 suspensions of elongated particles in silicone oils were used to investigate the role of crystal shape 4 the effects of saturation time and pressure were examined the rheology of both spherical and elongated particle bearing suspensions were characterized by concentric cylinder rotational rheometry the flow dynamics of the bubbly fluid from the process of bubble nucleation up to the development of a permeable bubble network were constrained using image analysis different fluid regimes were distinguished i nucleation ii foam build up and iii foam oscillation by comparing results obtained from the different series of experiments we were able to assess the primary role played by the presence of particles on the evolution of the gas volume fraction within the samples particle fraction has a dominant role at high concentration affecting the motion of the fluid finally particle shape influences the long term degassing efficiency of the fluid using scaling considerations such observations are applied to mafic to intermediate systems the results of our experimental investigation contribute to constraining vesiculation processes in magmas of various crystallinities at shallow depths

experimental constraints on permeable gas transport in crystalline silicic magmas the gas and fluid transport in magmas via permeable flow through interconnected bubble networks controls the rate of outgassing from magmas ascending in volcanic conduits and the fluid transport in the mushy boundary layer of magma reservoirs hence clarifying its mechanism and rate is crucial to understanding the explosivity of volcanic eruptions and the evolution and dynamics of a magma reservoir recent experimental studies have determined the gas permeabilities in crystal free rhyolite and basalt however no experimental study has investigated the effect of the crystal contents on the permeable gas transport in magmas in this study we performed decompression experiments for hydrous rhyolitic melts having crystallinities of 30 and 50 vol to examine the effect of crystals on the bubble microstructure and gas permeability during magma vesiculation size controlled 100 meshed corundum crystals were used as an analog of the phenocrysts in silicic magmas microstructural analyses using x ray ct showed that bubbles coalesce and their connectivity increases with a decrease in the final pressure after the decompression that is an increase in the vesicularity as long as the vesicularities of melt part in the crystal free basis melt vesicularity were similar no clear effect of the crystallinity on the degree of bubble coalescence and connectivity was observed at melt vesicularities 68 vol this result indicates that the permeable fluid transport through a deep volcanic conduit which has been proposed on the basis of the observations of volcanic gases and natural products is so slow that other processes like shear deformation or magma convection may be needed to explain the observations

formation of k feldspar megacrysts in granodioritic plutons by thermal cycling and late stage textural coarsening k feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases grown to large sizes when the magma was mostly liquid owing to their large size euhedral form and features that suggest deposition by magmatic sedimentation however phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that k feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50 crystallization and is thus largely incapable of flow and sedimentation megacryst size distributions compositions and textural relationships from the cretaceous tuolumne intrusive suite california reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones whereas linear growth rates during cooling are similar thus during thermal cycling material is transferred from small crystals to larger ones megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of k feldspar explains the presence of megacrysts in the inner parts of thetuolumne intrusive suite and elsewhere and may be a common process in formation of megacrystic granitic rocks

rheology of magmas with bimodal crystal size and shape distributions insights from analog experiments 1 magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts as often indicated by volcanic rock textures in this study we present two different experiments that investigate the flow behavior of these bidisperse systems in the first experiments rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller prolate particles microlites and larger equant particles phenocrysts in a bubble free newtonian liquid silicate melt our data show that increasing the relative proportion of prolate microlites to equant phenocrysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude consequently the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles we propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models in a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup the resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well organized strings with sphere depleted shear bands in high shear regions these observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio the implications for magma flow are discussed and rheological results and textural observations are compared with observations on natural samples

structural analysis of flow related textures in lavas abstract the textures of coherent volcanic rocks including lavas and volcanic intrusives commonly contain features that are attributed to flowage previous applications of structural analysis to volcanic rocks are expanded here to provide a framework for analysis textures defined as the crystallinity granularity and shapes and arrangements of the components crystals glass and voids of a rock together with structures defined as individual features composed of the disposition attitude arrangement or relative positions of the components of a rock are first described second the spatial fabrics shapes and arrangements of the components of a rock and the orientation of textures and structures are identified third textures structures and fabrics are placed in the spatial and temporal geological context finally detailed interpretations of the kinematics and rheology of structures and fabrics is made leading to an integrated flow history of the rock illustrative case studies include rhyolite from the basal part of the tertiary minyon falls dome northeastern new south wales australia which has a texture comprising planar domains of differing crystal abundance flow bands multiple folds of these domains relatively homogeneous crystal alignment parallel to the fold axes and microfolding of these domains in the zone of interaction between phenocrysts and matrix including retrorotation of phenocrysts on short limbs of inequant folds trachyte dykes on fraser island queensland australia have a texture comprising crystal alignment planar concentration domains banding two planar domains of crystal alignment interpreted to be conjugate shear zones phenocrysts influence the spacing and distribution of the domains and interacted with shear zones by undergoing small amounts of rotation the shear zones overprinted a homogeneous crystal alignment during the last stage of flow before solidification as a result of dilatant granular interactions lava from miocene submarine banded silicic mafic domes on katsurajima on the coast of the shimane peninsula southwestern japan has a texture comprising crystal alignment irregular domains interpreted as the result of quenching local internal deformation of crystals including brittle boudinage and elongate vesicles and amygdales dacite from the late stages of the 1991 1995 eruption of mount unzen in western kyushu japan has a texture comprising crystal alignment deformation of phenocrysts and their surrounding matrix producing voids by mechanical means with the possible involvement of vapour phases the pervasive mechanical failure of the flowing magma may have triggered frontal collapse and the ensuing pyroclastic eruptions a spectrum of increasingly intense flow related textures can be recognised from simple planar alignments of crystals and vesicles to overprinting effects and domainal textures such as banding and shear zones and deformed domains pervasive ductile tearing and ultimately brittle fracture record flow during the transition from ductile magma to brittle rock the development of these flow related textures is controlled by the interplay of driving forces magma pressure gravity and resisting forces viscosity where driving forces are strong flow related textures are well developed when viscous resistance exceeds the driving forces flow and the development of flow related textures ceases