gas evolution in eruptive conduits combining insights from high temperature and pressure decompression experiments with steady state flow modeling abstract in this paper we examine the consequences of bubble nucleation mechanism on eruptive degassing of rhyolite magma we use the results of published high temperature and pressure decompression experiments as input to a modified version of conflow the numerical model of mastin and ghiorso 2000 u s g s open file rep 00 209 53 pp and mastin 2002 geochem geophys geosyst 3 10 1029 2001gc000192 for steady two phase flow in vertical conduits synthesis of the available experimental data shows that heterogeneous nucleation is triggered at p 120 150 mpa and leads to disequilibrium degassing at extreme h2o supersaturation in this latter case nucleation is an ongoing process controlled by changing supersaturation conditions exponential bubble size distributions are often produced with number densities of 106 109 bubbles cm3 our numerical analysis adopts an end member approach that specifically compares equilibrium degassing with delayed disequilibrium degassing characteristic of homogeneously nucleating systems the disequilibrium simulations show that delaying nucleation until p 150 mpa restricts degassing to within 1500 m of the surface fragmentation occurs at similar porosity in both the disequilibrium and equilibrium modes 80 vol but at the distinct depths of 500 m and 2300 m respectively the vesiculation delay leads to higher pressures at equivalent depths in the conduit and the mass flux and exit pressure are each higher by a factor of 2 0 residual water contents in the melt reaching the vent are between 0 5 and 1 0 wt roughly twice that of the equilibrium model

shallow conduit processes during the ad 1158 explosive eruption of hekla volcano iceland hekla is one of the most frequently active felsic volcanic systems in the world with several known pre historic large plinian eruptions and 18 historical subplinian to small plinian eruptions a common view is that plinian eruptions of hekla are relatively short lived and purely explosive events in detail these events exhibit subtle differences in terms of deposit characteristics reflecting significant differences in eruption behaviour of the 18 historical eruptions two had bulk magma compositions with 66 wt sio2 a plinian eruption in ad 1104 and a smaller less well characterised but atypical subplinian eruption in ad 1158 the ad 1158 eruption was a relatively sustained dry magmatic eruption with a more powerful opening phase followed by a lower intensity waning phase accompanied by minor destabilisation and collapse of the conduit walls we examine here the dynamics of the ad 1158 eruption focussing on the role of shallow conduit processes in modulating eruption dynamics vesicularity data constrain the relative influence of bubble nucleation growth and coalescence the juvenile pyroclasts are composed of two types of microvesicular pumice white and grey with contrasting vesicle number density vesicle size distribution and phenocryst and microlite contents textural analysis shows that these pumices reflect heterogeneity developed pre to syn eruptively in the conduit and that entrainment of longer resident magma by faster ascending magma permitted magma of contrasting maturity to be fragmented simultaneously in this regard the mixed melt of the ad 1158 eruption contrasts with the compositionally homogeneous melt phase of the more powerful ad 1104 plinian event which was typified by more uniform conduit and eruption dynamics accompanying higher average ascent rates

tracking bubble evolution inside a silicic dike abstract pressure estimates from rapidly erupted crustal xenoliths constrain the depth of intrusion of the silicic lavas hosting them this represents an opportunity for tracking magmatic bubble s evolution and quantifying the variation in bubble volume during rapid magma ascent through a volcanic dike just prior to eruption the petrology stable isotope geochemistry and x ray micro tomography of dacites containing crustal xenoliths erupted from a neogene volcano in se spain showed an increase in porosity from 1 7 to 6 4 from 19 to 13 km depth at nearly constant groundmass and crystal volumes this result provides additional constraints for experimental and numerical simulations of subvolcanic magma crust degassing processes in silicic systems and may allow the characterization of volcanic eruptive styles based on volatile content

a scaling growth model for bubbles in basaltic lava flows abstract pahoehoe an and massive lavas from mount etna show common statistical properties from one sample to another which are independent of scale size over certain ranges the gas vesicle distribution shows two scale invariant regimes with number density n v v b 1 where v is the volume and empirically b 0 for small bubbles and b 1 for medium to large bubbles we introduce a bubble growth model which explains the b 1 range by a strongly non linear cascading growth regime dominated by a quasi steady state coalescence process the small bubble region is dominated by diffusion its role is to supply small bubbles to the coalescence regime the presence of measured dissolved gas in the matrix glass is consistent with the notion that bubbles generally grow in quasi steady state conditions the basic model assumptions are quite robust with respect to the action of a wide variety of processes since we only require that the dynamics are scaled over the relevant range of scales and that during the coalescence process bubble volumes are approximately conserved the model also predicts a decaying coalescence regime with b 1 associated with a depletion of the gas source or alternatively a loss of large vesicles through the surface of the flow our model thus explains the empirical evidence pointing to the coexistence of two different growth mechanisms in subsurface lava flows but acting over distinct ranges of scale with non linear coalescence as the primary growth process the total vesicularity of each sample can then be well estimated from the partial vesicularity of each growth regime without any outlier problems

alkaline lake systems with stromatolitic shorelines in the ediacaran volcanosedimentary ouarzazate supergroup anti atlas morocco abstract in the zenaga and saghro inliers of the anti atlas morocco the ediacaran ouarzazate volcanosedimentary supergroup preserves a record of the complex interplay between effusion of lava floods and explosive volcanic activity from numerous separate volcanic centres volcanic units consist of basaltic and andesitic lava flows and subvolcanic sills and dikes and dacitic rhyolitic ignimbrites and tuffs that represent a post collisional high k calc alkaline magmatism volcanic events are separated by erosive and low angle unconformities related to regional tilting which mark significant hiatii in the timing of deposition and are locally marked by carbonate production in palaeoweathered lowlands infill of scattered decametre scale carbonate bearing troughs less than 1 km across started with the dominantly local derivation of weathered and eroded lava debris followed by the migration of scattered subaqueous bottomset and foreset structures and final nucleation of carbonate productivity microbial mats developed along the shorelines of these ponds when bathymetry increased the stromatolites developed domal to columnar morphologies that coalesced laterally to form metre thick biostromes the lack of evidence for significant fluvial and deltaic deposits suggests that the streams that entered the ponds were intermittent and thus not a significant source of water it appears that groundwater rather than surface water was the dominant source of water to sustain the ephemeral ponds in the low lying weathered palaeodepressions preserved in the ouarzazate supergroup fluids passing through the surrounding volcanic landscape and the volcanic ashes interbedded within the lacustrine infill supplied silica enriched fluids giving the ponds the characteristics of alkaline lake systems their lacustrine affinity is supported by the subaerial character of many of the volcanic products that surround them ree composition of partly silicified dolomitized and undolomitized stromatolites is entirely consistent with freshwater lacustrine or shallow lagoonal with a strong freshwater input depositional settings but they also display a strong contamination from the rhyolitic tuffs that overly them

mechanisms of bubble coalescence in silicic magmas bubble coalescence is an important process that strongly affects magmatic degassing without coalescence bubbles remain isolated from one another in the melt severely limiting gas release despite this fact very little has been done to identify coalescence mechanisms from textures of magmatic rocks or to quantify the dynamics of bubble coalescence in melts in this paper we present a systematic study of bubble coalescence mechanisms and dynamics in natural and experimentally produced bubbly rhyolite magma we have used a combination of natural observations aided by high resolution x ray computed tomography petrological experiments and physical models to identify different types of bubble bubble interaction that lead to coalescence on the timescales of magma ascent and eruption our observations and calculations suggest that bubbles most efficiently coalesce when inter bubble melt walls thin by stretching rather than by melt drainage from between converging bubble walls orders of magnitude are more rapid than melt drainage bubble wall stretching produces walls thin enough that inter bubble pressure gradients may cause the melt wall to dimple further enhancing coalescence to put these results into volcanogical context we have identified magma ascent conditions where each coalescence mechanism should act and discuss the physical conditions for preserving coalescence structures in natural pumice the timescales we propose could improve volcanic eruption models which currently do not account for bubble coalescence although we do not address the effect of shear strain on bubble coalescence the processes discussed here may operate in several different eruption regimes including vesiculation of lava domes post fragmentation frothing of vulcanian bombs and bubbling of pyroclasts in conduits

the campanian ignimbrite southern italy geochemical zoning insight on the generation of a super eruption from catastrophic differentiation and fast withdrawal more than ca 100 km3 of nearly homogeneous crystal poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the campanian ignimbrite super eruption the most powerful in the neapolitan area partition coefficient calculations equilibrium mineral assemblages glass compositions and texture were used to reconstruct compositional thermal and pressure gradients in the pre eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption our petrologic data indicate that a wide sill like trachytic magma chamber was active under the campanian plain at 2 5 kbar before ci eruption thermal exchange between high liquidus 1199 c trachytic sill and cool country rocks caused intense undercooling driving a catastrophic and fast 102 years in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption this process culminated in an abrupt reservoir opening and in a fast single step high decompression sanidine phenocrysts crystal size distributions reveal high differentiation rate thus suggesting that such a sill like magmatic system is capable of evolving in a very short time and erupting suddenly with only short term warning

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

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

vesiculation in magmas from stromboli and implications for normal strombolian activity and paroxysmal explosions in basaltic systems 1 we performed a series of x ray tomographic experiments and lattice boltzmann permeability simulations on pyroclastic products from explosive activity at stromboli between december 2004 and may 2006 we reconstructed the 3 d textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behavior of basaltic volcanoes scoriae from normal strombolian explosions display remarkably consistent vesicle volume distributions fit by power laws with an exponent of 1 0 2 we ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady state shallow magma system that supplies normal strombolian activity volume distributions and textures of vesicles in pumice clasts from the 5 april 2003 and 15 march 2007 paroxysmal activity are markedly different from those in the scoriae besides a power law function with a higher exponent portions of these distributions can be also fit by an exponential function suggesting the attempt of the system to reach near equilibrium conditions the investigated pumice clasts also lack the large connecting vesicles responsible for the development of degassing pathways in the stromboli magma that erupts the scoriae this testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure buildup at depth by comparison with degassing experiments on basaltic melts we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at stromboli

abrupt shifts between wet and dry phases of the 1875 eruption of askja volcano microscopic evidence for macroscopic dynamics abstract the eruption of askja in 1875 was a powerful eruption with intervals of sustained activity yet also with abrupt shifts in eruption style shifts from dry to wet fall from buoyant to collapsing column conditions and sharp fluctuations of eruption intensity occurred throughout the duration of the eruption deep ascent processes including decompression bubble nucleation and early growth of bubbles were similar among the four main phases regardless of style or column conditions uniformly high vesicle number densities 8 0 108 to 2 109 cm 3 suggest that magma degassing throughout the eruption was a disequilibrium process with very high nucleation rates power law distributions of the subpopulations of small bubbles on cumulative number density plots suggest continuous nucleation of bubbles shallow processes of bubble growth and magma ascent were at times decoupled from deep ascent processes at the onset of each phase the erupted magma appears to have a mature vesicle signature suggesting extended residence times in the shallow conduit irrespective of decompression or magma ascent rate during each phase the textural and density data suggest more uniform conditions of ascent and shallow degassing two pauses in the eruption were accompanied by shifts in vent position and we suggest that magma during this eruption was intruded in an elongated dike like geometry and vent migrations between phases led to the eruption of magma that had slightly greater residence time and hence more mature textures the major yet reversible shifts between dry or magmatic and wet or phreatomagmatic eruption styles were driven by changing external conditions accompanying the shifts in vent position the migration of the vent into and out of water sources rather than by changes of mass flux the shift between buoyant phreatoplinian wet plume conditions to the production of wet density currents is inferred to be a function of vent widening during the high intensity phreatoplinian phase thereby decreasing the exit velocity of the wet mixture in terms of the role of external water in the phreatoplinian phase our qualitative textural observations and quantitative data suggest that the magma was a foam prior to fragmentation however presently we cannot resolve the exact role and timing of the interaction with the external water

circumpolar mapping of antarctic coastal polynyas and landfast sea ice relationship and variability abstractsinking of dense water from antarctic coastal polynyas produces antarctic bottom water aabw which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide very recent studies have suggested that landfast sea ice fast ice plays an important role in the formation and variability of the polynyas and possibly aabw however they have been limited to regional and case investigations only this study provides the first coincident circumpolar mapping of antarctic coastal polynyas and fast ice the map reveals that most of the polynyas are formed on the western side of fast ice indicating an important role of fast ice in the polynya formation winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation the blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor these effects on the variability in

inferring volcanic degassing processes from vesicle size distributions both power law and exponential vesicle size distributions vsds have been observed in many different types of volcanic rocks we present results of computer simulations and laboratory analogue experiments which reproduce these findings and show that the distributions can be interpreted as the product of continuous bubble nucleation resulting from non equilibrium degassing this ongoing nucleation causes the bubbles to evolve through an exponential size distribution into a power law size distribution as nucleation and growth progress these findings may explain the apparent contradiction between present models of bubble growth in magmas which predict that degassing in explosive eruptions is a non equilibrium process and models of conduit flow which assume perfect equilibrium degassing the process of continuous nucleation is the mechanism whereby the volcanic system maintains near equilibrium in the case of rapid depressurization and slow volatile diffusion

statistical analysis of bubble and crystal size distributions application to colorado plateau basalts abstract a new analytical technique for the statistical analysis of bubble populations in volcanic rocks proussevitch a a sahagian d l and tsentalovich e p 2007 this issue statistical analysis of bubble and crystal size distributions formulations and procedures j volc geotherm res has been applied to a collection of colorado plateau basalts 96 samples a variety of mono and polymodal distributions has been found in the samples all of which belong to the logarithmic family of statistical functions most samples have bimodal log normal distributions while the others are represented by mono or bimodal log logistic and weibull distributions we have grouped the observed distributions into 11 groups depending on distribution types mode location and intensity the nature of the curves within these groups can be interpreted as evolution of vesiculation processes we conclude that within bimodal log normal distributions the mode of smaller bubbles is the result of a second nucleation and growth event in a lava flow after eruption in the case of log logistic distributions the larger mode results from coalescence of bubbles coalescence processes are reflected in growth of a larger mode and decreasing bubble number density another style of population evolution leads to a monomodal weibull or exponential distribution as a result of superposition of multiple log normal distributions in which the modes are comparable in size and intensity these various population distribution styles can be interpreted with an understanding of vesiculation processes that can be gained through appropriate numerical models of coalescence and population evolution the applicable vesiculation processes include a a single nucleation growth event b continuous multiple nucleation growth events c coalescence and d ostwald ripening

in situ degassing study on crystal bearing stromboli basaltic magmas implications for stromboli explosions 1 three dimensional crystal and bubble sizes and distributions in experimentally produced vesicular crystal bearing stromboli basalts and natural scoria were studied with x ray microtomography ct at high 1 85 m and low 5 46 9 0 m resolution the permeabilities from lattice boltzmann lb simulations and experimental measurements are about 1 2 orders of magnitude higher than in aphyric stromboli basalts at porosity 31 6 55 3 we propose that the higher permeability in crystal bearing samples results in highly efficient degassing in shallow highly porphyritc hp magma as opposed to the deeper aphyric lp magma in paroxysmal explosions the lp magma flows up in a cylindrical conduit due to the density and viscosity difference between the two magmas this type of convection can cause the lp magma with exsolved gas to be efficiently transferred through the overlying hp magma potentially resulting in the more violent paroxysmal explosions

modeling the failure of magmatic foams with application to stromboli volcano italy abstract the failure of magmatic foams has been implicated as a fundamental process in eruptions occurring at open conduit basaltic volcanoes in order to investigate the failure of magmatic foams we applied the fiber bundle model using global load sharing the strengths of the fibers for the model were taken from bubble wall widths measured in four computer simulated foams of low porosity and from one very low porosity and two high porosity foams produced in the laboratory by heating hydrated basaltic glasses to 1200 c the relative strength of an individual fiber in the model was calculated from the square of a bubble wall s average width and absolute strengths of the foams were calculated based upon the correlation of the strength of one modeled foam with experimental data the fiber bundle model is shown to successfully reproduce measured tensile strengths of porous volcanic rocks studied by other researchers and confirms previous findings of the primary importance of foam porosity as well as the secondary importance of structural details that affect the number and size of bubble walls and permeability because of the success of the fiber bundle model in reproducing experimental foam failure its results are compared to infrasonic measurements associated with bubbles at stromboli italy and demonstrate that within uncertainty the power law exponents of the infrasonic energies and of the fiber bundle model energies are in agreement both show a crossover from an exponent of 5 2 associated with the bursting of small bubbles in the infrasonic measurements to an exponent of 3 2 for normal strombolian eruptions associated with infrasonic signals from meter scale bubbles the infrasonic signals for major explosions and a paroxysmal eruption at stromboli fall near the extrapolation of the power law defined by the low amplitude bubble bursting events and are interpreted to reflect the bursting of multitudes of small bubbles rather than a few large bubbles the measurement of small amplitude infrasonic events at stromboli appears useful in predicting the recurrence interval of paroxysmal eruptions at this volcano and may also provide a tool that uses common small amplitude infrasonic events to constrain the frequency of larger eruptions at other volcanoes

vesiculation of basaltic magma during eruption vesicle size distributions in vent lavas from the pu9u9o9o kupaianaha eruption of kilauea volcano are used to estimate nucleation and growth rates of h 2 o rich gas bubbles in basaltic magma nearing the earth9s surface 120 m depth by using well constrained estimates for the depth of volatile exsolution and magma ascent rate nucleation rates of 35 9 events cm 3 s 1 and growth rates of 3 2 x 10 4 cm s are determined directly from size distribution data the results are consistent with diffusion controlled growth as predicted by a parabolic growth law this empirical approach is not subject to the limitations inherent in classical nucleation and growth theory and provides the first direct measurement of vesiculation kinetics in natural settings in addition perturbations in the measured size distributions are used to examine bubble escape accumulation and coalescence prior to the eruption of magma

homogeneous bubble nucleation in rhyolitic melt experiments and nonclassical theory 1 the transfer of volatiles from the earth s interior to the atmosphere occurs through degassing of magma the dynamics of which assert a significant control on volcanic eruptions the first and most critical step in degassing is the nucleation of gas bubbles which requires that a sufficient number of volatile molecules cluster together to overcome the free energy associated with the formation of a new interface between nucleus and surrounding melt this free energy is a function of surface tension typically assumed to equate to the macroscopically measurable value surface tension estimates inferred from bubble nucleation experiments in silicate melts are however lower than direct macroscopic measurements making it difficult to accurately predict magma ascent and decompression rates from measured bubble number densities in pyroclasts we provide a potential resolution to this problem through an integrated study of bubble nucleation experiments and modeling thereof based on nonclassical nucleation theory we find that surface tension between critical bubble nuclei and the surrounding melt depends on the degree of supersaturation and is lower than the macroscopically measured value this is consistent with the view that far from equilibrium the interface between a nucleus and surrounding metastable bulk phase is diffuse instead of sharp as a consequence the increase in nucleation rate with supersaturation is significantly larger at high supersaturations than predicted by classical nucleation theory

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

magma degassing during the plinian eruption of novarupta alaska 1912 1 we have modeled the nucleation and isothermal growth of bubbles in dacite from the 1912 plinian eruption of novarupta alaska bubble growth calculations account for the exsolution of h2o and co2 beginning with bubble nucleation and ending when bubble sizes reproduced the observed size distribution of vesicles in novarupta pumice clasts assuming classical nucleation theory bubbles nucleated with a diameter of the order of 10 8 m and grew to sizes ranging from 10 6 m to greater than 10 3 m the typical range of vesicle sizes found in novarupta pumice the smallest vesicles in novarupta pumices are also the most abundant and bubbles with radii of 10 6 m to 10 5 m comprise almost 90 of the entire bubble population we find that these bubbles must have nucleated and grown to their final size within a few 100 milliseconds despite these extremely fast growth rates the pressures of exsolved volatiles contained within the bubbles remained high up to about 107pa in excess of ambient pressure assuming a closed system the potential energy of these compressed volatiles was sufficient to cause magma fragmentation even though only a fraction of the pre eruptive volatiles had exsolved unless the matrix glasses of novarupta pyroclasts retains a large fraction of pre eruptive volatiles the majority of magmatic volatiles 80 90 was likely lost by open system degassing between magma fragmentation and quenching

measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes abstract this paper outlines methods for determining a bubble size distribution bsd and the moments of the bsd function in vesiculated clasts produced by volcanic eruptions it reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes the analysis is based on a quantitative morphological stereological method for 2 dimensional imaging of cross sections of samples one method determines with some assumptions the complete shape of the bsd function from the chord lengths cut by bubbles the other determines the 1st 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area the surface area per unit volume and the volume fraction of bubbles comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former though the results coincide in absolute and relative magnitudes results of the analysis for vesiculated rocks from eleven subplinian to plinian eruptions show some interesting systematic correlations both between moments of the bsd and between a moment and the eruption column height or the sio2 content of magma these correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas this suggests that bubble coalescence does not predominate in sub plinian to plinian explosive eruptions the moment moment correlations put constraints on the style of the nucleation and growth process of bubbles the scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization which leads to an intermediate growth law between the diffusional growth r m t 2 3 at a constant depressurization rate and the ostwald ripening r m t 1 3 under a constant pressure where rm and t are the mean radius of bubble and the effective time of diffusion respectively occurred in the eruptions it is emphasized that the bsd in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

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

submarine silicic volcanism of the healy caldera southern kermadec arc sw pacific i volcanology and eruption mechanisms the submarine healy volcano southern kermadec arc with a 2 2 5 km wide caldera is pervasively mantled with highly vesicular silicic pumice within a water depth of 1 150 1 800 m pumices comprise type 1 white light grey pumice with 30 mm vesicles and weak moderate foliation type 2 grey pumice with millimetre scale laminae flow banded foliation including stretched vesicles 55 mm in length and a minor finely vesicular type 3 pumice all types are sparsely porphyritic with undevitrified glassy groundmass 68 70 sio2 which is microlite and lithic free coexisting pyroxenes yield magma temperatures of 950 c pumice density is 0 5 g cm 3 and vesicularity is 78 83 vesicle size distributions for types 1 and 2 pumice range from 20 m to 20 mm with a strong power law relation with d 2 5 0 4 for vesicles 9 mpa will limit vesicularity to less than the observed 78 83 whereas pressure 6 mpa require a more shallower reconstruction of the edifice and larger volume syn eruptive collapse uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts most pyroclasts cool condensing and ingesting water into steam inflated vesicles and then sink progression into pyroclastic mode would expand the eruption column displace ambient water reduce the hydrostatic load and further promote vesiculation and fragmentation pyroclasts within the column would quench at these reduced pressures we argue that healy eruptions deeper than 1 000 m cannot be pyroclastic volumes for the lower and upper bounds of edifice size are 2 36 and 3 58 km3 respectively but do not account for intra caldera pumice fill these volumes are considered to be predominantly primary eruption output as shown by a dearth of accessory lithics in all pumice yielding at an average 81 vesicularity eruptive pumice volumes of between 10 and 15 km3 some pyroclasts may have risen to the sea surface and be a correlative of the sea rafted loisels pumice the latter occurs in some new zealand holocene beach sequences and has a estimated age of 590 80 calendar years

bubble growth in slightly supersaturated albite melt at constant pressure abstract bubble growth experiments were performed in a piston cylinder by hydrating albite melt with 11 wt h2o at 550 mpa followed by rapid decompression at 1 mpa s 1 to pressures of 450 or 400 mpa at these conditions the melt was supersaturated with 0 5 or 1 5 wt h2o respectively which caused rapid exsolution and bubble growth results at 1200 c demonstrate that portions of the initial cumulative bubble area distributions may be characterized by a power law with an exponent near 1 but they rapidly evolve to exponential distributions and approach a unimodal distribution after 32 h of growth this evolution occurs by the growth of larger bubbles at the expense of smaller ones the growth rate of the average bubble radius in these experiments is described by a power law whose exponent is 0 35 close to the theoretical exponent of 1 3 for phase growth in which coalescence is dominated by ostwald ripening of the bubbles over the range of pressures and water contents investigated at 1200 c the bubble size distributions and growth rate are not significantly affected by changes in the amount of exsolved water or by splitting the decompression path into two steps similar decompression experiments at 800 c are dominated by smaller bubbles than in the 1200 c experiments and also demonstrate exponential cumulative size distributions but consistently contain a small fraction of larger bubbles the growth rate of these bubble radii cannot be fit with a power law but a logarithmic dependence of the bubble radii on time is possible suggesting a difference in the growth mechanisms at low and high temperatures this difference is attributed to the orders of magnitude changes in melt viscosity and water diffusion in the melt as the temperature varies from 800 to 1200 c at 1200 c the transport properties of albite melt resemble those of natural basaltic melts whereas at 800 c the properties are similar to those of andesitic to dacitic melts the decompression rate used in this study exceeds natural rates by one to two orders of magnitude thus these results indicate that natural mafic to intermediate magmas supersaturated with only a small excess of water should easily nucleate bubbles during ascent and that bubble growth in mafic magmas will proceed much more rapidly than in andesitic to dacitic magmas intermediate composition magmas also may be capable of forming bimodal bubble size distributions even in the case when only one nucleation event occurred the rapid evolution of the bubble size distribution from a power law to an exponential may be useful in constraining the time duration between bubble nucleation and the quenching of natural samples

linking variable explosion style and magma textures during 2002 at stromboli volcano italy microtextural characteristics of fresh ejecta from stromboli volcano were examined from three periods of differing eruption style and intensity in 2002 activity shifted from relatively weak and infrequent ash charged explosions during january through may into two broad cycles of waxing activity in june through late september and late september through december followed by the onset on 28 december of the 2002 2003 effusive eruption analyzed sets of lapilli from may september october and 28 december show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity three distinct textures are observed among the analyzed pyroclasts low density ld with an abundance of subspherical bubbles the presence of large irregularly shaped bubbles and a light to transparent glass matrix transitional texture tt with an intermediate number of subspherical bubbles a high frequency of large irregularly shaped bubbles and a honey colored glass matrix and high density hd with sparse relatively small bubbles conspicuous large irregular bubbles and a dark glass matrix observational and quantitative data density vesicle size indicate that these textures are linked through variable residence time in stromboli s shallow conduit with an ongoing evolution from ld to hd magma calculations suggest that residual ld magma will evolve to hd texture in a period of hours to days contrasting amounts of the ld tt and hd magmas are present in each sample with the most tt in may the most ld in september october and the most hd in december this implies that the shallow magma had a different rheology at each collection period the viscosity of ld and hd magmas are calculated to be in the range of 2 000 to 2 600 and 3 000 to 5 000 pa s respectively which with their changing proportions must have implications for rates of bubble slug ascent and processes of fragmentation this study suggests that an increasing maturity of magma in stromboli s shallow conduit with resultant increase in viscosity feeds back to reduce the intensity of explosions whereas a steady flux of ld magma favors more powerful explosions

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

giant rafted pumice blocks from the most recent eruption of taupo volcano new zealand insights from palaeomagnetic and textural data abstract giant blocks of pumice lie strewn along a former shoreline of intracaldera lake taupo new zealand and are the sole subaerial evidence of the most recent volcanism at the taupo supervolcano geochemically they are identical to material erupted during the complex and multiphase 1 8 ka taupo eruption which they post date by one to two decades the blocks some of which are 10 m long show complex jointing patterns indicative of both surface chilling and continued interior expansion as well as heterogeneous vesicularity with dense rims mean density 917 kg m3 grading via an intervening transition zone mean density 844 kg m3 into a more highly vesicular interior mean density 815 kg m3 analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures some parts of block rims cooled to below 580 c well before emplacement on the shore whereas other parts in the interior and transition zones which cooled more slowly acquired different orientations before stranding some block interiors cooled after blocks were finally deposited and record the direction of the 1 8 ka field the blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of lake taupo several decades after the climactic taupo eruption over the inferred vent area these and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile rich felsic magma can be erupted underwater with only limited thermal fragmentation furthermore the prolonged flotation of out sized fragments of vesiculated magma formed during subaqueous dome growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions the genesis of pumice forming the blocks is not entirely clear most simply the blocks may represent part of a vesiculated carapace of a growing lava dome broken loose as the dome grew and deformed then rising buoyantly to the surface parts of the carapace could also be released by local magma water explosions some textures of the pumice however suggest fresher magma released from beneath the carapace this may suggest that silicic dikes and pillows pods intruded into a growing mound of silicic hyaloclastite itself formed by quench fragmentation and thermal granulation of the dike margins this fragmental cover would have inhibited cooling of a still hot and actively vesiculating interior which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile following their formation the large fragments of pumice floated to the lake s surface where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling

scaling vesicle distributions and volcanic eruptions models of coalescence decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities n v are of the scaling form text n left text v right propto text v text b 3 1 where v is the volume of the vesicles and b3 the 3 dimensional scaling power law exponent we analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from 10 m to 3 cm they are well fit with b3 0 85 we show that to within experimental error this exponent is the same as that reported in the literature for basaltic lavas and other volcanic products the importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at packing bubbles allowing very high vesicularities to be reached before the critical percolation threshold a process which for highly stressed magmas would trigger explosion in this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics

percolating magmas and explosive volcanism 1 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 index terms 3220 mathematical geophysics nonlinear dynamics 8414 volcanology eruption mechanisms 8429 volcanology lava rheology and morphology citation gaonac h h s lovejoy and d schertzer percolating magmas and explosive volcanism geophys res lett 30 11 1559 doi 10 1029 2002gl016022 2003

the effect of magma flow on nucleation of gas bubbles in a volcanic conduit abstract we solve the dynamics of magma ascent and the kinetics of bubble nucleation and growth simultaneously which allow us to predict bubble sizes and number densities under ascent conditions as magma rises toward the surface the pressure decreases and eventually becomes less than the solubility pressure when the degree of supersaturation becomes great enough bubbles nucleate nucleation will stop as the concentration of volatiles in the melt decreases due to growth of existing bubbles and hence the degree of supersaturation decreases we show that a second nucleation event may occur just below the fragmentation level near that level the degree of supersaturation continuously increases as the magma is rapidly decompressed as a result nucleation will not stop until fragmentation occurs this second nucleation event should be taken into account when interpreting bubble size distribution measurements made on natural pumices the bubbles of the second nucleation event have high internal gas pressures up to 2 mpa greater than the liquid pressure suggesting that the second nucleation event may enhance fragmentation of magma we apply the model to the calculation protocol defined at the volcanic eruption mechanism modeling workshop durham 2002 we found that as a result of disequilibrium degassing fragmentation occurs higher in the conduit than under equilibrium degassing

magma vesiculation and infrasonic activity at stromboli open conduit volcano abstract explosive activity at stromboli is explained in terms of dynamics of large gas bubbles that ascend in the magma conduit and burst at the free surface generating acoustic pressure that propagates as infrasonic signals in the atmosphere the rate and the amplitude of the infrasonic activity is directly linked to the rate and the overpressure of the bursting gas bubbles and thus reflects the rate at which magma column degasses under non equilibrium pressure conditions we investigate the link between explosive degassing and magma vesiculation by comparing the rate of infrasonic activity with the bubble size distributions bsds of scoria clasts collected during several days of explosive activity at stromboli bsds of scoria show a characteristic power law distribution which reflect a gas bubble concentration mainly controlled by a combined process of bubble nucleation and coalescence the cumulative distribution of the infrasonic pressure follows two power laws indicating a clear separation between the frequent but weak bursting of small gas bubbles puffing and the more energetic explosions of large gas slugs the exponents of power laws derived for puffing and explosive infrasonic activity show strongly correlated 0 96 changes with time indicating that when the puffing rate is high the number of energetic explosions is also elevated this correlation suggests that both puffing and explosive activity are driven by the same magma degassing dynamics in addition changes of both infrasonic power law exponents are very well correlated 0 92 with puffing and 0 87 with explosions with variations of the bsd exponents of the scoria clasts providing evidence of the strong interplay between scoria vesiculation and magma explosivity our analysis indicates that variable magma vesiculation regimes recorded in the scoria correlate with the event number and energy of the explosive activity we propose that monitoring infrasound on active volcanoes may be an alternative way to look at the vesiculation process in open conduit systems

experimental constraints on degassing and permeability in volcanic conduit flow this study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions degassing and permeability in magmas we studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature from the data collected we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium equilibrium bubble growth occurs when the decompression rate is slower than 0 1 mpa s 1 while higher rates cause porosity to deviate rapidly from equilibrium defining the first disequilibrium regime of degassing if the deviation is large enough a critical threshold of super saturation is reached and bubble growth accelerates defining the second disequilibrium regime we studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions under these open conditions we observed that bubbles start to coalesce at 43 vol porosity regardless of decompression rate coalescence profoundly affects bubble texture and size distributions and induces the melt to become permeable we determined coalescence to occur on a time scale 180 s independent of decompression rate we parameterized and incorporated our experimental results into a 1d conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust compared to previous models that assume equilibrium degassing of the melt during ascent the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by 25 the acceleration at high porosities 50 vol by a factor 5 and the associated decompression rate by an order of magnitude the integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

bubble growth in rhyolitic melts experimental and numerical investigation bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically rhyolitic melts were hydrated at 150 mpa 780 850 c to uniform water content of 5 5 5 3 wt the pressure was then dropped and held constant at 15 145 mpa upon the drop bubbles nucleated and were allowed to grow for various periods of time before final rapid quenching of the samples the size and number density of bubbles in the quenched glasses were recorded where number densities were low and run duration short bubble sizes were in accord with the growth model of scriven 1959 for solitary bubbles however most results did not fit this simple model because of interaction between neighboring bubbles hence the growth model of proussevitch et al 1993 which accounts for finite separation between bubbles was further developed and used to simulate bubble growth

bubble growth in highly viscous melts theory experiments and autoexplosivity of dome lavas we examine the physics of growth of water bubbles in highly viscous melts during the initial stages diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential at later stages internal pressure falls radial growth decelerates and follows the square root of time at this stage it is controlled by diffusion the time of transition between the two stages is controlled by the decompression melt viscosity and the peclet number of the system the model closely fit experimental data of bubble growth in viscous melts with low water content close fit is also obtained for new experiments at high supersaturation high peclet numbers and high variable viscosity near surface degassed silicic melts are viscous enough so that viscosity controlled growth may last for very long times using the model we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation we suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation the presence of such bubbles is an indicator of the conditions at fragmentation the water content of lavas extruded at lava domes is a key factor in their evolution melts of low water content 0 2 wt are too viscid and bubbles nucleated in them will not grow to an appreciable size bubbles may grow in melts with 0 4 wt water the internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows

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

radiometric chronology of changme khangpu glacier sikkim the 32 si concentration in a sample of surface ice from the snout of changme khangpu glacier is 0 36 disintegrations per minute tonne compared to the fall out value of 0 7 d p m tonne if this decrease is assumed to be solely due to decay of 32 si an age off c 100 years is estimated for the surface ice of the snout leading to an average flow velocity of c 40 m year for the past century a vertical profile of 210 pb in a core taken at an altitude of 5040 m shows two horizons where this isotope is enriched one between 3 and 4 m and another between 11 and 12 m indicating that the primary concentration of 210 pb can change by physico chemical processes like adsorption on dust none the less a longitudinal profile along the glacier shows a systematic decrease of 210 pb activity with decreasing altitude the surface ice of the snout giving a value of 0 2 d p m l corresponding to an age of 100 years which is concordant with the 32 si age this surface flow rate of the glacier is much larger than the average contemporary flow rate c 13m year the difference can be understood in terms of the past history of advance and recession of the glacier as revealed by the geomorphic evidence

vesiculation path of ascending magma in the 1983 and the 2000 eruptions of miyakejima volcano japan the vesiculation of magma during the 1983 eruption of miyakejima volcano japan is discussed based on systematic investigations of water content vesicularity and bubble size distribution for the products the eruption is characterized by simultaneous lava effusion and explosive sub plinian dry eruptions with phreatomagmatic wet explosions the magmas are homogeneous in composition basaltic andesite and in initial water content h2o 3 9 0 9 wt and residual groundmass water contents for all eruption styles are low h2o 0 4 wt suggestive of extensive dehydration of magma