radial variations of melt viscosity around growing bubbles and gas overpressure in vesiculating magmas abstract the viscosity of silicic melts depends strongly on their water content as bubbles grow in a supersaturated melt water evaporates from the bubble melt interface a diffusive profile develops and leads to steep viscosity gradients across the melt shell here we investigate the effects of radial viscosity profiles on the dynamics of bubble growth we find that the effective melt viscosity resisting gas overpressure in the bubbles is close to the viscosity at the dehydrated rind and may be higher than that of the surrounding melt by more than an order of magnitude as a result bubbles may retain pressures that are higher than ambient pressure for longer times magma degassing is delayed to shallower depth and fragmentation of magma due to gas overpressure may occur over a wider range of conditions measured water content in eruption products yields information on the average melt viscosity however additional information about the concentration profiles is needed for estimating the effective viscosity that controlled the evolution of bubble growth in the ejecta

the cooling of frontal flow ramps a calorimetric study on the rocche rosse rhyolite flow lipari aeolian islands italy relaxation geospeedometry using differential scanning calorimetry dsc has been applied to quantify the cooling history across the glass transition of flow ramps at the front of the calcalkaline rhyolite rocche rosse flow of lipari aeolian islands italy modelled cooling rates for the obsidian retrieved from two profiles range between 0 2 and 0 03 k min 1 cooling at the flow front appears to be dominated by conductive heat loss of individual flow ramps forming individual cooling units cooling rates of tens of kelvins per day appear to have controlled the last stage of viscous deformation before the entire flow came to rest it is inferred that cooling rates slower than those modelled are required to sustain flow in highly viscous rhyolitic lavas

fiamme degassing structures and their implications for the post emplacement temperatures and h2o contents of high grade ignimbrites abstract a new process in post emplacement ignimbrite degassing is proposed from observations of unusual structures in a high grade ignimbrite in sardinia the structures consist of fiamme decorated above and below by vesicles termed here fiamme degassing structures fds the relative volumes of vesicles and fiamme have a positive linear correlation and the ratio between the volume of vesicles above and below fiamme asymmetry ratio is 1 wt at

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

the use of an outlier detecting method in time series of continuous daily measurements of underground water level and temperature in earthquake prediction investigation abstract continuous daily measurements of underground water level and temperature have been monitored in four selected shallow wells in a seismically active area close to thessaloniki greece the aim was to study the changes in the underground water level and temperature with respect to earthquakes occurring in the same area during 1 january 1988 to 31 december 1989 a graphical method has been applied to analyze the time series in order to detect outliers our results reveal that a strong correlation exists the earthquakes occurred in the area and the water level time series the correlation was found to be weak in the case of the water temperature time series most of the outliers found correspond to a time interval varying between 1 and 7 days before or after the earthquakes the magnitude of the underground water level change varies from 3 to 10 cm and that of temperature from 0 3 to 0 8 c the strongest outliers correlate to earthquakes of magnitude greater than 3 0 the software developed to detect outliers use immediate preceding observations in the time series of continuous daily measurements at the time of their occurrence this is very important in studying precursory phenomena of impending earthquakes

microlite textures and volatile contents of obsidian from the inyo volcanic chain california 1 we test the hypothesis that microlite textures record progressive degassing during magma ascent we compare microlite number density nv and h2o concentrations in pyroclastic and effusive obsidian obs from the inyo volcanic chain ca comparisons reveal a weak correlation between microlite nv and h2o however most obsidians obss of a given volatile content exhibit a wide range of nv factors other than degassing must also influence final groundmass texture we postulate that obs textures record different ascent rates and residence histories prior to eruption from the conduit we conclude that microlites do not sensitively indicate the extent of magmatic degassing rather they provide qualitative indications of magma ascent and residence time

equilibrium and disequilibrium degassing of a phonolitic melt vesuvius ad 79 white pumice simulated by decompression experiments abstract equilibrium and disequilibrium degassing of a volatile phase from a magma of k phonolitic composition was investigated to assess its behavior upon ascent decompression experiments were conducted in ar pressurized externally heated pressure vessels at superliquidus temperature 1050 c in the pressure range 10 200 mpa using pure water as fluid phase all experiments were equilibrated at 200 mpa and then decompressed to lower pressures with rates varying from 0 0028 to 4 8 mpa s isobaric saturation experiments were performed at the same temperature and at 900 950 c to determine the equilibrium water solubility in the pressure range 30 250 mpa the glasses obtained from decompression experiments were analyzed for their dissolved water content vesicularity and bubble size distribution all decompressed samples presented a first event of bubble nucleation at the capsule melt interface homogeneous bubble nucleation in the melt only occurred in fast decompressed experiments 4 8 and 1 7 mpa s for p 100 mpa for these decompression rates high water over saturations were maintained until a rapid exsolution was triggered at p 150 mpa for slower rates 0 0028 0 024 0 17 mpa s the degassing of the melt took place by diffusive growth of the bubbles nucleating at the capsule melt interface this process sensibly reduced water over saturation in the melt preventing homogeneous nucleation to occur for decompression rates of 0 024 and 0 17 mpa s low water over saturations were attained in the melt gradually declining toward equilibrium concentrations at low pressures a near equilibrium degassing path was observed for a decompression rate of 0 0028 mpa s experimental data combined with natural pumice textures suggest that both homogeneous and heterogeneous bubble nucleations occurred in the phonolitic magma during the ad 79 vesuvius plinian event homogeneous bubble nucleation probably occurred at a depth of 3 km in response to a fast decompression of the magma during the ascent

diffusive dehydration and bubble resorption during open system degassing of rhyolitic melts abstract degassing through open paths such as bubble and or fracture networks is considered to be the principal mode of degassing in silicic magmas however its detailed mechanisms remain unclear to investigate the behavior of bubbles in a hypothetical open system condition we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell which artificially maintains a pressure difference between its inside and outside the thick wall cell maintains a constant volume within the sample chamber while allowing water vapor to escape the cell during the experimental runs the cells containing obsidian cores with ca 0 66 wt initial water content were externally heated to 1000 c for a period of 1 288 h the run charges generally showed a zonal structure composed of two contrasting regions a central region within which the bubbles were concentrated bubble rich core bc and a bubble free melt region surrounding the bc bubble free margin bfm with increased heating duration the thickness of the bfm increased via dissolution of the outermost bubbles in the bc the water content was nearly uniform throughout the bc whereas an outward decreasing gradient was observed in the bfm we found that diffusive dehydration occurred from the sample surface and the bubbles were resorbed into the melt thus the bfm bc boundary moved inwards these processes were modelled numerically and the calculation results were in good agreement with the experimental data if a lifetime of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment then the paths have to be pinched off quickly 1 2 h at maximum pressure difference between the open path and the melt and thus the thickness of the bubble free layer reaches at most 0 1 mm for the formation of bubble free obsidian layers with a width of a few millimeters which are often observed in natural obsidian flows open paths should be maintained for at least a few hundred hours

the glass transition in hydrous granitic melts abstract the influence of water on the glass transition temperature of hydrous granitic melts is estimated on the basis of a variety of available sources of data that provide phenomenological data for the liquid glass transition and for which the relaxation timescale can be accurately estimated these sources include a dilatometric micropenetration viscosity determinations b temperature dependent spectroscopic investigations of hydrous melts and c densities of synthetic fluid inclusions in glasses all available evidence on the glass transition temperature of hydrous granitic melts when corrected for equivalent timescales yields a single consistent trend of decreasing tg with increasing water content that has powerful applications in the rheology and kinetics of processes involving late stage silicic intrusives and eruptive products the brittle ductile transition in hydrous silicic liquids can be accurately predicted using this trend combination of the presently derived relationship for the influence of water on the glass transition of granitic melts with presently available data for the high temperature viscosities of such systems permits the estimation of the temperature dependence of the viscosity of hydrous melts over the entire range of temperature relevant to granite petrogenesis such a comparison reveals that the viscosity temperature relationships of hydrous granitic melts are not arrhenian as is assumed by most existing calculational schemes for the prediction of hydrous melt viscosities the increasing deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian with increasing water content is broadly consistent with similar trends accompanying the addition of excess alkalies to metaluminous granitic melt base compositions in detail however the influence of water on the deviation of viscosity temperature relationships of hydrous granitic melts from arrhenian is not as strong as would be predicted from a molar comparison of the effects of h2o vs alkali oxide or alkaline earth oxide added to these melts the answer to this apparent discrepancy may lie in the incomplete dissociation of water in the melt structure

vesiculation rates of obsidian domes inferred from h2o concentration profiles 1 explosivity of rhyolite domes depends on bubble nucleation and growth to understand these processes we measure millimeter scale variations in h2o by synchrotron fourier transform infrared ftir spectromicroscopy on interlayered obsidian pumice samples the h2o contents of all samples are above the 1 atm solubility value 0 10 wt and decrease systematically towards vesicular zones indicating that gas bubbles were growing and that degassing of melt to atmospheric pressure was incomplete h2o profiles are compared with models for water diffusion in order to constrain the temporal scale of vesiculation diffusion timescales range from 0 4 to 15 days and represent the time between bubble nucleation and quenching we use these results to estimate the duration of volatile exsolution and evaluate the timing of the formation of explosion pits frequently found on the surfaces of obsidian domes though never witnessed

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

volcanic rock mass properties from snowdonia and tenerife implications for volcano edifice strength although volcano instability is increasingly recognized as a societal hazard numerical data on the relevant mechanical properties remain sparse we report new field data on the rock mass properties of volcanic materials from snowdonia north wales uk and tenerife canary islands using rock types that range in composition from phonolite to rhyolite we summarize a method for estimating the overall strength of a volcanic edifice based on the rock mass rating index rmr and the hoek brown criterion we show that the average rock mass compressive cm and cohesive c strengths decrease exponentially with rmr according to cm 0 652 0 0559rmr and c 0 035 0 0669rmr respectively and appear insensitive to both initial magma composition and relative age this exponential relationship provides a new predictive tool for directly estimating rock mass strength from the rmr our analysis further predicts a marked reduction of up to 96 in the rock mass compressive strength relative to the intact rock value based on laboratory tests and that overall the combined results from both study areas yield cohesive strength values from 4 8 to 0 44 mpa estimated values of rock mass angle of friction range from 28 to c 38 recent modifications to the hoek brown criterion in particular the inclusion of the disturbance factor d suggest that even these low values of rock mass strength and cohesion may be optimistic and the true values of rock parameters relevant for accurate predictions of volcano edifice strength may be up to 30 weaker still

the influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma volcanic rocks and magma display a wide range of porosity and vesicle size a result of their complex genesis while the role of porosity is known to exert a fundamental control on strength in the brittle field less is known as to the influence of vesicle size to help resolve this issue here we lean on a combination of micromechanical sammis and ashby s pore emanating crack model and stochastic rock failure and process analysis code modelling the models show for a homogenous vesicle size that an increase in porosity in the form of circular vesicles from 0 to 40 and or vesicle diameter from 0 1 to 2 0 mm results in a dramatic reduction in strength for example uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 the presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle emanating microcracks that grow in the direction of the applied macroscopic stress as strain increases these microcracks continue to grow and eventually coalesce leading to macroscopic failure vesicle clustering which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles and the increased ease of microcrack interaction is encouraged at higher porosity and reduces sample strength once a microcrack nucleates at the vesicle wall larger vesicles impart higher stress intensities at the crack tips allowing microcracks to propagate at a lower applied macroscopic stress larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture this explains the reduction in strength at higher vesicle diameters at a constant porosity the modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear the largest reductions are observed at low porosity and small vesicle diameters in detail we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 porosity vesicle clustering and stress lobe interaction are implicit at high porosity regardless of the vesicle diameter in the case of an inhomogeneous vesicle size the microcracks grow from the largest vesicles and brittle strength is closer to that of the largest vesicle end member the results of this study highlight the important role of vesicle size and the complex interplay between porosity and vesicle size in controlling the brittle strength of volcanic rocks and magma

pore pressure embrittlement in a volcanic edifice the failure mode of porous rock in compression dilatant or compactant is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock wong solid earth 102 3009 3025 1997 both of which are subject to change in space and time within a volcanic edifice while lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products pore pressures are prone to fluctuations during periods of volcanic unrest for example an increase in pore fluid pressure can result in rock fracture even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour a process termed pore fluid induced embrittlement we explore this phenomenon through a series of targeted triaxial experiments on typical edifice forming andesites from volcan de colima mexico we first show that increasing pore pressure over a range of timescales on the order of 1 min to 1 day can culminate in brittle failure of otherwise intact rock irrespective of the pore pressure increase rate we record comparable accelerations in acoustic emission and strain prior to macroscopic failure we further show that oscillating pore fluid pressures can cause iterative and cumulative damage ultimately resulting in brittle failure under relatively low effective mean stress conditions we find that macroscopic failure occurs once a critical threshold of damage is surpassed suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks finally we observe that inelastic compaction of volcanic rock as we may expect in much of the deep edifice can be overprinted by shear fractures due to this mechanism of embrittlement pore fluid induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and as a result may assist in the development of a fractured halo zone surrounding the conduit potentially explaining commonly observed near conduit outgassing at many active volcanoes further rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger scale regional fault systems our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system

shear viscosity of rhyolite vapor emulsions at magmatic temperatures by concentric cylinder rheometry abstract the viscosity of natural rhyolitic melt from lipari aeolian islands and melt bubble emulsions 30 50 vol porosity generated from lipari rhyolite have been measured in a concentric cylinder rheometer at temperatures and shear rates in the range 925 1150 c and 10 3 10 1 2 s 1 respectively in order to better understand the dependence of emulsion shear viscosity on temperature and shear rate in natural systems bubble free melt exhibits newtonian arrhenian behavior in the temperature range 950 1150 c with an activation energy of 395 30 kj mol the shear viscosity is given by log m 8 320 20624 t suspensions were prepared from natural rhyolite glass to which small amounts of na2so4 were added as a foaming agent reasonably homogeneous magmatic mixtures with an approximate log normal distribution of bubbles were generated by this technique suspension viscosity varied from 106 1 to 108 37 pa s and systematically correlates with temperature and porosity in the shear stress range 104 26 105 46 pa of the experiments the viscosity of melt bubble emulsions is described in terms of the relative viscosity r e m where e is the emulsion viscosity and m is the viscosity of melt of the same composition and temperature the dependence of relative viscosity on porosity for magmatic emulsions depends on the magnitude of the capillary number ca g rb 1 m 1 the ratio of viscous forces acting to deform bubbles to interfacial forces resisting bubble deformation for inviscid bubbles in magmatic flows three regimes may be identified for ca 10 an asymptotic regime is reached in which relative viscosity decreases with increasing porosity and is independent of ca our experiments were carried out for 30

the effect of water and fo2 on the ferric ferrous ratio of silicic melts new experiments on the effect of dissolved water on the ferric ferrous ratio of silicic melts have been performed at 200 mpa between 800 c and 1000 c and for fo2 between nno 1 35 and nno 6 6 water saturated conditions were investigated compositions studied include six metaluminous synthetic melts with feotot progressively increasing from 0 47 to 4 25 wt two natural obsidian glasses one peraluminous and the other peralkaline and a synthetic rhyolitic glass having the composition of the matrix glass of the june 15 1991 pinatubo dacite ferrous iron was analyzed by titration and feotot by electron microprobe variation of quench rate was found to have no detectable effect on the ferric ferrous ratio of the hydrous silicic melts investigated at nno no dependence of the ferric ferrous ratio with temperature is observed at fo2 nno 1 the experimental ferric ferrous ratios are equal or lower than calculated the peralkaline samples show the same type of behaviour a non linear relationship between xfe2o3 xfeo and fo2 implies that a term for dissolved water must be added to the kc equation if it is to be applied to the calculation of ferric ferrous ratios of hydrous silicic melts above nno 1 the ferric ferrous ratio is essentially controlled by the anhydrous melt composition and fo2 however differences exist between measured and calculated ferric ferrous ratios of silicic melts that are not all attributable to the effect of dissolved water additional work is needed to describe more precisely the dependence of the ferric ferrous ratio on anhydrous melt composition the oxidizing effect of water is restricted to relatively reduced magmatic liquids in oxidized calk alkaline magma series the presence of dissolved water will not largely influence melt ferric ferrous ratios

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

in situ confirmation of permeability development in shearing bubble bearing melts and implications for volcanic outgassing the ferocity of volcanic eruptions their penchant for either effusive or explosive behaviour is to a large extent a matter of the ease with which volatiles are able to escape the volcanic system of particular importance are the mechanisms by which permeable networks within magma are fabricated and how they permit gas escape thereby diffusing possibly calamitous explosions here we present a series of experiments that confirms sample scale fracture propagation and permeability development during shearing viscous flow of initially impermeable bubble bearing 2 10 4 s 1 at moderate shear strain rates 2 10 4 s 1 3 via en echelon mode i fractures produced by repeated fracture events at high shear strain rates 4 5 10 4 s 1 permeability develops shortly after the onset of inelastic deformation and is again established through a series of en echelon mode i fractures critically strain is not immediately localized on mode i fractures making them long lived and efficient outgassing channels that are ideally oriented for directing volatiles from the central conduit upward and outward toward the conduit rim indeed mode i fracture arrays may prove necessary for dissipating gas overpressures in the central regions of the magma column which are considered difficult to outgas these experiments highlight mechanisms that are likely active along conduit margins and constrain previously postulated processes under truly applicable conditions

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

the thermal history of a spatter fed lava flow the 8 ka pantellerite flow of mayor island new zealand the thermal history of the 8 ka spatter fed lava flow of mayor island new zealand has been investigated via relaxation geospeedometry cooling rates extant during the vitrification of the clastogenic pantellerite flow have been quantified cooling rates modelled for two obsidian layers at the top and the base of the flow vary over four orders of magnitude the flow base defined by a lower obsidian layer shows a gradual increase in cooling rates from 0 00072 k min adjacent to the crystalline core of the flow to 0 017 k min at the contact to a basal crumble breccia cooling rates of flow ridges formed by an upper obsidian layer vary between 0 001 and 6 k min based on field observation and thermal modelling cooling rates of kelvins to tens of kelvins per day for both upper and lower obsidians are difficult to interpret as simple cooling processes instead we propose a thermal history involving thermal annealing as an alternative explanation for these slow effective cooling rates such thermal annealing appears to have facilitated welding and rheomorphism of the spatter deposit leading to the development of a coherent lava sheet based on the results obtained here we present a quantitative scenario of the thermal history during the final stages of flow emplacement as a contribution to understanding the rheology of rheomorphic spatter deposits associated with explosive peralkaline rhyolitic volcanism and briefly address the hazard potential of such deposits

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

strain induced outgassing of three phase magmas during simple shear a major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability controlled outgassing we studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble 0 12 0 36 volume fraction and crystal bearing 0 0 42 volume fraction silicate melts experiments were performed under torsion at high temperature and pressure 723 873 k and 150 200 mpa in a paterson deformation apparatus at bulk shear strains between 0 and 10 the experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery three dimensional imaging and analysis of deformed bubbles was performed using x ray tomography the development of localized deformation in all samples enhanced by crystal content leads to brittle fracture at bulk strains 2 and sample wide fracturing in samples deformed to strains 5 a decrease in both bubble fraction and dissolved volatile content with increasing strain along with strain hardening rheological behavior suggests significant shear induced outgassing through the fracture networks applicable to shallow conduit degassing in magmas containing crystal fractions of 0 0 42 this study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive explosive transition

groundwater monitoring for earthquake prediction by an amateur network in japan since 1976 groundwater levels and the temperature of thermal water have been monitored in 100 wells distributed mostly in the southern kanto and tokai districts of japan in order to predict earthquakes good examples of groundwater precursors were recognized prior to the following destructive earthquakes the 1978 west off izu oshima earthquake m 7 0 the 1978 off miyagi earthquakes m 7 4 and the 1980 east off izu peninsula earthquake m 6 7 in the 1978 west off izu oshima earthquake groundwater levels and the japan meteorological agency jma volume strainmeters installed in the izu peninsula showed precursory changes at about the same time in the other cases however precursory behavior was observed only in water level and temperature monitoring empirical relations are given between earthquake magnitude and the epicentral distance of the farthest groundwater anomaly and between earthquake magnitude and the precursor time of groundwater

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

models for viscosity and shear localization in bubble rich magmas abstract bubble content influences magma rheology and thus styles of volcanic eruption increasing magma vesicularity affects the bulk viscosity of the bubble melt suspension and has the potential to promote non newtonian behavior in the form of shear localization or brittle failure here we present a series of high temperature uniaxial deformation experiments designed to investigate the effect of bubbles on the magma bulk viscosity the starting materials are cores of natural rhyolitic obsidian synthesized to have variable vesicularity 0 66 the foamed cores were deformed isothermally t 750 c at atmospheric conditions using a high temperature uniaxial press under constant displacement rates strain rates between 0 5 1 10 4 s 1 and to total strains of 10 40 the viscosity of the bubble free melt 0 was measured by micropenetration and parallel plate methods to establish a baseline for experiments on the vesicle rich cores at the experimental conditions rising vesicle content produces a marked decrease in bulk viscosity that is best described by a two parameter empirical equation log 10 bulk log 10 0 1 47 1 0 48 our parameterization of the bubble melt rheology is combined with maxwell relaxation theory to map the potential onset of non newtonian behavior shear localization in magmas as a function of melt viscosity vesicularity and strain rate for low degrees of strain i e as in our study the rheological properties of vesicular magmas under different flow types pure vs simple shear are indistinguishable for high strain or strain rates where simple and pure shear viscosity values may diverge our model represents a maximum boundary condition vesicular magmas can behave as non newtonian fluids at lower strain rates than unvesiculated melts thereby promoting shear localization and explosive or non explosive magma fragmentation the extent of shear localization in magma influences outgassing efficiency thereby affecting magma ascent and the potential for explosivity

strain induced magma degassing insights from simple shear experiments on bubble bearing melts experiments have been performed to determine the effect of deformation on degassing of bubble bearing melts cylindrical specimens of phonolitic composition initial water content of 1 5 wt and 2 vol bubbles have been deformed in simple shear torsional configuration in an internally heated paterson type pressure vessel at temperatures of 798 848 k 100 180 mpa confining pressure and different final strains micro structural analyses of the samples before and after deformation have been performed in two and three dimensions using optical microscopy a nanotomography machine and synchrotron tomography the water content of the glasses before and after deformation has been measured using fourier transform infrared spectroscopy ftir in samples strained up to a total of 2 the bubbles record accurately the total strain whereas at higher strains 10 the bubbles become very flattened and elongate in the direction of shear the residual water content of the glasses remains constant up to a strain of 2 and then decreases to about 0 2 wt at 10 results show that strain enhances bubble coalescence and degassing even at low bubble volume fractions noticeably deformation produced a strongly water under saturated melt this suggests that degassing may occur at great depths in the volcanic conduit and may force the magma to become super cooled early during ascent to the earth s surface potentially contributing to the genesis of obsidian

pre seismic responses of underground water level and temperature concerning a 4 8 magnitude earthquake in greece on october 20 1988 abstract it is widely known that level and temperature changes of the underground waters are associated with earthquake activity in this paper we present results obtained from a simple and indeed inexpensive underground water level and temperature monitoring network established for the first time in a seismic zone of greece sharp changes in the underground water level and temperature have been recorded mainly as pre seismic events of the earthquake of magnitude 4 8 which occurred in the area of the network on october 20 1988

flow banding in obsidian a record of evolving textural heterogeneity during magma deformation we perform a quantitative textural analysis of banding in obsidian from big glass mountain bgm medicine lake volcano california and from mayor island mi new zealand the samples are compositionally homogeneous with banding defined by variable microlite content bgm or vesicularity mi the spatial distribution of alternating light and dark bands has a 1 wavenumber power spectral density as well as multifractal characteristics through the use of simple mathematical models we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma our results indicate that changes in crystallinity vesicularity were concurrent with magma deformation and are random multiples of the total amount of crystallinity vesicularity that was already present at any given time banded obsidian from bgm and mi can be found in close spatial proximity to brittle deformational textures we therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation fragmentation can result in variable rates of degassing of fragments which in turn can lead to nonuniform crystal vesicle nucleation and growth rates repeated fragmentation welding and subsequent viscous deformation can thus lead to banding formation

coupled effect of magma degassing and rheology on silicic volcanism abstract explosive volcanism such as the 1991 mt pinatubo philippines and the 2008 mt chaiten chile eruptions is caused by violent vesiculation of hydrous magma however gas may efficiently separate from magma owing to the enhancement of gas permeability by shear deformation of magma flowing in a volcanic conduit this makes it difficult to maintain the driving force of explosive volcanism although explosive volcanism is actually common here we propose that shear localization in a volcanic conduit controls the eruption style and explosivity based on deformation experiments of vesicular magma linked with synchrotron radiation x ray radiography and computed tomography we observed for the first time in situ that the shear localization caused magma fracturing and formed a slip plane and thus inhibited deformation and outgassing elsewhere we also observed the compaction of vesicular magma into a dense lava as a result of outgassing when shear localization did not occur in a natural setting shear localizes along the edges of a volcanic conduit where the strain rate is high causing a highly permeable fracturing layer to form at the conduit s edge and leaving less sheared and less outgassed magma at its center the less outgassed magma in the center may ascend rapidly and cause explosive volcanism non explosive lava effusion may occur only when shear localization does not occur effectively this new view explains the rapid ascent of viscous magma and the formation of pyroclasts with contrasting vesicularity pyroclastic obsidian and highly vesiculated pumice

dynamics of obsidian flows inferred from microstructures insights from microlite preferred orientations the flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface microlite preferred orientations are used to investigate the emplacement dynamics strain history and structural evolution of obsidian dome eastern california measurements of three dimensional microlite trend and plunge in samples from the near vent region distal flow and flow front show 1 the flow directions along the dome margins 2 the deformation style e g pure versus simple shear at the dome margins and 3 the variation in strain as a function of position within the flow microlites form well developed lineations in the plane of flow banding in all samples stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin the radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent these results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow along most of the eastern periphery lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust marginal talus piles and topography orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and nearvent samples developed in a pure shear flow microstructures in a sample from near the distal flow base records a component of simple shear variance in microlite trend provides a measure of the amount of strain acquired during flow standard deviation in trend decreases from the near vent region to the flow margins reflecting progressive alignment of microlites during transport pure shear strain inferred from orientation distributions increases from approximately 0 3 near the vent to about 1 1 at the flow front the difference between these strains 0 8 is an estimate of the strain associated with flow emplacement such strain is similar in size to that estimated from mesoscopic structures v1 and for horizontal spreading of a fluid whose volume is equal to that of obsidian dome v1 6 this suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front these techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger scale structures indicative of flow directions are poorly preserved fl 2002 elsevier science b v all rights reserved

bubble development in explosive silicic eruptions insights from pyroclast vesicularity textures from raoul volcano kermadec arc critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation here we present data on densities and vesicle size and number characteristics for representative pyroclasts from six silicic eruptions of contrasting size and style from raoul volcano kermadec arc we use these data to evaluate histories of bubble nucleation coalescence and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material density vesicularity distributions show a scarcity of pyroclasts with 65 75 vesicularity however pyroclasts closest to this vesicularity range have the highest bubble number density bnd values regardless of eruptive intensity or style clasts with vesicularities greater than this 65 75 pivotal vesicularity range have decreasing bnds with increasing vesicularities interpreted to reflect continuing bubble growth and coalescence clasts with vesicularities less than the pivotal range have bnds that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite rich magma or vesiculation during slow ascent of degassing magma bubble size distributions bsds and bnds show variations consistent with 65 75 representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation consistent with the closest packing of spheres we consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit these processes can act in concert with multiple nucleation events generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed fragmentation preserves the range in vesicularity seen as well as any post fragmentation pre quenching expansion which may have occurred we demonstrate that differing density pyroclasts from a single eruption interval can have widely varying bnd values corresponding to the degree of bubble maturation that has occurred the modal density clasts the usual targets for vesicularity studies have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation

dynamic lithospheric response to megathrust and precursory seismicity features of megathrust abstract temporal variations of seismic properties in inner trench regions before and after three megathrusts with magnitudes greater than or equal to 8 8 since 2004 are investigated to understand the nature of megathrust earthquakes the seismicity was increased significantly and the fault type compositions changed after megathrusts the seismicity displays characteristic fault type dependent distribution on rupture planes the postseismic thrustal events were populated around the down dip rupture margins due to the concentration of shear stress after coseismic ruptures normal faulting earthquakes increased after the megathrusts particularly in shallow depth regions with large slips which may be associated with lithospheric rebound and development of splay faults the earthquake occurrence rate b value displays a characteristic slip dependent feature the earthquake occurrence rates decrease with the slip amount of forthcoming megathrust which may be caused by continuous accumulation of plate driven stress and tectonic loading around the future rupture planes on the slab surface the slip dependency of earthquake occurrence rates is enhanced with time until the occurrence of megathrust the level of seismicity after megathrust is inversely proportional to that before megathrust yielding comparable average seismicity over the rupture zone it was also observed that the dynamic lithospheric response is highly correlated with slip distribution on the rupture plane the tension axes of the normal faulting earthquakes for 100 days after the tohoku oki earthquake focus to an apparent pole suggesting a radial viscoelastic deformation of lithosphere the temporal changes of slip dependent b values fit well with an exponential function suggesting an exponential increase of normal stress on the locked region with time until the occurrence of the megathrust this observation suggests that the temporal variation of slip dependent b values may be useful for prediction of forthcoming megathrusts at least several tens years beforehand

an introduction to the application of x ray microtomography to the three dimensional study of igneous rocks imaging rocks in three dimensions through x ray microtomography enables routine visualization of structures in samples which can be spatially resolved down to the sub micron scale although x ray tomography has been applied in biomedical research and clinical settings for decades it has only recently been applied to studies of rocks and few geoscientists realize its value and potential this contribution provides an introduction to the principles and techniques of x ray microtomography to the study of igneous rock textures as well as reviewing the current state of the art we hope that this short review will encourage more geoscientists to apply x ray microtomography in their research and that this will lead to new insights into the processes that occur in magmatic as well as other geological systems

determining flow type shear rate and shear stress in magmas from bubble shapes and orientations we interpret the shear environments that produced bubble textures in obsidian samples using the results of theoretical numerical and experimental studies on the deformation of bubbles in shear flows in particular we use the shapes and orientations of bubbles vesicles in obsidian to estimate shear rates and shear stresses and assess flow type simple vs pure shear this technique can be used to determine shear rates in volcanic conduits the origin of pyroclastic obsidian and the emplacement history and dynamics of obsidian flows a spherical bubble in a viscous fluid subjected to a low reynolds number steady flow field deforms until it reaches a steady shape and orientation bubble deformation is governed by the competing stresses from shearing that deforms and surface tension that rerounds the ratio of these stresses is the capillary number ca because the relationships among ca bubble orientation and shape differ for pure and simple shear we can distinguish between these flow types using preserved bubble geometries furthermore because ca is a function of shear rate we can use relationships between ca and the magnitude of deformation to calculate shear rates when melt viscosity and surface tension are known to demonstrate the potential of the technique we examine three obsidian samples chosen for diversity of origin and texture two of the samples have low crystallinities and banding defined by layers of different vesicularity bubble geometries in a spatter fed obsidian flow sample record deformation by pure shear whereas a juvenile obsidian clast from a pyroclastic fall deposit records predominantly simple shear a third sample from an obsidian flow has banding marked by variable concentrations of microlites and shows bubble deformation most consistent with dominantly simple shear and some bubble relaxation the highest shear rate and shear stress are recorded by the pyroclastic obsidian shear rate 0 01 s 1 shear stress 60 kpa the spatter fed obsidian flow sample deformed by pure shear records the lowest shear stress 4 9 kpa the obsidian flow sample deformed by simple shear records the lowest shear rates 10 6 6 10 6 9 s 1 despite high shear stresses 22 36 kpa because of the high viscosity of the degassed rhyolite 0 13 wt water

regional and local controls on archean rock hosted cobalt mineralization at the mcara deposit southern superior province ontario canada the mcara deposit is in eastern ontario canada and is hosted in an archean inlier to the paleoproterozoic huronian basin it is currently estimated to contain 2 4 million pounds of cobalt at an