petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma the recent eruption of the soufriere hills volcano in montserrat july 1995 to present october 1997 has produced a hornblende bearing andesitic lava dome it is possible to petrologically estimate changes in ascent rates of amphibole bearing magmas for certain rates of decompression a breakdown rim of fine grained anhydrous reaction products forms where amphibole is in contact with melt the thickness of the rim varies with ascent rate most of the amphibole phenocrysts in the magma storage region lack breakdown rims about 10 have 200 400 m thick coarse grained breakdown rims that are interpreted to be relicts of a past heating event study of a time series of new dome andesites showed that ascent rate increased from december 1995 0 001 m s to july 1996 0 008 m s while eruptive style remained extrusive ascent rate increased to 0 012 m s in august 1996 and the first major explosive eruption occurred on 17 18 september 1996

waveform classification of volcanic low frequency earthquake swarms and its implication at soufriere hills volcano montserrat low frequency volcanic earthquakes are indicators of magma transport and activity within shallow conduit systems at a number of volcanoes these events exhibit a high degree of waveform similarity providing a criterion for classification using cross correlation techniques to quantify the degree of similarity we develop a method to sort events into families containing comparable waveforms events within a family have been triggered within one small source volume from which the seismic wave has then travelled along an identical path to the receiver this method was applied to a series of 16 low frequency earthquake swarms well correlated with cyclic deformation recorded by tiltmeters at soufriere hills volcano montserrat in june 1997 nine waveform groups were identified containing more than 45 events each the families are repeated across swarms with only small changes in waveform indicating that the seismic source location is stable with time the low frequency seismic swarms begin prior to the point at which inflation starts to decelerate suggesting that the seismicity indicates or even initiates a depressurisation process a major dome collapse occurred within the time window considered removing the top 100 m of the dome this event caused activity within some families to pause for several cycles before reappearing this shows that the collapse did not permanently disrupt the source mechanism or the path of the seismic waves

control on the emplacement of the andesite lava dome of the soufriere hills volcano montserrat by degassing induced crystallization lava solidification is controlled by two mechanisms external cooling and gas exsolution the latter inducing crystallization due to increasing liquidus temperature the andesite lava dome of the soufriere hills volcano montserrat is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement in the magma chamber the magma has an estimated viscosity of 7 106 pa s during ascent gas exsolution caused the magma to extrude in a highly crystalline state with only 5 15 residual melt viscosities in the range 1013 1014 pa s and mechanical strength 1 mpa deformation can be heterogeneous with extrusion along shear zones rheological stiffening in the upper conduit also causes large overpressures shallow seismicity and cyclic patterns of dome extrusion gas rich porphyritic andesites tend to be the least mobile kind of lava because transition from magma into hot crystalline material was reached during ascent

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

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

shallow vent architecture during hybrid explosive effusive activity at cordon caulle chile 2011 12 evidence from direct observations and pyroclast textures in june 2011 an eruption of rhyolite magma began at the puyehue cordon caulle volcanic complex southern chile by january 2012 explosive activity had declined from sustained pyroclastic plinian to sub plinian fountaining to mixed gas and ash jetting punctuated by vulcanian blasts this explosive activity was accompanied by synchronous effusion of obsidian lava in a hybrid explosive effusive eruption fortuitous climatic conditions permitted ground based observation and video recording of transient vent dynamics as well as real time collection of proximal juvenile ash as it sedimented from the active plume the main eruptive vent complex and site of lava effusion was represented by two loci of vulcanian blasts within a single tephra cone containing a pancake shaped proto lava dome these blast loci each consisted of clusters of sub vents that expressed correlated shifts in eruption intensity indicating the presence of partially connected and or branching zones of high permeability within the upper conduit pyroclast textures were examined by x ray computed microtomography and their permeability was modeled by lattice boltzmann simulations the porosity 39 to 67 and darcian permeability 3 1x10 15 m2 perpendicular to fabric to 3 8x10 11 m2 parallel to fabric of fine ash emitted during ash jetting indicates that the permeable zones comprised highly sheared tube like bubbly magma and contrast with the low porosity 17 and nul permeability of bombs ejected to hundreds of metres from the vent in vulcanian blasts residual h2o content of ash 0 14 wt and two bombs 0 2 0 25 wt determined by karl fischer titration indicate degassing of this pyroclastic material to near atmospheric pressures ash textures and simple degassing vesiculation models indicate the onset of permeability by ductile processes of shear enhanced bubble coalescence in the upper 1 to 1 5 km of the conduit repeated ash jetting and vulcanian blasts indicate that such ductile processes were not sufficiently effective to accommodate all degassing requirements and additional brittle e g fragmentation events transiently assisted in maintaining an open system degassing regime that permitted long lived hybrid explosive effusive activity the eruption at cordon caulle has provided an excellent opportunity to closely observe key processes of silicic volcanism

magmatic conditions and magma ascent as indicated by hornblende phase equilibria and reactions in the 1995 2002 soufriere hills magma a study of amphiboles and associated minerals in samples of soufri ere hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts a separate set of small pargasitic crystals in the groundmass and two types of reaction rims on the phenocrysts the composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma as many as seven zones 5200 mm wide in the hornblende phenocrysts begin with a sharp increase in mg and si and then change gradually to a more feand al rich hornblende a transition that is consistent with a temperature rise analyses of the hornblende phenocrysts and associated fe ti oxides verify previous conclusions that the pre eruption magma was at 130mpa and 830 10 c but was variably heated before eruption the heating occurred within 30 days of eruption for all magmas erupted based on the width of ti rich rims on titanomagnetite phenocrysts experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855 c at a ph2o of 130mpa and would be even more al rich if crystallized at higher pressure pargasite is not stable in the andesite and its presence along with high an plagioclase microphenocrysts requires mafic magma mingling and hybridization with pre existing andesite experimental melts of the andesite at 130mpa and 830 and 860 c compare well with melt inclusions in quartz and plagioclase respectively reaction rims on a few hornblende crystals in each andesite sample are rich in high ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende decompression induced breakdown rims occur in some samples and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830 860 c the average ascent rate of magma during much of the 1995 2002 eruption has been40 02 m s the rate that allows hornblende to erupt free of decompression induced reaction rims

conduit to localized scale degassing during plinian eruptions insights from major element and volatile cl and h2o analyses within vesuvius ad 79 pumice textural investigations of the ad 79 vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption which emitted two types of pumice white and gray associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively these studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale to validate this hypothesis analyses of volatiles cl h2o and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools microraman spectrometry and electron microprobe and combined with major element and volatile concentration profiles and maps the results indicate that the melt phase differentiated through degassing induced crystallization of leucite and that the gray pumice magma was efficiently homogenized prior to degassing because cl diffuses more slowly than h2o during fast ascent it behaves as an incompatible element and can be used as a tracer of crystallization and h2o degassing we emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases and establish degassing scenarios that incorporate the effects of shear zones

instability of magma flow from volatile dependent viscosity volatiles dissolved in silicic magma at depth exsolve as the magma nears the surface and cause an increase in viscosity of the magma a model of a volcanic conduit within an elastic medium and a viscosity dependent on the volatile content of the magma produces oscillatory magma flow for a critical range of steady input flow rates oscillatory flow is recognized as a fundamental mode of behavior at silicic volcanoes and understanding it allows improved short term forecasting of timing and eruption style

ground deformation measurements at the soufriere hills volcano montserrat ii rapid static gps measurements june 1996 june 1997 we describe measurements of ground deformation around the erupting soufriere hills volcano made between june 1996 and june 1997 using rapid static gps the measurements define a lateral displacement field which is approximately radially symmetrical about the growing dome no significant sustained vertical displacements have been identified radial displacements have generally increased steadily during this period of observation the rate of displacement decreases from 43 cm per year at a distance of 0 6 km from the centre of the dome to about 1 cm year at a distance of 3 8 km strains within one km of the dome itself are greater than 10 3 the rate of variation of strain with distance indicates a causative centre of pressure at a depth of not greater than 750 meters below the summit of the dome

magmatic gas percolation through the old lava dome of el misti volcano the proximity of the major city of arequipa to el misti has focused attention on the hazards posed by the active volcano since its last major eruption in the fifteenth century el misti has experienced a series of modest phreatic eruptions and fluctuating fumarolic activity here we present the first measurements of the compositions of gas emitted from the lava dome in the summit crater the gas composition is found to be fairly dry with a h2o so2 molar ratio of 32 3 a co2 so2 molar ratio of 2 7 0 2 a h2s so2 molar ratio of 0 23 0 02 and a h2 so2 molar ratio of 0 012 0 002 this magmatic gas signature with minimal evidence of hydrothermal or wall rock interaction points to a shallow magma source that is efficiently outgassing through a permeable conduit and lava dome field and satellite observations show no evolution of the lava dome over the last decade indicating sustained outgassing through an established fracture network this stability could be disrupted if dome permeability were to be reduced by annealing or occlusion of outgassing pathways continued monitoring of gas composition and flux at el misti will be essential to determine the evolution of hazard potential at this dangerous volcano

models of tremor and low frequency earthquake swarms on montserrat recent observations from soufriere hills volcano in montserrat reveal a wide variety of low frequency seismic signals we discuss similarities and differences between hybrid earthquakes and long period events and their role in explosions and rockfall events these events occur usually in swarms and occasionally merge into tremor an observation that can shed further light on the generation and composition of harmonic tremor we use a 2d finite difference method to model major features of low frequency seismic signatures and compare them with the observations a depth dependent velocity model for a fluid filled conduit is introduced which accounts for the varying gas content in the magma and the impact on the seismic signals is discussed we carefully analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in the gas content of the magma as well as in terms of a time dependent triggering mechanism of low frequency resonances in this way we explain the simultaneous occurrence of low frequency events and tremor with a spectral content comprising integer harmonics

dynamics of magma flow near the vent implications for dome eruptions abstract magma erupting out of a volcanic vent into a flow or a dome is subjected to a marked change of boundary conditions which strongly affects the distribution of gas pressure and stress within the flow a numerical code is implemented to study the role of degassing induced crystallization the 2 d governing equations for a compressible viscous liquid are solved with a fully axially symmetric finite element model for different crystal contents and vent geometrical shapes the potential effects of gas pressure deformation rates and elongational stresses on the stability of endogenous dome growth are studied for two shallow vent structures for common eruption conditions values of the internal gas pressure at the vent may be as large as 2 mpa the magnitude of gas pressure at the vent is sensitive to the amount of crystallization and to the shape of the shallow conduit beneath the vent high tensile stresses of 2 mpa are generated at the vent which may account for ring fracturing of the dome and gas escape through shallow fissures such failure conditions may also lead to a change of exit conditions from lateral spreading to the vertical protrusion of a lava spine

strain localization in vesicular magma implications for rheology and fragmentation the rheology of two or three phase magmas has been the focus of much interest because it controls magma ascent and eruption behavior research on magma rheology has typically considered homogeneous fl ow here we demonstrate based on natural examples that strain resulting from viscous fl ow preceding explosive fragmentation localizes into shear zones at a microscopic scale strain localization affects the rheological behavior of magmas modifying predictions based on experiments localization leads to high strain rate shear zones where elongated sheared vesicles and shear heating have a weakening effect surrounding regions of relatively low strain rate where subequant vesicles have a strengthening effect thus energy is dissipated more effi ciently into localized bands where strain rate increases through feedback effects and can lead to melt fragmentation

the influence of viscous and latent heating on crystal rich magma flow in a conduit the flow dynamics of crystal rich high viscosity magma is likely to be strongly influenced by viscous and latent heat release viscous heating is observed to play an important role in the dynamics of fluids with temperature dependent viscosities the growth of microlite crystals and the accompanying release of latent heat should play a similar role in raising fluid temperatures earlier models of viscous heating in magmas have shown the potential for unstable thermal runaway flow as described by a gruntfest number using an arrhenius temperature dependence for the viscosity but have not considered crystal growth or latent heating we present a theoretical model for magma flow in an axisymmetric conduit and consider both heating effects using finite element method techniques we consider a constant mass flux in a 1 d infinitesimal conduit segment with isothermal and adiabatic boundary conditions and newtonian and non newtonian magma flow properties we find that the growth of crystals acts to stabilize the flow field and make the magma less likely to experience a thermal runaway the additional heating influences crystal growth and can counteract supercooling from degassing induced crystallization and drive the residual melt composition back towards the liquidus temperature we illustrate the models with results generated using parameters appropriate for the andesite lava dome forming eruption at soufriere hills volcano montserrat these results emphasize the radial variability of the magma both viscous and latent heating effects are shown to be capable of playing a significant role in the eruption dynamics of soufriere hills volcano latent heating is a factor in the top two kilometres of the conduit and may be responsible for relatively short term days transients viscous heating is less restricted spatially but because thermal runaway requires periods of hundreds of days to be achieved the process is likely to be interrupted our models show that thermal evolution of the conduit walls could lead to an increase in the effective diameter of flow and an increase in flux at constant magma pressure

influence of extrusion rate and magma rheology on the growth of lava domes insights from particle dynamics modeling abstract lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit repeated cycles of growth are punctuated by collapse as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface here we explore lava dome growth and failure mechanics using a two dimensional particle dynamics model the model follows the evolution of fractured lava with solidification driven by degassing induced crystallization of magma the particle dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh based analyses due to mesh entanglement effects the deformable talus evolves naturally as a frictional carapace that caps a ductile magma core extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure this new model is calibrated against existing observational models of lava dome growth results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate the effects of extrusion rate on magma rheology are sensitive to material stiffness which in turn is a function of volatile content and crystallinity material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability degassing induced crystallization causes material stiffening and enhances material strength reflected in non newtonian magma behavior the increase in stiffness and strength of the injected magma causes a transition in the style of dome growth from endogenous expansion of a ductile core to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re direction of magma flow creating a new shear zone and the switching of dome growth from one side to the other our model shows similar dome growth dynamics as observed at soufriere hills volcano montserrat indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology

modelling shear bands in a volcanic conduit implications for over pressures and extrusion rates shear bands in a volcanic conduit are modelled for crystal rich magma flow using simplified conditions to capture the fundamental behaviour of a natural system our simulations begin with magma crystallinity in equilibrium with an applied pressure field and isothermal conditions the viscosity of the magma is derived using existing empirical equations and is dependent upon temperature water content and crystallinity from these initial conduit conditions we utilize the finite element method using axi symmetric coordinates to simulate shear bands via shear localisation we use the von mises visco plasticity model with constant magma shear strength for a first took into the effects of plasticity the extent of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for soufriere hills volcano montserrat although the model is generic in nature our model simulates shallow up to approximately 700 in shear bands that occur within the upper conduit and probably govern the lava extrusion style due to shear boundaries we also model the change in the over pressure field within the conduit for flow with and without shear bands the pressure change can be as large as several mpa at shallow depths in the conduit which generates a maximum change in the pressure gradient of 10 s of kpa m the formation of shear bands could therefore provide an alternative or additional mechanism for the inflation deflation of the volcano flanks as measured by tilt metres shear bands are found to have a significant effect upon the magma ascent rate due to shear induced flow reducing conduit friction and altering the over pressure in the upper conduit since we do not model frictional controlled slip only plastic flow our model calculates the minimum change in extrusion rate due to shear bands however extrusion rates can almost double due to the ton nation of shear bands which may help suppress volatile loss due to the paucity of data and large parameter space available for the magma shear strength our model results can only allow for a qualitative comparison to a natural system at this stage c 2007 elsevier b v all rights reserved

rheological behavior of bubble bearing magmas abstract the vesiculation of the rising magma in a volcanic conduit results in a large change in the viscosity of magma understanding the effect of bubbles on the viscous behavior of bubble melt mixtures over a wide range of bubble volume fractions is essential for accurate modeling of the volcanic processes in this study new equations for the relative viscosity of concentrated bubble bearing suspensions are developed using a differential scheme along with the solution of an infinitely dilute suspension of bubbles out of the four models developed in the paper two models predict the relative viscosity to be a function of only two variables capillary number and volume fraction of bubbles there are no adjustable parameters in these models the remaining two models include an additional parameter that is the maximum packing volume fraction of bubbles the proposed models are evaluated using published numerical and experimental data on the relative viscosity of concentrated suspensions of bubbles the viscosity models for bubble bearing suspensions developed in this study are useful in modeling of the volcanic processes

evidence for the development of permeability anisotropy in lava domes and volcanic conduits the ease at which exsolving volatiles can migrate though magma and outgas influences the explosivity of a volcanic eruption volcanic rocks often contain discrete discontinuities providing snapshots of strain localisation processes that occur during magma ascent and extrusion whether these features comprise pathways for or barriers to fluid flow is thus of relevance for volcanic eruption and gas emission modelling we report here on nine discontinuity bearing andesite blocks collected from volcan de colima mexico we present a systematic porosity and permeability study of fifty cores obtained from the blocks collected and interpret the genetic processes of the discontinuities through detailed microstructural examination bands in pumiceous blocks were inferred to be relicts of inhomogeneous bubble expansion which despite significantly increasing porosity do not markedly affect permeability other discontinuities in our blocks are interpreted to be shear strain induced flow banding cavitation porosity and or variably healed fractures in each of these cases an increase in permeability up to around three orders of magnitude was measured relative to the host material a final sample contained a band of lower porosity than the host rock characterised by variably infilled pores in this case the band was an order of magnitude less permeable than the host rock highlighting the complex interplay between dilatant and densifying processes in magma we therefore present evidence for significant permeability anisotropy within the conduit and or dome of a volcanic system we suggest that the abundance and distribution of strain localisation features will influence the escape or entrapment of volatiles and therefore the evolution of pore pressure within active volcanic systems using a simple upscaling model we illustrate the relative importance of permeable structures over different lengthscales strain localisation processes resulting in permeability anisotropy are likely to play an important role in the style magnitude and recurrence interval of volcanic eruptions

rheological properties of dome lavas case study of unzen volcano abstract transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology the common presence of crystals in dome lavas produces a complex non newtonian rheology thus models of such complex rheology are essential for volcanic eruption models here we have measured the rheology of natural unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 mpa and temperatures between 940 and 1010 c crystal rich unzen lavas are characterised by two essential rheological features which produce non newtonian effects the first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco elastic fluid that exhibits shear thinning the second effect takes the form of a time dependence of the viscosity at moderate to high stress 10 mpa in this regime the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding effects which in turn lower the apparent viscosity of natural dome lavas ultimately deformation may lead to complete rupture of the lava if the stress is sufficient cracking thus stands as an important process in natural dome lava rheology the ubiquitous non newtonian rheology of dome lavas observed experimentally here needs to be adequately treated in order to generate appropriate eruption models

catastrophic lava dome failure at soufriere hills volcano montserrat 12 13 july 2003 the lava dome collapse of 12 13 july 2003 was the largest of the soufriere hills volcano eruption thus far 1995 2005 and the largest recorded in historical times from any volcano 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows which reached the sea the evolution of the collapse can be interpreted with reference to the complex structure of the lava dome which comprised discrete spines and shear lobes and an apron of talus progressive slumping of talus for 10 h at the beginning of the collapse generated low volume pyroclastic flows it undermined the massive part of the lava dome and eventually prompted catastrophic failure from 02 00 to 04 40 13 july 2003 large pyroclastic flows were generated these reached their largest magnitude at 03 35 when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes the high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge which flowed inland a vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03 35 13 july and four further explosions occurred at progressively longer intervals during 13 15 july 2003 the dome collapse lasted approximately 18 h but 170 of the total 210 million m3 was removed in only 2 6 h during the most intense stage of the collapse

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

multi stage growth of the trachytic lava dome of the puy de dome chaine des puys france field geomorphological and petro geochemical evidence abstract understanding lava dome eruptions is a major concern in volcanology regarding the assessment of associated hazards this question is usually addressed through the study of active calc alkaline domes however alkaline domes remaining poorly known the puy de dome is the most prominent volcano of the chaine des puys quaternary intraplate volcanic field in the french massif central it is a complex trachytic lava dome ca 11 ka old field investigations geomorphological and petro geochemical studies were performed to understand the magmatic system of this dome and reconstruct its volcanic evolution the different lava facies were fully characterised and their relationships with the volcano building stages have been established a new four step evolution is proposed for the growth of this edifice mineralogical and geochemical variations document magmatic differentiation with self mixing and minor crustal contamination in a zoned reservoir within the upper crust magma mixing and crustal contamination through fluids interactions during magma ascent and the involvement of fumarolic activity throughout the edifice building are also documented further this work highlights the specific features of the puy de dome compared to calc alkaline domes lack of lava flows and almost complete magma degassing during ascent resulting in the lack of significant explosive activity until the last eruption step

controls on conduit magma flow dynamics during lava dome building eruptions 1 lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity we develop a transient model of the magma flow in a volcanic conduit from an open system magma chamber with continuous replenishment the model incorporates gas exsolution bubble growth gas escape through the magma and decompression induced crystallization and considers variations in magma temperature water content conduit diameter phenocryst size chamber volume and magma rheology calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime when discharge rate is low and crystals grow efficiently leading to high magma viscosity to another stable state when discharge rate is high and crystallization is negligible the difference in discharge rates between these regimes can be several orders of magnitude periods are similar to the observed timescales and mainly depend on the chamber volume the system shows strongly nonlinear responses to the variation of governing parameters if magma has a bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of newtonian rheology large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature water content phenocryst size distribution or conduit diameter the system can fluctuate between low and high discharge rates with transitions to explosive activity

cyclic extrusion of a lava dome based on a stick slip mechanism abstract lava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by vulcanian explosions and pyroclastic flows we consider a simple system of nonlinear equations describing a 1d flow of lava extrusion through a deep elastic dyke feeding a shallower cylindrical conduit in order to simulate this short period cyclicity stick slip conditions depending on a critical shear stress are assumed at the wall boundary of the cylindrical conduit by analogy with the behaviour of industrial polymers in a plastic extruder the elastic dyke acts like a barrel and the shallower cylindrical portion of the conduit as a die for the flow of magma acting as a polymer when we applied the model to the soufriere hills volcano montserrat for which the key parameters have been evaluated from previous studies cyclic extrusions with periods from 3 to 30 h were readily simulated matching observations the model also reproduces the reduced period of cycles observed when a major unloading event occurs due to lava dome collapse

volcanoes erupt stressed quartz crystals author s befus ks manga m stan c tamura n abstract 2019 american geophysical union all rights reserved volcanic eruptions are energetic events driven by the imbalance of magmatic forces the magnitudes of these forces remain poorly constrained because they operate in regions that are inaccessible either underground or dangerous to approach new techniques are needed to quantify the processes that drive eruptions and to probe magma storage conditions here we present x ray microdiffraction measurements of volcanic stress imparted as lattice distortions to the crystal cargo of magma from yellowstone and long valley eruptions elevated residual stresses between 100 and 300 mpa are preserved in erupted quartz multiple volcanic forces could be culpable for the deformation so we analyzed crystals from pyroclastic falls pyroclastic density currents and effusive lavas stresses are preserved in all quartz but cannot be attributed to differences in eruption style instead lattice deformation likely preserves an in situ measurement of the deviatoric stresses required for the brittle failure of viscous crystal bearing glass during ascent

geodetic constraints on the shallow magma system at soufriere hills volcano montserrat 1 ground surface tilts recorded proximal to a growing lava dome and sustained through repeated dome collapses provide unique geodetic constraints on the depth size and overpressures of the shallow magmatic system at soufriere hills volcano tilt records identify a single pressure source inflating with periods 3 30 h sustained tilt amplitude ratios for paired stations identify the pressure source at a centroid depth between 740 and 970 m this pressure source and its robust oscillatory response survived a dome collapse that removed the upper 200 m of the magmatic system shallow overpressures of 1 4 mpa are confirmed over a radius of 200 340 m this size suggests pressurization of a fluid saturated fractured rock mass annulus surrounding a narrow magma conduit

magma flow instabilities in a volcanic conduit implications for long period seismicity silicic volcanic eruptions are typically accompanied by repetitive long period lp seismicity that originates from a small region of the upper conduit these signals have the capability to advance eruption prediction since they commonly precede a change in the eruption vigour shear bands forming along the conduit wall where the shear stresses are highest have been linked to providing the seismic trigger however existing computational models are unable to generate shear bands at the depths where the lp signals originate using simple magma strength models presented here is a model in which the magma strength is determined from a constitutive relationship dependent upon crystallinity and pressure this results in a depth dependent magma strength analogous to planetary lithospheres hence in shallow highly crystalline regions a macroscopically discontinuous brittle type of deformation will prevail whilst in deeper crystal poor regions there will be a macroscopically continuous plastic deformation mechanism this will result in a depth where the brittle ductile transition occurs and here shear bands disconnected from the free surface may develop we utilise the finite element method and use axi symmetric coordinates to model magma flow as a viscoplastic material simulating quasi static shear bands along the walls of a volcanic conduit model results constrained to the soufriere hills volcano montserrat show the generation of two types of shear bands upper conduit shear bands that form between the free surface to a few 100 m below it and discrete shear bands that form at the depths where lp seismicity is measured to occur corresponding to the brittle ductile transition and the plastic shear region it is beyond the limitation of the model to simulate a seismic event although the modelled viscosity within the discrete shear bands suggests a failure and healing cycle time that supports the observed lp seismicity repeat times however due to the paucity of data and large parameter space available these results can only be considered to be qualitative rather than quantitative at this stage c 2007 elsevier b v all fights reserved

distributions of bomb 14 c biogeochemistry and elemental concentration in hani mire peat profiles ne china implications of environmental change abstract two 50 cm long peat cores from the hani mire on the western slope of the changbai mountains northeastern china were collected for investigating physical biological and geochemical processes of the peatland in response to recent climate change the 14c dating using accelerator mass spectrometry ams on the peat cores provides a nuclear bomb carbon curve which is used for peat chronological construction the sedimentation rate sr of core s1 from a sphagnum magenllanicum hummock in the hani mire was about 0 98 cm y and deposited from 1957 2008 ce whereas the sr of core s2 from s palustre hummock 1 km apart from s1 was 1 59 cm y and accumulated during 1976 2008 the discrepancy of the srs may mainly be attributed to the different inorganic material supplies from surface runoff which affected growth of sphagnum spp at different sites with lower rate corresponding to higher inorganic contents higher ti ca and mg contents thus lower values of loss on ignition loi on the other hand the two cores had similar values and variation trends in the ph c n n and p contents and dry bulk density dbd implying that the organic source and decomposition were similar under the same climatic conditions the variations of toc c n and pb content in the peat cores matched well with the 5 y running average of local annual precipitation record with higher toc and c n but lower pb content corresponding to higher rainfall and vice versa these properties in the meso oligotrophic peat sequences can reflect climatic changes the recent rates of carbon accumulation rerca for s1 and s2 calculated from the toc dbd and sr were averaged 121 6 24 3 and 175 5 35 1 g m2 y respectively

paleomagnetic constraints on the structure of the deccan traps abstract new paleomagnetic results from two widely separated areas of the deccan traps allow one to test the simple normal reversed normal nrn magnetostratigraphic model previously proposed the new data from the mandla region to the northeast of the main outcrop of the deccan traps together with earlier apparently conflicting results are all compatible with the nrn stratigraphy provided the flows are not strictly horizontal but have a synform antiform structure with 500 m amplitude and 150 km wavelength the structure parallels the paleozoic narmada son rift system with slopes never exceeding 0 5 sampling in rajahmundry a remote southeastern outlier of the deccan with well constrained age allowed identification of the rn boundary in an area where only normal polarities had been reported it has also been possible to correlate the detailed geochemical stratigraphy in the western ghats with the nrn magnetic stratigraphy a compilation of 35 magnetic studies together with the above findings confirms the general applicability of the nrn model and reveals the large scale topography of the rn boundary which is an isochron over most of the deccan the rn isochron surface displays a boomerang shape with topography ranging from sea level to above 1600 m this observed topography is consistent with a simple model in which doming and renewed rifting and dyke intrusion resulted from impingement 70 65 ma ago of the reunion plume under the indian lithosphere the narmada son rift was rejuvenated and a continental rift system with minor extension with a triple junction in the cambay graben resulted and was subsequently covered by the deccan flows although in places tectonic faulting and volcanism continued to interfere the rajahmundry outcrops constrain the original extent and southeastward dip away from the triple junction of much of the traps a significant part of which has now been removed by erosion and rifting away of the seychelles

viscosity of high crystal content melts dependence on solid fraction 1 the rheological properties of suspensions containing high solid fractions are investigated attention is focused on viscosity of silicate and magmatic melt systems a general empirical equation which describes the relative viscosity of suspensions as a function of suspended solid fraction is proposed in the limit of very dilute solid concentrations it reduces to the einstein equation the proposed relationship is satisfactorily applied to reproduce available experimental data relative to silicate melts moreover the extrapolation of the model to very high concentrations is compared with experimental observations on partially melted granite

the transition from endogenous to exogenous growth of lava domes with the development of shear bands the transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the formation of discontinuities within the dome such transitions and vice versa are an important characteristic of most long lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events for the purpose of this paper following recent experimental and observational evidence we assume that such a transition occurs when shear bands are generated a model for the formation of shear bands and therefore the growth transition within a dome and coupled conduit domain is presented shear bands are most likely to initiate at the junction of the conduit and base of the dome where the shear stress experienced between new lava entering the dome and existing lava is greatest stress accumulation within the shear bands is likely to lead to brittle shear resulting in the formation of fractures finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities similarly the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit we consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth the development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for soufriere hills volcano montserrat during october to december 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 m modelling indicates that the observed fall in magma extrusion rate from about 2 0 m3s 1 to 0 5 m3s 1 as a result of the increased pressure head from the dome and the evolution in viscosity could have subsequently changed the dome growth regime due to the development of shear bands our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development

rheology of bubble bearing magmas abstract the physical effects of air or argon bubbles on the rheology of a calcium aluminosilicate melt have been measured at temperatures ranging from 830 to 960 c at 1 bar pressure the melt composition is sio 2 64 al 2 o 3 23 and cao 13 wt while bubble volume fractions are 0 0 06 0 13 0 32 0 41 and 0 47 measured newtonian viscosities range from 10 10 to 10 14 dpa s melts with bubble fractions of 0 06 and 0 13 show with increasing temperature t an increasing relative viscosity for t t 850 c for all bubble fractions the viscosity decreases markedly with temperature the observed maximum decrease of the relative viscosity is 75 for a bubble fraction of 0 47 at 907 c at all bubble fractions the viscosity is independent of the applied stress which ranged from 11 to 677 bars no clear indications were observed of non newtonian rheological behavior under our experimental conditions the relative viscosity of the two phase liquid depends primarily on the bubble fraction physical and volcanological implications of these measurements are discussed

the fast response of volcano seismic activity to intense precipitation triggering of primary volcanic activity by rainfall at soufriere hills volcano montserrat abstract one minute resolution time series of rainfall and seismic data from the soufriere hills volcano montserrat are analysed to explore the mechanism of external forcing of volcanic eruptions by rainfall over three years of activity the real time seismic amplitude rsam shows a narrow statistically significant peak within 30 min after the start of intense rainfall events and a much broader peak with a lag of 6 40 h the classified seismic events indicate that the volcanic response to rainfall begins at the surface and gradually penetrates deeper into the dome as there is an increase in the pseudo magnitude of surface rockfall events including pyroclastic flows with lags from the first 30 min to 40 h long period rockfalls from shallow degassing at lags of 4 and 14 h and long period and hybrid events source depth approximately 1 km with lags at 14 and 24 h after the start of rainfall events there was no rainfall related change in deeper volcano tectonic activity there was no change in the frequency of any type of classified event indicating that the rainfall acts to modulate existing internal processes rather than generating new events itself these robust results are due to many 229 different rainfall events and not just to a few large magnitude cases the rainfall triggered volcanic activity examined here is consistent with a model of fast shallow interactions with rainfall at the dome surface after which a deeper dome collapse follows

shear stress along the conduit wall as a plausible source of tilt at soufriere hills volcano montserrat 1 surface deformations recorded in close proximity to the active lava dome at soufriere hills volcano montserrat can be used to infer stresses within the uppermost 1000 m of the conduit system most deformation source models consider only isotropic pressurisation of the conduit we show that tilt recorded during rapid magma extrusion in 1997 could have also been generated by shear stresses sustained along the conduit wall these stresses are a consequence of pressure gradients that develop along the conduit numerical modelling incorporating realistic topography can reproduce both the morphology and half the amplitude of the measured deformation field using a realistic shear stress amplitude equivalent to a pressure gradient of 3 5 104 pa m 1 along a 1000 m long conduit with a 15 m radius this shear stress model has advantages over the isotropic pressure models because it does not require either physically unattainable overpressures or source radii larger than 200 m to explain the same deformation

lava dome growth and evolution with an independently deformable talus subduction zone volcanism occurs due to partial melting from subducting slabs which generally results in high viscosity magma containing a large amount of volatiles such volcanic eruptions can form a lava dome from which collapse events are a common and important part of their evolution collapse events can have devastating consequences in the form of block and ash avalanche deposits pyroclastic ows surges and the generation of tsunamis if they enter the sea in addition to this once a mass of lava dome has been removed during a collapse event this results in a drop in pressure in the remaining volatile rich magma which may then erupt explosively the internal structure of a lava dome and the extent of the surrounding carapace talus is unknown but likely to be critical for hazard assessment and is the motivation for this research presented is a computational model for the growth and evolution of an endogenous lava dome including an independently deformable talus using the finite element method dome growth is modelled to occur under two time scales continuous dome expansion via the addition of new magma into the molten core interior and relatively instantaneous talus readjustments due to rockfalls and the disintegration of the solid surface the continuous deformation of the dome is modelled as a uid with a yield strength in the talus region while talus deformation is modelled as a granular material that rests at angles below its angle of repose both surfaces dome and core talus interface are displaced using the level set method the model is axi symmetric and assumes that solidi cation and therefore talus growth occurs due to gas exsolution which promotes crystallisation rather than from surface cooling appropriate for intermediate composition lava ows for the purpose of this paper we consider and apply the model to the soufriere hills volcano montserrat but the techniques used are generic allowing the model to be applied to other dome forming eruptions the model provides information on the shape of the dome with the growth and extent of the talus and core found to be predominantly governed by the lava extrusion rate degree of solidi cation i e a solidus pressure the friction angle associated with the talus and lava dome viscosity

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

ascent and decompression of viscous vesicular magma in a volcanic conduit during eruption lava domes and flows may become unstable and generate dangerous explosions fossil lava filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content these observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit bubbly magma behaves as a compressible viscous liquid involving three different pressures those of the gas and magma phases and that of the exterior to solve for these three different pressures one must account for expansion in all directions and hence for both horizontal and vertical velocity components we present a new two dimensional finite element numerical code to solve for the flow of bubbly magma even with small dissolved water concentrations gas overpressures may reach values larger than 1 mpa at a volcanic vent for constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure in the conduit and at the vent there are large horizontal variations of gas pressure and hence of exsolved water content such variations depend on decompression rate and are sensitive to the exit boundary conditions for the flow for zero horizontal shear stress at the vent relevant to lava flows spreading horizontally at the surface the largest gas overpressures and hence the smallest exsolved gas contents are achieved at the conduit walls for zero horizontal velocity at the vent corresponding to a plug like eruption through a preexisting lava dome or to spine growth gas overpressures are largest at the center of the vent the magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth dependent radii

using the level set method to model endogenous lava dome growth modeling volcanic phenomena is complicated by free surfaces often supporting large rheological gradients analytical solutions and analogue models provide explanations for fundamental characteristics of lava flows but more sophisticated models are needed incorporating improved physics and rheology to capture realistic events to advance our understanding of the flow dynamics of highly viscous lava in peleean lava dome formation axi symmetrical finite element method fem models of generic endogenous dome growth have been developed we use a novel technique the level set method which tracks a moving interface leaving the mesh unaltered the model equations are formulated in an eulerian framework in this paper we test the quality of this technique in our numerical scheme by considering existing analytical and experimental models of lava dome growth which assume a constant newtonian viscosity we then compare our model against analytical solutions for real lava domes extruded on soufriere st vincent w i in 1979 and mount st helens usa in october 1980 using an effective viscosity the level set method is found to be computationally light and robust enough to model the free surface of a growing lava dome also by modeling the extruded lava with a constant pressure head this naturally results in a drop in extrusion rate with increasing dome height which can explain lava dome growth observables more appropriately than when using a fixed extrusion rate from the modeling point of view the level set method will ultimately provide an opportunity to capture more of the physics while benefiting from the numerical robustness of regular grids

petrologic evidence for pre eruptive pressure temperature conditions and recent reheating of andesitic magma erupting at the soufriere hills volcano montserrat w i the recent eruption of the soufriere hills volcano in montserrat july 1995 to present september 1997 has produced an andesitic dome sio 2 59 61 wt the eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region p 130 mpa 5 km depth the composition of the andesite has remained essentially constant throughout the eruption but heating by the mafic magma increased the andesite temperature from 830 c to 880 c prior to being heated the stable mineral assemblage in the andesite was plagioclase amphibole orthopyroxene titanomagnetite ilmenite quartz the rise in temperature from 830 c to 880 c fo 2 1 log unit above nno has caused quartz to become unstable and has also caused changes in silicate and fe ti oxide mineral compositions the andesitic magma is likely saturated with an h 2 o rich vapor phase in the upper part of the magma storage region melt h 2 o content is 4 7 wt

volcano wide fringes in ers synthetic aperture radar interferograms of etna 1992 1998 deformation or tropospheric effect mount etna 3300 m is the volcano that has been first and most studied by differential synthetic aperture radar previous papers gave evidence for a large scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models also atmospheric effects on mountainous areas are known to be very significant accordingly interferograms may reflect both deformation and tropospheric effects we investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms from 38 ers synthetic aperture radar ascending scenes taken from 1992 to 1999 we have computed 238 interferograms the amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters from a simple analysis of phase elevation relation we give evidence for the clear difference in pattern between stratified tropospheric effects and large scale deformation produced by a source in a three dimensional elastic body with topography using combination of inversion processes we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms finally time evolution of the parameters and modeling probability estimation are retrieved for each sar image estimated relative change in tropospheric delay varies in time from 2 7 to 3 0 1 2 fringes they are consistent with other tropospheric estimations at etna based on ground met and global positioning system observations on the same period of time magmatic volume variations remain below error bars and thus cannot be estimated properly finally some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods without the need to invoking a deformation of the volcano