numerical simulation of strain within lava domes abstract a new analytical and numerical method is presented to assess 3d deformation within lava domes the method allows any simultaneous combination of strain components even that of a pure shear and six elementary simple shear components the method s practical use is presented model results provide the following information for lava dome strain patterns in the upper part of a dome stretching is perpendicular to flow direction and in the lower part it is parallel flinn parameter values indicate a flattening strain ellipsoid through the dome except in a small zone in the centre where it is a constriction ellipsoid

failure of volcano slopes magmatic intrusions can initiate and sustain massive and catastrophic volcano collapse their role is twofold involving both driving and resisting forces first flank stability is diminished by magmastatic and magma overpressures and steepened slopes that accompany intrusion second excess pore pressures in potential failure zones can be generated as a result of intrusion related mechanical or thermal straining of the rock fluid medium pressurized retrograde boiling in high level magma chambers or hydrothermal fluid circulation also earthquakes may aid collapse through inertial forces and shaking induced pore pressure generation these excess pore pressures reduce the sliding resistance as shown for wedge shaped slide blocks for selected cases the destabilizing influence of mechanically induced pore pressures is maximized as the intruded width or corresponding overpressure of the intrusion is increased the destabilizing influence of thermally induced pore pressures is conditioned by the seve

instability of exogenous lava lobes during intense rainfall on many volcanoes there is evidence of a relationship between dome collapse and periods of high precipitation we propose a mechanism for this relationship and investigate the conditions that optimize failure by this process observations of elongate lobes that evolve through exogenous growth of lava domes reveal that they commonly develop tensile fractures perpendicular to the direction of motion these cracks can increase in depth by localized cooling and volumetric contraction during periods of high rainfall water can fill these cracks and the increase in fluid pressure on the base of the lobes and within the crack can trigger the collapse of the hot exogenous lava domes using limit equilibrium analysis it is possible to calculate the water and vapor forces acting on the rear and base of the potentially unstable part of the lobe the model presented is rectangular in cross section with material properties representative of andesitic dome rocks vapor pressures at the base of cracks are sealed by the penetrating rainfall which forms a saturated cap within the lobe this leads to an increase in fluid pressurization both through the underlying gas pressure and the downslope component of the liquid water cap fluid pressurization increases as the penetration depth increases this rainfall penetration depth is dependent on the thermal properties of the rocks antecedent temperature lobe geometry and the intensity and duration of precipitation dominant parameters influencing the stability of the lobe are principally lobe thickness duration and intensity of rainfall and antecedent lobe temperature our modeling reveals that thicker lobes are intrinsically more unstable due to the amplification of downslope forces in comparison to cohesive strength the increase in the duration and intensity of rainfall events also increases the potential for collapse as it leads to deeper liquid penetration deeper penetration depths are also achieved through lower antecedent temperatures since less fluid is lost through vaporization thus the potential for rain triggered collapse increases with time from emplacement

growth of the lava dome and extrusion rates at soufriere hills volcano montserrat west indies 2005 2008 the third episode of lava dome growth at soufriere hills volcano began 1 august 2005 and ended 20 april 2007 volumes of the dome and talus produced were measured using a photo based method with a calibrated camera for increased accuracy the total dense rock equivalent dre volume of extruded andesite magma 306 51 mm3 was similar within error to that produced in the earlier episodes but the average extrusion rate was 5 6 0 9 m3s 1 dre higher than the previous episodes extrusion rates varied in a pulsatory manner from 0 5 m3s 1 to 20 m3s 1 on 18 may 2006 the lava dome had reached a volume of 85 mm3 dre and it was removed in its entirety during a massive dome collapse on 20 may 2006 extrusion began again almost immediately and built a dome of 170 mm3 dre with a summit height 1047 m above sea level by 4 april 2007 there were few moderate sized dome collapses 1 10 mm3 during this extrusive episode in contrast to the first episode of dome growth in 1995 8 when they were numerous the first and third episodes of dome growth showed a similar pattern of low 0 5 m3s 1 but increasing magma flux during the early stages with steady high flux after extrusion of 25 mm3

an overview of lava dome evolution dome collapse and cyclicity at soufriere hills volcano montserrat 2005 2007 1 the third episode of lava dome growth at soufriere hills volcano montserrat was characterised by higher average magma discharge rates than either previous dome growth episode at this volcano and yet fewer collapses during sustained dome growth at moderate high average rates 6 m3 s we identified 2 6 week discharge pulses that each supplied c 20 mm3 magma from depth our observations are consistent with some existing models but we explain discrepancies by a combination of higher volatile contents and higher ascent rates cycles of c 11 16 days were evident in rockfall lp rockfall and shallow lp earthquake counts related to dome growth and degassing we speculate that degassing at the conduit margins together with stick slip conduit flow may drive these cycles only one major collapse 10 mm3 occurred during the third episode on may 20 2006 as a new magma pulse entered the dome and coincided with heavy rainfall

anatomy of a lava dome collapse the 20 march 2000 event at soufriere hills volcano montserrat abstract a second extrusive phase of the currently ongoing 1995 2003 eruption of soufriere hills volcano shv montserrat commenced in mid november 1999 following 19 months during which no fresh lava had reached the surface by mid march 2000 a new andesite lava dome constructed within a collapse scar girdled by remnants of the 1995 1998 dome complex had attained an estimated volume of 29 3 million m3 mm3 on 20 march 2000 during a period of heavy rainfall on the island a significant collapse event ensued that removed 95 of the new lava dome 28 3 mm3 during 5 hours of activity that generated 40 pyroclastic flows and at least one magmatic explosion the associated ash cloud reached an altitude of 9 km and deposited ash on the island of guadeloupe to the southeast and a number of lahars and debris flows occurred in valleys on the flanks of shv a large quantity of observational data including contemporaneous field observations and continuous data from the broadband seismic network on montserrat allow a detailed reconstruction of this dome collapse event in contrast to most of the large dome collapses at shv the 20 march 2000 event is distinguished by a lack of short term precursory elevated seismicity at shallow depths beneath the lava dome broadband seismic amplitude data recorded during the event are used to infer the cumulative volume of collapsed dome as the collapse progressed these data indicate that the high velocity pyroclastic flows observed at the climax of the event removed by far the largest portion 68 of the lava dome at peak discharge rates estimated from the seismic record of 2 104 m3 s 1 following the 20 march 2000 collapse lava dome growth recommenced immediately and continued without significant interruption until another larger dome collapse occurred on 29 july 2001 the 29 july 2001 event also coincided with heavy rainfall on montserrat matthews et al 2002 geophys res lett 29 doi 10 1029 2002gl014863 and lacked precursory elevated seismic activity we attribute the initiation of the 20 march 2000 collapse to a prolonged spell of heavy rainfall on the lava dome prior to and during the event the precise causal mechanism remains controversial though some combination of mechanical erosion and or destabilization of a critically poised face of the lava dome the action of pressurized steam or water on potential failure surfaces within the dome rapid cooling of hot lava and small phreatic explosions seems likely anecdotal evidence exists for other rainfall induced activity on montserrat and the triggering of explosive or pyroclastic flow activity by rainfall has been noted at dome forming volcanoes elsewhere including merapi indonesia voight et al 2000 j volcanol geotherm res 100 69 138 unzen japan yamasato et al 1997 papers meteorol geophys 48 santiaguito guatemala smithsonian institution global volcanism network bull 15 1990 and mount st helens usa mastin 1994 geol soc am bull 106 175 185 hazard mitigation plans at dome forming volcanoes would therefore benefit from the inclusion of meteorological forecasting and rain monitoring equipment particularly in the tropics

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

theory of dike intrusion in a saturated porous solid a study of the pore pressure fields that develop around intruding dikes is described under the motivation that intruded geometry may be determined from pore pressures recorded in relatively remote monitoring wells the pore pressure fields induced around cylindrical and planar intrusions are described as analogues to moving point or line dislocations within an infinite saturated porous elastic medium the resulting transient pressure fields reduce to an equivalent steady state when viewed from the advancing front solutions for the moving point and line dislocations yield a dependence on common dimensionless groupings thus dimensionless pressure rise accompanying intrusion and recorded at a static location may be uniquely referenced to the dimensionless parameters representing emplacement velocity and time the resulting transient pressure response may be divided into two groups representing fast and slow emplacement where hydraulic parameters representing the host porous medium are known a priori both emplacement location and the cross sectional area may be determined uniquely for slow emplacement the intrusion rate may also be determined only a lower limiting intrusion rate may be discerned for fast emplacement field data are rare but two intrusive events at krafla iceland are examined using the proposed moving dislocation models predicted location and lower limiting intrusion rate of the krafla dikes compare favorably with field observation despite more than a 9 km separation to the monitoring well

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

lava production at soufriere hills volcano montserrat 1995 2009 1 we estimate that about 1 km3 of andesitic lava has been produced at soufriere hills volcano montserrat from 1995 to 2009 there were three major episodes of extrusion each lasting about 2 to 3 5 years and producing about 280 to 340 m m3 of lava and one minor episode our estimates account for the dense rock equivalent volumetric contributions from the core and talus components of the lava dome pyroclastic flow deposits and air fall deposits by 2005 at least two thirds of the erupted mass has already entered the sea the average lava flux across the major extrusion episodes has been 3 5 m3s 1 with short period 10 15 days pulses up to 10 20 m3s 1 the first and third episodes of extrusion show similar flux histories suggesting similar behaviour of the system ten years apart waning flux towards the end of each episode may be caused by declining overpressure in the magma reservoir

column on column structures as indicators of lava ice interaction ruapehu andesite volcano new zealand abstract lava flows of the mangawhero formation ca 15 60 ka on ruapehu volcano erupted during the last glaciation in a distal flow lobe at tukino on the east side of the mountain small secondary columns 10 20 cm thick have formed on the sides of large rectangular primary 0 5 3 m thick cooling columns thick 10 m zones of such small columns form a lateral and basal outer rind of the lobe as they do not mark glassy zones of quenching these secondary columns are interpreted as being formed by a second cooling event at temperatures below the boundary between the low creep and elastic regimes 600 c by rapid influx of copious amounts of water temperature drops deduced from extensional strains of the two sets of columns were used to gauge the viability of such a two stage process absence of reliable data on andesite contraction coefficients was overcome by using a sliding scale to assess a large range of values the estimates indicate that two stage chilling is feasible after flowing across relatively ice poor terrain the lava flow must have interacted with a valley glacier that provided water for further chilling the already formed primary columns and formation of the outer rind small columns given this evidence for lava ice interaction it is likely that prominent thick flows elsewhere in the mangawhero formation may have been constrained to their ridge top locations by ice conditions similar to those described by lescinsky and sisson lescinsky d t sisson t w 1998 ridge forming ice bounded lava flows at mount rainier washington geology 26 351 354

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

numerical modelling of the growth dynamics of a simple silicic lava dome 1 two dimensional axisymmetrical computational models have been developed using finite element techniques to represent the rheological stiffening due to degassing induced crystallisation and the effects of a brittle carapace within a simple lava dome the dome is modelled numerically considering the effects of temperature pressure and rate of extrusion using these state variables the bulk viscosity is derived from empirical equations and the bulk rheology approximated using a solidification viscosity dividing the dome into solid dominated and fluid dominated domains elastoplastic equations for the solid component and fluid mechanical equations for the fluid domain are solved simultaneously allowing continuous stress across the fluid solid interface to ensure the two components move in unison we apply this model a hookean solid carapace model and a purely fluid lava dome model to the case of the october 1980 lava dome at mount st helens the results show that dynamic growth models involving a solid carapace are required to simulate this lava dome

solidifying bingham extrusions a model for the growth of silicic lava domes in a previous study of the effects of cooling and solidification on flows issuing onto a horizontal plane and spreading under gravity we considered the case of a viscous fluid that solidifies to form a thin surface crust with a finite yield strength in that case the coupling of solidification and viscous stresses in the flow led to a sequence of flow regimes or styles of flow and crustal deformation here we study the spreading from a small source of a plastic material having a yield strength before cooling in this case the fluid again begins to freeze as it spreads radially under gravity and forms a dome having a surface crust which is stronger than the extruded fluid if cooling is sufficiently rapid compared to gravity driven spreading the flow is found to be controlled by solidification the flow again takes on one of a number of flow regimes depending on the pace of solidification relative to the rate of lateral flow or extrusion rate however these flow regimes are quite different from those for the viscous extrusions implying that the internal yield stress has a strong influence on the behaviour styles of flow ranged from inflation of an axisymmetric dome to irregular extrusion of lateral lobes and vertical spines these qualitatively different regimes have much in common with the eruption styles of volcanic lava domes produced by effusion of extremely viscous silicic magmas which may possess a yield strength and the model provides information about the factors influencing the morphology and hazards of such volcanic flows

on magma chamber evolution during slow effusive eruptions 1 we explore some of the controls on the evolution of a slow effusive eruption imposed by processes in a shallow magma reservoir we envisage that the emplacement of basaltic magma into a shallow evolved crustal reservoir generates sufficient overpressure to trigger the slow effusive eruption of the overlying layer of evolved magma the subsequent evolution of the system then depends on a number of factors including 1 the volatile contents of the two magmas 2 the rate of any continuing influx of basalt 3 the cooling heating rates of the two layers of magma and 4 the eruption rate of the silicic magma where we assume that the crystals and bubbles remain in the layer in which they formed any exsolved volatiles in the magma increase the compressibility of the magma in a waning system this tends to increase the volume of material which needs to be erupted to relieve a given overpressure by an amount of order 10 100 and hence increases the duration of the eruption typical calculations suggest that eruptions from magma chambers of size 1 10 km3 may persist for times of order 1 10 years if the main body of silicic magma is saturated in volatiles in addition cooling of the basalt leads to formation of dense crystals which lowers the pressure however if the magma becomes saturated exsolution of volatile gases occurs tending to increase the pressure again in a waning system this tends to extend the eruption duration and increase the mass erupted from the chamber for chamber volumes of 1 10 km3 cooling rates of 10 6 k s can increase the eruption volume by a factor of order 10 rapid cooling may also lead to an initial increase in the eruption rate while the chamber pressure is controlled by the rate of production of new bubbles if there is an influx of basalt to the chamber which is maintained during the eruption then the evolution of the eruption is primarily dependent on this input rate although the cooling of the basalt may increase the eruption rate to values in excess of this input rate especially at an early stage in the eruption the complex and nonlinear evolution of the eruption rate predicted by our simple model illustrates the sensitivity of the eruption rate to the magma volatile content and magma cooling rates such understanding is key in developing quantitative models for hazard assessments of slow effusive eruptions

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

discrete element simulations of bed force anomalies due to force chains in dense granular flows abstract surficial flows on volcanic edifices are often comprised of high concentrations of granular materials either derived from juvenile eruptive material or material sourced from an unstable edifice although much attention has been given to granular flow mobility the physics that govern flow internal behavior and substrate interaction are still poorly understood recent laboratory experiments have shown that force chains transmit extreme localized forces to the substrates of free surface granular flows in this work we combine computational and experimental approaches to investigate the forces at the bed of a simplified granular flow a photoelastic experimental approach is used to resolve discrete forces in the granular flows we also conduct discrete element model dem simulations using input parameters derived from measureable physical material properties of experimental and natural materials that successfully reproduce the experimental bed forces generated by filamentary force chain structures observed in free surface granular flows the dem results show particular sensitivity to values prescribed for contact stiffness which is in contrast with previous reports indicating that stiffness values have insignificant influence on flow behavior the transient forces at the bed generated by force chain structures can greatly exceed by several 100 the bed forces predicted from continuum approaches and our simulation results imply that natural materials are more prone to excessive bed forces than photoelastic materials due to their larger contact stiffnesses this work suggests that force chain activity may play an important role in the bed physics of dense volcanic flows by influencing substrate entrainment

large eddy simulation of gas particle kinematic decoupling and turbulent entrainment in volcanic plumes abstract in the framework of the iavcei international association of volcanology and chemistry of the earth interior intercomparison study on volcanic plume models we present three dimensional 3d numerical simulations carried out with the ashee ash equilibrium eulerian model the ashee model solves the compressible balance equations of mass momentum and enthalpy of a gas particle mixture and is able to describe the kinematic decoupling for particles characterized by stokes number i e the ratio between the particle equilibrium time and the flow characteristic time lower than 0 2 or particles smaller than about 1 mm the computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a large eddy simulation approach and is thus suited to analyze the role of particle non equilibrium in the dynamics of turbulent volcanic plumes the two reference scenarios analyzed correspond to a weak mass eruption rate 1 5 10 6 kg s and a strong volcanic plume mass eruption rate 1 5 10 9 kg s in absence of wind for each scenario we compare the 3d results averaged in space and time with theoretical results obtained from integral plume models such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid scale turbulence model and the influence of gas particle non equilibrium processes on the large scale plume dynamics we thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant of the order of 20 but still lower than that typically associated with input data and integral model approximations in the weak plume case 3d results are consistent with the predictions of integral models in the jet and plume regions with an entrainment coefficient around 0 10 in the plume region in the strong plume case the self similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable with an average value of 0 24 for both cases integral model predictions diverge from the 3d plume behavior in the umbrella region the presented analysis of 3d numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies in addition high resolution 3d runs allow reproduction of observable quantities such as infrasound signals which can be useful for constraining eruption dynamics during real events

experiments on internal strain in lava dome cross sections simple experiments have been conducted to study the strain evolution in lava dome cross sections a viscous fluid is injected vertically from a reservoir into a feeding conduit silicone putty is used as analogue magma two dimensional experiments allow the assessment of the internal strain within the dome particle paths are symmetrical on either side of a central line passing through the feeding conduit and display parabolic trajectories the highest strain zone is located above the extrusion zone in cross sections stretch trajectories show a remarkable concentric pattern wrapping around the extrusion zone of the analogue magma to the lateral margins a triple junction of stretch trajectories defines an isotropic point in the strain field in the main central part of the dome an intermediate zone of reversed sense of shearing is caused by a change in the sign of the velocity gradient with respect to that in the upper and lower zones knowledge of this evolving strain pattern can provide a better understanding of the evolution of natural domes also it can help to unravel the kinematic history of ancient domes partly removed by erosion

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

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

non newtonian rheological law for highly crystalline dome lavas volcanic eruption models are hampered by the lack of multiphase magmatic flow laws most rheological models estimate the viscosity of multiphase lavas via the einstein roscoe equation but this simplification cannot be used for high crystallinity and it does not consider the non newtonian strain rate dependence of viscosity we carried out parallel plate experiments on natural samples to simulate multiphase lava deformation under various stresses and strain rates multiphase lavas exhibit an important component of shear thinning and appear to invalidate the adequacy of einstein roscoe based formulations for highly crystalline lava rheology the remarkable singular dependence of viscosity on strain rate yields a novel universal rheology law at eruptive temperatures t i e log 0 993 8974 t 0 543 log our work reveals the importance of considering microcracking and viscous dissipation at very high strain rate 10 3 s 1 explaining the occurrence of seismic swarms along the conduit margins and consequently supporting plug like magma ascent models

discrete element simulations of gravitational volcanic deformation 2 mechanical analysis 1 particle dynamics simulations of granular piles constructed upon planar substrates with different shear strengths model the gravitational deformation of natural volcanoes results reveal direct correlations among pile morphology layer stratigraphy deformation structures and stress field dependent upon the shear strength along the base of the pile piles constructed above a strong cohesive substrate that prevents basal slip develop outward dipping layers through particle avalanching down angle of repose slopes in the absence of basal cohesion slip occurs along a basal decollement if the topographically induced shear stresses exceed the basal shear strength for friction coefficients decreasing from 0 3 to 0 1 fault planes migrate progressively inward defining a transition in deformation mode from shallow slumping to deep seated landsliding and finally to full gravitational spreading the variations in deformation mode reflect the dependence of internal fault geometry on basal strength in accordance with critical coulomb wedge theory in order for slip to occur both internal and basal failure criteria must be met simultaneously topographically induced shear stresses increase outward along the base of the pile enabling decollement sliding once the basal failure criterion is met normal faults arise from the inside edge of the slipping decollement accommodating downward and outward displacements of the flanks surface slopes evolve to maintain equilibrium between gravitational driving and basal resisting stresses this simple mechanical understanding of gravitational deformation of granular piles provides insight into the controls on modes of volcanic deformation providing a predictive tool for understanding volcanic evolution and hazards in many settings

the pre eruption conditions for explosive eruptions at merapi volcano as revealed by crystal texture and mineralogy abstract we analyze the textures and mineralogy of merapi tephra generated during explosive vei 3 4 eruptions over the past 2000 years and compare these data with those observed for merapi dome and flow lavas we find that the merapi pumiceous tephra and lava textures differ significantly with respect to small size crystal populations but that phenocryst textures are generally similar a similar initial phase of crystallization is indicated for tephras and lavas in mid crustal 10 km depth reservoirs subsequent textural differences are mainly affected by ascent rate and degassing during ascent and for dome lavas with temporary storage in shallower reservoirs these differences also correspond to different eruptive styles our analyses include study of pumices lava and breadcrust bomb samples including samples from some of the most recent explosive episodes prior to the 2010 eruption textural analyses of youthful breadcrust bomb samples yield insights on one type of transition between effusive and explosive eruptive styles involving pressure build up under a degassed crystalline shallow conduit plug in general comparison of the crystal size distributions and calculated residence times among the effusive and explosive eruptive styles suggests that the two main magma product types resided for similar lengths of time in a mid crustal reservoir before ascending toward the surface and either erupting explosively tephra or stagnating in a shallow magma chamber prior to extrusion lava the interpretation is supported by the occurrences of amphibole in pristine condition in tephra and in altered state in lava finally the 1872 and 2010 explosive eruptions are examined and compared with others over the past three millennia the 2010 bulk rock compositions overlap with products of other major explosive merapi eruptions such as the ngrangkah tegalsruni temusari and kepuharjo tephras the 2010 products show a gradational late stage mafic enrichment dissimilar to 1872 which showed little

discrete element simulations of gravitational volcanic deformation 1 deformation structures and geometries 1 we have carried out two dimensional particle dynamics simulations of granular piles subject to frictional coulomb failure criteria to gain a first order understanding of different modes of gravitational deformation within volcanoes under uniform basal and internal strength conditions granular piles grow self similarly developing distinctive stratigraphies morphologies and structures piles constructed upon cohesive substrates exhibit particle avalanching forming outward dipping strata and angle of repose slopes systematic decreases in basal strength lead to progressively deeper and steeper internal detachment faults and slip along a basal decollement landslide forms grade from shallow slumps to deep seated landslide and finally to axial subsidence and outward flank displacements or volcanic spreading surface slopes decrease and develop concave up morphologies with decreasing decollement strength depositional layers tilt progressively inward spatial variations in basal strength cause lateral transitions in pile structure stratigraphy and morphology this approximation of volcanoes as coulomb granular piles reproduces the richness of deformational structures and surface morphologies in many volcanic settings the gentle slopes of hawaiian volcanoes and olympus mons on mars suggest weak basal decollements that enable volcanic spreading high angle normal faults favored above weak decollements are interpreted in both settings and may explain catastrophic sector collapse in hawaii and broad aureole deposits surrounding olympus mons in contrast steeper slopes and shallow detachment faults predominate in the canary islands thought to lack a weak decollement favoring smaller more frequent slope failures than predicted for hawaii the numerical results provide a useful predictive tool for interpreting dynamic behavior and associated geologic hazards of active volcanoes

physical processes in volcanic eruptions mechanisms are modeled in terms of dynamic physico chemical processes in this respect volcanology shares with much of earth science in general the benefits that accrue from the application of the methods and concepts of other disciplines to its problems basic concepts from the fields of thermo dynamics fluid mechanics and rheology have been particularly impor tant in fostering new insights recent developments have been so numerous as to make it necessary to limit the scope of this review and thus we concentrate here on the basic physical processes involved in the rise of magma to the surface and the distribution of cruption products on thc ground or in the atmosphere the perspective of planetary volcanism wilson hcad 1983 underlines the fact that volcanoes can be regarded as probes of the composition and structure of a planetary interior as a function of position depth and time