multiple levels of magma storage during the 1980 summer eruptions of mount st helens wa transitions in eruptive style explosive to effusive sustained to pulsatory are a common aspect of volcanic activity and present a major challenge to volcano monitoring efforts a classic example of such transitions is provided by the activity of mount st helens wa during 1980 where a climactic plinian event on may 18 was followed by subplinian and vulcanian eruptions that became increasing pulsatory with time throughout the summer finally progressing to episodic growth of a lava dome here we use variations in the textures glass compositions and volatile contents of melt inclusions preserved in pyroclasts produced by the summer 1980 eruptions to determine conditions of magma ascent and storage that may have led to observed changes in eruptive activity five different pyroclast types identified in pyroclastic flow and fall deposits produced by eruptions in june 12 july 22 and august 7 1980 provide evidence for multiple levels of magma storage prior to each event highly vesicular clasts have h2o rich 4 5 5 5 wt melt inclusions and lack groundmass microlites or hornblende reaction rims characteristics that require magma storage at p 160 mpa until shortly prior to eruption all other clast types have groundmass microlites ph20 estimated from both h2o bearing melt inclusions and textural constraints provided by decompression experiments suggest pre eruptive storage pressures of 75 40 and 10 mpa the distribution of pyroclast types within and between eruptive deposits can be used to place important constraints on eruption mechanisms fall and flow deposits from june 12 1980 lack highly vesicular microlite free pyroclasts this eruption was also preceded by a shallow intrusion on june 3 as evidenced by a seismic crisis and enhanced so2 emissions our constraints suggest that magma intruded to a depth of 4 km beneath the crater floor fed the june eruption in contrast eruptions of july and august although shorter in duration and smaller in volume erupted deep volatile rich magma if modeled as a simple cylinder these data require a step wise decrease in effective conduit diameter from 40 50 m in may and june to 8 12 m in july and august the abundance of vesicular intermediate to deep clast types in july and august further suggests that this change was effected by narrowing the shallower part of the conduit perhaps in response to solidification of intruded magma remaining in the shallow system after the june eruption eruptions from july to october were distinctly pulsatory transitioning between subplinian and vulcanian in character as originally suggested by scandone and malone 1985 a growing mismatch between the rate of magma ascent and magma disruption explains the increasingly pulsatory nature of the eruptions through time recent fragmentation experiments spieler et al 2004 suggest this mismatch may have been aided by the multiple levels at which magma was stored and degassed prior to these events

transitions between explosive and effusive eruptions of silicic magmas water rich silicic magmas may erupt explosively giving rise to massive columns of fragmented ash1 5 or effusively as viscous bubbly lavas to form lava domes6 8 complex cycles of explosive and effusive eruptions are often observed8 12 although these two eruption mechanisms have been modelled independently1 8 the conditions under which a volcano exhibits a particular style of eruption are not known and the transitions between eruption styles are poorly understood by modelling the ascent of magma along a permeable conduit we show here that for small magma chamber overpressures the style of eruption explosive or effusive is dependent on the magma flow rate we also identify a critical overpressure above which only explosive eruptions occur complex eruption sequences follow naturally for example if a shallow magmatic system is supplied with magma at an intermediate flow rate the eruption will be slow and effusive until the chamber over pressure becomes too large an explosive eruption then relieves the overpressure and an effusive eruption style resumes we suggest that monitoring of temporal variations in chamber overpressure for example by measuring ground deformation can be used to assess whether a passively effusing volcano has the potential to erupt explosively

remarkable cyclic ground deformation monitored in real time on montserrat and its use in eruption forecasting telemetered high resolution tiltmeters were installed in montserrat in summer of 1995 in december 1996 and in may 1997 the 1995 installations several km from the soufriere hills vent were too distant to yield useful data however the 1996 and 1997 installations on the crater rim revealed 6 14 h inflation cycles caused by magma pressurization at shallow depths 0 6 km below the base of dome the tilt data correlated with seismicity explosions and pyroclastic flow activity and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

magma production and growth of the lava dome of the soufriere hills volcano montserrat west indies november 1995 to december 1997 from november 1995 to december 1997 a total volume of 246 106 dre m of andesite magma erupted partitioned into 93 106 m of the dome 125 106 m of pyroclastic flow deposits and 28 106 m of explosive ejecta in the first 11 weeks magma discharge rate was low 0 5 m s from february 1996 to may 1997 discharge rates have averaged 2 1 m s but have fluctuated significantly and have increased with time three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m s short term pulsations in growth lasting a few days reach discharge rates of over 10 m s and there are periods of days to a few weeks when dome growth is 0 5 m s discharge rate increased from may 1997 with an average rate of 7 5 m s to december 1997 the observations indicate an open magmatic system

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

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

effects of conduit geometry on magma ascent dynamics in dome forming eruptions abstract we develop a steady state two phase flow model of magma ascent through an axisymmetric conduit of variable radius r and length l in order to quantify relationships between conduit geometry and magma ascent dynamics holding boundary conditions and chamber magma properties constant we vary conduit geometry systematically and independently such that the upper conduit radius increases or decreases by a factor of r t r b radius ratio 0 4 r t r b 2 5 above a change initiation height h 0 1 h l 0 7 and over length l e l e l 0 2 where r t and r b are conduit radius above t and below b the radius change and h is the height above the top of the magma chamber conduit widening causes a drop in overpressure and corresponding increase in gas volume fraction and magma acceleration over the whole length of the conduit with all changes increasing in magnitude with increasing radius ratio magma ascent rate increases roughly as r 2 and volumetric flow rate subsequently increases as r 4 when r t r b r both increasing r t for a fixed r b increasing radius ratio and increasing r b for a fixed r t decreasing radius ratio increase volume flow and magma ascent rates compared to changes in geometry small changes in chamber pressure

periodic behavior in lava dome eruptions lava dome eruptions commonly display fairly regular alternations between periods of high activity and periods of low or no activity the time scale for these alternations is typically months to several years here we develop a generic model of magma discharge through a conduit from an open system magma chamber with continuous replenishment the model takes account of the principal controls on flow namely the replenishment rate magma chamber size elastic deformation of the chamber walls conduit resistance and variations of magma viscosity which are controlled by degassing during ascent and kinetics of crystallization the analysis indicates a rich diversity of behavior with periodic patterns similar to those observed magma chamber size can be estimated from the period with longer periods implying larger chambers many features observed in volcanic eruptions such as alternations between periodic behaviors and continuous discharge sharp changes in discharge rate and transitions from effusive to catastrophic explosive eruption can be understood in terms of the non linear dynamics of conduit flows from open system magma chambers the dynamics of lava dome growth at mount st helens 1980 1987 and santiaguito 1922 2000 was analyzed with the help of the model the best fit models give magma chamber volumes of 0 6 km3 for mount st helens and 65 km3 for santiaguito the larger magma chamber volume is the major factor in explaining why santiaguito is a long lived eruption with a longer periodicity of pulsations in comparison with mount st helens

dynamics of two phase conduit flow of high viscosity gas saturated magma large variations of sustained explosive eruption intensity the ascent of gas saturated magma in a conduit can lead to the transition from the laminar flow of a bubble rich melt to the turbulent flow of particle rich hot gas in upper part of the conduit this process is investigated with a help of a disequilibrium two phase flow model for steady and unsteady conditions for the description of different gas magma flow regimes in the conduit separated sets of equations are used the main difference from previous conduit flow models is the consideration of the pressure difference between a growing bubble and the surrounding melt for a bubbly liquid to evolve into a gas particle flow a critical overpressure value must be exceeded this transition region is simulated by means of a discontinuity namely by a fragmentation wave the magma flow calculations are carried out for a given conduit length and overpressure between the magma chamber and the atmosphere the steady solution of the boundary problem is not unique and there are up to five distinct steady regimes corresponding to fixed eruption conditions within the framework of the quasi steady approach governing parameters being varied monotonically the transition from one regime to another can take place suddenly and is accompanied by the fundamental restructuring of the conduit flow resulting in rapid or abrupt changes in the intensity of explosive eruptions abrupt intensification of an explosive eruption occurs when the chamber pressure becomes sufficiently less than saturation pressure and therefore corresponds to the case of shallow depth magma chamber and high initial water content a regime with minimum flow rate may relate to the growth of a lava dome following the explosive phase of eruption these models can explain geological observations that imply large and sudden changes of discharge rate in large magnitude explosive eruptions particularly at the transition between plinian phase and ignimbrite formation

temporal evolution of flow conditions in sustained magmatic explosive eruptions abstract the temporal evolution of fundamental flow conditions in the magma chamber plus conduit system such as pressure velocity mass flow rate erupted mass etc during sustained magmatic explosive eruptions was investigated to this aim simplified one dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together the chamber model assumed an homogeneous composition of magma and a vertical profile of water content the chamber could have a cylindrical elliptical or spherical rigid geometry inside the chamber magma was assumed to be in hydrostatic equilibrium both before and during the eruption since the time scale of pressure variations at the conduit inlet of the order of hours is much longer than the travel time of magma in the conduit of the order of a few minutes the flow in the conduit was assumed as at steady state the one dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked flow conditions at the exit bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law above the nucleation level bubbles and liquid magma were considered in mechanical equilibrium the same equilibrium assumption was made above the fragmentation level between gas and pyroclasts due to the hydrostatic hypothesis the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and through the conduit model as a function of time simulation results pertaining to rhyolitic and basaltic magmas defined at the volcanic eruption mechanism modeling workshops durham nh 2002 nice france 2003 are presented important flow variables such as pressure density velocity shear stress in the chamber and conduit are discussed as a function of time and magma chamber and conduit properties results indicate that vent variables react in different ways to the pressure variation of the chamber pressure density and mass flow rate show relative variations of the same order of magnitude as the conduit inlet pressure whereas velocity is more constant in time sill like chambers produce also significantly longer and more voluminous eruptions than dike like chambers water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption duration and volume maximum erupted mass fractions of about 0 2 are computed for small water saturated and shallow chambers

historical eruptions of merapi volcano central java indonesia 1768 1998 abstract information on merapi eruptive activity is scattered and much is remotely located a concise and well documented summary of this activity has been long needed to assist researchers and hazard mitigation efforts and the aim of this paper is to synthesize information from the mid 1700s to the present a descriptive chronology is given with an abbreviated chronology in a table that summarizes events by year assigns preliminary volcanic explosivity index vei ratings and hartmann classifications and provides key references the history of volcano monitoring is also outlined the study reveals that a major difference in eruption style exists between the twentieth and nineteenth centuries although the periodicity between larger events seems about the same during the twentieth century activity has comprised mainly the effusive growth of viscous lava domes and lava tongues with occasional gravitational collapses of parts of oversteepened domes to produce the nuees ardentes commonly defined as merapi type in the 1800s however explosive eruptions of relatively large size occurred to vei 4 and some associated fountain collapse nuees ardentes were larger and farther reaching than any produced in the twentieth century these events may also be regarded as typical eruptions for merapi the nineteenth century activity is consistent with the long term pattern of one relatively large event every one or two centuries based on the long term eruptive record deduced by others from volcanic stratigraphy it is uncertain whether or not a recurrence time model continues to apply to merapi but if so merapi could soon be due for another large event and its occurrence with only modest or inadequately appreciated precursors could lead to a disaster unprecedented in merapi s history because the area around the volcano is now much more densely populated

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

erosion processes in volcanic conduits and application to the ad 79 eruption of vesuvius abstract the flow of gas magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall erosion may be produced by the impact of pyroclasts on the conduit wall fluid shear stress at the wall conduit wall collapse and volcanic tremor using a two phase flow non equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process this abrasion process was found to be the largest near the conduit exit where the gas magma velocities are the largest the erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit depending on the wall roughness and yield strength of wall rocks this shear stress is the largest near the magma fragmentation level where the gas magma viscosity and velocity gradients are very large the collapse of conduit wall due to the difference between the gas magma and lithostatic pressures can occur below and above the magma fragmentation level causing the production of lithics directly when the wall collapses inward and indirectly when the wall collapses outward the effectiveness of different erosion mechanisms was tested with the magma characteristics conduit geometry and wall rock properties pertaining to the ad 79 eruption of vesuvius it was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit the conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments

from vulcanian explosions to sustained explosive eruptions the role of diffusive mass transfer in conduit flow dynamics abstract magmatic explosive eruptions are influenced by mass transfer processes of gas diffusion into bubbles caused by decompression melnik and sparks melnik o e sparks r s j 2002 modelling of conduit flow dynamic during explosive activity at soufriere hills volcano montserrat in druitt t h kokelaar b p eds the eruption of soufriere hills volcano montserrat from 1995 to 1999 geological society london memoirs 21 307 317 proposed two end member cases corresponding to complete equilibrium and complete disequilibrium in the first case diffusion is fast enough to maintain the system near equilibrium and a long lived explosive eruption develops in the latter case pre existing bubbles expand under conditions of explosive eruption and decompression but diffusive gas transfer is negligible this leads to a much shorter eruption here we develop this model to consider the role of mass transfer by investigating transient flows at the start of an explosive eruption triggered by a sudden decompression the simulations reveal a spectrum of behaviours from sustained to short lived highly non equilibrium vulcanian style explosions lasting a few tens of seconds through longer lasting eruptions that can be sustained for tens of minutes and finally to eruptions that can last hours or even days behaviour is controlled by a mass transfer parameter which equals n 2 3 d where n is the bubble number density and d is the diffusivity the parameter is expected to vary between 10 5 and 1 s 1 in nature and reflects a time scale for efficient diffusion the spectrum of model behaviours is consistent with variations in styles of explosive eruptions of silicic volcanoes in the initial stages peak discharges occur over 10 20 s and then decline to low discharges if a critical bubble overpressure is assumed to be the criterion for fragmentation then fragmentation may stop and start several times in the declining period causing several pulses of high intensity discharge for the cases of strong disequilibria the fluxes can decrease to negligible values where other processes such as gas escape through permeable magma prevents explosive conditions becoming re established so that explosive activity stops and dome growth can start for cases closer to the equilibrium the eruption can evolve towards a quasi steady sustained flow never declining sufficiently for gas escape to become dominant

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

crustal deformation in the mid may 1991 crisis preceding the extrusion of a dacite lava dome at unzen volcano japan abstract tilt observations at fg1 station located about 680 m west of the central crater of unzen volcano southwestern japan revealed details about the process of magma ascent during the mid may 1991 crisis just before the appearance of a lava dome crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike modeled by a vertical line source and a planer tensile dislocation in an elastic half space marked change in tilt was observed at first in the ew direction at fg1 during may 11 14 suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater after the interruption of the rise of the magma column magma began to intrude laterally as a dike on may 15 toward approximately the n80 w direction this resulted in a large tilt change essentially in the ns component at fg1 the horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m on may 17 the magma column started to rise again probably because the impediment to magma ascent was fractured by the growth of the dike the top of the magma column finally reached to the crater bottom probably on may 19 and a new lava dome was actually observed on may 20 after which the remarkable upward tilt to the south at fg1 due to the dike intrusion decreased rapidly the deformation after may 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of may 1991 although large errors are inevitable in estimated values because of a lack of sufficient data the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions

control of magma flow in dykes on cyclic lava dome extrusion 1 lava dome eruptions are commonly characterized by large fluctuations in discharge rate with cyclic behaviour on time scalesranging from hours to decades examples include bezymianny volcano russia merapi java santiaguito guatemala mt st helens usa mt unzen japan and soufriere hills volcano montserrat previous models have assumed simple cylindrical conduits for magma transport but extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels the widths of dykes embedded in an elastic medium are influenced by local magma pressure affecting flow rates and system dynamics strongly we develop a model for magma flow in dykes which predicts intense pulsations of magma extrusion for the case of a constant source pressure the period time scale is determined by the elastic deformation of the dyke walls and the length to width ratio of the dyke the dyke acts like a volumetric capacitor storing magma as pressure increases and then releasing magma in a pulse of extrusion for the soufriere hills volcano cyclic extrusions with time scales of a few weeks are predicted for dykes 300 500 m long and 3 6 m wide matching observations the model explains the sharp onset of tilt pulsations and seismic swarms citation costa a o melnik r s j sparks and b voight 2007 control of magma flow in dykes on cyclic lava dome extrusion geophys res lett 34 l02303 doi 10 1029 2006gl027466

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

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

transient two dimensional dynamics in the upper conduit of a rhyolitic eruption a comparison of closure models for the granular stress to elucidate the role of particle collisions in redistributing momentum after fragmentation a numerical study was performed comparing the behavior of an inviscid or collision less granular material with a granular material whose viscosity and pressure were modeled using kinetic theory the granular viscosity calculation is sensitive to particle size and three particle sizes were considered 0 0002 m 0 002 m and 0 02 m a critical volume fraction of gas 0 75 identifies the onset of fragmentation dividing the region of rhyolitic magma with dispersed bubbles from the region of turbulent gas with dispersed particles the transient simulations can be divided into two dynamic regimes an initial shock followed by a transition to steady state during the initial shock phase treatment of the granular pressure and viscosity led to greater particle velocities relative to the inviscid calculation due to the development of higher gas pressure at fragmentation however as steady state is approached the viscosity slows the particle phase relative to the inviscid counterpart for particles greater than a millimeter the modeled sub millimeter particle velocity was insensitive to the treatment of the granular pressure and viscosity centimeter scale particles have a much higher granular viscosity up to 10 1 pa s and the kinetic theory calculation is thus relevant in these conditions after reaching steady state the differential velocity between gas and particles at the conduit exit correlates with particle size for both the inviscid and granular viscosity calculations negligible differential velocity develops for sub millimeter particles but centimeter scale particles exit the conduit with 5 differential velocity between phases

controls of conduit geometry and wallrock elasticity on lava dome eruptions abstract many lava dome building eruptions show periodic to complex non periodic pulsatory activity typical time scales associated with this activity range from hours to decades previous studies modelled the ascent of magma using a set of transient 1 d transport equations accounting for degassing induced crystallization kinetics gas exsolution and viscosity increase due to crystal growth these models assumed flow in a cylindrical conduit with a fixed cross section area since several observations suggest that extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi axes changing with depth quasi static elastic deformation of the dyke is accounted by an analytical solution that couples cross section area with the magmatic overpressure the effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated the presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time scales and cyclicity patterns with respect to a uniform cylindrical conduit there is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber intermediate regimes are possible periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke our study emphasizes the strong non linearities and complex behaviours of lava dome eruptions from a forecasting and hazard perspective intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable on the other hand lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while

textures water content and degassing of silicic andesites from recent plinian and dome forming eruptions at mount pelee volcano martinique lesser antilles arc previous petrological and phase equilibrium experimental studies on recent silicic andesites from mount pelee volcano have evidenced comparable pre eruptive conditions for plinian and dome forming pelean herein eruptions implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the mount pelee magmas during eruption to further investigate the degassing conditions of plinian and pelean magmas of mount pelee we study here the most recent mount pelee s products p1 at 650 years b p 1902 and 1929 eruptions which cover a range of plinian and pelean lithologies for bulk rock vesicularities glass water contents glass inclusions in phenocrysts and matrix glasses and microtextures water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally deduced pre eruptive melt water content i e 5 3 6 3 wt whereas they are much lower in the dominant pelean lithologies crystalline poorly vesicular lithics and dome samples this indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre eruptive water contents the water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation determination of the water contents of both the pre eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn eruptive degassing we find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones typically 90 100 and 65 70 of the initial water content respectively the vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices consistent with a closed system near equilibrium degassing up to fragmentation by contrast the low residual water contents low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open system degassing microtextural features including silica bearing and silica free voids in the pelean lithologies may represent a two stage vesiculation