physical models of volcanic eruptions abstract our present understanding of the dynamics of volcanic eruptions is summarized a few important facts about volcanic eruptions are reviewed and outstanding problems are identified current research efforts are aimed at explaining why an eruption may alternate between and pass through different dynamical regimes the basic physics of eruptions are explained from a dynamical point of view with emphasis on the processes acting to fix the gas content of the erupted material the coupling between flow processes and pressure release leads to large changes of decompression rate during ascent in a volcanic conduit the decompression rate may reach very high values which suggests that the kinetics of degassing play an important role accurate values of the solubility of the major magmatic volatile species are required at pressures of

in the volcano s shadow earlier this year unzen volcano erupted killing 42 people what follows is an account of how japan s volcano coordinating committee dealt with this and similar events

mobility of pyroclastic flows and surges at the soufriere hills volcano montserrat the soufriere hills volcano on montserrat has produced avalanche like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity pyroclastic flows associated with dome collapse generate overlying dilute surges which detach from and travel beyond their parent flows the largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall h run out distance l area inundated a and volume transported v dome collapse flow mobilities characterised by either l h or a v2 3 resemble those of terrestrial and extraterrestrial cold rockfalls dade and huppert 1998 in contrast fountain fed pumice flows and fine grained secondary pyroclastic flows travel slower but for comparable initial volumes and heights can inundate greater areas

the eruptive rate and history of kuju volcano in japan during the past 15 000 years abstract the eruptive history of kuju volcano on kyushu japan during the past 15 000 years has been determined by tephrochronology and 14c dating kuju volcano comprises isolated lava domes and cones of hornblende andesite together with aprons of pyroclastic flow deposits on its flanks kuju volcano produced tephras at roughly 1000 yr intervals during the past 5000 years and 70 of the domes and cones have formed during the past 15 000 years the youngest magmatic activity of kuju volcano was the 1 6 km3 andesite eruption about 1600 years ago which emplaced a lava dome and block and ash flow kuju volcano shows a nearly constant long term eruption rate 0 7 0 4 km3 for 1000 years during the past 15 000 years this rate is within the range of estimated average eruption rates of late quaternary volcanoes in the japanese arc but is about one order of magnitude higher than the eruption rate of unzen volcano kuju volcano has been in phreatic eruption since october 1995 the late quaternary history of kuju indicates that it poses a significant volcanic hazard primarily due to block and ash flows from collapsing lava domes

generation of pyroclastic flows and surges by hot rock avalanches from the dome of mount st helens volcano usa several hot rock avalanches have occurred during the growth of the composite dome of mount st helens washington between 1980 and 1987 one of these occurred on 9 may 1986 and produced a fan shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic flow deposit laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash cloud surge swept beyond the limits of the pyroclastic flow plumes that rose 2 3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event but no explosion occurred all the facies observed originated from a single avalanche erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater and a small flood that carried juvenile and other clasts north of the crater a second broadly similar event occured in october 1986 larger events of this nature could present a significant volcanic hazard

exogenous and endogenous growth of the unzen lava dome examined by satellite infrared image analysis abstract phase 1 of the most recent eruption of unzen volcano japan occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome the temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events however such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the mount st helens dome in that case using detailed photogrammetry here we investigate these competing dome growth processes at unzen via analysis of their thermal emission signals present in shortwave infrared swir satellite imagery we find that both the endogenous and exogenous growth styles exhibit identifiable swir signatures whose magnitude can be related to their respective magma supply rates we find that the swir radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate analysis of time series imagery indicates that like mount st helens unzen experienced a temporally decreasing magma discharge rate during phase 1 as time progressed and the discharge rate fell the growth style of the unzen dome changed from exogenous only through exogenous decreasing and endogenous increasing to endogenous only thus relative growth rates at the endogenous and exogenous areas of the unzen dome appear to be at least partly controlled by the overall magma discharge rate as with mount st helens the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent which impeded exogenous growth more effectively as time elapsed and the discharge rate fell these results suggest the similarity of the processes operating at the unzen and mount st helens domes though important differences remain in that dome growth was basically continuous at unzen whilst at mount st helens growth episodes were interspersed by significant periods of repose

stress tensor analysis of the 1998 1999 tectonic swarm of northern quito related to the volcanic swarm of guagua pichincha volcano ecuador the phreatic activity and the subsequent dacitic dome growth in 1998 1999 at guagua pichincha volcano ecuador were associated with two seismic swarms one located in the northern part of quito population 1 500 000 and another one just below the active volcano about 15 20 km sw from the first one quito swarm tectonic events have high frequencies from 1 to 10 15 hz we registered more than 3200 events among which 2354 events of 1 4 m l 4 2 between june 1998 and december 1999 at the 2 and 17 km depth the volcanic events below the guagua pichincha caldera have high from 1 to 10 15 hz and low less than 3 hz frequencies approximately 130 000 events were registered between september 1998 and december 1999 at the 2 4 and 3 5 km depth here we study the stress tensors of these two swarms deduced from the polarities of p first motions and compare them to the regional stress tensor deduced from cmt harvard focal mechanisms the quito swarm stress tensor is relatively close to the regional stress tensor the 1 axis was oriented n117 e close to the n102 e direction of the plate motion found by the gps measurement and 3 is nearly vertical the difference may be due to the action of the closely active guagua pichincha volcano the guagua pichincha stress tensor is very different from the regional tectonic one the 1 axis of the volcano is oriented n214 e almost perpendicular to the 1 of the swarm of quito and 3 is almost horizontal even if these two tensors are different they can be explained in a more general tectonic scheme the almost horizontal direction of 3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the guagua pichincha volcano the movement of the fluids magma gas and or groundwater produced by the closely active guagua pichincha volcano seems to have an influence in the acceleration of the generation of seismic events

using a discrete element approach to model lava dome emplacement and collapse abstract lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity the mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement we use a discrete element method implemented in the modelling software particle flow code to investigate lava dome growth but also go further to test the stability of the dome under the following conditions increased internal pressure switch in extrusion direction caused by partial cooling of the dome and extrusion of lava onto variable underlying topography we initially show the morphology development of a growing lava dome and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography whereas large deep seated failures occur when the dome is exposed to internal overpressures we find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation we demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome we can use numerical modelling to understand features that elude field observations

the 15 september 1991 pyroclastic flows at unzen volcano japan a flow model for associated ash cloud surges large scale collapse of a dacite dome in the late afternoon of 15 september 1991 generated a series of pyroclastic flow events at unzen volcano pyroclastic flows with a volume of 1 106 m3 as dre descended the northeastern slope of the volcano changing their courses to the southeast due to topographic control after they exited a narrow gorge an ash cloud surge rushed straight ahead detaching the main body of the flow that turned and followed the topographic lows to the east the surge swept the kita kamikoba area which had been devastated by the previous pyroclastic flow events and transported a car as far as 120 m following detachment the surge lost its force after it moved several hundred meters but maintained a high temperature the deposits consist of a bottom layer of better sorted ash unit 1 a thick layer of block and ash unit 2 and a thin top layer of fall out ash unit 3 unit 2 overlies unit 1 with an erosional contact the upper part of unit 2 grades into better sorted ash at distal block and ash flow deposits the bottom part of unit 2 also consists of better sorted ash and the contact with the unit 1 deposits becomes ambiguous video footage of cascading pyroclastic flows during the 1991 1995 eruption traveling over surfaces without any topographic barriers revealed that lobes of ash cloud protruded intermittently from the moving head and sides and that these lobes surged ahead on the ground surface this fact together with the inspection by helicopter shortly after the events suggests that the protruded lobes consisted of better sorted ash and resulted in the deposits of unit 1 the highest ash cloud plume at the oshigadani valley exit and the thickest deposition of fall out ash over kita kamikoba and ohnokoba indicate that abundant ash was also produced when the flow passed through a narrow gorge in the model presented here the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration main body an overriding and intermediate fluidization zone and an overlying dilute cloud release of pressurized gas in lava block pores due to collisions among blocks and the resulting upward current caused a zone of fluidization just above the main body the mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current where the ash was deposited as surge deposits the main body which had high internal friction and shear near its base then overran the surge deposits partially eroding them when the upward current of gas fluidization waned better sorted ash suspended in the fluidization zone was deposited on block and ash deposits in the distal places of block and ash deposits unit 2 probably was deposited in non turbulent fashion without any erosion of the underlying layer unit 1

numerical modelling of stress generation and microfracturing of vesicle walls in glassy rocks abstract in the absence of stress concentrating flaws such as microfractures vesicular glassy materials can withstand gas pressures within vesicles in excess of 100 mpa however vesicles within such materials are known to decrepitate explosively at much lower internal gas pressures both in natural systems and in the laboratory here we present a model that quantitatively predicts the generation of microfractures in vesicle walls during cooling cooling of gas bearing vesicles in glassy rock has little effect on water solubility in the glass but leads to a rapid decrease in gas pressure in the vesicles the drop in pressure causes disequilibrium between the water in the glass and in the vesicle dehydration of the glass in a diffusive boundary layer around the vesicle leads to elastic shrinkage the resulting strain generates large tensile tangential stresses which can exceed the strength of the glass causing microfracturing such microfractures present a possible means by which glassy rock surrounding vesicles could be weakened enough to permit explosive decrepitation at low pore vapor pressures the results have implications wherever hydrous vesicular glasses are formed for example rocks formed in shallow subvolcanic intrusions or vent plugs may spontaneously disintegrate with explosive emission of vapor glassy submarine lavas spontaneously decrepitate upon dredging from the ocean floor popping rock vesicular glasses produced in laboratory experiments investigating vapor melt phase equilibria have been observed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates

textural constraints on effusive silicic volcanism beyond the permeable foam model transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit one way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows in this paper we present textural observations and isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions after magma has reached the earth s surface this view is in contrast to the widely promoted permeable foam model which states that all volatiles escape during ascent of the magma and that all dense glassy material in lava flows forms from the collapse of pumiceous lava i e that silicic lavas emerge as highly inflated foam flows such interpretations which claim that silicic lavas are completely degassed upon extrusion and that all degassing must take place on the time scale of the eruption neglect several important pieces of evidence including the presence of obsidian in extremely small domes and of vesicular zones in the interiors of silicic flows the copious loss of volatiles through eruption plumes between eruptive phases and direct observations of surface vesiculation during growth of the mount st helens lava dome the permeable foam model also implies the unlikely requirement that explosive to effusive transitions be associated with an increase in eruption rate we present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent as well as late stage vesiculation we then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days weeks or months after the lava emerges from the vent

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 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

growth destruction and volcanic facies architecture of three volcanic centres in the miocene usak gure basin western turkey subaqueous subaerial volcanism in a lacustrine setting abstract early to mid miocene extension in western anatolia related to plate tectonic motions resulted in the development of a number of normal fault bounded sedimentary basins as well as different styles and compositions of volcanic activity the usak and gure basins accumulated a thick fluvio lacustrine fill in which three distinct volcanic edifices elmadag itecektepe and beydagi and their deposits can overlap with each other and with the sediments produced by the background sedimentation in addition complete facies architectures of small volume monogenetic volcanoes have been recognised in association with the three large complex polygenetic volcanoes providing a complex mixed siliciclastic and volcaniclastic basin infill in the respective basins where volcanism took place all three volcanic centres display a complex succession of effusive and explosive volcanisms and their reworked deposits with abundant evidences of magma water interaction such as peperites for non explosive magma water interaction with the lacustrine water saturated sediment and standing water body in a large alkaline lake during the constructive phase proximal successions of pyroclastic flows pyroclastic falls and rarely surge deposits are associated with distally emplaced debris flow deposits sometimes of mixed volcanogenic and terrestrial origins and are interbedded with lacustrine sediments of the inay group all three volcanic centres then experienced a phase of volcano growth and degradation between 17 and 15 ma ago most likely related to a combination of tectonic movements on ne sw trending basement faults which triggered multiple flank collapses and volcanic debris avalanches elmadag and voluminous ignimbrite eruptions that triggered caldera formation itecektepe and beydagi volcanic centres lacustrine conditions persisted during the destruction and post destruction stages of the volcano s evolution as evidenced by indications of magma water interactions within the central collapse structures which also contain uplifted basement metamorphic rocks now exhumed in their eroded cores

a model for calculating the viscosity of natural iron bearing silicate melts over a wide range of temperatures pressures oxygen fugacites and compositions a new general model that takes into account the pressure and redox state effect is presented to calculate melt viscosities of natural fe bearing melts this new model is applicable to melts that span a wide range of temperatures from 733 to 1873 k pressures 0 001 15 kbar h2o content from 0 to 12 3 wt and compositions from ultramafic mafic to silicic melts the accuracy of the model is calculated to be 0 23 log units of viscosity which is within or close to experimental uncertainty the transport properties including glass transition temperature and melt fragility can also be calculated from this model a spreadsheet to calculate the viscosity is provided in an electronic supplement

the association of lava dome growth with major explosive activity vei 4 domehaz a global dataset investigation of the global eruptive records of particular types of volcanoes is a fundamental and valuable method of understanding what style of activity can be anticipated in the future and can highlight what might be expected or unusual in particular settings this paper investigates the relationship between large explosions volcanic explosivity index vei 4 and lava dome growth from 1000 ad to present and develops the domehaz database domehaz contains information from 397 dome forming episodes including duration of dome growth duration of pauses in extrusion extrusion rates and the timing and magnitude vei of associated large explosions major explosive activity when associated with dome growth is more likely to occur before dome growth rather than during or at the end of dome forming eruptions in most cases where major explosive activity has been associated with dome growth the eruptions occurred at basaltic andesite to andesitic volcanoes the most common type of dome forming volcano but a greater proportion of dacitic and rhyolitic dome growth episodes were associated with large explosions high extrusion rates 10 m3 s 1 seem to be associated with large explosions and may inhibit degassing or destabilize existing domes leading to explosive decompression large explosions may alternatively be followed by dome growth which represents the clearing of residual magma from the conduit relationships extracted from the global record can be used to construct probability trees for new and ongoing dome forming eruptions or can be used in conjunction with other types of event trees to aid in forecasting volcanic hazards during a crisis especially for volcanoes where data are sparse

discrimination of lava dome activity styles using satellite derived thermal structures abstract thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes including those characterised by active lava domes to be most effective the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified ideally using information contained within the remote sensing data itself whilst using any ancillary field data to guide analysis and the necessary assumptions this study investigates a method by which such discrimination maybe accomplished for activity at lava domes for this purpose we use three landsat thematic mapper tm scenes of unzen volcano japan to determine the temperature structure of the active lava dome existing during 1991 1993 these data are of particularly high quality since the tm scenes were obtained during night time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface the results are matched to near contemporaneous geological sketch maps of the then current situation in order to identify the tm pixel groupings corresponding to the different styles of dome activity namely fumarolic degassing exogenous and endogenous dome growth and collapse deposits resulting from block and ash flows small scale pyroclastic flows and rockfalls the spatial and statistical characteristics of the resultant tm derived temperature distributions are then investigated to determine rule based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data results indicate that fumarolically active locations regions of active dome growth and areas of collapse deposition on the unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of tm imagery the statistics derived for hotspots at the fumarolically active areas show them to be in general significantly hotter and smaller than those found at the areas of active dome growth whereas hotspots retrieved on the areas of collapse deposition are cooler and larger we find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front these features are absent at locations showing only endogenous growth it is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended tm time series available for unzen and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

the mechanics of harmonic gas pressurization and failure of lava domes summary solutions are developed for the harmonic interior pressurization of idealized hemispherical lava domes to determine the evolution of instability with time integrated gas pressure distributions are evaluated to determine uplift on inclined planes of potential detachment that traverse the dome and to evaluate the limit equilibrium stability of blocks isolated by these planes average pressure distributions show an initial transient rise to peak dynamic gas pressures that corresponds to a harmonic steady state progress towards potential instability mirrors this rise and oscillates with the harmonic steady state the timing of pressurization of the dome lavas is modulated by a diffusive time the magnitude of this pressurization is modulated by dome geometry and the magnitude and non dimensional frequency of the interior inflation two distinct pressurization behaviours are apparent the first at high relative inflation frequencies is where the dynamic steady state corresponds to static pressurization at the mean interior pressure and the second at low relative inflation frequencies is where the dynamic state synchronously responds to the oscillating interior pressure boundary condition and peak uplift is proportional to the mean pressure plus the maximum harmonic overpressure these two conditions are separated by a frequency shift or change in fluid diffusivity of only two orders of magnitude and are significant because the destabilizing uplift force may increase by a factor of two where all other parameters except excitation frequency or diffusivity remain fixed these results are applied to stability evaluations of the lava dome at the soufriere hills volcano to examine failure mechanisms potentially consistent with the observed delay in collapse linear models incorporating time invariant strength and transport parameters are only able to represent collapse for interior gas pressures at the upper limit of observed magnitudes conversely behaviours incorporating fatigue weakening or hysteretic permeability enhancement of the dome rocks are able to approach failure for modest pressurization magnitudes to match observed delays in collapse and potentially to explain why some domes collapse explosively whereas others do not

tephra dispersal from myojinsho japan during its shallow submarine eruption of 1952 1953 a new and detailed bathymetric map of the myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952 1953 eruption especially how the silicic pyroclasts both primary and reworked enlarged the volcano and were dispersed into the surrounding marine environment myojinsho 420 km south of tokyo along the izu ogasawara arc was the site of approximately 1000 phreatomagmatic explosions during the 12 5 month eruption these explosions shattered growing dacite domes producing dense clasts that immediately sank into the sea minor amounts of pumice floated on the sea surface after some of these events the myojinsho cone has slopes of almost precisely 21 in the depth range 300 700 m we interpret this to be the result of angle of repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions many of these gravity flows resulted from explosions and associated dome collapse but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1 2 km of the volcano s summit entering water as deep as 400 m because the fall velocity of single particles decreased by a factor of 30 in passing from air into the sea we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra gravitational instabilities within these tephra choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained upon reaching the sea floor many of these currents probably continued to move downslope along myojinsho s submarine slopes fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption ocean currents carried this tephra to distal areas where it presumably forms a pyroclastic component of deep sea sediment

iron titanium oxide minerals in block and ash flow deposits implications for lava dome oxidation processes abstract the ikeshiro pyroclastic flow deposit at yufu volcano kyushu in japan consists of typical block and ash flows generated by collapse of the ikeshiro lava dome erupted ca 2000 years ago lava clasts in the ikeshiro pyroclastic flow deposit were previously found to consist to two types of rock sample with different magnetic mineral assemblages established by rock magnetic experiments saito et al 2003 j volcanol geotherm res 126 127 142 we examined iron titanium oxides in samples from the ikeshiro pyroclastic flow deposit with an optical microscope and with an electron microprobe analyzer as a result samples were classified into two types with different iron titanium oxide mineral assemblages type a oxides are characterized by homogeneous titanomagnetite and titanohematite type b oxides are exsolved and composed of two or three phases ti poor titanomagnetite titanohematite pseudobrookite and rutile the reconstituted compositions of type b oxides show the same fe ti ratio as type a oxides this indicates that type b oxides are produced by oxidation of type a oxides type a oxides yield an equilibrium temperature of about 800 850 c at an oxygen fugacity of nno 2 using a two oxide geothermobarometer this indicates that deuteric oxidation in the lava dome separated samples of each type type a rocks originated in oxygen poor parts of the lava dome with temperatures of about 800 850 c while type b rocks originated in oxygen rich parts of the lava dome type a oxides remained unoxidized while type b oxides oxidized and were transformed into complex grains by intense deuteric oxidation the collapse of the lava dome generated the ikeshiro pyroclastic flow all oxides were quenched from about 800 850 c and preserved their compositions by rapid cooling

satellite thermal analyses of lava dome effusion rates at unzen volcano japan this paper presents analyses of infrared remote sensing data of the 1992 1994 lava dome growth episode of unzen volcano japan the primary aim is to determine whether such data can indicate trends in the rate of magma supply to the dome since this is one of the principal factors controlling the frequency of pyroclastic flow spaceborne low spatial resolution infrared radiance data correctly delimit the two discrete phases of magma supply to the dome and during phase 1 the short wavelength data are found to be strongly correlated with ground based estimates of lava effusion rate the correlation is however significantly weaker during phase 2 analysis of high spatial resolution data from airborne and satellite based systems allows us to determine the principal source of shortwave infrared flux as fumarolically heated dome surfaces these being significantly hotter than the actively growing lava lobes variations in the low spatial resolution shortwave infrared signal are therefore directly attributed to fluctuations in the size and temperature of the fumarolically heated area these fumarolic variations are largely dependent upon the flux rate of magmatic gas which in turn is partly dependent upon the rate of magma supply gas fluxes measured using correlation spectrometer cospec measurements of so2 concentration indicate that the relationship between fumarolic discharge and the effusion rate of lava was relatively stable during phase 1 but highly variable during phase 2 we suggest these variations as an explanation for the differing correlations observed between the emitted shortwave infrared flux and the effusion rate of lava during phases 1 and 2 we conclude that appropriate analysis of high and low spatial resolution shortwave infrared remote sensing data may assist in elucidating trends in lava supply at volcanoes exhibiting similar dome growth episodes to that experienced at unzen

transition from dome forming to plinian eruptive styles controlled by h2o and cl degassing the transition from a plinian pumice to an effusive dome forming eruptive style is frequently observed in volcanic systems and is generally attributed to the progressive loss of volatiles from magma stored in a superficial reservoir this explosive effusive transition has been explained by the evolution from a closed to an open system of degassing1 2 3 4 but in this context an eruption at mt pelee martinique french west indies dated at 650 years ago which exhibited a rarely observed5 6 succession from dome forming to plinian activity in a short interval of time7 is at odds with such an explanation in this eruption near surface explosions of the dome produced two peleean turbulent pyroclastic flows whose deposits are similar to those of the effusive 1902 eruption and then plinian activity produced pumice fallouts and flows the reconstruction of the degassing paths of both eruptive regimes using the densities and the h2o and cl contents of the clasts shows that the interaction of rising magma with hydrothermal fluids at shallow depth may play a critical role in determining eruptive style

experimental fragmentation of crystal and vesicle bearing silicic melts we experimentally investigate the effect of crystals on the fragmentation behavior of a three phase melt gas crystals system under rapid decompression starting materials are cylinders of hydrated haplogranite melts containing alumina crystals that are placed at 6 30 mpa and 600 800 c in a fragmentation bomb for foaming subsequently these bubble and crystal bearing melts are rapidly decompressed within 30 mpa for phenocryst and microlite bearing samples

emplacement of pyroclastic flows during the 1998 1999 eruption of volcan de colima mexico after three years of quiescence volcan de colima reawakened with increasing seismic and rock fall activity that reached its peak on november 20 1998 when a new lava dome forced its way to the volcano s summit the new lava rapidly reached the s sw edge of the summit area beginning the generation of merapi type pyroclastic flows that traveled down la lumbre and the el cordoban western and eastern ravines reaching distances of 3 4 5 and 3 km respectively on december 1 1998 the lava flow split into three fronts that in early 1999 had reached 2 8 3 1 and 2 5 km in length advancing down the el cordoban ravines the lava flow fronts disaggregated into blocks forming pyroclastic flows one of the best examples occurred on december 10 1998 as the lava flow ceased moving in early 1999 activity became more explosive strong blasts were recorded on february 10 may 10 and july 17 1999 the last event developed a 10 km high eruptive column from which a pyroclastic flow developed from the base traveling 3 3 km sw from the summit into the san antonio montegrande ravines regardless of the mechanism of pyroclastic flow generation each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension when the basal avalanche lost velocity and eventually stopped the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic flow deposit and an upper dune bedded surge deposit the dilute pyroclastic flow scorched and toppled maguey plants and trees and sandblasted vegetation in the direction of the flow at the end of the dilute pyroclastic flow path the suspended particles lifted off in a cloud from which a terminal ash fall was deposited the basal avalanche emplaced block and ash flow deposits up to 8 m thick that filled the main ravines and consisted of several flow units each flow unit was massive monolithologic matrix supported and had a clast supported steep front ca 1 5 to 2 m thick composed of boulders up to 1 7 m in diameter the juvenile lithic clasts had an average density of 1800 kg m3 the dilute pyroclastic flow emplaced overbank deposits found on valley margins or beyond the tip of block and ash flow deposits they consist from bottom to top of a massive medium to coarse sand size flow layer 2 4 cm thick a dune bedded surge layer 2 10 cm thick and a massive silt size layer 0 5 cm thick the total estimated volume of the pyroclastic flow deposits produced during the 1998 1999 eruption is 24 105 m3

crumbling of dacite dome lava and generation of pyroclastic flows at unzen volcano recent modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1 2 slow uprise of magma is often accompanied by effective degassing leading to the extrusion of lava flows and domes although lava dome extrusion is one of the less explosive modes of eruption it is often accompanied by explosive pyroclastic activities3 5 the 1991 eruption of unzen volcano provided an opportunity to observe at close range several types of small scale pyroclastic flow glowing avalanches originating from lava domes most of the pyroclastic flows are of merapi type caused by blocks falling from a collapsing dome others are of pelean type originating in an explosion from the side of a dome the lavas apparently show variable degrees of self explosivity we suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes resulting in both merapi type and pelean type pyroclastic flows from the domes

hazards from pyroclastic density currents at mt etna italy abstract despite the recent recognition of mount etna as a periodically violently explosive volcano the hazards from various types of pyroclastic density currents pdcs have until now received virtually no attention at this volcano large scale pyroclastic flows last occurred during the caldera forming ellittico eruptions 15 16 ka ago and the risk of them occurring in the near future is negligible however minor pdcs can affect much of the summit area and portions of the upper flanks of the volcano during the past 20 years small pyroclastic flows or base surge like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of etna four different mechanisms of pdc generation have been identified during these events 1 collapse of pyroclastic fountains as in 2000 and possibly in 1986 2 phreatomagmatic explosions resulting from mixing of lava with wet rock 2006 3 phreatomagmatic explosions resulting from mixing of lava with thick snow 2007 4 disintegration of the unstable flanks of a lava dome like structure growing over the rim of one of the summit craters 1999 all of these recent pdcs were of a rather minor extent maximum runout lengths were about 1 5 km in november 2006 and march 2007 and thus they represented no threat for populated areas and human property around the volcano yet events of this type pose a significant threat to the lives of people visiting the summit area of etna and areas in a radius of 2 km from the summit craters should be off limits anytime an event capable of producing similar pdcs occurs the most likely source of further pdcs in the near future is the southeast crater the youngest most active and most unstable of the four summit craters of etna where 6 of the 8 documented recent pdcs originated it is likely that similar hazards exist in a number of volcanic settings elsewhere especially at snow or glacier covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration

mechanisms of oxidation and degassing in the takanoobane rhyolite lava of aso volcano japan abstract vertical variation of the oxidation state as recorded by fe ti oxides of takanoobane rhyolite lava tr lava is revealed on the basis of petrographic and rock magnetic studies petrographic observations of the interior of tr lava showed that fe ti oxides are preferentially oxidized along the flow structure that is characterized by the light colored band composed of aggregates of deformed minute cavities whereas both the top and bottom parts of the lava are not oxidized rock magnetic studies measuring thermal demagnetization of a three component isothermal remanent magnetization irm and magnetic susceptibility revealed that hematite of a highly oxidized state is formed preferentially along the light colored band consistent with the petrographic observations the observed preferential oxidation of the light colored band is adequately explained by post eruptive degassing of hydrogen from the interior of the lava rather than migration of atmospheric oxygen into the hot interior of the cooling lava against the back streaming volcanic gas the magmatic volatiles emitted from the interior can be transported effectively through the light colored band and microcracks which ubiquitously develop nearly vertical to the flow direction the degassing mechanism should be limited to highly viscous silicic lavas such as rhyolite because the mechanism requires development of the flow structure and the microcracks as the pathways for gases these structures are achieved only in highly viscous silicic lavas

magma fragmentation speed an experimental determination abstract the propagation speed of a fragmentation front combined with the ascent velocity of magma is in all likelihood a controlling factor in the dynamics of explosive volcanic eruptions direct measurement of the fragmentation speed in natural systems appears to be impossible at present fortunately laboratory experiments can provide information on the propagation speed of the fragmentation front here we present the results of fragmentation speed determinations using a so called fragmentation bomb these are to the best of our knowledge the first in situ fragmentation speed determinations performed on magma natural magma samples merapi basaltic andesite mount st helens dacite and unzen dacite have been investigated in the temperature range of 20 950 c and at pressures up to 25 mpa two techniques have been employed firstly in experiments at 20 c dynamic pressure transducers were placed above and below the magma samples and the fragmentation speed of the magma sample was derived from an analysis of the decompression curves secondly at elevated temperatures an alternative technique was introduced and successfully employed this involved the severing via fragmentation of conducting wires placed within the samples at various heights fragmentation speeds are very low falling in the range of 2 70 m s and increasing with an increase in the magnitude of the decompression step responsible for the fragmentation the first high temperature determination seems consistent with low temperature results implications for explosive volcanism are discussed briefly

generation of block and ash flows during the 1990 1995 eruption of unzen volcano japan processes generating block and ash flows by gravitational dome collapse merapi type pyroclastic flow were observed in detail during the 1990 1995 eruption of unzen volcano japan two different types were identified by analysis of video records and observations during helicopter flights most of the block and ash flows erupted during the 1991 1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes the mass around the crack became unstable locally decreasing in tensile strength finally a slab separated from the lobe front fragmented progressively from the base to the top within a few seconds and became a block and ash flow rock falls immediately followed in response to local instability of the lobe front clasts in these rock falls fragmented and merged with the preceding flow in contrast block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows

instability and collapse of hazardous gas pressurized lava domes an important type of gravitational lava dome failure involves significant internal gas overpressure that influences the failure process for the first time this paper presents a rigorous analysis of the mechanics of gas pressurized dome failure we develop diffusion models to calculate gas overpressures in a lava dome and embed these pressure data into stability analyses to demonstrate how gas pressurization can initiate instability the results bear on the mechanistic understanding of hazardous dome failures gas pressurization can promote deep seated failure and the timing of conduit pressurization can control the timing of dome collapse theory explains why dome collapse can be delayed for cases of oscillating effusion dome collapses and explosive events linked to collapse of lava domes can be hazardous monitoring criteria are considered to anticipate whether or not such collapses are likely to be explosive

hydrogen isotope evidence for extrusion mechanisms of the mount st helens lava dome from october 1980 to october 1986 the dacite lava dome at mount st helens grew through the accumulation of more than 15 separate lobes during extrusive episodes that lasted from 1 to 24 days except for an anomalous period of continuous growth throughout most of 19831 over these six years the growth rate was generally volume predictable that is the amount of lava added to the dome in a given eruption was roughly proportional to the time since the preceding extrusion2 these well documented eruptions were separated by repose intervals lasting from several months to more than a year2 in an attempt to identify the triggering mechanisms for these periodic eruptions we have used hydrogen isotope analyses to determine water content and deuterium content for 18 samples of the mount st helens dome dacite these isotope data the first ever collected from an active lava dome suggest a steady state process of magma evolution combining crystallization induced volatile production in the chamber with three different degassing mechanisms closed system volatile loss in the magma chamber open system volatile release during ascent and kinetically controlled degassing upon eruption at the surface the data suggest that future dome building eruptions may require a new influx of volatile rich magma into the chamber

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

non equilibrium and unsteady fluid degassing during slow decompression decompression experiments were performed on corn syrup water solutions in order to investigate the eect of viscosity on processes of vesiculation and degassing at low to moderate degrees of volatile supersaturation repeat experiments demonstrated similar long term vesiculation be havior at moderate decompression rates despite highly vari able initial nucleation styles results suggest that mag mas may not necessarily achieve chemical equilibrium by vapor exsolution and may require viscosity dependent criti cal supersaturations in order to vesiculate vesiculation also increased the ambient pressure and decreased supersatura tions resulting in unsteady degassing

the transition from explosive to effusive eruptive regime the example of the 1912 novarupta eruption alaska the shift from explosive to effusive silicic volcanism seen in many historical eruptions reflects a change in the style of degassing of erupted magma this paper focuses on such a transition during the largest eruption of the twentieth century the 1912 eruption of novarupta the transition is recorded in a dacite block bed which covers an elliptical area of 4 km 2 around the vent approximately 700 studied blocks fall into four main lithologic categories 1 pumiceous 2 dense 3 flow banded dacites and 4 welded breccias textural analyses of the blocks indicate portions of the melt underwent highly variable degrees of outgassing vesicle populations show features characteristic of bubble coalescence and collapse a decrease in measured vesicularity and increased evidence for bubble collapse compared with pumice from earlier plinian episodes mark the transition from closed to open system degassing block morphology and textures strongly suggest the magma was first erupted as a relatively gas rich lava dome plug but incomplete out gassing led to explosive disruption heterogeneous degassing of ascending magma began in plinian episode iii and resulted in instability during episode iv dome growth and a series of vulcanian explosion s modeling of the dynamics of explosion initiation and ejecta dispersal indicates that a significant concentration in gas is required to produce the explosions responsible for the observed block field dispersal the amount of gas available in the hot pumiceous dome material appears to have been inadequate to drive the explosion s therefore external water most likely contributed to the destruction

volcanic evolution of volcan aucanquilcha a long lived dacite volcano in the central andes of northern chile volcan aucanquilcha northern chile has produced 37 km3 of dacite 63 66 wt silica mainly as lavas with ubiquitous magmatic inclusions 59 62 wt silica over the last 1 million years a pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche deposit occurs on the eastern side the 6 000 m high edifice defines a 9 km e w ridge and lies at the center of a cluster of more than 15 volcanoes the aucanquilcha volcanic cluster that has been active for at least the past 11 million years the e w alignment of vents is nearly orthogonal to the arc axis a majority of volcan aucanquilcha was constructed during the first 200 000 years of eruption whereas the last 800 000 years have added little additional volume the peak eruptive rate during the edifice building phases was 0 16 km3 ka and the later eruptive rate was 0 02 km3 ka comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and punctuated rather than continuous activity volcan aucanquilcha lavas are dominated by phenocrysts of plagioclase accompanied by two populations of amphibole biotite clinopyroxene fe ti oxides and or orthopyroxene accessory phases include zircon apatite and rare quartz and sanidine one amphibole population is pargasite and the other is hornblende the homogeneity of dacite lava from volcan aucanquilcha contrasts with the heterogeneity 52 66 wt silica at nearby volcan ollague which has been active over roughly the same period of time we attribute this homogeneity at aucanquilcha to the thermal development of the crust underneath the volcano resulting from protracted magmatism there whereas volcan ollague lacks this magmatic legacy

fractal analysis of the fracture strength of lava dome material based on the grain size distribution of block and ash flow deposits at unzen volcano japan abstract a fractal theory of rock fragmentation is applied to block and ash flow deposits from the fugendake dome unzen volcano kyushu japan in order to analyze the material strength and the energy required for size reduction of the source lava dome two fractal dimensions h and ds which are mutually interchangeable represent the relative strength and energy for particles reduced to a given size they can be theoretically estimated from the power relations of a reference grain size to the cumulative mass and number of fragments smaller than that size the unzen fugendake block and ash flow deposits have been further modified by size sorting and secondary fragmentation that occurred during flowage so that the h value decreases or ds value increases with increasing distance from the source coarse reversely graded deposits are however found to retain the original size population relatively well the ds values estimated from deposits of this type are compatible with those previously reported from decompression fragmentation experiments conducted on the same dome material the employed fractal approach could thus give insights into the potential mode of dome collapse that generates block and ash flows

h2o content and temperature limit the explosive potential of rhyolite magma during plinian eruptions abstract recent rhyolite eruptions on earth have demonstrated their capacity to produce a multitude of hazards including ash formation lasting months and impacting the large reaches of the southern hemisphere nevertheless the underlying mechanisms driving these eruptions are not yet fully understood magmatic volatiles especially h2o dictate whether volcanic eruptions proceed explosively or effusively experimental evidence for the role played by h2o in driving explosive fragmentation is rare in particular in the eruption of rhyolitic magma here we show that when hydrous rhyolitic obsidians from chaiten volcano chile are experimentally heated above their glass transition temperatures at ambient 1 atm conditions two different behaviors result depending on starting h2o concentration and temperature obsidians vesiculate to stable or quasi steady state foams when h2o is 1 wt for a wide range of temperatures 728 1032 c but will explode within just tens of seconds 874 c explosive activity occurs above chaiten s estimated eruption temperature but for lower temperatures only foaming occurs whether a foaming sample remains coherent or explodes depends on two interrelated factors the peclet number pe a dimensionless ratio of diffusive and viscous timescales and the timescale or rate of decompression which is dictated in part by the h2o vapor pressure gradient between bubbles and atmosphere at or above 1 4 wt h2o and for a range of permissible chaiten eruption temperatures 780 825 c pe is large 10 meaning viscous deformation aiding vapor expansion is the dominant mode of growing bubbles consequently vesiculation can proceed rapidly due to high initial overpressure and low melt viscosity until the point is reached that the melt deforms at a rate greater than its relaxation rate resulting in fragmentation below 1 4 wt h2o and for temperatures equal to or lower than chaiten s eruption decompression and deformation rates inferred from experimental clast expansion and bubble growth behavior are at least an order of magnitude smaller than the explosive cases which is insufficient for critical melt rupturing pe estimations highlight the first order influence of temperature and h2o content on physical degassing behavior since the starting h2o content and temperature dictate many the parameters that govern bubble growth pressure gradient melt viscosity and chemical diffusivity our experiments shed light on what limits fragmentation in natural eruptions and offer an alternative explanation for h2o contents measured in plinian fall deposits from chaiten and other rhyolite centers in essence pyroclastic obsidian h2o contents could reflect temperature and p h2o limitations on fragmentation furthermore since foam expansion under low p high t h2o conditions can foster strain rates in excess of the melt s relaxation rate and thus drive fragmentation we contend that in addition to rapid magma ascent and high magma supply rates rapid clast expansion under low pressure conditions may also be important for explosive magma fragmentation

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

co ignimbrite ash fall deposits of the 1991 eruptions of fugen dake unzen volcano japan abstract fugen dake the main peak of unzen volcano began a new eruption sequence on november 17 1990 on may 20 1991 a new lava dome appeared near the eastern edge of the fugen dake summit small scale 10 4 10 6 m 3 in volume merapi type block and ash flows were frequently generated from the growing lava dome during may june 1991 these pyroclastic flows were accompanied by co ignimbrite ash plumes that deposited ash fall deposits downwind of the volcano three examples of co ignimbrite ash fall deposits from unzen pyroclastic flows are described the volume of fall deposits was estimated to be about 30 by volume of the collapsed portions of the dome that formed pyroclastic flows this proportion is smaller than that described for other larger co ignimbrite ash fall deposits from other volcanoes grain size distributions of the unzen co ignimbrite ash fall deposits are bi modal or tri modal most ashes are finer than 4 phi and two modes were observed at around 4 7 phi and 9 phi they are composed mainly of groundmass fragments fractions of another mode at around 2 phi are rich in crystals derived from dome lava some of the fine ash component fell as accretionary lapilli from the co ignimbrite ash cloud indicating either moisture or electrostatic aggregation we believe that the co ignimbrite ash of unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope these ashes were entrained into the convective plumes generated off the tops of the moving flows

3d electrical imaging of the inner structure of a complex lava dome puy de dome volcano french massif central france abstract lava domes result from extrusion of massive lava frequent explosions and collapses this contribution focuses on a complex trachytic lava dome the puy de dome volcano located in the chaine des puys volcanic field french massif central france we performed electrical resistivity tomography ert acquisitions on the entire edifice in order to investigate its overall inner structure as well as to detail its summit area the resulting large ert dataset integrated a recently developed 3d inversion code based on an unstructured discretization of the geometrical model the 3d inversion models obtained refine the existing geological model of the puy de dome s inner structure obtained by previous geophysical studies these results also highlight the strong fracturing and fumarolic alteration that affect the summit part of the volcano

source mechanism of seismic waves excited by pyroclastic flows observed at unzen volcano japan a number of pyroclastic flows have occurred at unzen volcano unzendake in kyushu japan since the end of may 1991 seismic waves which are excited by the pyroclastic flows contain low frequency components in their initial phases they were recorded by two long period seismographs in the seismic network deployed around the volcano this paper is an attempt to clarify the source mechanism of the seismic waves triggered by the pyroclastic flows visual data show that the pyroclastic flow which excites these seismic waves occurs in three successive steps first fall of a lava dome front dome collapse then its collision with the mountain slope and its fragmentation into small blocks and finally descent of the blocks along the slope we can build a simple mechanical model which gives theoretical time histories of vertical and horizontal components of a force acting on the spot of dome collapse on the mountain slope the model of a mechanism generating these seismic waves was constructed through analysis of the low frequency components with the aid of inversion methods which can estimate source time function of the force acting on the source the estimated time histories are close to those predicted by the simple mechanical model we conclude that the three steps mentioned above are essential in exciting the low frequency seismic signals this idea is supported by the fact that the volume of fallen material estimated from the visual data is proportional to the maximum vertical amplitude of seismic waves triggered by the pyroclastic flow this relation holds for the two pyroclastic flows of june 22 1991 and may 28 1992 which were studied in this paper these findings seismically confirm a visual observation that the pyroclastic flow is a result of dome collapse

lava dome growth and mass wasting measured by a time series of ground based radar and seismicity observations range and intensity measurements of the change in summit lava 1 5 10 6 m 3 22 2 avtis range measurements to measure the talus growth 3 9 10 6 m 3 57 and 3 rockfall seismicity to measure the pyroclastic flow deposit volumes 1 4 10 6 m 3 21 which gives an overall dense rock equivalent extrusion rate of about 7 m 3 s 1 these figures demonstrate how efficient nonexplosive lava dome growth can be in generating large volumes of primary clastic deposits a process that by reducing the proportion of erupted lava stored in the summit region will reduce the likelihood of large hazardous pyroclastic flows

analytical models for bubble growth during decompression of high viscosity magmas analytical models for decompressional bubble growth in a viscous magma are developed to establish the influence of high magma viscosity on vesiculation and to assess the time scales on which bubbles respond to decompression instantaneous decompression of individual bubbles analogous to a sudden release of pressure e g sector collapse is considered for two end member cases the infinite melt model considers the growth of an isolated bubble before significant bubble interaction occurs the shell model considers the growth of a bubble surrounded by a thin shell and is analogous to bubble growth in a highly vesicular magmatic foam results from the shell model show that magmas less viscous than 109 pa s can freely expand without developing strong overpressures the timescales for pressure re equilibration are shortened by increased ratios of bubble radius to shell thickness and by larger decompression time scales for isolated bubbles in rhyolitic melts infinite melt model are significantly longer implying that such bubbles could experience internal pressures greater than the ambient pressure for at least a few hours following a sudden release of pressure the shell model is developed to assess bubble growth during the linear decompression of a magma body of constant viscosity for the range of decompression rates and viscosities associated with actual volcanic eruptions bubble growth continues at approximately the equilibrium rate with no attendant excess of internal pressure the results imply that viscosity does not have any significant role in preventing the explosive expansion of high viscosity foams however for viscosities of 109 pa s there is the potential for a viscosity quench under the extreme decompression rates of an explosive eruption it is proposed that the typical vesicularities of pumice of 0 7 0 8 are a consequence of the viscosity of the degassing magmas becoming sufficiently high to inhibit bubble expansion over the characteristic time scale of eruption for fully degassed silicic lavas with viscosities in the range 1010 to 1012 pa s time scales for decompression of isolated bubbles can be hours to many months

volcanic eruptions prediction hazard assessment remote sensing and societal implications in volcanology the period 1991 1994 was very busy with many active volcanoes keeping pace with the higher than average frequency of eruptions about 60 70 per year recorded during the late 1970 s to the late 1980 s simkin 1993 some large and violent eruptions occurred in well populated areas but these passed without the higher death tolls caused by eruptions in the previous decade a reflection perhaps of increased volcanic hazard awareness improved prediction and monitoring and better communication between volcanologists and public officials peterson and tilling 1993 the loss of human life was limited to a little over 1000 with perhaps 750 of these attributable to the mount pinatubo eruption on luzon in the philippines many were killed by post eruption phenomena such as mudflows and by disease in the displaced populations this large eruption which had a world wide atmospheric impact dominates the picture of volcanism in the early 1990 s however significant eruptions also occurred at hudson unzen spurr redoubt hekla mayon galeras rabaul kliuchevskoi etna the most voluminous lava outpourings this century and many other volcanoes here we assess the current state of volcano monitoring and hazard awareness against a backdrop of the eruptions of the past four years and take a look at future developments stressing new techniques in the field of remote sensing

sliding episodes during the 2002 2003 stromboli lava effusion insights from seismic volcanic and statistical data analysis 1 repeated phenomena of flank instability accompanied the 28 december 2002 to 21 july 2003 eruption of stromboli volcano the major episodes were two tsunamigenic landslides on 30 december 2002 2 d after the volcano unrest after 30 december sliding processes remodeled the area affected by slope instability we propose analyses of 565 sliding episodes taking place from december 2002 to february 2003 we try to shed light on their main seismic features and links with the ongoing seismic and volcanic activity using variogram analysis as well a characterization of the seismic signals in the time and frequency domains is presented for 185 sliding episodes their frequency content is between 1 hz and 7 hz on the basis of the dominant peaks and shape of the spectrum we identify three subclasses of signals one of which has significant energy below 2 hz low frequency signatures were also found in the seismic records of the landslides of 30 december which affected the aerial and submarine northwestern flank of the volcano accordingly we surmise that spectral analysis might provide evidence of sliding phenomena with submarine runouts we find no evidence of sliding processes induced by earthquakes additionally a negative statistical correlation between sliding episodes and explosion quakes is highlighted by variogram analysis variograms indicate a persistent behavior memory of the flank instability from 5 to 10 d we interpret the climax in the occurrence rate of the sliding processes between 24 and 29 january 2003 as the result of favorable conditions to slope instability due to the emplacement of nw se aligned dike fed vents located near the scarp of the landslide area afterward the stabilizing effect of the lava flows over the northwestern flank of the volcano limited erosive phenomena to the unstable loose slope not covered by lava

an exceptional case of endogenous lava dome growth spawning pyroclastic avalanches the 1999 bocca nuova eruption of mt etna italy abstract during an eruption at the bocca nuova one of the summit craters of mt etna in october november 1999 a part of the crater floor near its wnw rim was uplifted to form a dome shaped feature that consisted of older lava and pyroclastics filling the crater this endogenous dome grew rapidly over the crater rim thus being perched precariously over the steep outer slope of the bocca nuova and near continuous collapse of its steep flanks generated swiftly moving pyroclastic avalanches over a period of several hours these avalanches advanced at speeds of 10 20 m s and extended up to 0 7 km from their source on top of lavas emplaced immediately before their deposits were subsequently covered by lava flows that issued from vents below the front of the dome and from the bocca nuova itself growth of the dome was caused by the vertical intrusion of magma in the marginal western part of the crater which deformed and uplifted previously emplaced still hot and plastically deformable eruptive products filling the crater the resulting avalanches had all the characteristics of pyroclastic flows spawned by collapse of unstable flanks of lava domes but in this case the magma involved was of mafic hawaiitic composition and would under normal circumstances have produced fluid lava flows the formation of the dome and the generation of the pyroclastic avalanches owe their occurrence to the rheological properties of the eruptive products filling the crater which were transformed into the dome and to the morphological configuration of the bocca nuova and its surroundings the density contrast between successive erupted products may also have played a role although events of this type are to be considered exceptional at etna their recurrence might represent a serious hazard to visitors to the summit area

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

explosive volcanism on venus transient volcanic explosions as a mechanism for localized pyroclast dispersal it is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on venus the influence of the venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions however it may be possible for explosive activity to occur in a fashion analogous to vulcanian eruptions on earth as a result of the accumulation of hot pressurized gas under a coherent rock lid the explosion may be initiated by the failure of this retaining caprock causing the catastrophic release of the high pressure gas which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere on earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near surface crust whereas on venus where the presence of crustal stores of volatile compounds is uncertain the latter option only is favored prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion this paper presents the results of a numerical model describing the explosion process under boundary conditions representing the venusian physical environment this involves treatments of the acceleration of the driving gas caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock in this way predictions of the eruption velocities and of the resulting distribution of large solid ejecta can be obtained for likely conditions on venus deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on venus compared with distances of several kilometers commonly attained by ejecta from transient explosions on earth more typical blocky deposits may extend for only a few hundred meters which implies that they would not be detected in the magellan radar data however the possible presence of associated pyroclastic flow and fine grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on venus

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

newly discovered fish faunas from the early triassic karoo basin south africa and their correlative implications recent collecting in exposures of the lowermost burgersdorp formation beaufort group of the karoo basin of south africa has revealed a previously unknown fish fauna from the early triassic scythian lowermost cynognathus assemblage zone caz which forms an important component of the total vertebrate assemblage the newly discovered fish material includes lungfish saurichthyids and a large microfauna that includes numerous isolated chondrichthyan teeth two fin spine fragments and actinopterygian scales and teeth the latest fish finds together with the lowermost cynognathus assemblage zone vertebrate faunas make this karoo basin assemblage zone one of the most diverse early triassic faunal assemblages comparable in faunal diversity to those from the czatkowice formation poland and the arcadia formation australia the presence of the lungfish ptychoceratodus phillipsi in the early middle triassic cynognathus assemblage zone subzone b and in the underlying latest early triassic cynognathus assemblage zone subzone a indicates that these lungfish could serve as range index fossils within the caz and thus are potentially useful biostratigraphic markers across the early middle triassic boundary furthermore the new fish fauna provides a vital marine realm link in particular with the faunas of madagascar and australia that is unavailable using the tetrapod faunal elements of the lower caz

bubble growth during decompression of magma experimental and theoretical investigation a model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt the model accounts for the interplay of three dynamic processes decompression rate of the magma deformation of the viscous melt around the growing bubble and diffusion of volatiles into the bubble generally these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically for a better understanding of the physics of the processes we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance diffusion or linear decompression rate we show that the solutions are a function of time and two dimensionless numbers which are the ratios of either the diffusive or viscous time scales over the decompression time scale the conditions for each growth regime are provided as a function of the two governing dimensionless parameters analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts the model solutions including the division to the growth regimes as function of the two parameters provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure supersaturation and gas volume fraction the model results are in agreement with the conditions of plinian explosive eruption e g mount st helens 18 may 1980 where high gas overpressure is expected the conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions mostly in equilibrium degassing and partly in overpressure conditions we show that in a situation of quasi static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble insensitive to the diffusivity profile

welded block and ash flow deposits from mount meager british columbia canada abstract the 2360 years b p eruption of mount meager british columbia produced a succession of rarely observed welded block and ash flow deposits and non welded equivalents we report on these rhyodacitic block and ash flow deposits by describing the deposits documenting the nature of the welding process and establishing the origins of these pyroclastic deposits variations in welding intensity are tracked by image analysis of field texture maps petrographic study of shard morphology and measurements of sample density and porosity pyroclast oblateness from image analysis of texture maps provides a qualitative record of welding intensity but underestimates the amount of compactional e g volume strain when compared to estimates from physical property measurements e g density porosity unconsolidated deposits have an average matrix porosity of 41 38 45 and clasts have an average porosity of 32 24 40 in unconsolidated deposits isolated porosity is pervasive 50 m shortening of the thickest deposit from 162 m to the current 112 m the pyroclastic flow deposits result from accumulation in a narrow confined river valley the accumulation was sufficiently rapid to keep the deposits above their glass transition temperature thereby allowing the succession to weld as a single cooling unit the nature and distribution of isolated porosity in the juvenile clasts of rhyodacite suggest an explosive rather than effusive origin for these block and ash flow deposits that may be analogous to some vulcanian eruption triggered dome collapse processes observed at soufriere hills volcano montserrat rather than gravitational collapse

deposits associated with the 1990 1995 eruption of unzen volcano japan abstract the 1990 1995 eruption of unzen volcano in japan was characterized by lava dome growth and block and ash flows triggered by dome collapse the repeated dome collapse events produced block and ash flow ash cloud surge and fine grained ash fall deposits block and ash flow deposits which are confined to valleys are massive to reversely graded very poorly sorted and contain non vesiculated to poorly vesiculated blocks up to 10 m across the deposits often display finer grained basal parts coarse clast depleted flow units which have intermediate grain size distribution between main flow and surge deposits occur as marginal part of block and ash flow deposits ash cloud surge deposits which spread more widely are finer grained and better sorted than block and ash flow deposits ash fall deposits which were derived from ash clouds of block and ash flows and were dispersed around the volcano are very fine grained and are as well sorted as the surge deposits lahars resulting from remobilization of pyroclastic debris devastated the base of the volcano and their deposits are moderately fines depleted and occasionally display two different lahar facies debris flow and hyperconcentrated flow units

holocene block and ash flows from summit dome activity of citlaltepetl volcano eastern mexico a major eruption produced several block and ash flows about 4 100 years b p at citlaltepetl volcano pico de orizaba an ice capped 5670 m high andesitic active stratovolcano located at the eastern end of the mexican volcanic belt repetitive gravitational collapse of a dacitic dome at the summit crater produced a series of block and ash flows lahars and floods which were channeled through two main river valleys on the west and south flanks of the volcano the total erupted volume is estimated to be at least 0 27 km3 the deposits in both areas are similar in composition and size but they differ in the area covered distribution and structure the western deposits form a large fan cover a larger area and include numerous laharic and fluviatile deposits in contrast the southern deposits form prominent terraces where confined in narrow channels and have associated laharic units in distal areas where the flows reach a maximum distance of 30 km from the vent directed disruptions of a central summit dome occurred possibly first to the west and then to the southeast perhaps due to minor modifications of the summit dome morphology producing the voluminous block and ash flow deposits documented here the flows were strongly controlled by topography influencing the deposition of the moving particles grain size variations along the flow paths are hardly detectable suggesting no evident lateral downstream transformations because sudden changes in dome morphology may cause significant variations in the direction of future dome collapse specific areas of potential affectation cannot be predicted therefore about 350 000 inhabitants living within a radius of 35 km from the vent could be potentially impacted if catastrophic block and ash flows were to recur in the future from similar summit dome activity recognition of these deposits is therefore important for hazard assessment because some seemingly safe areas may be at high risk

dome growth and destruction during the 1989 1990 eruption of redoubt volcano abstract much of the six month long 1989 1990 eruption of redoubt volcano consisted of a dome growth and destructive phase in which 14 short lived viscous silicic andesite domes were emplaced and 13 subsequently destroyed the life span of an individual dome ranged from 3 to 21 days and volumes are estimated at 1 10 6 to 30 10 6 m 3 magma supply rates to the vent area averaged about 5 10 5 m 3 day for most of the dome building phase and ranged from a high of 2 2 10 6 m 3 per day initially to a low of 1 8 10 5 m 3 per day at the waning stages of the eruption the total volume of all domes is estimated to be about 90 10 6 m 3 and may represent as much as 60 70 of the volume for the entire eruption the site of 1989 1990 dome emplacement like that in 1966 was on the margin of a north facing amphitheatre like summit crater the domes were confined on the east and west by steep cliffs of pre eruption cone building volcanic rocks and thus were constrained to grow vertically rapid upward growth in a precarious site caused each dome to spread preferentially to the north resulting in eventual gravitational collapse as long as the present conduit remains active at redoubt volcano any dome formed in a new eruption will be confined to a narrow steeply sloping gorge leading to rapid vertical growth and a tendency to collapse gravitationally repetitive cycles of dome formation and failure similar to those seen in 1989 1990 are probably the norm and must be considered in future hazard analyses of redoubt volcano

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

magma fragmentation dynamics experiments with analogue porous low strength material experiments were conducted on the fragmentation of analogue low strength porous material plastiprin by rapid decompression in a shock tube type apparatus the porous samples length 365 mm cross section dimensions 40 40 mm pressurized by air to pressures up to 0 9 mpa were rapidly decompressed to 0 1 mpa rapid decompression of samples caused fragmentation and ejection of the fragmentation products into a large volume tank the process of analogue material fragmentation was documented using high speed cinematography and dynamic pressure measurements the duration of the fragmentation event is significantly shorter than that of the ejection event the fragmentation of material precedes the acceleration of fragments as a result of fragmentation sub parallel fractures are generated the characteristic fragment size decreases as the initial pressure differential increases the ejected fragments obtain velocities of 60 m s the mechanisms of material fragmentation during unloading and fragmentation wave propagation are discussed the experimental results provide insight into the fragmentation dynamics of highly viscous magmas in which brittle failure at high strain rate is possible