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

3 d reconstruction of ash vesicularity insights into the origin of ash rich explosive eruptions abstract explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation even during a single eruptive event the increasing amount of fine pyroclasts is often uncritically related to explosive magma water interaction i e hydromagmatic fragmentation here we report examination of two examples of major explosive eruptions from the quaternary vulsini volcanic district central italy in which the fine grained nature of deposits even in near vent settings indicates negligible effect of transport and implies the eruption of highly fragmented magmas sem morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation we apply a recently developed stereo scanning electron microscopy ssem technique proussevitch et al 2011 to determine vesicularity features e g bubble size distribution and bubble number density hereafter bsd and bnd respectively of ash particles ssem analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash rich eruptions we conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs

conduit vent structures and related proximal deposits in the las canadas caldera tenerife canary islands the las canadas caldera wall and the outer slopes of the caldera provide three dimensional exposures of numerous proximal welded fallout deposits and have been mapped in detail as a result some parts of the ucanca and guajara formations of the stratigraphy of marti et al 1994 have been divided into members that correspond to individual eruptions mapping has also revealed the occurrence of conduit vent structures associated with proximal welded fallout deposits conduit vent structures consist of an upper flaring area and a lower narrow conduit conduit vent geometry and dimensions include cylindrical plugs and eruptive fissures steeply dipping towards the caldera depression and elongated vents the flaring area can be rather asymmetric and is usually filled by down vent rheomorphic flow of the proximal fallout deposit the lower conduits are filled by lava plug agglutination of juveniles onto conduit walls and dyke intrusion with eventual dome extrusion the eruption dynamics of welded fallout deposits and magma fragmentation within the conduit are consistent with an evolution from explosive to effusive in this context conduit flow regimes evolve from turbulent to annular flow in which the conduit is progressively choked and laminar flow leading to the final conduit closure

experimental study of bubble growth in stromboli basalt melts at 1 atm abstract volcanic eruptions are characterized by intense degassing thus it is imperative to have high quality laboratory data to constrain degassing mechanisms in order to investigate bubble formation and growth at 1 atm degassing experiments using a stromboli basalt were performed on the gsecars x ray beamline at the advanced photon source volatile bearing glasses were synthesized at 1250 c and 1000 mpa in a piston cylinder with h 2 o or mixtures of h 2 o co 2 they were then heated in situ on the x ray beamline at 1 atm bubble growth was observed in situ using x ray radiography the 3d bubble size distributions in the quenched samples and a natural stromboli pumice were studied by synchrotron x ray microtomography the results show that bubble nucleation and growth in basaltic melts are volatile concentration dependent bubbles can easily form in melts initially containing high volatile concentrations the effect of co 2 on bubble nucleation and growth becomes significant at large co 2 concentrations of 880 to 1480 ppm but is not important at lower concentrations multiple nucleation events occur in most of these degassing experiments and they are more pronounced in more supersaturated melts bubble growth is controlled by viscosity near glass transition temperatures and by diffusion at higher temperatures bubbles begin to pop 10 to 20 s after a foam is developed at vesicularities of 65 to 83 bubble size distributions follow power law relations at vesicularities of 1 to 65 and bubble size distributions evolve from power law relations to exponential relations at vesicularities of 65 to 83 this evolution is associated with the change from far from equilibrium degassing to near equilibrium degassing during far from equilibrium degassing multiple nucleation events are pronounced and possibly account for the generation of power law relations when the system reaches near equilibrium degassing coalescence is dominant and leads to the formation of bubbles of similar size therefore bubble size distributions are described by exponential relations

rhyolite magma degassing an experimental study of melt vesiculation the vesiculation of a peralkaline rhyolite melt initially containing 0 14 wt h2o has been investigated at temperatures above the rheological glass transition tg 530 c by a in situ optical observation of individual bubble growth or dissolution and b dilatometric measurements of the volume expansion due to vesiculation the activation energy of the timescale for bubble growth equals the activation energy of viscous flow at relatively low temperatures 650 790 c but decreases and tends towards the value for water diffusion at high temperatures 790 925 c the time dependence of volume expansion follows the avrami equation

influence of decompression rate on fragmentation processes an experimental study a key for an effective early warning or even a forecasting system for volcanic eruptions lies in the understanding of volcanic processes and the correct interpretation of monitoring data here we investigate the processes occurring in the upper conduit at the phase transition between melt and the surrounding rock characterizing the fragmentation threshold and speed fragmentation experiments using a shock tube were performed to address the following questions 1 how does sample conduit size affect fragmentation processes 2 what is the impact of different decompression rates and differing stress distributions in the sample on fragmentation threshold and speed the observed fragmentation behaviour is analyzed in terms of energy density and compared with recent related studies a 2d fragmentation mechanism is presented based on the arrangement of cracks in fragmented samples a numerical model for the estimation of the first principle stress distribution before the magma failure is provided

a complex quaternary ignimbrite forming phonolitic eruption the poris member of the diego hernandez formation tenerife canary islands abstract the poris member 0 28 ma of the diego hernandez formation tenerife canary islands is a widespread succession of plinian fall surge and non welded ignimbrite deposits it was erupted from the northeastern sector of the multicyclic las canadas caldera the summit caldera complex of the las canadas edifice we present new stratigraphic data which allow the evolution of the eruption to be divided into six main stages 1 an initial surge producing phreatomagmatic phase 2 generation of a buoyant plinian column culminating in vent wall collapse and temporary vent blockage 3 a resumption of phreatomagmatic activity producing accretionary lapilli bearing ash surges and a phreatomagmatic ignimbrite 4 sustained column collapse resulting in the progressive aggradation of a complex compositionally layered ignimbrite sequence 5 a late plinian phase marked by sharp chemical zonation 6 final vent destruction and the generation of multiple surges and pyroclastic flows volume calculations indicate a total erupted volume in the order of 13 14 km 3 3 4 km 3 dense rock equivalent which together with facies and lithic analysis implies that a small scale caldera collapse occurred contributing to the incremental enlargement of the las canadas caldera the geochemistry and mingling inter relationships of four magmatic endmembers ranging from alkaline mafic to evolved phonolitic liquids indicate that two magma chambers fed stages 4 and 5 of the eruption sequence the eruption was triggered when mafic magma intruded the two chamber system

formation of tube pumice structure under pure shear insights from extension tests of solidifying foam abstract tube pumice the bubbles of which are highly elongated in one direction is a common product of explosive silicic eruptions especially caldera forming eruptions to understand the conditions necessary to generate tube like bubbles we performed extensional experiments on polyurethane foam which experiences vesiculation deformation and solidification at room temperature the strain and strain rate of pure shear deformation were controlled by the pull length and pull rate of the extension respectively the shapes of bubbles in the solidified polyurethane foam were analyzed in three dimensions by x ray computed tomography the experimental results demonstrate that strain and capillary number control the degree of bubble deformation highly deformed tube like bubbles were found in the experiments with large strain and large capillary number in each experimental sample there is scatter in the relation between the degrees of bubble deformation and expected capillary numbers which we interpret to be the result of the inhomogeneous shear field around individual bubbles and bubble coalescence despite the large scatter the average deformation degree is predicted well by the dynamics of a single bubble where the viscous stress acting on the bubble surface is taken as the product of bulk strain rate and effective viscosity using this approach the average deformation degree of experimental bubbles in each sample was approximately calculated from the deformation model of a single bubble at least in the range of pure shear strain smaller than 1 5 small bubbles in the samples extended at low viscosities tend to be less elongated than those calculated from the deformation model due to shape relaxation the effect of shape relaxation is quantified by a new dimensionless number i e the quench number which represents the ratio of the timescale of increasing viscosity and shape relaxation our experiment suggests that bubble shapes in highly vesicular pumice can be used to analyze their viscosity and strain rate during the elongation process an analysis of the published data was presented as a test case

rheological transitions in high temperature volcanic fault zones silicic magma experiences shear induced brittle fracturing during its ascent resulting in the formation of a magmatic fault at the conduit margin once the fault is formed frictional behavior of the fault controls the magma ascent process we observed torsional deformation of a magmatic fault gouge in situ at temperatures of 800 and 900 c using synchrotron radiation x ray radiography the torsional deformation rate was set at 0 1 10 rpm corresponding to equivalent slip velocities of 2 27 10 5 1 74 10 3 m s 1 and shear strain rates of 0 014 1 16 s 1 the normal stresses used were 1 5 and 10 mpa the magmatic fault showed frictional sliding as well as viscous flow even above the glass transition temperature the transition between frictional sliding and viscous flow depends on temperature deformation rate and normal stress on the fault at 900 c the fault showed viscous deformation at a normal stress of 10 mpa while frictional sliding was predominant at 800 c we propose the ratio of timescales of fault healing and deformation as a criterion for transition between frictional sliding and viscous flow the experimentally calibrated criterion infers that frictional sliding is predominant from 500 m in depth during explosive eruption this may explain rapid magma ascent without efficient outgassing frictional heating would in turn enhance fault healing resulting in the reverse transition from frictional sliding to viscous flow followed by deceleration of magma ascent therefore cyclic transitions between frictional sliding and viscous flow are a possible explanation for the cyclic behavior of lava effusion

bubble deformation in magma under transient flow conditions abstract we investigate the transient behavior of bubble deformation in magma previous studies have used the relationships between capillary number and equilibrium shape for a single bubble under either steady simple shear or steady pure shear to estimate strain rates in pyroclasts and to calculate bubble shapes in a conduit flow we developed a deformation model for a bubble by adjusting the empirical functions used in an existing model for a droplet to fit experimental data for bubble deformation the modified model was used to calculate transient large deformations of a single bubble in arbitrary shear flows in this paper we demonstrate the significance of the modified model through two different applications firstly using this model we are able to simulate the entire history of the evolution of bubble shapes in a time dependent flow using the computational code conflow in this simulation we further include the effect of gas expansion on the bubble shape from this simulation we show that bubble deformation in conduit flow is controlled by the history of both capillary number and strain secondly by using this model to analyze bubble shapes observed in natural magma samples we are able to better estimate the strain rates and deformation durations we apply this model to certain transient features in the relationship between bubble shape and radius in the published data of obsidian clasts from newberry volcano and estimate a deformation duration of 2 7 4 0 min

flow and fracturing of viscoelastic media under diffusion driven bubble growth an analogue experiment for eruptive volcanic conduits abstract to visualize the behavior of erupting magma in volcanic conduits we performed shock tube experiments on the ductile brittle response of a viscoelastic medium to diffusion driven bubble expansion a sample of shear thinning magma analogue is saturated by gas ar under high pressure on rapid decompression ar supersaturation causes bubbles to nucleate grow and coalesce in the sample forcing it to expand flow and fracture experimental variables include saturation pressure and duration and shape and lubrication of the flow path bubble growth in the experiments controls both flow and fracturing and is consistent with physical models of magma vesiculation two types of fractures are observed i sharp fractures along the uppermost rim of the sample and ii fractures pervasively diffused throughout the sample rim fractures open when shear stress accumulates and strain rate is highest at the margin of the flow a process already inferred from observations and models to occur in magma pervasive fractures originate when wall friction retards expansion of the sample causing pressure to build up in the bubbles when bubble pressure overcomes wall friction and the tensile strength of the porous sample fractures open with a range of morphologies both types of fracture open normally to flow direction and both may heal as the flow proceeds these experiments also illustrate how the development of pervasive fractures allows exsolving gas to escape from the sample before the generation of a permeable network via other processes e g bubble coalescence this is an observation that potentially impact the degassing of magma and the transition between explosive and effusive eruptions

incorporation of trace metals cu zn and cd into gypsum implication on their mobility and fate in natural and anthropogenic environments abstract the coexistence of calcium ca2 sulfate so42 with trace metal cations m ii can possibly lead to m ii gypsum coprecipitation and solid solution formation however gypsum s role in the fixation of m ii is still largely unknown this study investigated the precipitation of ca2 and so42 in the presence of m ii i e cu2 zn2 or cd2 and the incorporation of the metal cations into the gypsum structure at different environmental conditions trace metals in two natural gypsum samples yunnan and neimeng china and one hydrometallurgical byproduct gypsum sample from a cu refinery were also assessed x ray diffraction scanning electron microscopy energy dispersive x ray spectroscopy and fourier transform infrared spectroscopy were employed to characterize the solid samples the lab scale results showed that the m ii content in the coprecipitates increased with ph and the initial m ii concentration at lower ph zn2 and cd2 were dominantly fixed through isomorphic substitution for ca2 in the gypsum structure while cu2 was likely incorporated into the channel structure of the gypsum where the water molecules reside in contrast at higher ph thermodynamic modeling results indicated that m ii hydroxides and base metal sulfates mx oh y so4 z were formed and constituted an appreciable fraction of m ii bearing species in the host solids our study further showed that trace amounts of cu zn and cd were also substitutianly incorporated in the two natural and one hydrometallurgical gypsum samples the present study may have important implications for understanding the geochemical cycle of trace metals in ca2 and so42 coexisting systems as well as possible attenuation mechanisms that may occur along their supersaturation and coprecipitation

cooling dynamics of spatter fed phonolite obsidian flows on tenerife canary islands abstract relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on tenerife to determine their thermal history across the glass transition the phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits the detailed sampling resolution within the two series enabled an accurate quantification of their thermal history cooling rates within the investigated spatter fed flows vary over more than two orders of magnitude the highest cooling rates of 0 39 k min were modeled for the central vesiculated part of one flow the dense basal obsidian layers of both flows were cooled at substantially lower rates of 0 0042 and 0 0028 k min respectively there appears to be an influence of in situ vesiculation processes on the thermal budget of the investigated flows in addition the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with quasi isothermal annealing within this decollement numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows the effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data in addition viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition

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

dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling we have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity volatile concentration and diffusivity on the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems we conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2 4 orders of magnitude our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions an analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics r t1 2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship r log t is more appropriate during active bubble growth under certain conditions a second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression model results for degassing of initially water saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent on the basis of decompression history explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates 1 5 m s 1

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

synchrotron x ray computed microtomography studies on vesiculated basaltic rocks synchrotron x ray computed microtomography cmt was applied to five basalt samples from different locations in order to determine the microgeometrical structures and physical properties of vesicles two vesiculated basaltic lavas and one calcite filled vesiculated basaltic lava were collected from the lesotho highland south africa and two basaltic scoria tephra were taken from the lungkuan volcano group in northeast china they were examined with cmt on the x27c beam line of the brookhaven national synchrotron light source the voxel size varied from 10 to 20 m depending on the chosen field of view of the tomographic instrument a fast filtered back transform ffbt algorithm was employed in the tomographic reconstruction a 3d image geometry analysis package was used to measure the vesicularity specific surface area and interconnectivity of the vesicles the results of the analyses showed that the bubbles have roughly spheroidal forms of different sizes vesicularity ranged from 45 for lavas to 80 for scoria at least 90 of the vesicles are interconnected specific surface area was determined both by counting voxel faces and by use of two point correlation functions

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

pele s hairs and tears natural probe of volcanic plume we present a detailed petrographic study of pele s hairs and tears sampled from masaya volcano nicaragua this study provides new observations of these little known pyroclastic objects using both secondary electron images sei and back scattering electron images bsei our work shows that pele s tears can be associated with pele s hairs after their formation tears can be trapped on the walls and or in the cavities of pele s hairs moreover chemical investigations of the pele s hairs and tears highlight the presence of a chemical zonation the edge of these tears and hairs show a siliceous enrichment allowing us to quantify the interaction time of the silicate glass with acid gases in the volcanic plume this study confirms the syneruptive and post eruptive volatile exsolution from pele s hairs and tears

fragmentation versus transportation mechanisms in the pyroclastic sequence of monte pilato rocche rosse lipari italy abstract monte pilato is a rhyolitic pumice cone formed about 1400 years ago during the last volcanic activity which occurred at lipari its activity consisted of numerous explosive episodes ending with the extrusion of the rocche rosse obsidian lava flow we have identified nine different pyroclastic lithofacies of which four main lithofacies represent 90 of the erupted products they include 1 deposits consisting of decimeters to meters thick well sorted graded beds of pumice lapilli which are of fall origin 2 massive deposits consisting of decimeters thick beds of pumice lapilli in an ash matrix interpreted as being of pyroclastic flow origin 3 deposits of centimeters thick ash beds of surge origin 4 decimeter thick fine ash layers with scattered subcentimetric accretionary lapilli of turbulent flow origin the complex grain size distributions of the main lithofacies were distinguished using a multivariate statistic analysis r mode factor analysis variations of the grain size characteristics in relation to the distance from the vent area have been investigated in order to discriminate the roles played by fragmentation and transportation processes in the lithofacies 1 fall deposits the grain size distribution is always unimodal and varies from a rosin type distribution near the vent to a gaussian one in the distal outcrops this is caused by the transport process and the different particle trajectories in lithofacies 2 3 and 4 the grain size distributions are always polymodal the polymodality is not caused by preferential processes occurring during transportation but seems rather to be connected with the original fragmentation of the pyroclastic material the pumice lapilli present both in lithofacies 1 and 2 have mean vesicularity index values always in excess of 65 and the value is homogeneous throughout the sample these data suggest that for these two lithofacies the pumice lapilli originated from exsolution of magmatic gases in contrast the fine ash particles in lithofacies 2 and also the main mode of lithofacies 3 and 4 deposits show typical features of hydromagmatic fragmentation processes the grain size population is always centered at 4 5 0 032 0 064 mm and interestingly the same grain size has been obtained recently zimanowski et al 1991 for fragments produced by experimental magma water interaction field and laboratory data suggest that during monte pilato rocche rosse eruptions magmatic and hydromagmatic mechanisms were frequently operative at the same time

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

magma degassing and crystallization processes during eruptions of high risk neapolitan volcanoes evidence of common equilibrium rising processes in alkaline magmas abstract compositional textural and experimental data on products from explosive and effusive eruptions of neapolitan volcanoes campi flegrei and somma vesuvio allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas significant differences in compositional and textural features have been recognized between lavas scoria and pumice resulting respectively from effusive moderately and extremely explosive eruptions pumice samples have highly vesicular glassy matrix low microlite number density and moderate to high water content crystal size distributions csd are steep with high intercept values the narrow microlite size range indicates single nucleation event scoria products are characterized by moderate vesicularity and water content they have high number density of microlites which are bimodal in size csd show distinct inflections that are explained as two crystal populations growing in distinct time lava samples generally have low vesicularities moderate to high microcrystalline groundmass and low glass water content the comparison between textural and compositional features of natural rocks with samples obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near equilibrium conditions even at high decompression rate moderate to long magma rise times calculated in the order of a few days produce open degassing responsible for moderately explosive to effusive activity short magma rise times calculated in the order of a few hours result in closed system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold vesicularity and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10 30 mpa in these conditions degassing fragmentation and in turn the eruptive style is strongly influenced by initial conditions in the magma chamber volatile content temperature pressure instead of decompression rate in contrast with that observed for rhyolitic melts these differences have important consequences in terms of volcanic hazards and risk the low viscosity alkaline magma is able to maintain efficient degassing even during the final stage of magma ascent favoring in the case of closed system fragmentation and explosive activity

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

bubbledrive 1 a numerical model of volcanic eruption mechanisms driven by disequilibrium magma degassing abstract we have developed a numerical model for magma dynamics in volcanic conduits that involves conduit flow driven by a detailed treatment of bubble growth the bubbledrive 1 model can be characterized by several unique aspects including treatment of the transient problem specific eruption triggers disequilibrium degassing a full scale bubble growth module and steady state eruption in the case of constant recharge from below the general conditions for the model are homogeneous two phase flow a simple fragmentation threshold pre existing bubbles and parameters corresponding to magma compositions of dacite and rhyolite the advantage of using a full scale bubble growth model in our treatment of conduit flow makes it possible to track the level of oversaturation throughout the conduit in space and time the model accounts for dynamic decompression in which the evolving interaction between decompression rate due to magma ascent and diffusive addition of gas into bubbles due to decreased solubility with decreasing pressure serves as the driving force of magma flow in the conduit the ability of the model to run the transient case enables us to explore specific triggers to initiate an eruption which proceeds with duration and eruption energy depending on conduit geometry and several other factors our numerical study revealed several notable features of volcanic systems that may shed additional light on natural systems each of the model runs was directed at elucidating specific aspects of eruptions and the suite of runs indicated some common features that may play a key role in all silicic eruptions we find that conduit geometry is critical in determining the nature of eruption conduits lacking a magma chamber have smooth and relatively quiet eruption styles conduits that include a magma chamber involve eruptions that start out like those without chambers but then evolve into a long and uniform eruption that ends with a violent explosion as the chamber is emptied the uniform phase is maintained by vesiculation within the chamber and the blast occurs when the fragmentation level descends down into the magma chamber resistance to flow imposed by the narrow conduit above the chamber acts as a buffer stabilizing the eruption during the steady state phase until the chamber is depleted in material discharge and vent velocity are proportional to the conduit chamber depth and are controlled by the initial concentration of dissolved volatiles maximum oversaturation is observed immediately below the fragmentation level of the magma column oversaturation at the vent remains remarkably constant at 1 wt of h2o for all cylindrical conduits with no recharge from below oversaturation at the vent is much more complex for conduits that contain magma chamber at 2 km depth our numerical study indicates that the primary factors that control eruption behavior are magma composition and conduit shape this suggests that future observation programs could provide valuable information by determining sub edifice structure so that better constraints on geometry can be derived

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

the constitutive equation and flow dynamics of bubbly magmas 1 a generalized constitutive equation for bubbly liquids is presented which successfully reproduces the expected viscosity response for both steady flows with varying capillary number ca a measure of the bubble deformation and unsteady flows with varying dynamic capillary number cd a measure of the steadiness of the flow previously given in separate studies the constitutive equation is given in terms of observable material and flow parameters and is

drill core based facies reconstruction of a deep marine felsic volcano hosting an active hydrothermal system pual ridge papau new guinea odp leg 193 abstract pual ridge is a deep marine felsic volcanic edifice in the eastern manus back arc basin papua new guinea with an estimated volume of 6 to 9 km 3 it is 1 1 5 km wide 20 km long and rises 500 600 m above the surrounding ocean floor the active pacmanus hydrothermal field on the crest of pual ridge at 1640 1690 m below sea level was the target of ocean drilling program leg 193 variably altered dacite lavas have been recovered from the subsurface of a low t discharge site snowcap and a high t black smoker site roman ruins reaching a maximum depth of 380 m below seafloor mbsf volcanic facies interpretation of these cores is difficult due to incomplete recovery and widespread pseudoclastic textures generated by fracturing and multi phase incomplete fluid dacite interaction however distinction of genuine volcaniclastic facies and facies with alteration related clastic appearance is important in order to define paleo seafloor positions within the volcanic stratigraphy that may be prospective for massive sulfide mineralization this has been accomplished using remnant primary characteristics indicative of transportation such as polymictic composition grading or textural evidence for differential movement of individual clasts three phases of volcanic activity can be distinguished and a proximal facies association dominated by coherent facies of dacite lavas exists below snowcap at roman ruins a medial facies association consists of lava flows with about equal proportions of coherent and volcaniclastic facies endogenous growth was an important process during lava flow emplacement and groundmass textures such as perlite flow banding and spherulites indicate that cooling rates were variable locally allowing for high temperature devitrification a tube pumice breccia unit is interpreted as the resedimented facies of a quench fragmented highly vesicular dacite lava carapace sulfide accumulations in the subsurface are restricted to roman ruins suggesting that the abundance of volcaniclastic facies favored ascent of metal bearing fluids due to the inherent high permeability evidence for mineralization at paleo seafloor positions at 110 and 195 mbsf indicate a complex interrelationship between volcanic and hydrothermal activity a facies model has been developed which may help to locate subaqueous felsic lava dominated volcanic centers and associated sulfide deposits in ancient volcano sedimentary successions

scaling vesicle distributions and volcanic eruptions models of coalescence decompressive expansion of the later stages of bubble growth predict that for diverse types of volcanic products the vesicle number densities n v are of the scaling form text n left text v right propto text v text b 3 1 where v is the volume of the vesicles and b3 the 3 dimensional scaling power law exponent we analyze cross sections of 9 pumice samples showing that over the range of bubble sizes from 10 m to 3 cm they are well fit with b3 0 85 we show that to within experimental error this exponent is the same as that reported in the literature for basaltic lavas and other volcanic products the importance of the scaling of vesicle distributions is highlighted by the observation that they are particularly effective at packing bubbles allowing very high vesicularities to be reached before the critical percolation threshold a process which for highly stressed magmas would trigger explosion in this way the scaling of the bubble distributions allows them to be key actors in determining the rheological properties and in eruption dynamics

explosive rhyolite tuya formation classic examples from kerlingarfjoll iceland rhyolite eruptions in iceland mostly take place at long lived central volcanoes examples of which are found associated with each of the present day rift zone ice caps subglacial eruptions at kerlingarfjoll central volcano produced rhyolite tuyas that are notable for their exposures of early erupted pyroclastic material observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation eruptions begin with violent phreatomagmatic explosions that generate massive tuff mt but the influence of water quickly declines leading to the formation of massive lapilli tuffs mlt containing magmatically fragmented vesicular pumice and ash these are deposited rapidly near the vent probably by moist pyroclastic density currents confined by ice but not within a meltwater lake the explosive effusive transition is controlled by the ascent rate and gas content of the magma an unusual obsidian rich massive lapilli tuff lithofacies omlt is identified and interpreted as pyroclastic material that was intruded into gas fluidised deposits at the explosive effusive transition the effusive phase of eruption involves the emplacement of intrusions and lava caps intrusions of lava into the early erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores large lava bodies with columnar jointed margins cap the tuyas and have intrusive basal contacts with the tephras the main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region this is in contrast to subglacial basaltic tuya forming eruptions which are characterised by the formation of meltwater lakes phreatomagmatic fragmentation and subaqueous deposition the lack of meltwater storage may reduce the potential for large jokulhlaups

on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc abstract large silicic explosive eruptions are the most catastrophic volcanic events yet the intratelluric mechanisms underlying are not fully understood here we report a field and laboratory study of the kos plateau tuff kpt 161 ka aegean volcanic arc which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate to large scale caldera forming eruptions a prominent feature of the kpt is the occurrence of quite unusual platy shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse on macroscopic and sem observations flat clast faces are elongated parallel to tube vesicles while transverse surfaces often occur at 45 to vesicle elongation this peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls which induced vesiculation and fragmentation of the ascending magma either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre climactic kpt eruption phases this mechanism implies that the closer to the conduit walls where the stronger are the velocity gradients the larger was the proportion of plate vs conventional lensoid juvenile fragments in the ascending gas pyroclast mixture consequently plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained plinian type eruption column as well as in occasional lateral blast currents generated at the vent as a whole plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions the plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic rich breccias related to extensive caldera collapse at the eruption climax as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents along with fissure erosion concomitant to the disruption of the collapsing block

decompression rate of magma at fragmentation inference from broken crystals in pumice of vulcanian eruption abstract decompression rate of magma at fragmentation immediately before or after fragmentation was determined using broken crystals found in the pumices ejected during the vulcanian explosion of sakurajima volcano japan the combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7 0 10 3 7 8 10 4 pa s for the ejecta produced during the later phase of vulcanian explosion this result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent an understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation

bubble growth in rhyolitic melts experimental and numerical investigation bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically rhyolitic melts were hydrated at 150 mpa 780 850 c to uniform water content of 5 5 5 3 wt the pressure was then dropped and held constant at 15 145 mpa upon the drop bubbles nucleated and were allowed to grow for various periods of time before final rapid quenching of the samples the size and number density of bubbles in the quenched glasses were recorded where number densities were low and run duration short bubble sizes were in accord with the growth model of scriven 1959 for solitary bubbles however most results did not fit this simple model because of interaction between neighboring bubbles hence the growth model of proussevitch et al 1993 which accounts for finite separation between bubbles was further developed and used to simulate bubble growth

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

magma fragmentation in eruptions a process known as fragmentation is how erupting magma from a volcano breaks up into fine particles and ash and it determines the type of eruption gentle or violent that occurs theoretical and laboratory studies show that treating magma as a brittle material can explain its behaviour at the point of fragmentation when it is transformed from a foam into a gassy spray

effects of bed structure on erosion of cohesive sediments data from two recent papers are used to illustrate the effects of change of structure of fine sediment on erosion rates the paper by roberts et al presents erosion rates as functions of bed bulk density and shear stress ranges of densities were obtained by allowing various times for consolidation and the erosion tests were made where the density was nearly constant with depth a marked change in the linear relation between erosion rate and density is presented that is attributed to a change of bed structure the paper by zreik et al presents erosion rate data for a thin layer near the surface of a bed formed by sedimentation the linear relation between amount of overburden removed and bed strength also has a marked change that can be attributed to a change of bed structure the initial structures are described as aggregates of aggregates and the changes are shown to be progressive collapse of the larger to smaller interaggregate pores due to thermal motion biased by overburden pressure to a nearl

facies architecture emplacement mechanisms and eruption style of the submarine andesite el barronal complex cabo de gata se spain abstract el barronal complex consists of a succession of andesite lavas and andesite volcaniclastic facies interbedded with carbonate and siliciclastic sedimentary rocks carbonate and siliciclastic rocks were deposited in a shallow marine environment during periods of volcanic quiescence lavas consist of an inner coherent core grading outward into hyaloclastite breccia made of dense clasts that in turn grade into hyaloclastite breccia made of vesicular clasts in massive to layered zones volcaniclastic facies contain clasts produced during explosive eruptions and reworked clasts from sources above wave base volcaniclastic facies were deposited from cold granular flows with different grain size populations stratigraphy and facies architecture at el barronal suggest that a succession of several discrete eruptive events occurred with a similar cyclic pattern made of an initial explosive phase followed by effusive emplacement of lavas in turn followed by a period of quiescence of volcanic activity hyaloclastic fragmentation of magma took place in the final stages of lava emplacement allowing only for local disorganization of the jigsaw fit texture

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

tube pumices as strain markers of the ductile brittle transition during magma fragmentation magma fragmentation the process by which relatively slow moving magma transforms into a violent gas flow carrying fragments of magma is the defining feature of explosive volcanism yet of all the processes involved in explosively erupting systems fragmentation is possibly the least understood1 2 several theoretical and laboratory studies on magma degassing3 4 5 6 7 and fragmentation8 9 10 11 have produced a general picture of the sequence of events leading to the fragmentation of silicic magma12 13 14 but there remains a debate2 over whether magma fragmentation is a consequence of the textural evolution of magma to a foamed state where disintegration of walls separating bubbles becomes inevitable due to a foam collapse criterion or whether magma is fragmented purely by stresses that exceed its tensile strength here we show that tube pumice where extreme bubble elongation is observed is a well preserved magmatic strain marker of the stress state immediately before and during fragmentation structural elements in the pumice record the evolution of the magma s mechanical response from viscous behaviour foaming and foam elongation through the plastic or viscoelastic stage and finally to brittle behaviour these observations directly support the hypothesis that fragmentation occurs when magma undergoes a ductile brittle transition and stresses exceed the magma s tensile strength

sustained blasts during large volcanic eruptions we carried out numerical simulations to investigate magma ascent in wide conduits during large explosive eruptions wide conduits allow high discharge rates low frictional pressure losses and shallow levels of explosive fragmentation of the magma within the conduit in contrast with the commonly modeled lower rate eruptions during which magma fragments inside the conduit at depth and feeds a vertical eruptive jet we find that for sufficiently high discharge rates 10 10 kg s 1 the fragmentation level may rise up to the surface gas rich unfragmented magma reaches the surface at high pressure and feeds a sustained volcanic blast geologic evidence for very high discharge rate eruptions wide conduits and shock waves in large pyroclastic flows supports the occurrence of this type of explosive eruption

the corte blanco garnetiferous tuff a distinctive late miocene marker bed in northwestern argentina applied to magnetic polarity stratigraphy in the rio yacones salta province abstract we introduce the corte blanco tuff a white garnetiferous air fall unit as a distinctive neogene marker bed three whole rock k ar ages from rocks at the source of this unit indicate that it was erupted 8 73 0 25 ma from the la pava ramadas caldera on the argentine puna ash spread eastward across the foreland provinces of the eastern cordillera and sierras subandinas recognition of this dated marker unit in these provinces provides the first easily identified late miocene time line in the vast densely vegetated region to the east of the puna we encountered the unit in seven localities from all three morphostructural provinces in nw argentina a depositional gap in the air fall material is present between 20 and 150 km to the east of the caldera recognition of the corte blanco tuff in the rio yacones near salta allowed an interpretation to be made of a magnetic polarity stratigraphy section erected in marginally suitable neogene detrital strata of the rio guanaco formation we interpret the strata we examined to have been deposited between 10 5 and 6 4 ma and report a 10 clockwise rotation since the strata were deposited this is the first numerically dated section in the rio guanaco formation of nw argentina these results reveal that uplift of the eastern cordillera was in progress at this time