decrypting silicic magma plug fragmentation at azufral crater lake northern andes insights from fine to extremely fine ash morpho chemistry azufral sw colombia is a dangerous silicic volcano hosting a crater lake which serves as an excellent example of an incipient plug disruption through phreatomagmatism we studied the youngest succession of dilute pyroclastic density currents pdcs onlapping the north eastern crater rim scanning electron microscopy coupled with energy dispersive x ray spectroscopy was used to carry out an automated single particle analysis of fine to extremely fine ash we were able to obtain fast and accurate chemical analysis and imaging of 15 098 particles within the 250 63 m and the 63 32 m size ranges the 2d form and roughness parameters were determined for 4895 juvenile glassy particles and validated by 3d micro x ray computer tomography there are two end members of high group 1 and low group 2 roughness juvenile glassy particles group 1 comprises high roughness glass particles with solidity values as low as 0 34 in 2d and 0 33 in 3d and convexity values as low as 0 33 in 2d and 0 26 in 3d group 2 comprises low roughness glass particles with 2d solidity values 0 79 and 3d solidity values typically 0 58 in this group 2d convexity values are 0 68 and 3d convexity values are 0 71 both end members are mostly discriminated by the 2d concavity index 0 14 to 0 77 in group 1 vs 0 05 0 35 in group 2 the remaining group 3 comprises particles of intermediate roughness values in this study we show how an incipient plug developed over a short repose time might be subjected to only a few cycles of vesicle nucleation collapse and densification retaining the characteristics of juvenile glass each glassy juvenile ash type defined by a particular morphology roughness and microtexture can be linked to a density stratified conduit model in azufral the capping and conduit lining dense regions and the permeable zones of the incipient plug likely cracked the newly formed cracks could allow hydraulic forcing caused by external water and induce phreatomagmatic interaction this interaction favoured the fine fragmentation of the plug while enhancing ongoing magmatic processes finally the variations of bulk componentry provided clues on dilute pyroclastic density current transport and physical fractionation processes by secondary fragmentation elutriation and interaction with the crater rim

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

explosive origin of silicic lava textural and δd h2o evidence for pyroclastic degassing during rhyolite effusion a long standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava a widely cited idea is that the explosive to effusive transition manifests a two stage degassing history whereby lava is the product of non explosive open system gas release following initial explosive closed system degassing direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent this explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava here we use precise h isotope and magmatic h2o measurements and textural evidence to demonstrate that effusion results from explosion s lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels tuffisites that repetitively and explosively vent from effusing lava our measurements show specifically that d h ratios and h2o contents of a broad suite of explosive and effusive samples from chaiten volcano hydrous bombs plinian pyroclasts tuffisite veins and lava define a single and continuous degassing trend that links wet explosive pyroclasts 1 6 wt h2o d 76 4 to dry obsidian lavas 0 13 wt h2o d 145 7 this geochemical pattern is best fit with batched degassing model that comprises small repeated closed system degassing steps followed by pulses of vapour extraction this degassing mechanism is made possible by the action of tuffisite veins which by tapping already vesicular or brecciated magma allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed system domains and large tracts of conduit magma by giving them long range connectivity even though tuffisite veins render magma degassed and capable of effusing they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut thus the effusion of silicic lava traditionally thought to be relatively benign process presents a particularly hazardous form of explosive volcanism

mercury radar imaging evidence for polar ice the first unambiguous full disk radar mapping of mercury at 3 5 centimeter wavelength with the goldstone 70 meter antenna transmitting and 26 antennas of the very large array receiving has provided evidence for the presence of polar ice the radar experiments conducted on 8 and 23 august 1991 were designed to image the half of mercury not photographed by mariner 10 the orbital geometry allowed viewing beyond the north pole of mercury a highly reflective region was clearly visible on the north pole during both experiments this polar region has areas in which the circular polarization ratio pt was 1 0 to 1 4 values 0 1 are typical for terrestrial planets such high values of have hitherto been observed in radar observations only from icy regions of mars and icy outer planet satellites

fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts abstract products of magma fragmentation can pose a severe threat to health infrastructure environment and aviation systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be observed directly and their deposits undergo transport related sorting however enhanced knowledge is required for hazard assessment and risk mitigation laboratory experiments on natural samples allow the precise characterization of the generated pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes they hold the promise of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation we performed a series of rapid decompression experiments on three sets of natural samples from unzen volcano japan the analysis comprised grain size analysis and surface area measurements the grain size analysis is performed by dry sieving for particles larger than 250 m and wet laser refraction for smaller particles for all three sets of samples the grain size of the most abundant fraction decreases and the weight fraction of newly generated ash particles up to 40 wt increases with experimental pressure potential energy for fragmentation this energy can be estimated from the volume of the gas fraction and the applied pressure the surface area was determined through argon adsorption the fragmentation efficiency is described by the degree of fine particle generation results show that the fragmentation efficiency and the generated surface correlate positively with the applied energy

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

generation of leucogranites via fractional crystallization a case from the late triassic luoza batholith in the lhasa terrane southern tibet abstract leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution but how these rocks form remains a matter of much debate the luoza batholith in the central lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc alkaline s type granodiorites group 1 and highly fractionated s type leucogranites group 2 laser ablation inductively coupled plasma mass spectrometry zircon u pb dating indicates that these rocks were emplaced at ca 221 205 ma the group 1 samples ca 205 ma have low sio2 content 64 67 wt and low differentiation index values di 75 76 and are slightly depleted in ba p sr and ti compared to the group 2 samples ca 221 ma and 213 ma which have high sio2 content 74 75 wt high differentiation index values 91 94 and are significantly depleted in ba p sr and ti samples from group 1 and group 2 exhibit similar zircon ehf t values 15 6 to 1 4 both of which are significantly higher than melts from the pure ancient basement in the central lhasa subterrane 20 5 to 10 6 the group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central lhasa subterrane with contributions from mantle or juvenile crust derived components whereas the group 2 samples may have formed via varying degrees of fractional crystallization of different minerals e g plagioclase k feldspar biotite zircon allanite monazite titanite and apatite from the parental magmas represented by the granodioritic samples in group 1 in shallow crustal magma chambers in combination with sedimentary records the high temperature and low pressure conditions indicated by the group 1 samples can be geodynamically linked to a back arc extensional setting in response to the southward subduction of the bangong nujiang tethys ocean seafloor our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation

experimental and textural investigation of welding effects of compaction sintering and vapor phase crystallization in the rhyolitic rattlesnake tuff abstract the abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification we conducted two series of experiments using ash from the nonwelded base of the rhyolitic rattlesnake tuff of oregon usa to examine conditions of welding one series of experiments was conducted at atmospheric pressure 1 at in a muffle furnace with variable run times and temperature and another series was conducted at 5 mpa and 600 c in a cold seal apparatus with variable run times and water contents we compared the results to a suite of incipiently to densely welded natural samples of the rattlesnake tuff experiments at 1 at required a temperature above 900 c to produce welding which is in excess of the estimated pre eruptive magmatic temperature of the tuff the experiments also yielded globular clast textures unlike the natural tuff during the cold seal experiments the gold sample capsules collapsed in response to sample densification textures and densities that closely mimic the natural suite were produced at 5 mpa 600 c and 0 4 wt h2o over run durations of hours to 2 days clast deformation and development of foliation in 2 week runs were greater than in natural samples both more and less water reduced the degree of welding at otherwise constant run conditions for 5 mpa experiments changes in the degree of foliation of shards and of axial ratios of bubble shards and non bubble mainly platy shards are consistent with early densification related to compaction and partial rotation of shards into a foliation subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards sintering local fusion of shard shard contacts was increasingly important with longer run times higher temperatures and greater pressures during runs with high water concentrations sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces a few tenths wt h2o in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor phase sublimates seal permeability in the tuff

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

geochemistry of self burning coal mining residues from el bierzo coalfield nw spain environmental implications abstract the geochemical composition of unburned and burned burning coal waste piles from past mining activities in el bierzo coalfield nw spain was determined for the comprehensive characterization of the coals waste materials for the identification of the mode of occurrence of environmental sensitive trace elements and for the identification of geochemical changes induced by the thermal effects resulting from the combustion process the methodologies used for the geochemical characterization of the coal and coal waste material included proximate analysis ultimate analysis inductively coupled plasma mass spectrometry for the determination of the inorganic composition of major and trace elements and scanning electron microscopy with energy dispersive x ray spectrometry considering the potentially hazardous trace elements the results demonstrated that the unburned material are enriched in pb sb and th comparatively with the background of black shales bbs considering the enrichment depletion behavior of the elements in burned vs unburned material it was observed that fe k na s and ti are enriched in burned material while ca mg and p are depleted the trace elements nb re ta te and w are enriched in burned waste material while cd cu mn zn and ree are depleted higher contents of oxygen and total sulfur and lower content of carbon were determined in the burned burning materials the enrichment depletion behavior of the elements is attributed to the self burning process and to the changes caused by thermal effects the enrichment of some elements may result from the elements concentration due to the volatilization of other elements and organic matter consumption during the combustion process the depletion of some elements may be associated with their volatilization during combustion however other reasons for this behavior such as the elements mode of occurrence weathering and or leaching should be considered the investigation about the mode of occurrence of the worldwide potentially hazardous trace elements in coal wastes reveals that as cd pb sb and u are preferentially organically associated however as and cd also presents some affinity with mineral mater the elements be co cr mn ni se and th exhibit a significant association with mineral matter

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

scales of columnar jointing in igneous rocks field measurements and controlling factors columnar jointing is a common feature of solidified lavas sills and dikes but the factors controlling the characteristic stoutness of columns remain debated and quantitative field observations are few in number in this paper we provide quantitative measurements on sizing of columnar joint sets and our assessment of the principal factors controlling it we focus on 1 chemistry as it is the major determinant of the physical mechanical and thermal properties of the lava and 2 geology as it influences the style of emplacement and lava geometry setting boundary conditions for the cooling process and the rate of heat loss in our analysis we cover lavas with a broad range of chemical compositions from basanite to phonolite for six of which we provide new geochemical analyses and of geological settings our field measurements cover 50 columnar jointing sites in three countries we provide reliable manually digitized data on the size of individual columns and focus the mathematical analysis on their geometry 23 889 data on side length of which 17 312 are from full column sections and 3 033 data on cross sectional area and order of polygonality the geometrical observations show that the variation in characteristic size of columns between different sites exceeds one order of magnitude side length ranging from 8 to 338 cm and that the column bounding polygons average order is less than 6 the network of fractures is found to be longer than required by a minimum energy hexagonal configuration indicating a non equilibrium geologically quick process in terms of the development and characteristic sizing of columnar joint sets our observations suggest that columns are the result of an interplay between the geological setting of emplacement and magma chemistry when the geological setting constrains the geometry of the emplaced body it exerts a stronger control on characteristic column stoutness at unconstrained geometries e g unconfined lava flows chemistry plays the major role resulting in stouter columns in felsic lavas and slenderer columns in mafic lavas

the influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma volcanic rocks and magma display a wide range of porosity and vesicle size a result of their complex genesis while the role of porosity is known to exert a fundamental control on strength in the brittle field less is known as to the influence of vesicle size to help resolve this issue here we lean on a combination of micromechanical sammis and ashby s pore emanating crack model and stochastic rock failure and process analysis code modelling the models show for a homogenous vesicle size that an increase in porosity in the form of circular vesicles from 0 to 40 and or vesicle diameter from 0 1 to 2 0 mm results in a dramatic reduction in strength for example uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 the presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle emanating microcracks that grow in the direction of the applied macroscopic stress as strain increases these microcracks continue to grow and eventually coalesce leading to macroscopic failure vesicle clustering which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles and the increased ease of microcrack interaction is encouraged at higher porosity and reduces sample strength once a microcrack nucleates at the vesicle wall larger vesicles impart higher stress intensities at the crack tips allowing microcracks to propagate at a lower applied macroscopic stress larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture this explains the reduction in strength at higher vesicle diameters at a constant porosity the modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear the largest reductions are observed at low porosity and small vesicle diameters in detail we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 porosity vesicle clustering and stress lobe interaction are implicit at high porosity regardless of the vesicle diameter in the case of an inhomogeneous vesicle size the microcracks grow from the largest vesicles and brittle strength is closer to that of the largest vesicle end member the results of this study highlight the important role of vesicle size and the complex interplay between porosity and vesicle size in controlling the brittle strength of volcanic rocks and magma

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

permeability control on magma fragmentation fragmentation of porous magma that is subject to gas overpressure is considered to be a crucial process in the generation of explosive volcanic eruptions a decompressive event e g rapid magma ascent landslide dome collapse disrupts the stress equilibrium between the gas phase and the surrounding melt when the gas in the pores is exposed to a pressure gradient it may either fragment the surrounding magma or escape from the magma along an existing pathway of cracks and interconnected bubbles therefore magma permeability can be a decisive parameter in determining if an eruption experiences fragmentation i e whether it is explosive or effusive or exhibits a temporal transition between the two eruptive styles despite the central role that gas permeability may play in the fragmentation of volcanic rocks previous studies have not experimentally verified or quantified this influence based on a comprehensive database of combined permeability and fragmentation experiments we show that high permeability substantially increases the overpressure required to fragment porous volcanic rocks our results allow us to deduce a fragmentation criterion that incorporates gas permeability as well as porosity and internal overpressure this criterion implies that the energy required for fragmentation is less dependent on the actual pore geometry than on the way the void space is interconnected and thus on the contribution of permeable gas flow to decompression

gum rosin acetone system as an analogue to the degassing behaviour of hydrated magmas solutions of gum rosin and acetone reproduce the volatile and temperature dependent viscosity together with the phase behaviour of hydrated magmas a range of experimental exsolution conditions was investigated including the variation of supersaturation rate of decompression solution temperature and volatile content degassing processese were controlled by the formation of an exsolution interface above a supersaturated liquid the end products ranged from a mildly degassed liquid to a solid foam which preserved strained vesicles solutions of gum rosin and acetone are proposed as a suitable analogue system with which to study magma degassing processes

the rheology of two phase magmas a review and analysis abstract we consider the current state of our understanding of the rheology of two phase magmas that is suspensions of either bubbles or crystals in a viscous silicate melt the discussion is restricted to strain rates at which the suspending melt can be considered newtonian we start by considering the range of textures found in magmas and the bubble deformation and particle motions caused by shearing we then review proposed models for suspensions focussing on those functions of the form r f or f that have been most widely used to describe magmatic systems r is the relative apparent viscosity of the suspension is the volume fraction of the suspended phase is the driving stress and is the strain rate both theoretical and empirical methods are presented and then compared against the available analogue i e non magmatic and magmatic data the paper contains new data and significant re analysis of previously published data we present a new semi empirical constitutive model for bubble bearing magmas that is valid for steady and unsteady flow and large strains and strain rates this equation utilises a new parameter the capillarity cx that encapsulates the combined effect of shearing and unsteadiness on bubble suspensions we also present a new scheme for dealing with polydispersivity of bubble suspensions new data on the rheology of particle suspensions undergoing forced oscillations are presented these data show that the cox merz rule only holds for dilute particle suspensions 0 25 a re analysis of all available experimental data that relate rheology to particle aspect ratio provides distinct curves of maximum packing as a function of aspect ratio for smooth and rough particles with magmatic data lying on the curve appropriate for rough particles we analyse several rheological datasets of crystal bearing basaltic magmas and find that they are in good agreement with the constitutive equations derived from analogue data by contrast the same equations do not agree well with data for high viscosity haplogranitic melts this may be an effect of fracturing or viscous dissipation within these samples the paper concludes with a practical rheological recipes section giving a step by step method for calculating a constitutive equation for a two phase magmatic suspension and assessing its likely accuracy

spatio temporal evolution of volcano seismicity a laboratory study we report a laboratory and microstructural study of a suite of deformation experiments in which basalt from mount etna volcano is deformed and fractured at an effective confining pressure representative of conditions under a volcanic edifice 40 mpa particular attention was paid to the formation of a fracture and damage zone with which to stimulate coupled hydro mechanical interactions that create the various types of seismicity recorded on volcanic edifices and which usually precede eruption location of ae events through time shows the formation of a fault plane during which waveforms exhibit the typical high frequency characteristics of volcano tectonic vt earthquakes we found that these vt earthquakes were particularly pronounced when generated using dry samples compared to samples saturated with a pore fluid water vt events generated during deformation of water saturated sample are characterised by a distinctive high frequency onset and a longer low frequency coda exhibiting properties often seen in the field as hybrid events we present evidence that hybrid events are in fact the common type of volcanic seismic event with either vt or low frequency lf events representing end members and whose proportion depend on pore fluid being present in the rock type being deformed as well as how close the rock is to failure we find a notable trend of reducing instances of hybrid events leading up to the failure stage in our experiments suggesting that during this stage the pore fluid present in the rock moves sufficiently quickly to provide a resonance seen as a lf coda our data supports recent modeling and field studies that postulate that hybrid events generated in volcanic areas are likely to be generated through the interaction of hydrothermal fluids moving through a combination of pre existing microcrack networks and larger faults such as those we observe in forensic post test examination

textures formed during emplacement and cooling of a palaeoproterozoic small volume rhyolitic sill the 52 m thick rhyolitic sill at onedin in northwestern australia intruded wet unconsolidated sediment of the koongie park formation during the palaeoproterozoic the sill is composed of five main zones a thin 1 2 m thick basal peperitic contact zone with sparse large 61 cm amygdales and abundant spherulites grades up over 1 m into the lower zone the lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric quartz and feldspar and has less than 5 amygdales in the overlying middle zone 20m thick amygdales increase in abundance from 5 near the base to 20 near the top this zone is dominated by spherulitic rhyolite the 18 m thick upper zone is complex with alternating 10 cm thick layers containing abundant spherulites lithophysae amygdales and perlite amygdales make up to 40 of the rock a pumiceous 2 3 cm thick layer occurs at the upper contact fluidal and blocky peperite marks the 1 2 m thick upper contact zone the five zones formed in response to different cooling rates within the sill the lower zone experienced the slowest cooling rate whereas the upper and basal contact zones with abundant glass cooled rapidly there is an overall upward increase in the amygdale abundance in many cases early formed bubbles became nucleation sites for lithophysae spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded irregularly shaped bubbles formed relatively late in crystalline zones textural zones in the rhyolitic sill at onedin are most similar to those in large volume rhyolitic lavas even though it is a small volume intrusion this similarity may be related to efficient heat retention beneath covering sediments for the sill or beneath a thick crust for large volume lavas the presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite and the lack of autobreccia and sub vertical flow banding near the top distinguish this rhyolite as a sill rather than a lava

magmatic cristobalite and quartz in the nwa 856 martian meteorite silica rich late stage crystallization pockets in the martian meteorite northwest africa nwa 856 were investigated by transmission electron microscopy tem the pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene they consist of elongated grains of cristobalite and quartz embedded in a silica rich glass interstitial to the amorphous phase and silica minerals a number of small accessory minerals have been identified typical for late stage crystallization products they are ilmenite tranquillityite fayalite troilite baddeleyite apatite and chloroapatite cristobalite and quartz are shocked as revealed by the occurrence of numerous amorphous lamellae this assemblage suggests metastable dendritic crystallization under hydrous conditions cristobalite crystallization was probably facilitated by the presence of impurities such as na or h2o our observations show that silica minerals can be formed under magmatic conditions on mars

rapid reinflation following the 2011 2012 rhyodacite eruption at cordon caulle volcano southern andes imaged by insar evidence for magma reservoir refill cordon caulle is a large fissural volcano that has erupted rhyodacitic magma of the same composition in its past three historical eruptions in 1921 1960 and 2011 2012 there was significant ground deformation observed before and during the 2011 2012 eruption here we use c and x band insar time series results to document post eruptive uplift up to 0 8 m between march 2012 and may 2015 with los rates up to 45 cm yr that have been largely aseismic along with subsidence in the 2011 2012 lava flow the 2012 uplift rate is one of the largest for silicic systems and was likely produced by the intrusion of 0 125 km3 of magma in the same tectonically controlled plumbing system that has been active during the historical eruptions nevertheless the uplift ended before the reservoir refilled with the erupted volume maybe due to a change in the pressure gradient produced by the 2011 2012 eruption

explosive fragmentation of erupting magma the magma responsible for explosive volcanic eruptions has both a volatile and an inert phase deep in the conduit of an active volcano bubbles nucleate as the volatiles exsolve as the magma rises the bubbles grow through depressurization and continued exsolution it is thought that when the pressure in the bubbles exceeds that in the overlying material the magma undergoes a rapid transformation from a continuous magmatic phase with bubbles to a continuous gas phase with fragmented pyroclastic material the fragmentation process is complex and poorly understood to understand better how the transport of fragmented material is coupled to exsolution and vaporization we have performed depressurization experiments on a two phase system designed to simulate the eruption process we identify a new explosive vaporization process in which a fragmentation front propagates downwards through a mixture of volatile liquid and inert particulate material suppressing the growth of nucleated bubbles by compressing the material ahead of it this process is distinct from and may complement previously identified fragmentation mechanisms such as non nucleate vaporization and fragmentation induced by an expanding magmatic foam

permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit to trap the volcanic gases in the conduit the lava dome or plug must be impermeable however silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization since fractured magma has high permeability the reduction of permeability is a key process that allows gases to accumulate leading to an explosion we performed uniaxial compression experiments on rhyolite fragments at temperatures of 700 900 c to investigate the permeability reduction of fractured rhyolite experimental results show that permeability is controlled by porosity and permeability reduction can be caused by compaction of fractured magma based on the observed relationship between permeability and porosity we calculate the time scale of permeability reduction i e the transition from permeable to impermeable magma the calculated time scale 100 1000 s is much shorter than the eruption repose 1 10 h observed at sakurajima volcano japan this result implies that an impermeable lava dome and plug formed during the eruption repose in contrast the repose time 20 40 min of santiaguito volcano guatemala is shorter than the time scale of permeability reduction 10 h this indicates that gases could not have accumulated before volcanic explosions and that other mechanisms could control eruption cyclicity

pseudopillow fracture systems in lavas insights into cooling mechanisms and environments from lava flow fractures abstract detailed field observations of structures within the flow front of a holocene trachyandesite lava from snaefellsnes iceland are presented the lava provides exceptional three dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water the flow front interior is characterised by structures consisting of a large metre scale curviplanar master fracture with many smaller centimetre scale subsidiary fractures perpendicular to the master fracture such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures we propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together all documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant i e liquid water ice or snow we use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures master fracture surface textures used to identify fracture mechanisms include chisel marks striae cavitation dimples river lines and rough smooth fracture surface textures these indicate both brittle and ductile fracture happening on different types of master fracture chisel marks on subsidiary fractures indicate comparative cooling rates cooling directions and isotherm orientations at the time of fracture we propose a model for pseudopillow fracture system formation taking into account all the various fracture types textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants

nonisothermal viscous sintering of volcanic ash volcanic ash is often deposited in a hot state volcanic ash containing glass deposited above the glass transition interval has the potential to sinter viscously both to itself particle particle and to exposed surfaces here we constrain the kinetics of this process experimentally under nonisothermal conditions using standard glasses in the absence of external load this process is dominantly driven by surface relaxation in such cases the sintering process is rate limited by the melt viscosity the size of the particles and the melt vapor interfacial tension we propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering we apply our model to viscous sintering scenarios for cooling crystal poor rhyolitic ash using the 2008 eruption of chaiten volcano as a case example we predict that moderate linear cooling rates of 0 1 c min 1 can result in the common observation of incomplete sintering and the preservation of pore networks

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