grain size distribution patterns of a point bar system in the usri river india grain size distribution patterns in a point bar system of the usri river india were critically analysed in the light of log normal log hyperbolic and log skew laplace distribution models sand samples were collected from the cross bedding foreset of different sizes of bedform the objectives were to i study whether bedform heights have any role in grain size distribution patterns ii offer a best fit statistical model iii study the downstream variation of size sorting in a point bar system and iv study the mechanism of grain sorting the results indicate that the bedform heights have no role in grain size distribution patterns quantitatively when the errors in three distribution models were analysed it was observed that the log normal distribution is the best fit statistical model and the next one is the log skew laplace however in the upper reaches of the river log normal distribution is the best fit model in the case of large bedforms whereas in the lower reaches the log normal model is the best fit one in the case of small bed forms it is also observed that within a point bar for large and small bedforms there is a tendency for mean grain size to decrease downstream between point bars for large bedforms there is no consistency in decreasing grain size downstream whereas for small bed forms the decrease of grain size downstream is observed except near the confluence at palkia with distance of transport the coarser and finer fractions of sediments are gradually chopped off the coarser fractions are buried below the advancing bedforms on the lee sides and the finer ones are transported further downstream thus the finer admixture giving rise to the fining upward sequence overlies a carpet of coarser materials this mechanism provides a clue to the process of grain sorting in the fluvial environment an interpretation has been offered for the log normality of the grain size distribution pattern during prolonged transportation in a fluvial environment the larger grain size fractions are gradually chopped off and buried below the advancing bedforms on their lee sides on the other hand the finer fractions are transported further downstream in suspension thus the narrow intermediate size fraction takes active part in the distribution patterns leading to the generation of unimodality and a symmetric distribution pattern downstream which are the main criteria for log normality similarly increase of bedform size is the effect of increase of stream power and froude number leading to the selective segregation of bed materials thus the intermediate size fractions take a more active part than the coarser and the finer size fractions in developing log normality besides the hydrodynamic parameters of the usri coarsening of grain size downstream has been attributed to i the aggrading nature of the usri downstream and ii the contribution of coarser materials to the usri by its tributaries and bank erosion copyright 2006 john wiley sons ltd

melting of the glacier base during a small volume subglacial rhyolite eruption evidence from blahnukur iceland abstract although observations of recent volcanic eruptions beneath vatnajokull iceland have improved the understanding of ice deformation and meltwater drainage little is known about the processes that occur at the glacier base we present observations of the products of a small volume effusive subglacial rhyolite eruption at blahnukur torfajokull iceland lava bodies typically 7 m long have unusual conical morphologies and columnar joint orientations that suggest emplacement within cavities melted into the base of a glacier cavities appear to have been steep walled and randomly distributed these features can be explained by a simple model of conductive heat loss during the ascent of a lava body to the glacier base the released heat melts a cavity in the overlying ice the development of vapour escape pipes in the waterlogged permeable breccias surrounding the lava allows rapid heat transfer between lava and ice the formed meltwater percolates into the breccias recharging the cooling system and leaving a steam filled cavity the slow ascent rates of intrusive rhyolitic magma bodies provide ample time for a cavity to be melted in the ice above even during the final 10 m of ascent to the glacier base an equilibrium cavity size is calculated at which melting is balanced by creep closure this is dependent upon the heat input and the difference between glaciostatic and cavity pressure the cavity sizes inferred from blahnukur are consistent with a pressure differential of 2 4 mpa suggesting that the ice was at least 200 m thick this is consistent with the volcanic stratigraphy which indicates that the ice exceeded 350 m in thickness although this is the first time that a subglacial cavity system of this type has been reconstructed from an ancient volcanic sequence it shares many characteristics with the modern firn cave system formed by fumarolic melting within the summit crater of mount rainier washington at both localities it appears that localised heating at the glacier base has resulted in heterogeneous melting patterns despite the different rheological properties of ice and firn similar patterns of cavity roof deformation are inferred the development of low pressure subglacial cavities in regions of high heat flux may influence the trajectory of rising magma with manifold implications for eruptive mechanisms and resultant subglacial volcanic landforms

an explosive intrusive subglacial rhyolite eruption at dalakvisl torfajokull iceland this paper describes unusual rhyolitic deposits at dalakvisl torfajokull iceland that were emplaced during a quaternary subglacial eruption despite its small volume 0 2 km3 the eruption mechanisms were highly variable and involved both explosive and intrusive phases the explosive phase involved vesiculation driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures in contrast the intrusive phase of the eruption generated vesicle poor quench hyaloclastites associated with a variety of peperitic lava bodies the presence of juvenile rich fluvio lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable in order to explain the variable eruption mechanisms a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent when the cavity became completely filled by volcanic deposits the vent became blocked and rising magma was forced to intrude through poorly consolidated debris this led to arrested fragmentation and welding of foam domains to form vesicle poor obsidian lava the transition to an intrusive eruption has taken place although this vent blocking mechanism is particularly relevant to subglacial eruptions it may also apply to subaerial rhyolitic eruptions where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone meanwhile the variable style of a small volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano ice interactions

bubble coalescence in basaltic lava its impact on the evolution of bubble populations morphological properties bubble number density porosity mean radius specific surface area of vesicular rocks provide quantitative information on the rates of bubble nucleation growth and coalescence in magmas when measured as a function of time such data are useful in constraining the timing and style of gas release during volcanic eruptions volcanic rocks commonly show strong zonation with respect to bubble size and porosity indicating a variation in the amount of bubble growth and coalescence preserved within a single sample morphological properties and bubble size distributions bsd s were measured in a suite of zoned alkali basalts using image analysis and the data were compared to theoretical predictions our data indicate that at porosities greater than 35 extensive coalescence occurred during the growth of bubbles with restricted nucleation at lower porosities vesiculation is dominated by nucleation and diffusion with no coalescence the interiors of many of our samples have undergone 4 7 binary coalescence events after eruption in a time of around 15 min the ostwald ripening effect has not significantly modified the bsd s

spherulites quench fractures and relict perlite in a late devonian rhyolite dyke queensland australia abstract a late devonian rhyolite dyke displays perlitic and other fracture sets as well as textures generated by crystallisation of the glass the dyke is less than a metre wide and has sharp contacts with ignimbrite although originally glassy no glass is preserved aligned magnetite after pyroxene microlites and trains of small 0 5 1 mm spherical spherulites crystallised early at temperatures above the glass transition temperature and before formation of the fracture sets long subplanar fractures oriented perpendicular to the dyke walls extend almost the full dyke width and end by merging with adjacent long fractures short subplanar cross fractures are perpendicular to and terminate at the long fractures well defined perlitic fractures are present within the volumes of rock generally

evoluton of polygonal fracture patterns in lava flows cooling induced fractures also known as columnar joints divide basaltic lava flows into prismatic columns with polygonal cross sections the regularity and symmetry of the fracture patterns have long fascinated naturalists in view of the recent selection of two candidate nuclear waste sites in areas where polygonally fractured volcanic rocks are located a better understanding of the fracture patterns is required field data indicate that the tetragonal networks at flow surfaces evolve systematically to hexagonal networks as the joints grow inward during solidification of lava this evolution occurs by the gradual change of most orthogonal intersections to nonorthogonal intersections of about 120 degrees the surface features and intersection geometries of columnar joints show that joint segments at any given level form sequentially yet harmoniously

statistical analysis of bubble and crystal size distributions application to colorado plateau basalts abstract a new analytical technique for the statistical analysis of bubble populations in volcanic rocks proussevitch a a sahagian d l and tsentalovich e p 2007 this issue statistical analysis of bubble and crystal size distributions formulations and procedures j volc geotherm res has been applied to a collection of colorado plateau basalts 96 samples a variety of mono and polymodal distributions has been found in the samples all of which belong to the logarithmic family of statistical functions most samples have bimodal log normal distributions while the others are represented by mono or bimodal log logistic and weibull distributions we have grouped the observed distributions into 11 groups depending on distribution types mode location and intensity the nature of the curves within these groups can be interpreted as evolution of vesiculation processes we conclude that within bimodal log normal distributions the mode of smaller bubbles is the result of a second nucleation and growth event in a lava flow after eruption in the case of log logistic distributions the larger mode results from coalescence of bubbles coalescence processes are reflected in growth of a larger mode and decreasing bubble number density another style of population evolution leads to a monomodal weibull or exponential distribution as a result of superposition of multiple log normal distributions in which the modes are comparable in size and intensity these various population distribution styles can be interpreted with an understanding of vesiculation processes that can be gained through appropriate numerical models of coalescence and population evolution the applicable vesiculation processes include a a single nucleation growth event b continuous multiple nucleation growth events c coalescence and d ostwald ripening

using dissolved h2o in rhyolitic glasses to estimate palaeo ice thickness during a subglacial eruption at blahnukur torfajokull iceland the last decade has seen the refinement of a technique for reconstructing palaeo ice thicknesses based on using the retained h2o and co2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses the method is here applied to blahnukur a subglacially erupted rhyolitic edifice in iceland a decrease in water content from 0 7 wt at the base to 0 3 wt at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption as blahnukur rises 350 m above the surrounding terrain this implies that the eruption occurred entirely within ice which corroborates evidence obtained from earlier lithofacies studies this paper presents the largest data set 40 samples so far obtained for the retained volatile contents of deposits from a subglacial eruption an important consequence is that it enables subtle but significant variations in water content to become evident in particular there are anomalous samples which are either water rich up to 1 wt or water poor 0 2 wt with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile poor prior to eruption the large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo ice thicknesses and highlights many of the uncertainties involved by using examples from blahnukur the quantitative use of this technique is evaluated however the relative pressure conditions which have shed light on blahnukur s eruption mechanisms and syn eruptive glacier response show that despite uncertainties in absolute values the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions which have never been observed

grain size distributions of bed load inferences from flume experiments using heterogeneous sediment beds abstract the grain size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments the existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain size fractions in the transport layer under given hydraulic conditions the efficiency of these models was tested with the observed data subsequently the patterns of observed grain size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions pdf such as log normal log hyperbolic and log skew laplace tests indicated that a log skew laplace distribution fitted best for 49 a log hyperbolic for 31 and a log normal for 20 out of forty five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness the results of this study would be useful to specify the grain size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments

cooling process recorded in subglacially erupted rhyolite glasses rapid quenching thermal buffering and the formation of meltwater the thermal histories of two subglacial rhyolite glasses from torfajokull complex of south central iceland have been determined from changes in the relaxation of enthalpy in the calorimetric glass transition interval heat capacity measurements of bulk glass samples from rau ufossafjoll sample number js 1 1 and blahnukur sample number js 2 1 by differential scanning calorimetry show marked differences in the temperature of the onset of glass transition tg on reheating the average tg is 750 k for js 1 1 whereas it is 850 k for js 2 1 these differences in the onset of tg are related to quench rate through considerations of the activation energy and timescales for structural relaxation modeled using the kohlrausch williams watt function multiple glass samples from the two torfajokull locations recover different average values and distributions of a fictive temperature t f which we interpret in terms of differences in the thermal histories of the glasses in both glass samples the range of fictive temperatures and by implication apparent cooling rates is lower than those estimated for a glass rapidly quenched by contact with cold glacial melt water and requires that the glass was annealed at temperatures in the glass transition interval associated with relaxation times of the order of 1000 10 000 s the complex cooling histories of the torfajokull samples show that the products of subglacial eruptions may be held at elevated temperatures for several hours and as a result may drive vigorous convection in the ice bound vaults above erupting rhyolite deposits

false pyroclastic textures in altered silicic lavas with implications for volcanic associated mineralization there is strong emphasis in recent literature that zn cu pb massive sulfide deposits associated with felsic volcanism the kuroko type deposits are directly related to submarine calderas formed by large submarine pyroclastic eruptions altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits volcaniclastic debris flows and massive to thin bedded tuffaceous rocks form a major component of a silurian succession that hosts zn cu pb massive sulfide deposits at benambra southeastern australia however critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures including apparent pumice fiamme and glass shards intercalated sediments indicate that most of the silicic volcanics were emplaced in subaqueous settings ranging from relatively shallow water to deep marine the false textures were produced mainly in originally glassy lava by the combined effects of devitrification perlitic fracture and pervasive hydrothermal alteration these processes are related to the emplacement and cooling history of individual lavas and to a regional essentially synvolcanic hydrothermal system tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics thereby further enhancing clastic appearance subsequent to recognition of widespread false pyroclastic textures at benambra similar features have been identified in three other australian massive sulfide districts these results suggest that a reinvestigation of host rocks is warranted at other deposits where submarine pyroclastics are documented noncaldera settings combining subsidence in an extensional volcano tectonic basin passive submarine lava effusion within the basin and ambient sedimentation derived from basin margins including resedimented pyroclastic debris may be more relevant models in a number of massive sulfide districts

measurement of bubble size distributions in vesiculated rocks with implications for quantitative estimation of eruption processes abstract this paper outlines methods for determining a bubble size distribution bsd and the moments of the bsd function in vesiculated clasts produced by volcanic eruptions it reports the results of applications of the methods to 11 natural samples and discusses the implications for quantitative estimates of eruption processes the analysis is based on a quantitative morphological stereological method for 2 dimensional imaging of cross sections of samples one method determines with some assumptions the complete shape of the bsd function from the chord lengths cut by bubbles the other determines the 1st 2nd and 3rd moments of distribution functions by measurement of the number of bubbles per unit area the surface area per unit volume and the volume fraction of bubbles comparison of procedures and results of these two distinct methods shows that the latter yields rather more reliable results than the former though the results coincide in absolute and relative magnitudes results of the analysis for vesiculated rocks from eleven subplinian to plinian eruptions show some interesting systematic correlations both between moments of the bsd and between a moment and the eruption column height or the sio2 content of magma these correlations are successfully interpreted in terms of the nucleation and growth processes of bubbles in ascending magmas this suggests that bubble coalescence does not predominate in sub plinian to plinian explosive eruptions the moment moment correlations put constraints on the style of the nucleation and growth process of bubbles the scaling argument suggests that a single nucleation event and subsequent growth with any kind of bubble interaction under continuous depressurization which leads to an intermediate growth law between the diffusional growth r m t 2 3 at a constant depressurization rate and the ostwald ripening r m t 1 3 under a constant pressure where rm and t are the mean radius of bubble and the effective time of diffusion respectively occurred in the eruptions it is emphasized that the bsd in vesiculated rocks from terrestrial volcanoes can be used to estimate quantitatively eruption processes such as the initial saturation pressure and magma ascent velocity in a volcanic conduit

fault fracture density and mineralization a contouring method for targeting in gold exploration abstract a widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore forming process in archaean greenstone hosted deposits high density fracturing was accompanied by enhanced fluid flow during fault fracture network development producing regional scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas fracture density is both increased and decreased during faulting and fault healing and fracture density accumulates over time in zones of high palaeo fluid flow localised zones where the density of fracturing is increased by deformation become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation the ora banda mining centre in western australia contains significant gold deposits that appear to demonstrate a close association between high density fracturing and gold precipitation fracture density in the ora banda mines was enhanced by fault fault intersections fault contact intersections and changes in fault geometry the mine scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales hence these relationships may be used in targeting for gold exploration contouring the density of fracturing in a region provides a semi quantitative way to rank areas for exploration and uses data from mapping drilling and high quality geophysical data as a basis for analyses fracture density contouring is complementary to other prospectivity analysis methods

variations in hydration within perlitised rhyolitic lavas evidence from torfajokull iceland perlitic fractures form due to the hydration of glassy rhyolitic lavas perlitised lavas are also an important industrial commodity yet there has been little study on the mechanisms of outcrop perlitisation here the fracture populations perlitisation and volatile concentrations of subglacial rhyolitic glassy facies have been studied adding a quantitative dimension to previous qualitative studies samples include hyaloclastite perlitised and non perlitised obsidian and microcrystalline rhyolite which are all present in lava lobes at torfajokull iceland fractures formed through cooling increase in spacing with increased distance inwards from the margins of lobes the size distribution of perlitic beads is shown to follow a log normal distribution the degree of perlitisation and the total volatile content measured using thermogravimetric analysis tga of samples are shown to decrease with distance inwards from lobe margins in general increased perlitisation is accompanied by increased hydration although complexity exists within single outcrops the elevated total volatile content of perlite is shown to be due to hydration by environmental water with the structural location of the water changing as total volatile content increases

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

palaeoclimate the riddle of the sediments the ratio of oxygen isotopes contained in the signal in deep sea sediments can tell us a great deal about past ice volume variations the challenge is to disentangle the different contributions to the signal the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

microearthquakes on and near the east pacific rise 9 10 n records from a seismic network deployed as part of an active tomography experiment at 9 30 n on the east pacific rise epr provide an opportunity to characterize local microearthquake activity over an 8 day period with the exception of the region around the 9 03 overlapping spreading center osc no events were located on the rise axis one microearthquake located from p and s wave arrival times lies adjacent to the network 18 km to the west of the rise axis five more events located from p wave and t phase data are to the south of the network three cluster around the western arm of the 9 03 osc while two are located 25 30 km to the east of the osc the seismic moments of the microearthquakes range from 5 1019 to 4 1020 dyn cm several orders of magnitude larger than those reported in other studies of shallow events along the axis of the epr these results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the epr are active off axis to distances of at least 20 km

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

products of an effusive subglacial rhyolite eruption blahnukur torfajokull iceland we present field observations from blahnukur a small volume 400 m thick lithofacies associations attest to complex patterns of volcano ice interaction erosive channels at the base of the subglacial sequence are filled by both eruption derived material and subglacial till which show evidence for deposition by flowing meltwater this suggests that meltwater was able to drain away from the vent area during the eruption much of the subglacial volcanic deposits consist of conical to irregularly shaped lava lobes typically 5 10 m long set in poorly sorted breccias with an ash grade matrix a gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface driven by magma water interaction as the lava flowed over poorly consolidated waterlogged debris sets of columnar joints on the upper surfaces of lobes are interpreted as ice contact features the morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2 5 m high avalanche deposits mantling the flanks of blahnukur appear to have been generated by the collapse of lava lobes and surrounding breccia a variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias much larger lava flows were emplaced in the latter stages of the eruption columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100 200 m in length there is little evidence for magma water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater as rhyolite magma rose to the base of the glacier the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated we present evidence that the cavity system evolved during the eruption reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate