explosive subglacial rhyolitic eruptions in iceland are fuelled by high magmatic h2o and closed system degassing rhyolitic eruptions beneath icelandic glaciers can be highly explosive as demonstrated by quaternary tephra layers dispersed throughout northern europe however they can also be small and effusive a subglacial rhyolitic eruption has never been observed so behavioral controls remain poorly understood and the influence of pre eruptive volatile contents is unknown we have therefore used secondary ion mass spectrometry to characterize pre eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the torfajokull central volcano formed in contrasting styles of eruption under ice 400 m thick this includes the products of the largest known eruption of icelandic subglacial rhyolite of 16 km 3 at ca 70 ka we find pre eruptive water contents in melt inclusions h 2 o mi of up to 4 8 wt which indicates that icelandic rhyolite can be significantly more volatile rich than previously thought our results indicate that explosive subglacial rhyolite eruptions correspond with high h 2 o mi closed system degassing and rapid magma ascent whereas their effusive equivalents have lower h 2 o mi and show open system degassing and more sluggish ascent rates volatile controls on eruption style thus appear similar to those for subaerial eruptions suggesting that ice plays a subsidiary role in influencing the behavior of subglacial rhyolitic eruptions

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

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

new evidence for degassing processes during explosive eruptions measurements of pore size distribution in five samples of pumice formed during explosive eruptions of plinian type show three distinct sizes of vesicle the characteristics of the three pore size populations are similar in the samples although they were collected from plinian deposits of five different volcanoes we interpret the pore size populations as representing the degassing of silicic magma on three very different time scales the coarse population 60 m represents slow vesiculation within the magma chamber over periods of weeks months or years at small levels of gas supersaturation the medium sized population 50 5 m represents rapid vesiculation of magma on ascent to the surface during the plinian eruption over periods of minutes to hours the fine population 5 0 5 m represents bubbles formed during explosive disruption of magma and rapid ejection into the atmosphere involving short time scales a few seconds but large decompression rates

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

physical volcanology of a subglacial to emergent rhyolitic tuya at rauðufossafjoll torfajokull iceland we present the first modern volcanological study of a subglacial to emergent rhyolite tuya at se rau ufossafjoll torfajokull iceland a flat topped edifice with a volume of 1 km3 was emplaced in upper pleistocene time beneath a glacier 350 m thick although it shares morphological characteristics with basaltic tuyas the lithofacies indicate a very different eruption mechanism field observations suggest that the eruption began with vigorous phreatomagmatic explosions within a well drained ice vault building a pile of unbedded ash up to 300 m thick this was followed by a subaerial effusive phase in which compound lava flows were emplaced within ice cauldrons small volume effusive eruptions on the volcano flanks created several lava bodies with a variety of features columnar jointed sides subaerial tops peperitic bases that are used to reconstruct spatially heterogeneous patterns of volcano ice interaction volcaniclastic sediments exposed in a stream section provide evidence for channelised meltwater drainage and fluctuating depositional processes during the eruption we develop models for the evolution of se rau ufossafjoll and discuss the differences between subglacial rhyolitic and basaltic eruption mechanisms which are principally caused by contrasting hydrological patterns

andesitic magma degassing investigated through h2o vapour melt partitioning of halogens at soufriere hills volcano montserrat lesser antilles magma degassing at soufriere hills volcano shv is characterised by an almost permanent so2 flux and a hcl production rate which mainly depends on dome growth rate degassing processes have been studied through textural h2o and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits cl br and i are strongly depleted in melts during h2o degassing with no significant cl br i fractionation whereas f is almost unaffected all magmas erupted at shv have followed a multi step degassing path from the magma chamber up to a shallow depth 1 km p 20 mpa from that depth however effusive and explosive paths are distinct vulcanian eruptions are the result of closed system degassing csd while effusive dome growth is the result of csd up to a very shallow depth 200 m p 5 2 mpa followed by open system degassing osd csd is modelled using the h2o solubility law the perfect gas law and partition coefficients of halogens between a rhyolitic melt and h2o vapour dv li gas loss characteristic of osd is modelled using a rayleigh law degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates which ranges from 150 500 dv lcl for osd ranges between 50 300 increasing with melt cl content for csd the lower effective dv lcl 20 is attributed to kinetic effects dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because osd is much more efficient at extracting halogens the model shows that hcl flux is a good proxy for the dome forming eruption rate comparison between model and measured gas compositions suggests a high hbr bro conversion rate bro total br 1 3 in the shv gas plume the degassing behaviour of cl br and i implies similar cl br 160 and br i 90 in initial melts volcanic clasts and high temperature gases the low cl br at shv compared to other island arcs 250 300 is attributed to a shallow pre eruptive br enrichment the almost permanent dome extrusion at shv since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume

h2o and halogen f cl br behaviour during shallow magma degassing processes abstract degassing paths of silicic h2o rich magmas during explosive plinian and dome forming eruptions may be represented in simple evolution diagrams relating the residual volatile content of the melt to the volume ratio of gas over melt vg vl these parameters are measurable in erupted magma fragments they are used for interpreting the h2o and halogen contents determined in a series of variously degassed volcanic clasts from the 650 y b p eruption at mt pelee martinique fwi which is characterized by the succession of a dome extrusion and a plinian activity h2o degassing of plinian pumice clasts are modeled by an equilibrium closed system evolution which allows to calculate the partition coefficients of halogen between the aqueous fluid and the rhyolitic melt d v l i 1 for f 10 1 5 for cl and 3 7 1 for br at around 900 c the strong increase of the dv li values of cl and br during dome extrusion is explained by an open system degassing model with a simultaneous crystallization of the melt some significant br enrichments relative to cl in dome fragments are attributed to interactions with an hydrothermal fluid close to seawater in composition the models allow calculation of cl and br contents of aqueous fluids generated by the different degassing paths cl contents are 0 5 mol l for closed system evolution and vary between 0 2 and 1 mol l during open system evolutions these fluids are able to extract significant amounts of chloride complexed metals and silica from the magma during dome forming eruptions they also may generate mineralization of the wall rocks or the rising dome itself e g silica precipitation which may seal the volcanic system and be responsible for the dome explosivity and the transition from dome forming to plinian eruptive style the cl br ratio of initial melts of p1 eruption 250 is much lower than the basaltic and exospheric ratios 400 suggesting that strong cl br fractionation may occur during the production or the differentiation of subduction related magmas

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

experimental constraints on bubble interactions in rhyolite melts implications for vesicle size distributions we have studied interactions between bubbles of two distinct size classes in rhyolite melts experimentally decompressed between 200 and 80 mpa the first set of decompression bubbles has a size range rdec of 1 11 m and is formed from nucleation and growth upon isothermal decompression of the melt the larger populations of hydration bubbles are on average 30 40 m in radius rhyd and are formed from pore spaces present that were filled with water vapor during the saturation runs prior to the decompression experiments the first type of interaction results in the elongation of decompression bubbles oriented radially around the larger hydration bubbles the degree of elongation increases both as a function of distance and with increasing ratio of hydration to decompression bubble size rhyd rdec the second type of interaction studied results in a reduction of the size of decompression bubbles located within a range of distances from 10 to 65 m from a hydration bubble surface relative to the modal size of the unaffected bubbles in the same sample in addition within an average distance of 10 m melt next to the hydration bubble surface is depleted in decompression bubbles our results indicate that concentration gradients in the melt are probably responsible for bubble size reduction and the depleted zones because the predicted time scales for ostwald ripening are much longer than those of the experiments these effects persist even to the lowest ending pressures studied 80 mpa which indicates that size distributions of small bubbles may be affected by concentration gradients in the depleted melt shell surrounding large bubbles large bubbles present in an ascending magma prior to a subsequent nucleation event could therefore affect the growth of the smaller bubble population occurring within the depleted melt shell of the larger bubbles and produce a bimodal vesicle size distribution elongated decompression bubbles may be strained as a result of melt flowing away from the much larger hydration bubbles as they grow estimates of capillary number ca plotted against deformation df indicate that bubbles in water rich rhyolite melts are deformable even at small sizes 1 m and small values of ca our results show a different trend of df with ca than previous studies in non geological systems predict indicating that viscosity effects may be important the preservation of deformation textures depends strongly on relaxation time explaining the lack of deformation textures in less viscous natural lavas

norwegian sea tephrostratigraphy of marine isotope stages 4 and 5 prospects and problems for tephrochronology in the north atlantic region abstract this paper presents an account of the tephrostratigraphy of south eastern norwegian sea core md99 2289 for the interval spanning marine isotope stage mis 5 and part of mis 4 from 70 000 to 130 000 years ago tephra deposits from in the order of 100 mainly basaltic volcanic eruptions in iceland and on jan mayen are described in terms of their stratigraphic context and geochemical composition representing the most comprehensive record of iceland and jan mayen volcanism over this time span the most important transport mechanism of tephra to the core site is considered to have been iceberg rafting which is thought to be responsible for a centennial to millennial scale time lag between the volcanic eruption events and the subsequent deposition of tephra in deep sea sediments while this clearly limits the use of some of the tephras as isochronous markers twelve discrete tephra deposits show stratigraphical and geochemical characteristics that suggest them to have been deposited near contemporaneously with their source volcanic events and hence suitable candidates as regional isochrones four of these accords with the geochemical signatures of tephra deposits previously described from the nordic seas region potential problems for the application of tephrochronology highlighted by this work are discussed and recommendations for strategies to overcome them are proposed

heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure abstract the nucleation of h 2 o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 800 c and either 50 or 125 mpa and then decompressed by 20 91 mpa at temperatures that ranged between 550 and 700 c and held at the lower pressures for 10 720 s before being quenched bubbles nucleated in number densities n b that vary between 3 10 7 and 2 10 8 cm 3 blocky shaped magnetite or the ends of needle shaped hematite acted as sites for nucleation but only if a minimum super saturation was exceeded which increases with increasing melt viscosity bubbles did not nucleate along the lengths of hematite needles nor on plagioclase both the beginning and ending times of the nucleation event increases with increasing melt viscosity using nucleation theory predictions neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature water diffusivity or viscosity instead the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor resulting from the increasing polymerization of the melt at lower temperatures and water contents because only 10 8 bubbles cm 3 nucleate on magnetite in the rhyolite melt used almost regardless of experimental conditions results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions

water solubility and chlorine partitioning in cl rich granitic systems effects of melt composition at 2 kbar and 800 c abstract experiments have been conducted in the system na2o k2o al2o3 sio2 h2o cl2o 1 at 2 kbar and 800 c to determine the effects of melt composition on the solubility of water in haplogranite melt and on the distribution of cl between aqueous fluid and haplogranite melt the melts studied include peraluminous subaluminous and peralkaline haplogranites and quartz albite and quartz orthoclase compositions the solubility of water at 2 kbar and 800 c in water saturated haplogranite melts varies strongly as a function of melt composition water solubility in melt decreases in the order strongly peralkaline haplogranite quartz albite composition moderately peralkaline haplogranite peraluminous haplogranite subaluminous haplogranite quartz orthoclase composition d cl wt cl in fluid wt cl in melt is a strong function of system composition d cl increases as the concentration of cl in the system increases a comparison of the distribution of cl between haplogranite melts of subaluminous peraluminous and peralkaline compositions and aqueous fluids shows that with similar concentrations of cl in the system d cl is largest for subaluminous melts a nk 1 having relatively high k2o concentrations as the a nk of melt increases or decreases from 1 or as the n nk of the melt increases both d cl and the concentration of cl in the fluid decrease the experimental data imply that complexes involving cl and na in hydrous subaluminous granite melts are dominant furthermore the data imply that in hydrous peraluminous granite melts cl complexes with network modifying a13 and in hydrous peralkaline granite melts cl complexes with network modifying na k on a molar basis cl apparently complexes with network modifying a13 in peraluminous melts as strongly as it does with na and k in peralkaline melts experimental studies of cl partitioning in granitic systems suggest that the strongest enrichment of magmatic hydrothermal fluids in cl and ore metals complexed with cl will most likely occur in fluids exsolved from subaluminous granite magmas that are relatively enriched in k2o sio2 and cl and relatively depleted in f co2 and na2o

gaining insight into regional coastal changes on la reunion island through a bayesian data mining approach abstract recent works have highlighted the interest in coastal geographical databases collected for coastal management purposes for obtaining insight into current shoreline changes on la reunion a tropical volcanic high island located in the southern indian ocean a dataset is available which describes shoreline changes the coastal geomorphology and the presence of anthropic structures this database is first supplemented with information on the exposure of each coastal segment to energetic waves and to estuarine sediment inputs to incorporate relative sea level changes along the coast in the database levelling data are analysed in combination with gps satellite altimetry and sea level reconstructions finally a method based on bayesian networks is used to assess the probabilistic relationships between the variables in the database the results highlight the high degree of dependency between variables a retrospective model is able to reproduce 81 of the observations of shoreline mobility importantly we report coastal ground motions for la reunion island of the order of 1 to 2 mm year along the coast however the resulting differing rates of relative sea level rise do not significantly impact on shoreline changes instead the results suggest a major control of geological processes and local coastal geomorphic settings on shoreline evolution while any exploration of a coastal database needs to be complemented with human reasoning to interpret the results in terms of physical processes this study highlights the significance of revisiting other datasets to gain insight into coastal processes and factors causing shoreline changes including sea level changes

climate effects on volcanism influence on magmatic systems of loading and unloading from ice mass variations with examples from iceland pressure influences both magma production and the failure of magma chambers changes in pressure interact with the local tectonic settings and can affect magmatic activity present day reduction in ice load on subglacial volcanoes due to global warming is modifying pressure conditions in magmatic systems the large pulse in volcanic production at the end of the last glaciation in iceland suggests a link between unloading and volcanism and models of that process can help to evaluate future scenarios a viscoelastic model of glacio isostatic adjustment that considers melt generation demonstrates how surface unloading may lead to a pulse in magmatic activity iceland s ice caps have been thinning since 1890 and glacial rebound at rates exceeding 20 mm yr 1 is ongoing modelling predicts a significant amount of additional magma generation under iceland due to ice retreat the unloading also influences stress conditions in shallow magma chambers modifying their failure conditions in a manner that depends critically on ice retreat the shape and depth of magma chambers as well as the compressibility of the magma an annual cycle of land elevation in iceland due to seasonal variation of ice mass indicates an annual modulation of failure conditions in subglacial magma chambers

pre eruptive volatile content degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of dalakvisl south iceland many volcanoes express a variety of contrasting eruptive styles with behavioural transitions between explosive and effusive activities understanding eruptive mechanisms is of key importance but we currently have little insight into the behavioural controls for rhyolitic eruptions beneath ice this paper addresses a subglacial rhyolitic eruption at dalakvisl in south iceland that involved both explosive and effusive activities we present measurements of pre and post eruptive magmatic volatile contents that we use to track magma degassing from the chamber to the surface and reconstruct pressure conditions during the eruption explosive and effusive phases involve significant differences in 1 pre eruptive volatile contents 2 the openness of magma degassing and 3 inferred confining pressure this final factor is potentially associated with a jokulhlaup induced decompression due to the combination of these factors dalakvisl experienced a change from effusive to explosive behaviour although we cannot currently resolve cause from effect our work points towards complex interactions between the dynamics of magma ascent and the response of overlying ice and meltwater during rhyolitic eruptions beneath ice

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

increased capture of magma in the crust promoted by ice cap retreat in iceland retreating ice caps overlying volcanoes are thought to relieve surface pressure causing enhanced magma generation and volcanic activity analysis of crustal stress during a dyke intrusion event associated with retreat of iceland s largest ice cap indicates that ice retreat could instead promote storage of magma in the crust rather than eruption at the surface

rhyolitic volcano ice interactions in iceland iceland contains an abundance and diversity of rhyolitic edifices produced during volcano ice interactions glaciovolcanism that is unmatched by any other volcanic province with an estimated 350 km3 erupted during the past 0 8 ma from at least fifteen volcanic systems this review summarises research undertaken during the past decade and provides a new summary of the distribution of rhyolitic glaciovolcanic rocks erupted during the past 0 8 ma descriptions of effusion dominated edifices focus on the two best studied edifices the c 0 6 km3 and 570 m high prestahnukur edifice and the smaller volume 0 1 km3 drape of blahnukur both show little or no evidence of magmatic fragmentation i e driven by volatile exsolution but both show abundant evidence of quench fragmentation and meltback of ice walls a particular feature of prestahnukur is its sheet like lava bodies which are interpreted as sills that intruded the ice edifice interface rhyolitic tuyas are products of sustained eruptions into thick ice and form distinctive steep sided edifices with flat or broad dome like tops they are 300 700 m high with volumes up to 2 km3 though 0 5 1 0 km3 is more typical initial vigorous phreatomagmatic eruptions within well drained ice vaults build steep sided piles of tephra confined by ice walls gradual upwards increases in inflated clasts point to the decreasing involvement of meltwater and the increasing ability of the magma to vesiculate as the eruption progresses and terminates non explosive degassing produces lava caps up to 300 m thick oraefajokull is a stratovolcano illustrating complex volcano ice interactions generated when rhyolitic lavas erupt onto steeper slopes and encounter ice of variable thickness and properties e g fracturing eruptive units record dramatic ice thickness fluctuations of up to 500 m in valley glaciers between eruptions which emphasises that useful palaeoenvironmental information can be gleaned from detailed studies of volcano ice interactions considerable value is added when palaeoenvironmental information is combined with ar ar dating for example ar ar dating of two rhyolitic tuyas at torfajokull reveal that they erupted prior to the last glacial maximum when temperatures were at least 4 c cooler than the present day ar ar dating of prestahnukur suggests it erupted during the last interglacial glacial transition into over 700 m of ice which corroborates studies arguing for rapid accumulation of land based ice during global cooling

constraints on the dynamics of subglacial basalt eruptions from geological and geochemical observations at kverkfjoll ne iceland the kverkfjoll area ne iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in iceland the water contents of slightly vesiculated glassy rims of pillows in six localities range from 0 85 0 03 to 1 04 0 03 wt the water content measurements allow the ice thickness to be estimated at between 1 2 and 1 6 km with the range reflecting the uncertainty in the co2 and water contents of the melt the upper estimates agree with other observations and models that the ice thickness in the centre of iceland was 1 5 2 0 km at the time of the last glacial maximum many of the pillows in the kverkfjoll area are characterised by vesiculated cores 40 60 vesicles surrounded by a thick outer zone of moderately vesicular basalt 15 20 vesicles the core contains 1 mm diameter spherical vesicles distributed uniformly this observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling the cores are attributed to a jokulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4 4 and 4 7 mpa depressurization equivalent to a drop in the water level in the range 440 470 m consideration of the thickness of solid crust around the molten cores at the time of the jokulhlaup indicates an interval of 1 3 days between pillow emplacement and the jokulhlaup upper limits for ice melting rates of order 10 3 m s are indicated this interpretation suggests that jokulhlaups can reactivate eruptions

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