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When volcanic eruptions involve interaction with external water (hydrovolcanism), the result is an ash-rich and energetic volcanic plume, as illustrated dramatically by the January 2022 Tonga eruption. The origin of the high explosive energy of these events remains an important question. We investigate this question by studying Prince Rupert’s Drops (PRDs)—tadpole-shaped glass beads formed by dripping molten glass into water—which have long fascinated materials scientists because the great strength of the head contrasts with the explosivity of the metastable interior when the tail is broken. We show that the fragment size distribution (FSD) produced by explosive fragmentation changes systematically with PRD fragmentation in air, water, and syrup. Most FSDs are fractal over much of the size range, scaling that can be explained by the repeated fracture bifurcation observed in three-dimensional images from microcomputed tomography. The shapes of constituent fragments are determined by their position within the original PRD, with platey fragments formed from the outer (compressive) shell and blocky fragments formed by fractures perpendicular to interior voids. When molten drops fail to form PRDs, the glass disintegrates by quench granulation, a process that produces fractal FSDs but with a larger median size than explosively generated fragments. Critically, adding bubbles to the molten glass prevents PRD formation and promotes quench granulation, suggesting that granulation is modulated by heterogeneous stress fields formed around the bubbles during sudden cooling and contraction. Together, these observations provide insight into glass fragmentation and potentially, processes operating during hydrovolcanism.

The breaking apart of magma into fragments is intimately related to the eruptive style and thus the nature and footprint of volcanic hazards. The size and shape distributions of the fragments, in turn, affect the efficiency of heat transfer within pyroclastic plumes and currents and the settling velocity, and so the residence time, of particles in the atmosphere. Fundamental work relating the glass transition to the fragmentation of magmas remains at the heart of conceptual and numerical models of volcanic eruptions. Current fragmentation criteria, however, do not predict the sizes and shapes of the resulting fragments, or fully account for the multiphase nature of magmas or ways in which magma can break in a fluidal manner or by thermal stress. The pulsatory, non-steady state nature of some eruptions, and related interactions with these fragmentation criteria, also requires further investigation. Here, we briefly review some recent advances in the field of magma fragmentation and provide a perspective on how integrated field, experimental and numerical modelling studies can address key outstanding challenges.

Summary Aquifers are poroelastic bodies that respond to strain by changes in pore pressure. Crustal deformation due to volcanic processes induces pore pressure variations that are mirrored in well water levels. Here, we investigate water level changes in the Belham valley on Montserrat over the course of 2 yr (2004–2006). Using finite element analysis, we simulate crustal deformation due to different volcanic strain sources and the dynamic poroelastic aquifer response. While some additional hydrological drivers cannot be excluded, we suggest that a poroelastic strain response of the aquifer system in the Belham valley is a possible explanation for the observed water level changes. According to our simulations, the shallow Belham aquifer responds to a steadily increasing sediment load due to repeated lahar sedimentation in the valley with rising aquifer pressures. A wholesale dome collapse in May 2006 on the other hand induced dilatational strain and thereby a short-term water level drop in a deeper-seated aquifer, which caused groundwater leakage from the Belham aquifer and thereby induced a delayed water level fall in the wells. The system thus responded to both gradual and rapid transient strain associated with the eruption of Soufrière Hills Volcano (Montserrat). This case study gives field evidence for theoretical predictions on volcanic drivers behind hydrological transients, demonstrating the potential of hydrological data for volcano monitoring. Interrogation of such data can provide valuable constraints on stress evolution in volcanic systems and therefore complement other monitoring systems. The presented models and inferred results are conceptually applicable to volcanic areas worldwide.

Interactions between lava flows and surface water are not always considered in hazard assessments, despite abundant historical and geological evidence that they can create significant secondary hazards (e.g., floods and steam explosions). We combine contemporary accounts of the 1783–1784 Laki fissure eruption in southern Iceland with morphological analysis of the geological deposits to reconstruct the lava–water interactions and assess their impact on residents. We find that lava disrupted the local river systems, impounded water that flooded farms and impeded travel, and drove steam explosions that created at least 2979 rootless cones on the lava flow.Using aerial photographs and satellite-derived digital terrain models, we mapped and measured 12 of the 15 rootless cone groups on the Laki lava field. We have identified one new rootless cone group and provide data that suggest another cone group previously attributed to the 939–940 CE Eldgjá eruption was created by the Laki eruption. We then use contemporary accounts to estimate formation dates and environments for each cone group, which formed in wetland/lake areas, on riverbeds, and near areas of impounded water. Furthermore, comparison with previous field studies shows that assessments using remote sensing can be used to identify and map meter-scale and larger features on a lava flow, although remote mapping lacks the detail of field observations.Our findings highlight the different ways in which lava can interact with surface water, threatening people, property, water supplies, and infrastructure. For these reasons, anticipation of such interactions is important in lava flow hazard assessment in regions with abundant surface water; we further demonstrate that remote sensing can be an effective tool for identifying lava–water interactions in past lava flows.

Many volcanoes around the world are persistently active with continuous degassing for years or even centuries, sometimes exceeding historic records. Such long-term stability contrasts with short-term instability, reflected in eruptive episodes that punctuate passive degassing. These two aspects of persistent activity, long-term stability as opposed to short-term instability, are often conceptualized through two distinct model frameworks: Exchange-flow in volcanic conduits is commonly invoked to explain the long-term thermal balance and sustained passive degassing, while the ascent of large gas slugs is called upon to understand explosive eruptions. While typically considered separately, we propose here that both flow processes could occur jointly in the conduits of persistently active volcanoes and in transient connections between subvolcanic melt lenses. To understand the dynamic interplay between exchange flow and slug ascent, we link analogue laboratory experiments with direct numerical simulations. We find that the two flows superimpose without creating major disruptions when only considering the ascent of a single gas slug. However, the sequential ascent of multiple gas slugs is disruptive to the ambient exchange flow, because it may entail continual buildup of buoyant magma at depth. While our study focuses on the laboratory scale, we propose that the dependence of exchange-flow stability on sequential slug ascent is relevant for understanding why explosive sequences are sometimes followed by effusive eruptions. Taken together, our work suggests that integrating exchange flow and slug ascent could provide a more complete understanding of persistently active volcanoes than either model framework offers in isolation.

At the global scale, large tectonic earthquakes and volcanic eruptions are believed to be random and independent events. Here, we compare global time-series of large earthquakes (Mw ≥ 7) and explosive volcanic eruptions (VEI ≥ 2) spanning 1960–2019. Both time-series exhibit decadal timescale trends, over which annual earthquake and eruption rates vary by a factor of ≈2. Moreover, global seismic moment release is positively correlated with global eruption rate, with Monte Carlo permutation tests showing that this correlation is significant with a P-value of 1000 km) by earthquake-induced dynamic stress changes, modulation of global earthquake and eruption rates by variations in Earth's rotational velocity, or natural synchronization of events over repeating cycles, are therefore required to explain the correlation.

Generation of silicic magmas leads to emplacement of granite plutons, huge explosive volcanic eruptions and physical and chemical zoning of continental and arc crust1-7. While the time scales for silicic magma generation in the deep and middle crust are prolonged8 magma transfer into the upper crust followed by eruption is episodic and can be rapid9-12. Ages of inherited zircons and sanidines from four Miocene ignimbrites in the Central Andes indicate a gap of 4.6 Myr between the start of pluton emplacement and onset of super-eruptions, with a 1 Myr cyclicity. Here we show that inherited sanidine crystals were stored at temperatures

Large earthquakes alter the crustal stress field across great distances (hundreds to thousands of kilometers) over geologically short timescales (seconds to years). These stress changes can affect magmatic systems, triggering (or suppressing) volcanic unrest and eruption, along with other deeper processes. We use simple kinematic source models in an isotropic elastic half‐space to assess earthquake‐induced static stress changes (>1 kPa) over the entire thickness of the lithosphere and consider the implications for magma ascent and storage. Modeling subduction zone earthquakes, we calculate static normal stress changes with depth on three mutually perpendicular end‐member magma pathways: vertical arc‐parallel, vertical arc‐perpendicular, and horizontal. From this, we define seven stress change regimes within the adjacent volcanic arc. Three of these regimes may strongly encourage magma ascent in dykes by inducing unclamping (decreased compressive normal stress) of vertical pathways which increases in magnitude toward the surface and clamping of horizontal pathways. Two of the regimes may encourage stalling and storage of magma in sills near the base of the crust by inducing unclamping of horizontal pathways at depth. The spatial distribution of the regimes is largely dependent on earthquake magnitude, but also varies with slip distribution and interface dip. We show how the responses of magmatic systems to earthquakes also depends on the stress change magnitude and the state of the magmatic system, with a greater impact expected for larger stress changes acting on weaker, more thermally mature systems.

The structural form and elemental distribution of material originating from different Fukushima Daiichi Nuclear Power Plant reactors (Units 1 and 3) is hereby examined to elucidate their contrasting release dynamics and the current in-reactor conditions to influence future decommissioning challenges. Complimentary computed X-ray absorption tomography and X-ray fluorescence data show that the two suites of Si-based material sourced from the different reactor Units have contrasting internal structure and compositional distribution. The known event and condition chronology correlate with the observed internal and external structures of the particulates examined, which suggest that Unit 1 ejecta material sustained a greater degree of melting than that likely derived from reactor Unit 3. In particular, we attribute the near-spherical shape of Unit 1 ejecta and their internal voids to there being sufficient time for surface tension to round these objects before the hot (and so relatively low viscosity) silicate melt cooled to form glass. In contrast, a more complex internal form associated with the sub-mm particulates invoked to originate from Unit 3 suggest a lower peak temperature, over a longer duration. Using volcanic analogues, we consider the structural form of this material and how it relates to its environmental particulate stability and the bulk removal of residual materials from the damaged reactors. We conclude that the brittle and angular Unit 3 particulate are more susceptible to further fragmentation and particulate generation hazard than the round, higher-strength, more homogenous Unit 1 material.

Heating of water under hot pyroclastic flow deposits can drive hydroeruptions, forming craters and aprons of secondary deposits. According to the established conceptual model, steam pressure builds until failure of the pyroclastic overburden and a relatively low permeability (fine-grained) cap promotes secondary explosions. We explore a complementary model where the stress from drag related to gas flow up through the particle interstices is comparable in magnitude to the static pressure difference between the base and the top of the pyroclastic flow deposit. The drag force supports (part of) the weight of the particles and so reduces inter-particle friction; in a mono-sized bed this friction is effectively eliminated at the ‘minimum fluidisation velocity’, which depends on the size and density of the particles. Through analogue experiments we show that violent outbursts can be generated when there are vertical variations in the minimum fluidisation velocities of granular materials. We ran experiments with layers of particles with different sizes or size distributions (bi-modal with different proportions of fine and coarse particles) in a tank with a porous base that allowed a distributed upward airflow through them. A finer-grained layer capping a coarser layer does not generate jets of particles or craters; rather, increased gas flux leads to fluidisation of first the fine and then the coarse (lower) layer. However, when the upper layer is coarser, the bed domes upward as a gas pocket grows within the finer layer for some combinations of layer thicknesses and grain sizes. When the gas pocket penetrates the top of the bed, it forms a crater and erupts particles. The gas velocity when doming initiates is greater than that calculated for the weight of the top layer to be balanced by drag and the pressure difference across that layer. This discrepancy is explained by the layers having a strength (from inter-particle friction), which is consistent with the observed dependence of the initiation velocity on the absolute thickness of the layer. Using data from Mt St Helens 1980 deposits, we show that the drag-related trigger observed in the laboratory is a feasible mechanism for secondary hydroeruptions through pyroclastic flow deposits.

Physical interactions between bubbles and crystals affect gas migration and may play a major role in eruption dynamics of crystal-rich magmas. Strombolian eruptions represent an end member for bubble-crystal interactions, in which large bubbles (significantly larger than the crystal size) rise through a crystal-rich near-surface magma. Indeed, volcanoes that produce Strombolian eruptions often generate ejecta with > 30 vol% (often > 45 vol%) average crystallinity. At Stromboli Volcano, Italy, average crystallinity can reach 55 vol%, which is approaching the eruptibility limit for magmas. At such high crystallinities the solids interact mechanically with each other and with bubbles. This complex rheology complicates the two-phase (liquid-gas) slug flow model often applied to Strombolian eruptions. To examine the effect of crystals on bubble rise, we performed analogue experiments in which large bubbles rise in a vertical tube filled with silicone oil and polypropylene particles. The particles have a slightly lower density than the oil, and therefore form a layer of oil + particles at the upper surface. We varied surface pressure, particle volume fraction, length of the particle-bearing cap, and bubble size to examine the ways in which these parameters influence Strombolian-type eruptions. We show that in experiments, suspended solids begin to affect bubble rise dynamics at particle volume fractions as low as 30 vol% (or, when divided by the random close packing value, a normalized particle fraction φ = 0.64 ). Bubbles in experiments with higher particle contents deform as they rise and burst through a small aperture, generating surface fountains that begin abruptly and decay slowly, and longer-lasting acoustic signals of lower amplitude than in particle-poor experiments. Particle fractions > 38 vol% ( φ > 0.80 ) generated strong deformations on fast-expanding bubbles that applied a high stress on the cap, but they trapped bubbles that were less overpressured. Qualitatively, the gas release behavior observed in particle-rich experiments is consistent with observations of Strombolian eruptions. Moreover, we estimate that the observed crystallinity of pyroclasts at Stromboli volcano represents φ > 0.8 . From this we suggest a “weak plug” model for Strombolian eruptions that evolves towards a low-viscosity equivalent of Vulcanian-style plug failure with a more crystalline, stronger, and less permeable plug. Importantly, this model allows the rise of several bubbles in the conduit at the same time and suggests that longer-lasting, more pulsatory and complex eruptions may reveal an increase in near-surface crystallinity, shedding some light on changing conduit conditions that could help determine the different gas rise regimes involved in passive degassing, puffing, and different expressions of Strombolian explosions.

Lava-water interactions (LWIs) are rarely considered in lava flow hazard assessments or emergency planning scenarios, though they can generate a range of secondary hazards, including tephra blasts, rootless eruptions, disruption to water supplies, and flooding. These hazards may endanger life, damage property, and hinder evacuation or rescue efforts, so identifying the signs of LWI in the products of past eruptions may help emergency planners identify potential hazards for future eruptions. The physical products of LWI, such as abundant hyaloclastite, high proportions of fine ash, lava pillows, and irregular columnar jointing, have long been recognized in the field. However, remote sensing offers the opportunity to assess whole lava fields relatively quickly and cheaply, and allows investigation of inaccessible lava fields and planetary volcanism. In addition, the large-scale view can reveal features that are not immediately visible in the field, and tools like LiDAR can be used to strip away vegetation to show hidden morphology and structure. We present features indicative of LWI that can be identified by remote sensing techniques and discuss what they can and can’t tell us about LWI in past eruptions. We illustrate these with data from the well-documented 1783-84 Laki fissure eruption, supplemented with other case studies from Iceland, Hawai’i and the Pacific NW. In particular, the size, type and spacing of rootless cones can tell us about the availability of water and intensity of rootless eruptions. When examined in conjunction with lava flow morphology and local topography, we can learn about the local lava flux and the likely water sources and pre-eruptive landscape. In the absence of rootless cones, dendritic textures on the lava flow surface may indicate passive LWI. These textures are found across the Laki lava field, commonly in areas where the lava encountered rivers or floods, and match those at other lava basaltic flows known to have interacted with water, including the 2018 eruption of Kilauea. Together, these features are useful indicators for identifying and interpreting past LWIs, both as a complement to field observations and when field studies are not feasible.

Volcanic ash is dispersed in the atmosphere according to meteorology and particle properties, including size and shape. However, the multiple definitions of size and shape for non‐spherical particles affect our ability to use physical particle properties to understand tephra transport. Moreover, although particles

A new approach to automating droplet freezing assays is demonstrated by comparing the ice nucleating efficiency of a K-feldspar glass and a crystal with the same bulk composition. The method uses a pyroelectric polymer PVDF (polyvinylidene fluoride) as a thermal sensor. PVDF is highly sensitive, cheap and readily available in a variety of sizes. As a droplet freezes latent heat is released, which is detected by the sensor. Each event is correlated with the temperature at which it occurred. The sensor has been used to detect microlitre volume droplets of water freezing, from which frozen proportion curves and nucleation rates can be quickly and automatically calculated. Our method shows glassy K-feldspar to be a poor nucleator compared to the crystalline form.

Estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale. This study uses the tephra from the caldera forming eruption of Mount Mazama (Crater Lake, OR, USA) to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude. We combine > 250 Mazama tephra occurrences, reported by a range of disciplines (including archaeology, paleoclimatology and volcanology), with new field studies to better understand the extent of the distal tephra. We find that the Mazama tephra has been remobilised to varying degrees over the past 7000 years, so each tephra locality was appraised for the likelihood that it records primary tephra fallout. We designated 45 of the distal (> 100 km from source) tephra sites as suitable for use in the production of isopachs using a spline fitting method. The new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic Mazama eruption to ~ 176 km3 (~ 61 km3 dense-rock equivalent; DRE). This study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit. Interpreting remobilised deposits can provide insight into post-eruptive processes that could potentially pose secondary hazards following large magnitude eruptions. We also show that in some circumstances, remobilised deposits preserve important physical properties such as grain size.

Aeroacoustic jet noise theory describes the relationship between acoustic signals and jet operating parameters. Based on empirical aeroacoustic relationships, a link between peak frequency and amplitude and jet diameter and velocity has been used by volcano acousticians to return the operating parameters of column generating volcanic eruptions. There are however significant differences between the nozzle geometries and structures of flows studied in the aeroacoustic literature and flows emitted from volcanic vents. These differences raise questions as to the validity of applying engineering relationships to volcanic eruptions. Combining particle image velocimetry and acoustic data we investigated the flow structures, acoustic signals and dominant source locations of jet flows emitted from three simple nozzle geometries (convergent, straight and divergent). Flows from the straight and divergent nozzles exited in a more developed flow regime than those emitted from convergent nozzles. While for the convergent nozzles dominant sound sources were located within the jet flow, the dominant signals from the straight and divergent nozzles were located in the region of the nozzle exit. Consequently, for the straight and divergent nozzles it was not possible to differentiate between signals generated by the jet flow and those generated by internal sound sources without beamforming analysis. In order to better replicate the volcanic case and in contrast to studies for industrial applications, our experimental test section was not engineered to minimise internal noise. Without acoustic treatment such as screens, choke plates and paddles, jet noise signals were contaminated by signals generated within the test rig and so, even for the convergent nozzles, the aeroacoustic relationships derived from engineering studies could not be used to retrieve the jet operating parameters. Our results indicate that in the volcanic case, the raw acoustic spectra will contain signals generated by both in-conduit and above-vent processes. Therefore inversions for volcanic jet parameters such as mass eruption rate from standard aeroacoustic relationships may be unreliable.

Modelling atmospheric volcanic ash dispersion is a critical tool in mitigating the impact of large explosive eruptions; it is also useful for understanding and reconstructing past events. Most atmospheric dispersion models include a sedimentation velocity term that is sensitive to the physical properties of the particle, but many do not use particle shape as an input parameter; instead particles are assumed to be spherical. There are many empirical and semi-empirical shape-dependent drag laws. We measure the velocity of scaled analogue particles over the range of flow conditions anticipated for volcanic ash dispersion to test published formulae against an independent dataset. We use a semi-empirical formula we determined to be accurate for non-spheres to investigate the sensitivity of the modelled transport of an ash cloud to particle shape, using the atmospheric dispersion model NAME and a shape parameter we measure for non-spherical ash particles from Katla volcano, Iceland. We find that model particle trajectories are sensitive to particle shape for particles >1–3 μm diameter; the sedimentation velocity of smaller particles is low compared to atmospheric vertical velocities. Sensitivity to shape increases with size such that 100 μm particles can travel 44% further from the source when they are highly non-spherical (sphericity = 0.5). Despite the sensitivity of the fall velocity of large particles to their shape, however, forecasts of distal ash concentration using particle size distributions of 0.1–100 μm and 0.1–250 μm show relatively good agreement between a spherical and non-spherical case for the first 36 h after an eruption. The vertical structure of an ash cloud is more sensitive to particle shape than the horizontal extent. Model particle trajectories are also sensitive to particle size, and we find a discrepancy between different particle size parameters for non-spherical ash: particle long axis L, used in cryptotephra studies, was on average twice the equivalent-volume sphere diameter dv, used in dispersion modelling, for tephra samples from Katla volcano, Iceland. This discrepancy in size measurements could explain the observed travel distance of large distal cryptotephra shards.

The 4·0–3·6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry-style copper mineralization. This study uses petrographic, petrological, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence for progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3·5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas. Mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite, testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.

In the current paradigm, magma primarily exists in the crust as a crystalline mush containing distributed melt lenses. If a melt-rich (or fluid) lens is less dense than the overlying mush, then Rayleigh-Taylor (RT) instabilities will develop and could evolve into spheroids of ascending melt. Due to contrasting melt-mush rheologies, the theoretical RT instability wavelength can be orders of magnitude larger than the magmatic system. We explored how this confinement affects the gravitational stability of melt lenses through laboratory experiments with pairs of liquids with one layer much thinner and up to 2.2·105 times less viscous than the other; we extended the viscosity ratio to 106 with linear stability analysis. We found the growth rate of a bounded RT instability is approximately (Formula presented.), where Δρ is the difference in density between the fluids, g is gravity, D is the container diameter, and μ2 is the viscosity of the thicker viscous layer. This differs from the unbounded case, where the growth rate also depends on the thickness and viscosity of the thin, low-viscosity layer. Applying the results to melt lenses in magmatic mushes, we find that for the ranges of expected rheologies, the timescales for development of the instability, and the volumes of packets of rising melt generated span very wide ranges. They are comparable with the frequencies and sizes of volcanic eruptions and episodes of unrest and so suggest that RT instabilities in mush systems can cause episodic volcanism.

International audience; Interactions between clasts in pyroclastic density currents (PDCs) generate volcanic ash that can be dispersed to the atmosphere in co-PDC plumes, and due to its small size, is far-travelled. We designed a series of experiments to determine the effects of pyroclast vesicularity and crystal content on the efficiency and type of ash generated by abrasion. Two different pyroclastic materials were used: (1) basaltic-andesite pyroclasts from Fuego volcano (Guatemala) with ~ 26–46% vesicularity and high groundmass crystallinity and (2) tephri-phonolite Avellino pumice (Vesuvius, Italy) with ~ 55–75% vesicularity and low groundmass crystallinity.When milled, both clast types produced bimodal grain size distributions with fine ash modes between 4 and 5φ (32–63 μm). Although the vesicular Avellino pumice typically generated more ash than the denser Fuego pyroclasts, the ash-generating potential of a single pyroclast was independent of density, and instead governed by heterogeneous crystal and vesicle textures. One consequence of these heterogeneities was to cause the vesicular Avellino clasts to split in addition to abrading, which further enhanced abrasion efficiency. The matrix characteristics also affected ash shape and componentry, which will influence the elutriation and transport properties of ash in the atmosphere. The experimental abrasion successfully replicated some of the characteristics of natural co-PDC ash samples, as shown by similarities in the Adherence Factor, which measures the proportion of attached matrix on phenocrysts, of both the experimentally generated ash and natural co-PDC ash samples. Our results support previous studies, which have shown that abrasion is an effective mechanism for generating fine ash that is similar in size (~ 5φ; 30 μm) to that found in co-PDC deposits. We further show that both the abundance and nature (shape, density, components, size distribution) of those ash particles are strongly controlled by the matrix properties of the abraded pyroclasts.

To quantify the size of tephra, two practical challenges must be addressed: the wide range of particle sizes (10−8 - 101 m) and the diversity of particle shape, density and optical properties. Here we use dynamic image analysis (DIA) to simultaneously characterise the size and shape of tephra samples from Mount Mazama, Krafla, Mount St. Helens and Campi Flegrei. The Camsizer X2 instrument used in this study, which has a measurement range of 0.8 μm – 8 mm, avoids the need to overlap different measurement methods and principles for fine (125 μm) particle sizes. Importantly, DIA does not require an assumption of particle properties. DIA also allows the measurement of grain size distributions (GSDs) using multiple size definitions. Quantification by particle long axis and the area equivalent sphere diameter, for example, make DIA GSDs compatible with the outputs of other methods such as laser diffraction and sieving. Parallel mass-based (sieving) and volume-based (DIA) GSDs highlight the effects of particle density variations on methods of size analysis; concentrations of dense crystals within a narrow size range, in particular, can affect mass-based GSDs and their interpretations. We also show that particle shape has an important effect on the apparent grain size of distal tephra. Extreme particle shapes, such as the platy glass shards typical of the distal Campanian Ignimbrite deposits, can appear coarser than other distal tephras if size is quantified according to the particle long axis. These results have important implications for ash dispersion models, where input GSDs assume that reported measurements are for volume-equivalent sphere diameters. We conclude that DIA methods are not only suitable for characterising, simultaneously, the size and shape of ash particles but also provide new insights into particle properties that are useful for both ash dispersion modelling and studies of explosive volcanism.

The transition to a low-carbon economy will increase demand for a wide range of metals, notably copper, which is used extensively in power generation and in electric vehicles. Increased demand will require new, sustainable approaches to copper exploration and extraction. Conventional copper mining entails energy-intensive extraction of relatively low-grade ore from large open pits or underground mines and subsequent ore refining. Most copper derives ultimately from hot, hydrous magmatic fluids. Ore formation involves phase separation of these fluids to form copper-rich hypersaline liquids (or ‘brines') and subsequent precipitation of copper sulfides. Geophysical surveys of many volcanoes reveal electrically conductive bodies at around 2 km depth, consistent with lenses of brine hosted in porous rock. Building upon emerging concepts in crustal magmatism, we explore the potential of sub-volcanic brines as anin situsource of copper and other metals. Using hydrodynamic simulations, we show that 10 000 years of magma degassing can generate a Cu-rich brine lens containing up to 1.4 Mt Cu in a rock volume of a few km3at approximately 2 km depth. Direct extraction of metal-rich brines represents a novel development in metal resource extraction that obviates the need for conventional mines, and generates geothermal power as a by-product.

The Don Manuel igneous complex and associated porphyry copper mineralization in the Andean Cordillera of central Chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics. The Don Manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes, which intrude the older units. Mineralization is associated with the biotite tonalite and intermediate porphyries, which also contain the greatest abundance of mafic enclaves. Zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing. New zircon U-Pb ages and whole-rock Ar-Ar ages indicate that the Don Manuel igneous complex was emplaced between ca. 4 and 3.6 Ma. The time scale for the episodic intrusion of the Don Manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes. Observations from the Don Manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement. The observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation.

Tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies. However, their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit. To assess the reliability of data from lake cores, we compare tephra deposited on land during the 2015 eruption of Calbuco volcano in Chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits. In general, the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar. As anticipated, however, cores sampled close to (here, within 300 m of) fluvial inflows were affected by sediment deposition from the lake’s catchment; they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading. Cores 850 m away from the inlet were not affected. We consider our results in the context of the particle settling regime as well as each lake’s location, bathymetry and catchment area. We find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles. We conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux, as well as lake features such as bathymetry and catchment area.

Ground deformation and gravity changes in restless calderas during periods of unrest can signal an impending eruption and thus must be correctly interpreted for hazard evaluation. It is critical to differentiate variation of geophysical observables related to volume and pressure changes induced by magma migration from shallow hydrothermal activity associated with hot fluids of magmatic origin rising from depth. In this paper we present a numerical model to evaluate the thermo-poroelastic response of the hydrothermal system in a caldera setting by simulating pore pressure and thermal expansion associated with deep injection of hot fluids (water and carbon dioxide). Hydrothermal fluid circulation is simulated using TOUGH2, a multicomponent multiphase simulator of fluid flows in porous media. Changes in pore pressure and temperature are then evaluated and fed into a thermo-poroelastic model (one-way coupling), which is based on a finite-difference numerical method designed for axi-symmetric problems in unbounded domains.Informed by constraints available for the Campi Flegrei caldera (Italy), a series of simulations assess the influence of fluid injection rates and mechanical properties on the hydrothermal system, uplift and gravity. Heterogeneities in hydrological and mechanical properties associated with the presence of ring faults are a key determinant of the fluid flow pattern and consequently the geophysical observables. Peaks (in absolute value) of uplift and gravity change profiles computed at the ground surface are located close to injection points (namely at the centre of the model and fault areas). Temporal evolution of the ground deformation indicates that the contribution of thermal effects to the total uplift is almost negligible with respect to the pore pressure contribution during the first years of the unrest, but increases in time and becomes dominant after a long period of the simulation. After a transient increase over the first years of unrest, gravity changes become negative and decrease monotonically towards a steady-state value.Since the physics of the investigated hydrothermal system is similar to any fluid-filled reservoir, such as oil fields or CO2 reservoirs produced by sequestration, the generic formulation of the model will allow it to be employed in monitoring and interpretation of deformation and gravity data associated with other geophysical hazards that pose a risk to human activity.

Assessing and communicating the risks posed by natural hazards requires not only a thorough understanding of the hazards themselves but also an understanding of the spatial and temporal impact of successive events from the same source(s). This approach is enhanced by the lens of time, which is often missing from modern responses to recent hazards. Archaeological studies, in contrast, examine the long-term consequences of hazardous events, but often lack details of the event chronology and immediate human impact that are available for recent events. Here we show ways in which historical records can bridge the gap between modern and prehistoric studies. We focus on the volcanically and seismically active region that hosts the capital city of Guatemala, a city that has been relocated twice in response to hazardous events since its original founding in 1527. More specifically, we examine documents - "Autos Hechos Sobre el Lastimoso, Estrago y Ruina que Padecio esta Ciudad de Guatemala..." [Autos] - that were collected in response to a cascading sequence of volcanic, seismic and mudflow events in 1717 to support a request to the Spanish government to relocate the capital city for a second time. These documents provide exceptional detail about the location of the witnesses, the nature of the hazardous activity and the response of local communities. This detail allows us not only to reconstruct the sequence of events but also to link volcanic activity at Volcán de Fuego to local seismicity and mudflows from Volcán de Agua, which we interpret as triggered by magma intruded after the eruption of Fuego ceased. At the same time, the long and well documented history of the region allows us to examine ways in which a single catastrophic event - in this case a large rainfall-triggered debris flow from Agua in 1541, which destroyed the first capital city - can reverberate through the centuries and affect the response of the local community almost 200 years later, in 1717. The long-term effects are particularly apparent because both the origin and affected area of the mudflow hazard were very different in the two cases. This example thus illustrates the importance of "memorability" (e.g., Slovic, 2000) in both the perception of, and response to, hazardous events.

Accurate measurements of volcanic ash morphology are critical to improving both our understanding of fragmentation processes and our ability to predict particle behaviour. In this study, we present new ways to choose and apply shape parameters relevant to volcanic ash characterisation. First, we compare shape measurements from different imaging techniques, including cross-sectional (2-D) and projected area images, and discuss their respective applications. We then focus on specific information that can be obtained from shape analysis of 2-D images. Using cluster analysis as an unbiased method to identify key controls on particle morphology, we find that four shape parameters – solidity, convexity, axial ratio, and form factor – can effectively account for the morphological variance within most ash samples. Importantly, these parameters are scaled to values between 0 and 1, and therefore contribute evenly to discrimination diagrams. In particular, co-variation in convexity and solidity can be used to distinguish different juvenile ash components based on characteristic bubble properties. By reducing observations of natural samples to simplified ash geometries, we quantify morphological changes associated with variations in the relative size and shape of bubbles and particles. Using this relationship, we assess the potential application of size-dependent shape analysis for inferring the underlying bubble size distribution, and thus the pre-fragmentation conditions. Finally, we show that particle shape analysis that includes the full range of available grain sizes can contribute not only measurements of particle size and shape, but also information on size-dependent densities.

Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use finite element analysis to demonstrate the response of aquifers to volumetric strain induced by pressurized magma reservoirs. Two different aquifers are invoked – an unconsolidated pyroclastic deposit and a vesicular lava flow – and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurization and the resulting hydraulic head changes as well as flow through the porous aquifer, i.e. porous flow. The simulated strain leads to centimetres (pyroclastic aquifer) to metres (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system. Well level changes are particularly sensitive to chamber volume, shape and pressurization strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time. We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do indeed mirror volumetric strain, and wells are therefore a valuable addition to monitoring systems that could provide important insights into pre-eruptive dynamics.

The size distribution of volcanic ash is rarely measured in real time and Volcanic Ash Advisory Centres (VAACs) often rely on a default particle size distribution (PSD) to initialise their dispersion models when forecasting the movement of ash clouds. We conducted a sensitivity study to investigate the impact of PSD on model output and consider how best to apply default PSDs in operational dispersion modelling. Compiled grain size data confirm that, when considering particles likely to be in the distal ash cloud (&lt, 125 &micro, m diameter), magma composition and eruption size are the dominant controls on grain size distribution. Constraining the PSD is challenging but we find that the grain size of deposits from large hydromagmatic eruptions remains relatively constant with distance, suggesting that total (whole-deposit) grain size distributions (TGSDs) for these eruptions could be estimated from a few samples. We investigated the sensitivity of modelled ash mass loadings (in the air and on the ground) to input PSDs based on coarse to fine TGSDs from our dataset. We found clear differences between modelled mass loadings and the extent of the plume. Comparing TGSDs based on ground-only and ground-plus-satellite data for the Eyjafjallaj&ouml, kull 2010 eruption, we found that basing input PSDs on TGSDs from deposits alone (likely missing the finest particles) led to lower modelled peak ash concentrations and a smaller plume.

The sedimentation rate of volcanic ash through the atmosphere influences its travel distance, with important implications for aviation and health. The fall velocity of a particle depends on its size and density, but also shape, and volcanic ash is not spherical. To capture the sedimentation of ash, atmospheric dispersion models use empirical drag equations calibrated using geometric shape descriptors. However, particle shape data are scarce and there is no standard method of shape measurement. In addition, shape measurements are not always available during an eruption, when dispersion models are used operationally to forecast ash hazard. We assess the variability in the shape of volcanic ash from Icelandic eruptions using X-ray computed tomography. To consider how good different drag equations and shape descriptors are at representing the sedimentation of volcanic ash we compare calculated fall velocities to measured fall velocities of volcanic ash in air in a settling column. We then suggest the best drag equations and shape descriptors for use in atmospheric dispersion models. We find that shape-dependent drag equations produce more accurate results than a spherical approximation. However, accurate drag calculations based on the shape descriptor sphericity, which is a function of surface area, require the imaging resolution to be within the range of 102 - 105 voxels per particle (where a voxel is a volumetric pixel) as surface area is sensitive to imaging resolution. We suggest that the large-scale form of the particle impacts sedimentation more than small-scale surface roughness. Shape descriptors based on ratios between principal axis lengths are more practical as they are less variable among particle size classes and much less sensitive to imaging resolution. Finally, we use particle shape data from this study and literature sources to make recommendations on default values for use with atmospheric dispersion models where no shape data are available.

Publisher's version (útgefin grein), It is well established that rootless cones and associated deposits are the result of explosive interactions between lava flows and environmental water, but there is substantial uncertainty about the dynamics of rootless eruptions, particularly the relative importance of lava supply, water availability and the conditions under which they meet. Here we present a case study of the Younger Laxá Lava in NE Iceland, and the >6500 rootless cones that it created. Critically, this long (63 km) lava flow interacted with water along its length, from flow through/around a large lake (Mývatn), down a narrow river gorge (Laxárdalur) and across a broad glacial valley with wetlands and rivers (Aðaldalur). Using high-resolution digital terrain models and aerial photographs, we map the flow surface morphology, and classify, measure and analyse the rootless cone type, size and spatial distribution in the context of both lava and water availability. We find that rootless cone size is controlled by the combined availability of lava and water: large rootless cones require sustained, high volumes of lava (related to proximity to vent) and water (e.g., Lake Mývatn), whereas limited supplies of lava (with distance from the vent and from the centre of the flow) and water (particularly in dammed river valleys) build smaller cones. Where we infer that where water was distributed in sediment, the lava–water interaction style changed to low-energy and distributed bubble bursts that created >3000 hornitos in the lower reaches of Aðaldalur. The distribution of rootless cones around Mývatn also defines the pre-eruption extent of the lake and suggests substantial lake level changes during the course of the eruption. By looking at the variation in rootless cone type and size in the context of the parent lava flow and the local environment, we explain how the availability of water and local mass flow rate of lava affect the dynamics of rootless eruptions., This work was supported by the Natural Environmental Research Council [grant no: NE/L002434/1], and on AXA Research Fund and Wolfson Merit Award to KVC. DTMs provided by the Polar Geospatial Center under NSF OPP awards 1043681, 1559691 and 1542736.

Aluto is a silicic volcano in central Ethiopia, flanked by two large population centers and home to an expanding geothermal power plant. Here we present data from two lake sediment cores sampled 12 km and 25 km from the volcano, which record at least 24 distinct eruptions in the Holocene. Tephra layers from the two cores are correlated using a variety of techniques, including major and trace element geochemistry as well as textural and morphological features from SEM‐BSE imaging. The purpose is to provide a Holocene reference section for further tephrostratigraphic studies of the volcano as well as to provide information on eruption frequency. The lake cores suggest that Aluto has had a variable eruption rate, with three eruption ‘clusters' in the Holocene at ~3, 6.5 and 11 ka, with small Vulcanian‐to sub‐Plinian eruptions separated by larger, Plinian eruptions. We infer that the smaller tephras are likely the product of pumice cone‐ and dome‐ forming eruptions. In addition, modern wind data suggest the likely direction of an ash cloud from Aluto is to the west and south west, which is towards population centres and is in agreement with thickness data from the cores. We conclude that current records underestimate the volcano's eruptive history and that hazard assessments should be updated accordingly.

The style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact, their relative viscosities, the depth of subsurface explosions, the substrate properties, and the vent geometry. Fundamental questions remain, however, regarding the role of magmatic vesiculation in determining the dynamics of magma-water interaction (MWI). Petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre- and syn-eruptive conditions during ‘dry’ magmatic eruptions, but the application of similar techniques to hydromagmatic activity has not yet been fully explored. In this study, we integrate glass volatile measurements (S, Cl, H2O and CO2) with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the Hverfjall Fires fissure eruption, Iceland. Here, coeval fissure vents produced both ‘dry’ magmatic (Jarðbaðsholar scoria cone complex) and variably wet hydromagmatic (Hverfjall tuff ring) activity, generating physically distinct pyroclastic deposits with contrasting volatile signatures. Matrix glass volatile concentrations in hydromagmatic ash (883 ± 172 [1σ] ppm S; 0.45 ± 0.03 [1σ] wt% H2O; ≤20 ppm CO2) are consistently elevated relative to magmatic ash and scoria lapilli (418 ± 93 [1σ] ppm S; 0.12 ± 0.48 [1σ] wt% H2O; CO2 below detection) and overlap with the range for co-erupted phenocryst-hosted melt inclusions (1522 ± 127 [1σ] ppm S; 165 ± 27 [1σ] ppm Cl). Measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70% of its initial sulfur prior to premature quenching at variably elevated confining pressures. By comparing volatile saturation pressures for both magmatic and hydromagmatic glasses, and how these vary through the eruptive stratigraphy, we place constraints on the conditions of MWI. Crucially, our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100–200 m, and that the external water supply was sufficient to maintain MWI throughout the eruption with no significant vertical or lateral migration of the fragmentation surface. We propose that development of an in-vent water-sediment slurry provides a mechanism through which the elevated confining pressures of ~1.6–2.6 MPa (or up to 6 MPa accounting for uncertainty in CO2 below analytical detection) could be maintained and buffered throughout the eruption, whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths. Importantly, these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of (1) the environment of MWI (e.g., groundwater versus surface water, vertical migration of the fragmentation level) and (2) the state of the magma at the time of fragmentation and quenching. We further suggest that the volatile content of tephra glasses provides a reliable alternative (or additional) indicator of a hydromagmatic origin, particularly for reduced Ocean Island Basalts where late-stage volatile saturation and degassing (S, H2O) occurs over a pressure range relevant to typical MWI environments.

Long-lived, persistently active lava lakes are rare but have been studied extensively because they may provide a natural laboratory for studying magmatic convection in basaltic and other low-viscosity volcanoes. However, lava lakes differ from other volcanic systems in various ways, particularly geometry: the presence of the lava lake requires a sudden and significant flaring of the conduit. The goal of this paper is to advance our understanding of the effect that a sudden increase in the conduit width has on the stability of large gas slugs and bubbles ascending from the plumbing system. We investigate this question by linking analog laboratory experiments to direct numerical simulations in two dimensions. We find that the rapid change in the geometry associated with the transition from the conduit to the lava lake causes large gas slugs to break up. The nondimensional regime over which we observe the breakup depends sensitively on the flare angle of the lava lake and the importance of inertial effects in the flow, implying that degassing-related eruptive activity at open-system volcanoes is sensitive to the upper conduit and lake geometries. Applying this idea to the likely geometries and viscosities of actual lava lakes, our study suggests that all lava lakes except Erebus fall well within the regime where large gas slugs are prone to breakup.

The freezing temperatures of hundreds of water drops with radii 20–50 lm containing known average concentrations of suspended, mostly micron- to submicron-sized, volcanic ash particles composed of SiO2-rich glass were recorded using optical microscopy. As expected, the ash suppresses supercooling, and in contrast to earlier studies of much larger ash particles, the median freezing temperature clearly scales with the available ash surface area per drop. The heterogeneous nucleation rate coefficient per unit mass of ash (jm) increases exponentially with decreasing temperature (T) (increasing supercooling) with a possible change in the slope of a plot of log jm against T at T ¼ 245 � 1 K. Although uncertainties in the ash surface area limit quantitative comparisons, we conclude that volcanic glass is a less effective ice-nucleating agent than feldspar crystals and more similar to other minerals previously studied. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

Large tube filling bubbles rising up through quiescent fluid in a vertical tube are commonly known as Taylor bubbles. Their apparent simplicity of form and behaviour has led to them being viewed and modelled as a paradigm for both large bubble dynamics, where there is no continuous gas flow, and slug flow for the case of continuous gas flow. Central to this approach is the question: what diameter tubes support stable Taylor bubbles? In this paper we examine the case of low viscosity Taylor bubbles through experiments and theory and show that they exist in much wider diameter tubes than had previously been reported. In order for the bubbles to be stable a settling period is required to allow the column to sufficiently quiesce. This settling period is compared favourably with the classical stability analysis of Batchelor (1987). We also observe such bubbles rising in an oscillatory manner if the gas input is abruptly curtailed. The oscillations match theoretical predictions well.

The abundant fine ash produced in the 2011 subglacial eruption of Grimsvotn, Iceland, highlights the fragmentation efficiency of mafic hydromagmatic eruptions, which is considerably higher than for comparable “dry” eruptions. Ash from the 2011 eruption can be divided into three morphological components—vesicular particles, shards, and dense fragments—distinguished by the size and abundance of constituent vesicles. We use the vesicle characteristics to define a new shape factor, the concavity index, which provides an unbiased way to classify individual ash particles as either bubbly (vesicular particles and shards) or dense. The relative proportion of bubbly and dense particles varies systematically with grain size, with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter. Measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from Hawaiian fountains and suggest a comparable degassing history during magma ascent. Yet concordance between the size distributions of ash and of bubbles in the Grimsvotn samples stands in contrast to the size distributions in Hawaiian fountains, where pyroclasts are orders of magnitude larger than individual bubbles. We propose that the Grimsvotn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water. The strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation.

Twenty-six new 40Ar/39Ar plateau ages for 23 lavas and domes from the Uturuncu volcano in the Altiplano of SW Bolivia reveal a protracted eruptive history from 1050±5 to 250±5 ka. Eruptions have been exclusively effusive, producing some 50 km3 of high-K dacites and silicic andesites. Bimodal mineral compositions, complex mineral textures, the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at Uturuncu. Post-458 ka, distinct high and low MgO–Cr magmas are resolved. These magmas erupt during similar times, suggesting that eruptions are tapping different parts of the magma system, albeit from the same vent system. Volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath Uturuncu, and calculated buoyancy forces are sufficient to drive effusive eruptions. Eruptive activity is episodic, with six eruptive periods separated by hiatuses of >50 kyr. Cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka. The episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying Altiplano–Puno Magma Body.

Local (100 s of metres from vent) monitoring of volcanic infrasound is a common tool at volcanoes characterized by frequent low-magnitude eruptions, but it is generally not safe or practical to have sensors so close to the vent during more intense eruptions. To investigate the potential and limitations of monitoring at near-regional ranges (10 s of km) we studied infrasound detection and propagation at Mount Erebus, Antarctica. This site has both a good local monitoring network and an additional International Monitoring System infrasound array, IS55, located 25 km away. We compared data recorded at IS55 with a set of 117 known Strombolian events that were recorded with the local network in January 2006. 75% of these events were identified at IS55 by an analyst looking for a pressure transient coincident with an F-statistic detection, which identifies coherent infrasound signals. With the data from January 2006, we developed and calibrated an automated signal-detection algorithm based on threshold values of both the F-statistic and the correlation coefficient. Application of the algorithm across IS55 data for all of 2006 identified infrasonic signals expected to be Strombolian explosions, and proved reliable for indicating trends in eruption frequency. However, detectability at IS55 of known Strombolian events depended strongly on the local signal amplitude: 90% of events with local amplitudes > 25 Pa were identified at IS55, compared to only 26% of events with local amplitudes

Phreatic eruptions, although posing a serious threat to people in crater proximity, are often underestimated and have been comparatively understudied. The detailed eruption catalogue for Ruapehu Volcano (New Zealand) provides an exceptional opportunity to study the statistics of recurring phreatic explosions at a crater lake volcano. We performed a statistical analysis on this phreatic eruption database, which suggests that phreatic events at Ruapehu do not follow a Poisson process. Instead they tend to cluster, which is possibly linked to an increased heat flow during periods of a more shallow-seated magma column. Larger explosions are more likely to follow shortly after smaller events, as opposed to longer periods of quiescence. The absolute probability for a phreatic explosion to occur at Ruapehu within the next month is about 10%, when averaging over the last 70 years of recording. However, the frequency of phreatic explosions is significantly higher than the background level in years prior to magmatic episodes. Combining clast ejection simulations with a Bayesian event tree tool (PyBetVH) we perform a probabilistic assessment of the hazard due to ballistic ejecta in the summit area of Ruapehu, which is frequently visited by hikers. Resulting hazard maps show that the absolute probability for the summit to be affected by ballistics within the next month is up to 6%. The hazard is especially high on the northern lakeshore, where there is a mountain refuge. Our results contribute to the local hazard assessment as well as the general perception of hazards due to steam-driven explosions.

Growing evidence for significant magmatic vesiculation prior to magma-water interaction (MWI) has brought into question the use of ‘diagnostic’ features, such as low vesicularities and blocky morphologies, to identify hydromagmatic pyroclasts. We address this question by quantifying co-variations in particle size, shape and texture in both magmatic and hydromagmatic deposits from the Hverfjall Fires fissure eruption, Iceland. Overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth, with substantial vesiculation prior to MWI. Hydromagmatic fragmentation occurred principally by brittle mechanisms, where the length scale and geometry of fracturing was controlled by the bubble population. This suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven, at least in part, by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling. In this way, the shape and size distributions of hydromagmatic pyroclasts, both critical input parameters for ash dispersion models, are strongly influenced by the dynamics of vesiculation prior to MWI. This result underlines the need to analyse multiple grain-size fractions to characterise the balance between magmatic and hydromagmatic processes. During the Hverfjall Fires eruption, the external water supply was sufficient to maintain MWI throughout the eruption, with no evidence for progressive exhaustion of a water reservoir. We suggest that both the longevity and the spatial distribution of MWI were determined by the pre-existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas., This work was completed with support for KVC from the AXA Research Fund and a Royal Society Wolfson Merit Award, a Royal Society URF to ACR and a University of Bristol postgraduate Scholarship to EJL. Fieldwork was supported by a New Researchers Award from the Geologists’ Association to EJL. We thank S. Kearns and B. Buse for their assistance with SEM analysis.

Vertical cylinders of bubble-enriched, chemically evolved volcanic rock are found in many inflated pahoehoe lava flows. We provide a putative theoretical explanation for their formation, based on a description of a crystallising three-phase (liquid, solid, gas) crystal pile in which the water-saturated silicate melt exsolves steam and becomes more silica-rich as it crystallises anhydrous minerals. These cylinders resemble pipes that form in solidifying binary alloys as a result of sufficiently vigorous porous medium convection within the mush. A convection model with the addition of gas bubbles that provide the buoyancy source indicates that the effective Rayleigh number is too low for convection to occur in the mush of a basalt lava flow. However, the formation of gas bubbles during crystallisation means that the base state includes fluid migration up through the crystal mush even without convection. Stability considerations suggest that it is plausible to form a positive feedback where increased local p...

We present a method to estimate the rheology of a lava flow and reconstruct variations of eruption parameters from measurements of flow dimensions together with petrological analysis of samples from the solidified flow. The model assumes a Herschel–Bulkley lava rheology, which we demonstrate is a reasonable approximation from new analyses of samples and rheological measurements from past eruptions of Etna volcano, as well as published data for other lava compositions and crystal contents. We present a simplified 2-D model for the flow of a Herschel–Bulkley fluid over an inclined plane with a constant flux from a vent, which we validate with analogue experiments scaled to typical conditions of lava flows. Further analogue experiments with two fluids of different rheologies demonstrate the dominance of the rheology at the flow front in controlling the advance rate of the flow. The Herschel–Bulkley flow model was applied to a lava flow of the 2002 eruption on Etna volcano. Flow velocity and effusion rate were calculated from the post-eruption flow dimensions and rheology estimates along the length of the flow were based on the crystal content and glass compositions of samples. The results compare well with the observations during the eruption, with a mean flow rate of 19.5 m3/s, which is in the range of the 15–20 m3/s calculated by previous authors.

Crystal-rich intermediate magmas are subjected to both primary and secondary fragmentation processes, each of which may produce texturally distinct tephra. Of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes. One way to address this question is by fragmenting pyroclasts under controlled conditions. We fragmented pumice samples from Soufriere Hills Volcano (SHV), Montserrat, by three methods: rapid decompression in a shock tube-like apparatus, impact by a falling piston, and milling in a ball mill. Grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns, and that the fractal dimension increases from a minimum of ~ 2.1 for decompression fragmentation (primary fragmentation) to a maximum of ~ 2.7 by repeated impact (secondary fragmentation). To assess the details of the fragmentation process, we quantified the shape, texture and components of constituent ash particles. Ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts. We also quantify the extent to which the matrix is separated from the crystals, which shows that secondary processes efficiently remove adhering matrix from crystals, particularly during milling (abrasion). Furthermore, measurements of crystal size distributions before (using x-ray computed tomography) and after (by componentry of individual grain size classes) decompression-driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution, but also that free crystals are smaller in the fragmented material than in the original pumice clast. Taken together, our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation. Critically, however, our extension of previous analyses to characterisation of shape, texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin. We illustrate this application with examples from SHV deposits.

The emerging concept of a magma reservoir is one in which regions containing melt extend from the source of magma generation to the surface. The reservoir may contain regions of very low fraction intergranular melt, partially molten rock (mush) and melt lenses (or magma chambers) containing high melt fraction eruptible magma, as well as pockets of exsolved magmatic fluids. The various parts of the system may be separated by a sub-solidus rock or be connected and continuous. Magma reservoirs and their wall rocks span a vast array of rheological properties, covering as much as 25 orders of magnitude from high viscosity, sub-solidus crustal rocks to magmatic fluids. Time scales of processes within magma reservoirs range from very slow melt and fluid segregation within mush and magma chambers and deformation of surrounding host rocks to very rapid development of magma and fluid instability, transport and eruption. Developing a comprehensive model of these systems is a grand challenge that will require close collaboration between modellers, geophysicists, geochemists, geologists, volcanologists and petrologists. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.