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We combine global detections of volcanic lightning with acoustic and hydroacoustic data to investigate novel indications of plume electrification in ground-based, geophysical data streams during the 2016–2017 eruption of Bogoslof volcano, Alaska. Such signals offer additional ways to diagnose the occurrence of volcanic lightning and confirm whether eruptive activity is producing significant amounts of ash. We discuss three signatures of lightning activity: volcanic thunder, electromagnetic pulses arising from lightning-induced voltages in cabling, and hydroacoustic signals associated with volcanic lightning. Observations of these phenomena provide additional insights into volcanic lightning activity and reveal several periods of electrical activity that were not otherwise detected during the Bogoslof eruption. [ABSTRACT FROM AUTHOR]

We use satellite imagery to investigate the shoreline changes associated with volcanic activity in 2018-2019 at Anak Krakatau, Indonesia, spanning a major lateral collapse and period of regrowth through explosive activity. The shoreline changes have been analyzed and validated through the adaptation of an existing methodology based on Sentinel-2 multispectral imagery and developed on Google Earth Engine. This work tests the results of this method in a highly dynamic volcanic environment and validates them with manually digitized shorelines. The analysis shows that the size of the Anak Krakatau Island increased from 2.84 km2 to 3.19 km2 during 15 May 2018-1 November 2019 despite the loss of area in the 22 December 2018 lateral collapse. The lateral collapse reduced the island area to ~1.5 km2 but this was followed by a rapid increase in area in the first two months of 2019, reaching up to 3.27 km2. This was followed by a period of little change as volcanic activity declined and then by a net decrease from May 2019 to 1 November 2019 that resulted from erosion on the SW side of the island. This history of post-collapse eruptive regrowth and coastal erosion derived from the shoreline changes illuminates the potential for satellite-based automated shoreline mapping to provide databases for monitoring remote island volcanoes. [ABSTRACT FROM AUTHOR]

Explosive eruptions involve the fragmentation of magma that changes the flow regime from laminar to turbulent within the volcanic conduit during ascent. If the gas volume fraction is high, magma fragments and the eruption style is explosive, but if not, the magma flows effusively out of the vent. Gas escape processes depend on how the magma can rupture, and recent experimental studies measured rupture stress thresholds of the order of a few megapascals. It is thus critical to model the gas content and state of stress evolution in the flowing magma within the conduit. We present a new self-consistent model of an explosive eruption from the magma chamber to the surface, based on a critical gas volume fraction. Our model allows to explore irregular geometries below the fragmentation level (2-D). We first compare our model with classical 1-D models of explosive eruptions and find that in the case of straight conduits and fragmented flows, 1-D models are accurate enough to model the gas pressure and vertical velocity distribution in the conduit. However, in the case of an irregular conduit shape at depth, 2-D models are necessary. Despite a certain conduit radius visible at the surface, very different stress fields within the flow could be present depending upon the position and shape of any conduit irregularities. Stresses of the order of more than 1 MPa can be attained in some locations. High tensile stresses are located at the centre of the conduit, while high shear stresses are located at the conduit walls leading to several potential rupture locations. Due to the interplay between the velocity field and decompression rate, similar conduit radius visible at the surface might also lead to very different fragmentation depths with a difference of more than 1500 m between an enlarged conduit shape at some depth and a straight conduit. At depth, different conduit sizes might lead to the same order of magnitude for the mass flux, depending on the conduit geometry. [ABSTRACT FROM AUTHOR]

Abstract We investigate whether the disproportionate contribution of individual volcanoes in the Large Magnitude Explosive Volcanic Eruption database (LaMEVE) potentially compromises the treatment of LaMEVE as a globally representative database of volcanic activity. We find that 41% of volcanoes which contribute at least one eruption to LaMEVE only contribute one eruption (10% of all eruptions), and the six most prolific volcanoes contribute 11% of eruptions. However, there is no systematic bias with respect to the eruption magnitude or date for volcanoes contributing one eruption. Also, no bias can be discerned for when the smallest or largest eruption at a volcano occurs in its eruptive record. Half of the volcanoes contributing one or more eruptions to the LaMEVE database had their first eruption prior to 36.4 ka. We find LaMEVE is representative – while there are well-known issues of eruption under-reporting, LaMEVE is not overly biased by the activity of a few volcanoes.

Seismic signals during explosive eruptions have been correlated to eruption size or eruption volume flux for individual eruptive episodes. However, the universality of these correlations has yet to be confirmed. We quantified the sources of high-frequency seismic signals associated with sub-Plinian and Vulcanian eruptions at Kirishima (Japan), Tungurahua (Ecuador) and other volcanoes in Japan using a simple approach based on highly scattered seismic waveform characteristics. We found that eruption plume heights scale to seismic source amplitudes and are described by two relations depending on the value of source amplitudes: power-law and exponential relations for plume height >6 km and <6 km, respectively. Though conceptually similar, our scaling relations differ from the previously proposed relation based on reduced displacement. By comparing seismic and geodetic data during sub-Plinian eruptions at Kirishima, we found that the source amplitude is proportional to eruption volume flux. Combining these relations, we show that our scaling relation for Plinian eruptions is consistent with predictions from plume dynamics models. We present a source model to explain the proportionality between the source amplitude and eruption volume flux assuming a vertical crack or a cylindrical conduit as the source. The source amplitude can be estimated in seconds without any complicated data processing, whereas eruption plumes take minutes to reach their maximum heights. Our results suggest that high-frequency seismic source amplitudes are useful for estimating plume heights in real time. [ABSTRACT FROM AUTHOR]

Volcanic lightning shows considerable promise as a monitoring and research tool to characterize explosive eruptions. Its key strengths are rapid and remote detection, because the radio signals produced by lightning can propagate thousands of km at the speed of light. Despite these tantalizing properties, the scientific work on volcanic lightning has only recently started gaining momentum. Much more is needed to understand what lightning reveals about the evolution of an eruption in near-real time. Here we examine the timing and energy release of lightning generated by the eruption of Kelud volcano in Indonesia on 13 February 2014, as detected by the World Wide Lightning Location Network (WWLLN). The eruption column reached at least 26 km above sea level, representing the highest plume since the advent of global lightning networks in the last decade. Therefore, it provides valuable constraints on the electrification of end-member, sustained Plinian columns. We investigate the lightning in context with satellite images, photographs, and other published studies. Results show that the earliest satellite-detected activity was a thermal anomaly at ~15:46 UTC, corresponding to a directed blast at the onset of eruption (and only a few lightning strokes). Following a brief pause, the eruption produced a sustained column and umbrella cloud that spread outward into the tropical stratosphere. Rates of umbrella expansion provide an average mass eruption rate (MER) in the range of 8 × 107–1 × 108 kg s−1. A more nuanced picture emerges from the time-varying MERs (determined between each satellite pass), which show rapid intensification during the first hour of eruption, followed by constant MER for about an hour, and waning toward the end (after ~17:50 UTC). At this stage, decreasing flux into the umbrella cloud coincides with column instability and formation of pyroclastic density currents, as recorded by photos from the ground ~17:45 UTC. We infer that some of the erupted mass partitioned into ground-hugging currents, leading to a lower apparent MER. Interestingly, there is not a 1:1 correlation between lightning intensity and MER over the course of eruption. Stroke rates increase sharply within the first 30–40 min (during rapid intensification of the plume), and then drop below 2 strokes per min once the MER remains constant. This suggests that electrification was controlled by the rate of increase in MER—in other words, the acceleration of particles out of the vent. We also show that lightning stroke-rates and energies are greatest within 50 km of the vent, even when the ash cloud extends >200 km downwind, indicating that lightning was focused in the regions of highest particle concentration and turbulence. Overall, we conclude that abrupt changes in lightning rates are clearly linked to changes in eruption behavior, and that rapid detection could aid monitoring efforts to characterize eruption rates or styles. • The 2014 eruption of Kelud produced abundant volcanic lightning detected by WWLLN • Rates of umbrella cloud expansion gave mass eruption rates from 8×107–1×108 kg/s • Lightning arose mainly in the turbulent core of the eruption column and umbrella • Stroke rates peaked during intensification of the eruption column (increasing MER) • These findings suggest particle acceleration may have enhanced electrification [ABSTRACT FROM AUTHOR]

We have developed a new two-layer pyroclastic density current (PDC) model that considers the effects of the entrainment and thermal expansion of ambient air in the upper dilute layer in order to investigate the dominant factors controlling the run-out distance of large-scale PDCs. Numerical simulations show a dilute current spreading radially from the collapsing eruption column edge and forming a thin, dense, basal current through particle settling; i.e., a two-layer PDC forms. Forward propagation of the two-layer PDC stops owing to lift-off of the dilute current and/or deposition of the dense current, and eventually the two-layer PDC converges to a steady state. Our parametric study classifies the flow patterns of steady-state two-layer PDCs into one of two flow regimes according to the relative magnitude of the run-out distances of the dilute and dense currents. This relative magnitude critically depends on the ratio of the deposition speed at the base of the dense current (D) to the speed of particles settling from the dilute current to the dense current (W s). The run-out distance of the whole two-layer PDC is determined by that of the dilute current when D / W s is large (≳4 × 10−3). Otherwise, when D / W s is small (≲4 × 10−3), the run-out distance of the dense basal current exceeds that of the dilute current; the former increases as D / W s decreases. The run-out distance of the dilute current is too short to explain some of the long run-out distances of PDCs observed in the field (e.g., the 1991 Pinatubo and 2014 Kelud eruptions). It is suggested that the dense current traveling beyond the lift-off point of the parent dilute current plays a significant role in the emplacement of PDCs with such long run-out distances. • We developed a new two-layer pyroclastic density current (PDC) model. • The model comprises a dilute current over a dense basal current. • Run-out distance of a PDC is determined from that of the dilute or dense current. • Run-out distances of dilute currents are greatly reduced by the entrainment of air. • Dense currents play a role in the emplacement of PDCs with long run-out distances. [ABSTRACT FROM AUTHOR]

Abstract Lightning is ubiquitous in large ash-rich eruptions. However, the quantification of the link between volcanic and electrical activities is still missing, hindering its potential for monitoring explosive eruptions. Here, we focus on vent lightning, i.e. discharges occurring within the ash-laden jet. We use a shock tube apparatus generating jets with variable mass of ash, grain size distribution and initial overpressure. The experimental jet is directed inside a Faraday cage, where the current (flux of electrical charges) is measured, allowing to estimate the total charge of the plume, and the number and magnitude of jet-to-ground discharges. Three mechanisms control the electrical structure of the jet: (i) the tribocharging of the ash particles against the shock tube walls; (ii) the particle–particle tribocharging in the jet flow; (iii) the particle and charge separation according to the particle size, leading to the formation of clusters of electrical charges. The number and magnitude of discharges mainly rely on the two latter mechanisms: while the particle–particle interactions define the total charge in all the clusters, which is linked to the observed cumulative magnitude of the discharges, the jet structure defines the size of the individual clusters, and, in turn, how the cumulative magnitude is partitioned on a number of discharges. Finally, our experimental relationships between eruptive and electrical parameters are compatible with field observations, suggesting that the pattern of discharges recorded by electrical monitoring systems can be used to interpret the structure of volcanic jets and their dynamic evolution. Highlights • Electrification of ash-laden jets is quantified by shock-tube experiments. • Charge partition between fine and coarse ash modulates the discharges. • Mass of fines and burst overpressure control total discharge magnitude. • Jet structure determines the number and size of the discharges. • Experimental observation can be scaled to natural eruptions. [ABSTRACT FROM AUTHOR]

Very-long-period (VLP) seismic events at Asama volcano in central Japan are characterized by a transient signal of 10–20 s duration. Associated with the transient motion, a high-frequency (HF) oscillation within the 5–10 Hz band is observed. We investigated the location and size of the oscillation source in the HF band (HF source) using an amplitude source location method and compared our results with those for the VLP band (VLP source) deduced in our previous waveform inversion study. We analysed 1437 VLP events recorded during an intense observation campaign during 2008–2009, and additional 571 events surrounding an eruptive activity in 2015 (including a VLP event that immediately preceded an eruption). The HF source locations of most events were deeper than the VLP source locations by ∼150 m, although there was almost no time lag between the signals. This suggests a strong connection between the two sources, with one source immediately responding to the other. The eruptive VLP event had an HF source location close to normal VLP events but had a greater event size. We surmise that the HF signal is caused by an inflow of volcanic gas from depth into a semi-vertical crack-like cavity (as imaged in our previous waveform inversion study), and the VLP signal is caused by resultant inflation of the same cavity. The centroid of inflation (VLP source) is near the roof of the cavity, where gas accumulates, whereas the HF oscillation is emitted more intensely from lower and narrower portions of the cavity. This results in source depth differences between the two signal bands. In this model, the inflation rate of the cavity is controlled by the volume flux of the gas inflow, producing similar temporal variations between the VLP and HF signals. According to this model, the eruptive VLP is associated with a greater amount of gas inflow and accumulation, which likely played a key role in the eruption. [ABSTRACT FROM AUTHOR]

Understanding the temporal variability of volcanic activity in the deep ocean is critical for estimating the fluxes of heat and chemicals between the lithosphere and the overlying ocean, and for characterizing the relationships between hydrothermal processes and the subseafloor biosphere. Because there is a growing interest in the development of mineral resources in seafloor hydrothermal settings, it is also important to understand the hydroacoustic environment near submarine volcanoes, which could be impacted by mining operations. To evaluate pre-industrialization sound levels at one proposed site of seafloor mining, a deep-sea hydrophone recording system was deployed for a period of 5 months in the Manus Basin, Bismarck Sea, in the territorial waters of Papua New Guinea. During the deployment the system recorded evidence of a long-lived (∼60-d) gas-driven explosive volcanic eruption. The signals observed were similar to those recorded at two other backarc volcanoes (West Mata and NW Rota-1), and considering the recent geological history of the region, are likely related to the continued growth of a volcanic cryptodome on the south side of North Su volcano. The data show that eruptive events can start and stop abruptly, can last for months, and can increase the local RMS sound pressure levels by about 12 dB on average. The estimated average acoustic source power of the volcanic eruption is roughly equivalent to that of a large commercial shipping vessel. [ABSTRACT FROM AUTHOR]

We implement an analytical model based on flexural deflection of a thin elastic disc to investigate the magnitude of lithospheric decompression caused by deglaciation at upper crustal magmatic reservoirs. Considering a published numerical climate model describing the space–time evolution of deglaciation after the Last Glacial Maximum (LGM) along the Southern Volcanic Zone (SVZ) of the Andes, we demonstrate that changes in pressure at upper crustal levels (<10 km depth) at the scale of several hundred years are of the order of 10–100 MPa. Total decompression and decompression rate (300–150 kPa yr−1) are 1–2 orders of magnitude larger than values previously estimated by other authors who assume that glacial loads are supported by an elastic half-space, that is, of infinite elastic thickness. The large decompression caused by flexural unbending of an elastic plate of finite thickness as assumed here can easily surpass the tensile strength of rocks (5–20 MPa), creating adequate conditions for failure of the reservoir walls, dike propagation inside and outside the reservoir and the eventual collapse of the reservoir accompanying an explosive eruption. We apply our results to the analysis of post-glacial eruptions of SVZ volcanoes, which erupted large volumes (>10 km3) of mafic ignimbrites hundreds to thousands of years after deglaciation onset. We show that this time lag is necessary to achieve a decompression of several tens of megapascals at depths of several kilometres that are consistent with the location of magmatic reservoirs as estimated by independent petrologic, seismic and/or geodetic studies. Moreover the northward increase of this time lag is in agreement with a smaller size of the Andean ice cap in the north than in the south during the LGM. For wet, volatile-rich magmas typical of subduction zones, the effect of large decompression at upper crustal reservoirs caused by flexural unbending of the lithosphere after deglaciation could play a major role in promoting large explosive eruptions through devolatization of the magma, during past deglaciation events as demonstrated here for the LGM along the SVZ and current accelerated ice retreat caused by climate change over large segments of subduction-related arcs at higher latitudes. [ABSTRACT FROM AUTHOR]

While gold partitioning into hydrothermal fluids responsible for the formation of porphyry and epithermal deposits is currently well understood, its behavior during the differentiation of metal-rich silicate melts is still subject of an intense scientific debate. Typically, gold is scavenged into sulfides during crustal fractionation of sulfur-rich mafic to intermediate magmas and development of native forms and alloys of this important precious metal in igneous rocks and associated ores are still poorly documented. We present new data on gold (Cu-Ag-Au, Ni-Cu-Zn-Ag-Au, Ti-Cu-Ag-Au, Ag-Au) alloys from iron oxide deposits in the Lesser Khingan Range (LKR) of the Russian Far East. Gold alloy particles are from 10 to 100 µm in size and irregular to spherical in shape. Gold spherules were formed through silicate-metal liquid immiscibility and then injected into fissures surrounding the ascending melt column, or emplaced through a volcanic eruption. Presence of globular (occasionally with meniscus-like textures) Cu-O micro-inclusions in Cu-Ag-Au spherules confirms their crystallization from a metal melt via extremely fast cooling. Irregularly shaped Cu-Ag-Au particles were formed through hydrothermal alteration of gold-bearing volcanic rocks and ores. Association of primarily liquid Cu-Ag-Au spherules with iron-oxide mineralization in the LKR indicates possible involvement of silicate-metallic immiscibility and explosive volcanism in the formation of the Andean-type iron oxide gold-copper (IOCG) and related copper-gold porphyry deposits in the deeper parts of sub-volcanic epithermal systems. Thus, formation of gold alloys in deep roots of arc volcanoes may serve as a precursor and an exploration guide for high-grade epithermal gold mineralization at shallow structural levels of hydrothermal-volcanic environments in subduction zones.

While long-term interactions of magma with carbonate wall-rock (a.k.a. carbonate assimilation) are well-studied, only recently some experimental studies focused on short-term interactions (seconds to minutes) at magma chamber conditions (0.5 GPa and 1200 °C). They have shown that carbonate assimilation can effectively release CO 2 and dissolve the ingested clast in syn -eruptive timescales. Carbonate wall-rock xenoliths in eruptive products can hence be seen as proof of even shallower ingestion (i.e., within the feeding dyke). To study these shallower interactions, we performed 66 experiments at atmospheric pressure (i.e., at the second endmember of the volcanic feeding system) and at 950–1230 °C with varying melt compositions and limestone compositions. Decarbonation was found to be mainly dependent on temperature and limestone composition while clast dissolution is largely dependent on magma composition, temperature, pressure and interaction time. In natural systems during magma ascent and with increasing quantities of assimilated wall-rock, the magma temperature would steadily decrease, limiting its own decarbonation and assimilation ability. But even in the 950 °C-experiments decarbonation (i.e., CO 2 release) remained a syn -eruptive process. We subsequently discussed the limits of carbonate assimilation as well as the potential effect of syn -eruptive addition of CO 2 to the magmatic mixture on magma ascent and eruption dynamics. • Carbonate assimilation within the whole volcanic plumbing system. • Controlling parameters and relative timescales of decarbonation and clast dissolution. • Decarbonation occurs in syn -eruptive timescales, independent of depth of clast ingestion. • Clast dissolution dominant at magma chamber, but only of very localized importance in shallow depths. • Additional CO 2 will form bubbles and increase eruption explosivity. [ABSTRACT FROM AUTHOR]

The Cenozoic Aldama Volcanic Field (8–0.24 Ma) is located on the southeastern margin of the Sierra de Tamaulipas and is the largest field of the Eastern Mexican Alkaline Province. It contains cinder cones, tuff rings, maars, and simple cones, as well as widespread lava flows. The volcanic rocks are geochemically characterized as alkali basalts, subalkaline basalts, trachybasalts (potassic and hawaiites), latites to trachytes, and grouped in six rock types according to their Mg-content and normative composition: (a) Alkaline near-primary rocks, (b) Alkaline basic rocks, (c) Alkaline intermediate rocks, (d) Subalkaline near-primary rocks, (e) Subalkaline basic rocks, and (f) Subalkaline intermediate rocks. Trace elements and isotopic ratios show that the source of the basaltic melts displays asthenospheric characteristics and stability conditions, indicating that the partial melting (<5%) occurred in lithospheric levels within the spinel stability depth. The coexistence in time and space of alkaline and subalkaline rocks could be explained by selective alkali metasomatism of a regional HFSE and REE-enriched OIB-like source. The magma generation occurred in a typical continental intraplate setting, associated with mantle disturbances produced by the eclogitization and detachment of the Hess conjugate subducted under NE Mexico. Some alkaline and subalkaline melts evolved to intermediate members by fractional crystallization. [Display omitted] • The Aldama Volcanic Field represents the biggest Cenozoic complex related to the Eastern Mexican Alkaline Province. • A geochemical characterization and the spatial distribution of the volcanic structures are presented. • An alkaline and a subalkaline series coexist in time and space, being the alkaline promoted by mantle metasomatism. • The evolution of basaltic magmas by fractional crystallization led to the formation of trachytic magmas. [ABSTRACT FROM AUTHOR]

We use satellite imagery to investigate the shoreline changes associated with volcanic activity in 2018−2019 at Anak Krakatau, Indonesia, spanning a major lateral collapse and period of regrowth through explosive activity. The shoreline changes have been analyzed and validated through the adaptation of an existing methodology based on Sentinel-2 multispectral imagery and developed on Google Earth Engine. This work tests the results of this method in a highly dynamic volcanic environment and validates them with manually digitized shorelines. The analysis shows that the size of the Anak Krakatau Island increased from 2.84 km2 to 3.19 km2 during 15 May 2018−1 November 2019 despite the loss of area in the 22 December 2018 lateral collapse. The lateral collapse reduced the island area to ~1.5 km2 but this was followed by a rapid increase in area in the first two months of 2019, reaching up to 3.27 km2. This was followed by a period of little change as volcanic activity declined and then by a net decrease from May 2019 to 1 November 2019 that resulted from erosion on the SW side of the island. This history of post-collapse eruptive regrowth and coastal erosion derived from the shoreline changes illuminates the potential for satellite-based automated shoreline mapping to provide databases for monitoring remote island volcanoes.

Explosive volcanoes from the southern Andes are able to disperse ash over wide areas of the Southern Hemisphere, potentially as far as Antarctica. With the aim of improving correlations between sources and tephra in southernmost South America and, possibly, Antarctica, this work presents new field, textural and geochemical data on tephra layers from southern Patagonia and Tierra del Fuego (Argentina and Chile). Major- and trace-element data, obtained on single glass shards allowed to identify tephra sources in Late Glacial-Holocene eruptions from Hudson, Reclus and Mt Burney volcanoes, located in the Southern and Austral Volcanic Zone of the Andean Cordillera. Twelve new radiocarbon age determinations of charcoals, peats and soils have further constrained the correlations between the studied tephra layers and known eruptions from Hudson, Mt Burney and Reclus volcanoes. Therefore, this study contributes to expand the geochemical dataset on volcanic glasses valuable for tephra correlations in South America, and improves the current tephrostratigraphic framework of this region. Furthermore, we revised literature data by compiling a database including Neogene-Quaternary volcanic tephra found in Antarctic ice cores, marine sediments, blue ice and continental outcrops as well as tephra produced by volcanic sources located in Antarctica and circum-Antarctic areas. This revision shows that Antarctic tephra can be correlated with confidence to Antarctic and circum-Antarctic (South Shetlands and South Sandwich Islands) volcanic sources, whereas correlations with South American sources are arguable, and a complete geochemical fingerprinting is needed for validation. [ABSTRACT FROM AUTHOR]

The subduction-related volcanic front in Nicaragua consists of the Tertiary “Coyol” member in the eastern highlands and the Quaternary to recent volcanic arc within the Nicaraguan depression. Although the Holocene to recent explosive volcanism has been studied extensively no detailed work has been done on the products of explosive volcanism from Quaternary volcanic complexes comprising also the Malpaisillo and Monte Galán Calderas, the focus of this study. The 11 km-wide Malpaisillo Caldera and ~3.5 km-wide Monte Galán Caldera, located ~50 km northwest of Managua, are surrounded by tens of meters of rhyolitic tephras. These pyroclastic flow and fall deposits extend proximally at least 11 km to the southeast and 23 km to the southwest, with observed depositional thicknesses of >16 m for a single ignimbrite unit (or >25 m for the entire section). Distal deposits are found as far as 350 km offshore in the Pacific. At least twelve highly explosive large-volume eruptive phases with corresponding tephra deposits (LPT = La Paz Centro Tephra, PPT = Punta de Plancha Tephra, LCbT = Lower Chibola Tephra, GT = Guacucal Tephra, UCbT = Upper Chibola Tephra, FeT = La Fuente Tephra, ST = Sabanettas Tephra, MgT = Miralago Tephra, ToT = Tolapa Tephra, LMT, MMT, UMT = Lower, Middle, and Upper Maderas Negras Tephras) are distinguished based on geochemical correlations and similar depositional characteristics. Radiometric 40 Ar/ 39 Ar ages indicate that most activity related to the large Malpaisillo Caldera occurred between ~570 and ~420 ka. The large Pleistocene Malpaisillo and Monte Galán Calderas are characterized by a long-lived history and, if evolved, a distinctly alkaline (K 2 O = 2.3–3.8 wt%; Na 2 O = 4.0–4.9 wt%) geochemical signature compared to the other Nicaraguan tephra deposits. As a result, the previously defined Malpaisillo Formation has been considerably extended and revised. Our findings contribute to fill a considerable gap in the long-term eruptive history of Nicaraguan volcanoes, with prominent implications for volcanic hazard evaluation for Nicaragua. [ABSTRACT FROM AUTHOR]

The size distribution of the population of particles injected into the atmosphere during a volcanic explosive eruption, i.e., the total grain-size distribution (TGSD), can provide important insights into fragmentation efficiency and is a fundamental source parameter for models of tephra dispersal and sedimentation. Recent volcanic crisis (e.g. Eyjafjallajökull 2010, Iceland and Córdon Caulle 2011, Chile) and the ensuing economic losses, highlighted the need for a better constraint of eruption source parameters to be used in real-time forecasting of ash dispersal (e.g., mass eruption rate, plume height, particle features), with a special focus on the scarcity of published TGSD in the scientific literature. Here we present TGSD data associated with Hekla volcano, which has been very active in the last few thousands of years and is located on critical aviation routes. In particular, we have reconstructed the TGSD of the initial subplinian–Plinian phases of four historical eruptions, covering a range of magma composition (andesite to rhyolite), eruption intensity (VEI 4 to 5), and erupted volume (0.2 to 1 km 3 ). All four eruptions have bimodal TGSDs with mass fraction of fine ash (<63 μm; m 63 ) from 0.11 to 0.25. The two Plinian dacitic-rhyolitic Hekla deposits have higher abundances of fine ash, and hence larger m 63 values, than their andesitic subplinian equivalents, probably a function of more intense and efficient primary fragmentation. Due to differences in plume height, this contrast is not seen in samples from individual sites, especially in the near field, where lapilli have a wider spatial coverage in the Plinian deposits. The distribution of pyroclast sizes in Plinian versus subplinian falls reflects competing influences of more efficient fragmentation (e.g., producing larger amounts of fine ash) versus more efficient particle transport related to higher and more vigorous plumes, displacing relatively coarse lapilli farther down the dispersal axis. [ABSTRACT FROM AUTHOR]

Satellite-based measurements of synthetic aperture radar amplitude provide a method for monitoring volcanoes during unrest and eruptions even when visual observations are not possible, for example due to poor weather or at night, and when radar phase measurements are noisy or decorrelated. Here, we use high resolution radar amplitude images from the TerraSAR-X and COSMO SkyMed satellites to investigate surface changes associated with explosive eruptions of Cotopaxi volcano, Ecuador in August 2015. We generate change difference and amplitude ratio maps spanning the start of explosive activity at Cotopaxi, which show complex spatial variations in radar amplitude both on and around the summit ice-cap that we attribute to a number of processes related to the eruption. Observed amplitude decreases are caused by crater deepening, ashfall onto ice and surface smoothing by ashfall onto slopes facing away from the satellite, while amplitude increases are due to deposition of coarse lapilli and wet tephra, increased soil saturation due to geothermally driven glacier melting, and smoothing of slopes facing towards the satellite. We discuss the potential applications of radar amplitude images for monitoring and hazard evaluation at active volcanoes. [ABSTRACT FROM AUTHOR]

Lightning often occurs during ash-producing eruptive activity, and its detection is now being used in volcano monitoring for rapid alerts. We report on infrasonic and sonic recordings of the related, but previously undocumented, phenomenon of volcanic thunder. We observe volcanic thunder during the waning stages of two explosive eruptions at Bogoslof volcano, Alaska, on a microphone array located 60 km away. Thunder signals arrive from a different direction than coeruptive infrasound generated at the vent following an eruption on 10 June 2017, consistent with locations from lightning networks. For the 8 March 2017 eruption, arrival times and amplitudes of high-frequency thunder signals correlate well with the timing and strength of lightning detections. In both cases, the thunder is associated with lightning that continues after significant eruptive activity has ended. Infrasonic and sonic observations of volcanic thunder offer a new avenue for studying electrification processes in volcanic plumes. [ABSTRACT FROM AUTHOR]

It is usually believed that volcanoes explicate their activity erupting magma, in which volatiles (mostly H 2 O) are dissolved in modest quantities. At Mount Etna, the maximum H 2 O found in olivine melt inclusions is 3.5 wt%, which would correspond to a moles H 2 O/mol basalt ratio of 0.14. A reappraisal of published data is proposed here, and a comparison is made between the gas flux and the volume of erupted lavas. The results are surprising: the moles H 2 O/mol basalt ratio is 1.41, which means that Mount Etna erupts 10 times the maximum H 2 O that could be dissolved in magma and 40% more moles of gas (H 2 O, CO 2 and S) than moles of basalt. By calculating the molar volume of the basaltic melt components (silicate tetrahedra and metallic cations) and of the gas phase at a pressure of 250 MPa, it is possible to envisage the magma within the deep plumbing system as a solution made of ~ 70% continuum gas phase (mostly H 2 O) at a supercritical state (density 360 kg/m 3 ) and 30% basaltic melt components. The transition from this low-density (1140 kg/m 3 ) water melt solution (WMS) to the high-density (2800 kg/m 3 ) basalt, usually erupted (defined as a continuous melt phase, CMP), occurs in the last 2 km and marks the boundary between a deep and a shallow plumbing system. The depth of this boundary varies with time, being driven by the rate at which the gas escapes the WMS to feed the persistent gas plume at the summit craters, leaving the CMP, which accumulates within the shallow plumbing system, until erupted. The overpressure of the gas phase in the WMS, acting like a piston cylinder, is fundamental in driving the eruption. In this work, the thermal contribution provided to the CMP by the large gas flux has also been considered, proving that it can supply the heat necessary to maintain the CMP. The volcano is here considered a dynamic system in which the eruptive activity is ruled by discontinuities in the flux of gas and heat. Negative fluctuations in the gas flux would decrease the heat supply, promote viscosity and trigger eruptions. Moreover, this view of the volcanic system, subverting the common paradigm in which the gas emitted is associated with an equivalent amount of degassing magma, explains the phenomenon, known as the ‘excess degassing problem’, which affects volcanoes of basaltic and andesitic nature worldwide. [ABSTRACT FROM AUTHOR]

Erosion has revealed a remarkable section through the heart of a volcanic island, Mason Spur, in the southwestern Ross Sea, Antarctica, including an unusually well-exposed section of caldera fill. The near-continuous exposure, 10 km laterally and > 1 km vertically, cuts through Cenozoic alkalic volcanic rocks of the Erebus volcanic province (McMurdo Volcanic Group) and permits the study of an ancient volcanic succession that is rarely available due to subsequent burial or erosion. The caldera filling sequence includes an unusual trachytic spatter-rich lapilli tuff (ignimbrite) facies that is particularly striking because of the presence of abundant black fluidal, dense juvenile spatter clasts of trachytic obsidian up to 2 m long supported in a pale cream-coloured pumiceous lapilli tuff matrix. Field mapping indicates that the deposit is an ignimbrite and, together with petrological considerations, it is suggested that mixing of dense spatter and pumiceous lapilli tuff in the investigated deposit occurred during emplacement, not necessarily in the same vent, with the mixed fragmental material emplaced as a pyroclastic density current. Liquid water was not initially present but a steam phase was probably generated during transport and may represent water ingested during passage of the current as it passed over either wet ground, stream, shallow lake or (possibly) snow. Well-exposed caldera interiors are uncommon and that at Mason Spur is helping understand eruption dynamics associated with a complex large island volcano. The results of our study should help to elucidate interpretations of other, less well exposed, pyroclastic density current deposits elsewhere in Antarctica and globally. [ABSTRACT FROM AUTHOR]

Lacustrine sequences in active volcanic settings preserve the record of fall-out products (tephras) from explosive volcanic activity from both proximal and distal sources. Sediments of Lake Chalco, located in the western part of the Trans Mexican Volcanic Belt, offer the opportunity to develop a detailed tephrostratigraphy of proximal and distal sources, and to provide stratigraphic marker horizons for the correlation of paleoclimate records. Here, we present major oxide glass and pumice data from 18 tephra layers interbedded in the lacustrine sediments of Chalco, from 11.5 to 31.3 cal ka BP. Tephra glass compositions range from basaltic trachyandesitic to rhyolitic. Two tephras were successfully correlated with the Tutti Frutti Plinian Eruption of Popocatépetl volcano; and two tephra layers from the Nevado de Toluca Plinian activity: the Upper Toluca Pumice and the Lower Toluca Pumice. Although the source of most of the tephras analyzed is unknown, their geochemical characterization, coupled with a robust chronology, contributes to establish a detailed tephrostratigraphy for the region. This tephra record also contributes to improving the estimated frequency of explosive volcanic activity for future hazards in the Basin of México and surrounding areas, where more than 29 million people live. Our findings estimate a recurrence interval of volcanic activity of ca. 1100 years in the interval between ca. 32 and 11.5 cal ka BP, shorter than previously estimated. [ABSTRACT FROM AUTHOR]

Abstract: Provenance studies of widely distributed tephras, integrated within a well‐defined temporal framework, are important to deduce systematic changes in the source, scale, distribution, and changes in regional explosive volcanism. Here, we establish a robust tephrochronostratigraphy for a total of 157 marine tephra layers collected during IODP Expedition 352. We infer at least three major phases of highly explosive volcanism during Oligocene to Pleistocene time. Provenance analysis based on glass composition assigns 56 of the tephras to a Japan source, including correlations with 12 major and widespread tephra layers resulting from individual eruptions in Kyushu, Central Japan, and North Japan between 115 ka and 3.5 Ma. The remaining 101 tephras are assigned to four source regions along the Izu‐Bonin arc. One, exclusively assigned to the Oligocene age, is proximal to the Bonin Ridge islands; two reflect eruptions within the volcanic front and back‐arc of the central Izu‐Bonin arc, and a fourth region corresponds to the Northern Izu‐Bonin arc source. First‐order volume estimates imply eruptive magnitudes ranging from 6.3 to 7.6 for Japan‐related eruptions and between 5.5 and 6.5 for IBM eruptions. Our results suggest tephras between 30 and 22 Ma reflect a subtly different Izu‐Bonin chemical signature compared to the recent arc. After a ∼9 Ma gap in eruption, tephra supply from the Izu‐Bonin arc predominated from 15 to 5 Ma, and finally a subequal mixture of tephra sources from the (palaeo)Honshu and Izu‐Bonin arcs occured within the last ∼5 Ma. [ABSTRACT FROM AUTHOR]

Explosive eruptions of trachytic to trachyandesitic composition produced thick and widespread pyroclastic deposits during the recent volcanic activity of Piton des Neiges (PdN), the largest volcano of La Réunion Island (France). However, without precise geochronology for these eruptions, the future behavior of the dormant volcano is difficult to assess. New high-resolution single crystal 40Ar/39Ar dates on alkali feldspar, together with petrochemical data on juvenile clasts, indicate that instead of multiple and small eruptions, PdN produced a single large-volume event ("Dalle Soudée" eruption) at 198.8 ± 2.5 kyrs in which its entire crustal plumbing system was reactivated by mafic injections in the silicic reservoir. After 60 kyrs of quiescence, PdN activity resumed with several small silicic eruptions. From a dome-forming trachytic event (Belouve eruption) represented by some of the most recent block-and-ash flow deposits near the present-day summit we report in-situ 238U/230Th disequilibrium dates on pristine zircons. Sanidine and zircon crystalized at ca. 875 °C in the trachytic magma. For this event, the corresponding 238U/230Th isochron age of the zircons overlaps with the youngest 40Ar/39Ar dates on single sanidine crystals and yields an age of 42.7 ± 3.8 kyr. The variability in trace elements of zircon argues for a prolonged crystallization during this phase prior to eruption. Similarly, the 40Ar/39Ar sanidine dates show a significant spread in both eruptions, covering ca. 80 kyrs, uncommon for a method with a predicted low closure temperature. Such a pattern of age distribution argues for the presence of inherited argon in sanidines and for long residence times in the magma reservoir. The recent eruptive history of Piton des Neiges, involving evolved magma compositions, long-living silicic reservoirs and millennia-long quiescence in volcanic activity, suggests that future potentially hazardous eruptions from this volcano cannot be excluded. [ABSTRACT FROM AUTHOR]

Data from the Gamma-Ray Spectrometer (GRS) that flew on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12-20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4-0.9 wt % CO from the reduction of FeO to Fe0-CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5-4.1 Ga timespan over which the planet was resurfaced through volcanic processes. [ABSTRACT FROM AUTHOR]

Geological evidences of volcanic conduit widening are common in most pyroclastic deposits (e.g. presence of lithic fragments from different depths), suggesting a continuous modification of the conduit geometry during volcanic eruptions. However, the controlling factors of the mechanisms driving conduit enlargement (e.g. erosion, local collapse) are still partially unclear, as well as the influence of conduit geometry on the eruptive dynamics. Although numerical models have been systematically employed to study volcanic conduits, their mechanical stability and the eruptive dynamics related to non-cylindrical conduits have been poorly addressed. We present here a 1D steady-state model which includes the main processes experimented by ascending magmas (i.e. crystallization, rheological changes, fragmentation, drag forces, outgassing and degassing), and the application of two mechanical stability criteria (Mohr–Coulomb and Mogi–Coulomb), in order to study the collapse conditions of volcanic conduits during a representative explosive rhyolitic eruption. It emerges that mechanical stability of volcanic conduits is mainly controlled by its radial dimension, and a minimum radius for reaching stable conditions can be computed, as a function of water content and inlet overpressure. Additionally, for a set of input parameters thought typical of explosive rhyolitic volcanism, we estimated a minimum magma flux for developing a mechanically stable conduit (~ 7 ∙ 10 7 − 3 ∙ 10 8 kg/s). Results are consistent with the unsteady character usually observed in sub-Plinian eruptions, opposite to mainly stationary Plinian eruptions, commonly characterized by higher magma discharge rates. We suggest that cylindrical conduits represent a mechanically stable configuration only for large radii. Because the instability conditions are not uniform along the conduit, the widening processes probably lead to conduit geometries with depth-varying width. Consequently, as our model is able to consider volcanic conduits with depth-dependent radius, two plausible and previously untested geometries have been studied, evidencing major and complex modifications in some eruptive parameters (particularly, exit pressure and mass discharge rate), and suggesting that the geometry acquired by the conduit as it is widened influences the eruptive dynamics. [ABSTRACT FROM AUTHOR]

We investigate whether the disproportionate contribution of individual volcanoes in the Large Magnitude Explosive Volcanic Eruption database (LaMEVE) potentially compromises the treatment of LaMEVE as a globally representative database of volcanic activity. We find that 41% of volcanoes which contribute at least one eruption to LaMEVE only contribute one eruption (10% of all eruptions), and the six most prolific volcanoes contribute 11% of eruptions. However, there is no systematic bias with respect to the eruption magnitude or date for volcanoes contributing one eruption. Also, no bias can be discerned for when the smallest or largest eruption at a volcano occurs in its eruptive record. Half of the volcanoes contributing one or more eruptions to the LaMEVE database had their first eruption prior to 36.4 ka. We find LaMEVE is representative - while there are well-known issues of eruption under-reporting, LaMEVE is not overly biased by the activity of a few volcanoes. [ABSTRACT FROM AUTHOR]

The volatile content in magmas is fundamental for the triggering and style of volcanic eruptions. Carbon dioxide, the second most abundant volatile component in magmas after H2O, is the first to reach saturation upon ascent and depressurization. We investigate experimentally CO2-bubble nucleation in trachybasalt and trachyte melts at high temperature and high pressure (HT and HP) through wetting-angle measurements on different (sialic, mafic or oxide) phenocryst phases. The presence of crystals lowers the supersaturation required for CO2-bubble nucleation up to 37 per cent (heterogeneous nucleation, HeN), with a minor role of mineral chemistry. Different from H2O-rich systems, feldspar crystals are effective in reducing required supersaturation for bubble nucleation. Our data suggest that leucite, the dominant liquidus phase in ultrapotassic systems at shallow depth (i.e. <100 MPa), facilitates late-stage, extensive magma vesiculation through CO2HeN, which may explain the shifting of CO2-rich eruptive systems towards an apparently anomalous explosive behaviour. [ABSTRACT FROM AUTHOR]

Volcanism on Montserrat (Lesser Antilles arc) has migrated southwards since the formation of the Silver Hills ~ 2.5 Ma, and has formed three successively active volcanic centres. The Centre Hills volcano was the focus of volcanism from ~ 1–0.4 Ma, before activity commenced at the currently active Soufrière Hills volcano. The history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes. Here, we describe the pyroclastic stratigraphy of subaerial exposures around central Montserrat; identifying 11 thick (> 1 m) pumiceous units derived from sustained explosive eruptions of Centre Hills from ~ 0.8–0.4 Ma. Over 10 other, less well- exposed pumiceous units have also been identified. The pumice-rich units are interbedded with andesite lava breccias derived from effusive, dome-forming eruptions of Centre Hills. The stratigraphy indicates that large (up to magnitude 5) explosive eruptions occurred throughout the history of Centre Hills, alongside effusive activity. This behaviour at Centre Hills contrasts with Soufrière Hills, where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano, from ~ 175–130 ka. Subsequent eruptions at Soufriere Hills have been dominated by andesitic effusive eruptions. The bulk composition, petrography and mineral chemistry of volcanic rocks from Centre Hills and Soufrière Hills are similar throughout the history of both volcanoes, except for occasional, transient departures to different magma compositions, which mark shifts in vent location or dominant eruption style. For example, the final recorded eruption of Centre Hills, before the initiation of activity at Soufrière Hills, was more silicic than any other identified eruption on Montserrat; and the basaltic South Soufrière Hills episode marked the transition to the current stage of predominantly effusive Soufrière Hills activity. The compositional stability observed throughout the history of Centre Hills and Soufrière Hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma, but may reflect local changes in long-term magma storage conditions that characterise individual episodes (on 10 5 year timescales) of volcanism on Montserrat. [ABSTRACT FROM AUTHOR]