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The French Massif Central (central-southern France) and the Eifel region (central-western Germany) are both young volcanic systems and considered dormant. They are part of the European Cenozoic Rift System (ECRIS) and show similar surficial manifestations of ongoing hydrothermal activity. For example, both areas exhibit numerous low flow rate CO2-rich springs, mainly occurring in concomitance of faults and fractures inherited from the Variscan orogeny.Here, the chemical and isotopic characterization of different fresh water bodies (springs, wells, rivers and volcanic lakes) has been provided. The composition of dissolved gases and the isotopic signatures of dissolved carbon indicate that meteoric water infiltrated and then interacted with a CO2-rich, mantle-related, component. The majority of studied water samples exhibit pCO2 between 0.3 and 1 bar and the total dissolved inorganic carbon (TDIC) is of the order of 0.01 mol/kg. At surface, most spring water samples are oversaturated with calcite, dolomite, chalcedony and quartz and are in equilibrium with amorphous silica. The correlation between the TDIC and its isotopic composition (δ13CTDIC) suggests that part of the analysed water samples experienced a degassing process prior to or immediately after emergence. The computed CO2 flux transported by groundwaters is of the same order of magnitude of the global baseline theorized for geothermal areas. This indicates that passive rifts systems contribute to the atmospheric CO2 content and highlights the importance of taking into account each carbon source in the study of the global carbon cycle.

Explosive volcanic eruptions eject a gas-particle mixture into the atmosphere. The characteristics of this mixture in the near-vent region are a direct consequence of the underlying initial conditions at fragmentation and the geometry of the shallow plumbing system. Yet, it is not possible to observe directly the sub-surface parameters that drive such eruptions. Here, we use scaled shock-tube experiments mimicking volcanic explosions in order to elucidate the effects of a number of initial conditions. As volcanic vents can be expected to possess an irregular geometry, we utilise three vent designs, two “complex” vents and a vent with a “real” volcanic geometry. The defining geometry elements of the “complex” vents are a bilateral symmetry with a slanted top plane. The “real” geometry is based on a photogrammetric 3D model of an active volcanic vent with a steep and a diverging vent side. Particle size and density as well as experimental pressure are varied. Our results reveal a strong influence of the vent geometry, on both the direction and the magnitude of particle spreading and the velocity of particles. The overpressure at the vent herby controls the direction of the asymmetry of the gas-particle jet. These findings have implications for the distribution of volcanic ejecta and resulting areas at risk.

Explosive volcanic eruptions can produce vast amounts of volcanic ash made up mainly of fragments of magmatic glass, country rock and minerals

Explosive volcanic eruptions are associated with a plethora of geophysical signals. Among them, acoustic signals provide ample information about eruptive dynamics and are widely used for monitoring purposes. However, a mechanistic correlation of monitoring signals, underlying source processes and reasons for short-term variations is incomplete. Scaled laboratory experiments can mimic a wide range of explosive volcanic eruption conditions. Here, starting (non-steady) compressible gas jets are created using a shock tube in an anechoic chamber and their acoustic signature is recorded with a microphone array. Noise sources are mapped in time and frequency using wavelet analysis and their dependence from pressure ratio, non-dimensional mass supply and exit-to-throat area ratio is deciphered. We observed that the pressure ratio controls the establishment of supersonic conditions and their duration, and influences the interaction between shock, shear layer, and vortex ring. The non-dimensional mass supply affects the duration of the discharge, the maximum velocity of the flow, and the existence of a trailing jet. Lower values of exit-to-throat area ratio induce a faster decay of the acoustic fingerprint of the jet flow. The simplistic experiments presented here, and their acoustic analysis will serve as an essential starting point to infer source conditions prior to and during impulsive volcanic eruptions.

After 50 years of volcanic quiescence, on 19 September 2021, an eruption started on the western flank of the Cumbre Vieja ridge of La Palma, Canary Islands, Spain. The eruption was characterised by simultaneous effusive and explosive activity from a several hundred-meter-long fissure, which later built up a cone and showed variable eruptive behaviour at different vents, suggesting a spatially complex plumbing system. Explosive eruptive activity ranged from mild ash emissions, Strombolian explosions to fire fountaining episodes.We carried out field measurements to study the variable explosive eruptive activity and associated acoustic signals. A single microphone initially deployed at about 2 km SW of the vents from 6 to 11 October was later replaced by an array of 3 microphones from 6 November to 13 December at about 300 m W of the microphone location in October. The microphones (PCB ½” free field acoustic sensors, 3.15-20 kHz frequency range) were each connected to an OptiMeas SmartPro digitiser continuously sampling at 5000 Hz. The digitisers are GPS synchronised for accurate acoustic array processing. The acoustic array was complemented by a thunderstorm detector continuously recording (since 11 October) lightning and electrical activity generated by the volcanic explosions (Vossen et al., 2022). Additionally, at the beginning of November 2021, thermal videos of the eruptive activity were acquired.Preliminary analysis of the large and unique acoustic dataset shows varying waveforms indicating evolving source conditions: eruption intensity, source mechanism, vent geometry, fragmentation depth and amount of ash ejected. Moreover, we observe a variability of frequency (peak and mean) and amplitude with time. Further analysis includes the characterisation of the acoustic source location within the growing volcanic edifice and the comparison and correlation with lightning and thermal infrared data to detail changes in explosive activity related to the evolving eruption sources. Caron E.J. Vossen et al. (2022), Electrical activity of the 2021 Cumbre Vieja eruption, EGU22-8052.

Between 19 September and 13 December 2021 (85 days), a flank eruption took place along the Cumbre Vieja ridge (La Palma Island), one of the most active volcanic centers of Canary Islands. The last 7000 years of Cumbre Vieja activity has been characterized by a combination of effusive and explosive eruptions. These generated both cinder cones and lava flows. The previous event occurred between October 26 and November 28, 1971 (eruption of Teneguía). The 2021 eruption was characterized by the alternate (and often simultaneous) emission of lava flows, lava fountains and tephra plumes along a ~1 km-length fissure consisting of about ten vents that built a cinder cone complex. The southern vents were mostly associated with the generation of tephra plumes and lava fountains, while the northern vents were mostly associated with the generation of lava flows. The ~12 km2 lava flow field on the west side of the island reached the sea on September 28, forming a new lava delta and interacting with sea water producing lava haze (i.e. laze). Even though tephra was sedimented all over La Palma and sometimes reached the neighboring islands, the cumulative tephra deposit is mostly elongated towards the southwest and the northeast due to the prevailing wind direction, reaching a maximum thickness southwest of the fissure. Both lava fountains and more explosive tephra plumes contributed to the formation of the tephra deposit. Tephra plumes were associated with variable intensity reaching a few kilometers of altitude (

Volcanic lightning is a common phenomenon observed during explosive eruptions of high magnitude and intensity. Lightning observations in milder explosive eruptions, generally of basaltic composition, are less frequent, arising the question of whether electrification may be a common feature over the whole spectrum of explosive styles and magma compositions.The 2021 eruption of Cumbre Vieja on the island of La Palma (Canary Islands, Spain) started on 19 September 2021, continuously producing lava flows and tephra of average basanite to tephrite composition during 85 consecutive days, eventually generating a >200 m tall scoria cone (about 1220 m a.s.l.) and a vast compound lava flow field. Lightning was frequently observed in the plume during different phases of the explosive activity. This eruption provided the rare opportunity to monitor variations in the electrical activity on various time scales continuously over several weeks. We measured such electrical activity using a lightning detector operating in the extremely low frequency range with a sample rate of 100 Hz (Vossen et al., 2021), installed about 2 km away from the active vents. The detector was deployed on 11 October 2021 and recorded continuously until the end of the eruption on 13 December 2021, thus providing a unique dataset of its kind.Lightning activity varied during the eruption with alternating hours-long periods of high intensity continuous lightning production as well as minutes-long isolated episodes with interposed periods of quiet. Stable fair-weather conditions over La Palma recorded by meteorological stations during the whole eruption (exception made for a thunderstorm episode on 26 November 2021), allow a confident attribution of the changeable lightning activity to the explosive activity of the scoria cone. Here, we present volcanic lightning and electrification timeseries as a function of the varying explosive activity as observed through thermal videography and acoustic recordings (Cigala et al., 2022). Vossen, C. E. J., Cimarelli, C., Bennett, A. J., Geisler, A., Gaudin, D., Miki, D., Iguchi, M., and Dingwell, D. B. (2021). Long-term observation of electrical discharges during persistent Vulcanian activity. Earth Planet. Sci. Lett., 570, 117084. https://doi.org/10.1016/j.epsl.2021.117084.

Formation of surficial sulfate– and halide–bearing salts by syn–eruptive ash–gas interactions is known to occur during volcanic eruptions. For reactions between aluminosilicates and the gas SO2, at...

In July and August 2019, two paroxysmal eruptions dramatically changed the morphology of the crater terrace that hosts the active vents of Stromboli volcano (Italy). Here, we document these morphological changes, by using 2259 UAS-derived photographs from eight surveys and Structure-from-Motion (SfM) photogrammetric techniques, resulting in 3D point clouds, orthomosaics, and digital surface models (DSMs) with resolution ranging from 8.1 to 12.4 cm/pixel. We focus on the morphological evolution of volcanic features and volume changes in the crater terrace and the upper part of the underlying slope (Sciara del Fuoco). We identify both crater terrace and lava field variations, with vents shifting up to 47 m and the accumulation of tephra deposits. The maximum elevation changes related to the two paroxysmal eruptions (in between May and September 2019) range from +41.4 to −26.4 m at the lava field and N crater area, respectively. Throughout September 2018–June 2020, the total volume change in the surveyed area was +447,335 m3. Despite Stromboli being one of the best-studied volcanoes worldwide, the UAS-based photogrammetry products of this study provide unprecedented high spatiotemporal resolution observations of its entire summit area, in a period when volcanic activity made the classic field inspections and helicopter overflights too risky. Routinely applied UAS operations represent an effective and evolving tool for volcanic hazard assessment and to support decision-makers involved in volcanic surveillance and civil protection operations.

By injecting a mixture of gas and pyroclasts into the atmosphere, explosive volcanic eruptions frequently generate vortex rings, which are toroidal vortices formed by the jet's initial momentum. Here, we report high‐speed imaging and acoustic measurements of vortex rings sourcing from gas‐rich eruptive jets at Stromboli volcano (Italy). Volcanic vortex rings (VVRs) form at the vent together with an initial compression acoustic wave, VVRs maximum rise velocity being directly proportional to the amplitude and inversely proportional to the duration of the compression wave. The axial rise and acoustic signature of VVRs match well those predicted by recent fluid‐dynamic experiments. This good match allows using the high‐frequency (80–1,000 Hz) component of the jet sound and the time‐dependent rise of VVRs to retrieve two key eruption parameters: the Mach number of the eruptive jets (, Key Points Volcanic vortex rings are formed by gas‐rich, jet‐forming Strombolian‐style explosionsThe explosion acoustic signals bear the signature of the vortex rings and reveal supersonic eruptive jets with Mach number up to 1.5Vent diameter can be estimated from the time‐dependent rise of vortex rings at 0.7 m, in agreement with direct observation from Uncrewed Aerial Vehicle

Volcanic ash (grain size The physico-chemical and phase state of ash influences substantially the thermal behavior of volcanic ash and our ability to predict that behavior and therefore depends on reliable characterization of this state. Here, we have investigated the chemical composition and mineralogy (phase state) of fresh volcanic ash from Tungurahua volcano, Ecuador, as a function of grain size (below 180 μm) in order to evaluate differences between ground-sampled ash and airborne ash, interacting with jet engines. For the sieved grain size fractions (down to Using an optical dilatometer image analysis, we have characterized the thermal behavior of the volcanic ash upon heating, which we parameterize in terms of four characteristic temperatures. Using these data the determination of the flow temperature is refined to improve measurement accuracy for fine-grained (

Some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water. In shallow water, “Surtseyan” eruptions are spectacular, and they efficiently fragment magma into fine ash particles. The aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions. To investigate both mechanisms, we analyzed ash-encased lapilli from the Surtseyan eruptions of Capelinhos (Azores, 1957–1958) and Hunga Tonga–Hunga Ha’apai (Tonga, 2014–2015) using X-ray computed microtomography and electron microscopy. We discovered pyroclasts that were not coated, sensu stricto, but had enveloping ash produced by in situ granulation of the particle surface. This was caused by thermal stress as pyroclasts briefly traveled through water and were quenched during eruption. In situ granulation is thus an important secondary disruption process in shallow subaqueous eruptions. Our results imply that ash encasement is not always evidence of particle aggregation and accretion, but it may also result from new ash formation. Shallow-water conditions produce the most efficient ash-generation conditions, leading to the greatest hazard to downwind populations and air traffic.

Funder: VUB Strategic Research Program, Exposure to volcanic ash is a long-standing health concern for people living near active volcanoes and in distal urban areas. During transport and deposition, ash is subjected to various physicochemical processes that may change its surface composition and, consequently, bioreactivity. One such process is the interaction with anthropogenic pollutants; however, the potential for adsorbed, deleterious organic compounds to directly impact human health is unknown. We use an in vitro bioanalytical approach to screen for the presence of organic compounds of toxicological concern on ash surfaces and assess their biological potency. These compounds include polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (dlPCBs). Analysis of ash collected in or near urbanised areas at five active volcanoes across the world (Etna, Italy; Fuego, Guatemala; Kelud, Indonesia; Sakurajima, Japan; Tungurahua, Ecuador) using the bioassay inferred the presence of such compounds on all samples. A relatively low response to PCDD/Fs and the absence of a dlPCBs response in the bioassay suggest that the measured activity is dominated by PAHs and PAH-like compounds. This study is the first to demonstrate a biological potency of organic pollutants associated with volcanic ash particles. According to our estimations, they are present in quantities below recommended exposure limits and likely pose a low direct concern for human health.

Mt. Pelée is a historically active volcano, situated on the island of Martinique (Lesser Antilles), that has shown a variety of explosive styles in the recent past, ranging from dome-forming (Pelean) to open-vent (Plinian) eruptions. The 1902-1905 eruption is infamous for the pyroclastic density currents (PDCs) that destroyed the towns of St. Pierre and Morne Rouge, killing 30 000 residents. Since the last eruption (dome-forming) in 1929-1932, Mt. Pelée was quiet and considered dormant until recently. In late 2020, the local Volcanological Observatory (OVSM) raised the alert level following a noticeable increase in seismicity, bringing into effect a reinforcement of monitoring resources. As St. Pierre is long since re-established, along with several other towns along the volcano’s flanks, it is of utmost importance to understand the possible range of eruptive activity to improve the preparedness strategies of local communities.The precise controls on eruption dynamics vary across volcanic systems and cannot be constrained via direct observation. However, crucial inferences can be made based on petrophysical properties and mechanical behaviours of erupted materials. For this study, we collected samples from PDC deposits of Mt. Pelée, from the two historic Pelean (1902-1905, and 1929-1932) and three pre-Columbian Plinian eruptions (1300 CE P1, 280 CE P2, and 79 CE P3). We measured petrophysical properties (density, porosity, permeability) of cylindrical samples drilled from bomb-sized clasts and investigated their fragmentation behaviour via grain size and high-speed video analysis. These results are used in comparison with field data of grain-size distribution (GSD) of individual outcrops and calculated total GSD data. We investigated the effects of transport-related sorting or fining.The “Pelean” samples are found to be denser (32-47% open porosity) than the pumiceous “Plinian” samples (55-66% open porosity). Moreover, these two classes are distinctly different in their crystallinity as samples underwent different ascent conditions. In our experiments, distinct fragmentation behaviour and resulting GSDs are observed for samples from each eruption style, regardless of experimental pressure conditions (5-20 MPa). Our results show the paramount importance of open porosity on fragmentation efficiency in pumiceous samples, alongside a strong influence of crystallinity. The fractal dimension of fragmentation calculated from weight fractions, independent of grain shape, shows clear differences in fragmentation efficiency as a function of sample properties and experimental starting conditions.Our results suggest that (i) the variability in porosity and permeability is too low to cause the increased explosivity exhibited during the 1902 eruption compared to the 1929 event, (ii) open porosity has a major control on fragmentation efficiency in pumiceous samples, (iii) fragmentation efficiency can be effectively evaluated by calculating the fractal dimension of the cumulative weight fractions of experimental products.The influence of crystallinity and pore textures on fragmentation efficiency must be further investigated to aid hazard model development for future eruptions of Mt. Pelée. Future work will constrain these textural parameters of naturally and experimentally fragmented materials from Mt. Pelée, to further elucidate the controls on eruptive dynamics at this hazardous volcano.

Explosive volcanic eruptions eject a mixture of gas and pyroclasts into the atmosphere at a range of velocities. Directly above the vent, in the gas-thrust region, a supersonic jet may be generated that strongly controls the eruptive dynamics. To improve our quantitative understanding of volcanic supersonic jets, the effect on particles within them, and their acoustic emission, we have performed small-scale explosive eruptions in the laboratory using a shock-tube. The shock-tube is composed of 3 parts, a bottom (5.6 m long, elevated pressure) and a top (48mm long, ambient pressure) plexiglass cylinder (5 mm inner diameter), separated by an electrovalve.We have run experiments using ambient air as gas and sand, with diameter between 0.1 and 0.3 mm, as particles. The gas volume was fixed while the pressure ratio (the shock-tube reservoir to ambient pressure ratio) was varied from about 4 to 8.4 to obtain supersonic flows. During the experiments, the jet was recorded with a high-speed camera operating at 34660 fps, and the resulting noise acoustic emission with microphones (6 Hz-140 kHz; 1000 kfps) positioned at 90° from the jet axis.Among the acoustic signals produced by a supersonic jet (jet noise) we have particularly focussed on the broad-band shock noise (BBSN) that is emitted by the interaction between shock cells and the turbulence in the jet. We estimated the jet velocity using an acoustic model based on the identification of the peak frequency of the BBSN. We also identified the BBSN frequency and its variation over time by applying the complex Morlet wavelet transformation. As expected, the BBSN frequency is inversely proportional to the gas velocity. Concerning the video recording, we analysed the shock cells behaviour and their temporal oscillation due to the presence of particles. Finally, the particle ejection rate was calculated in every video frame.We found that the acoustic signal and shock cells are influenced by the presence of particles. In fact, fluctuations in particle concentration are well visible and decelerate the flowing gas. As a consequence, there is a temporary decrease of the stand-off-distance between the vent and the first shock-cell and concurrent rise of the BBSN frequency. We noticed, in some cases, that the shock-cells disappear during a short time interval. The BBSN frequency and the stand-off-distance behaviour over time follow the oscillation of the particle ejection rate confirming their sensitivity to particle load variation.The future prospectives of this embryonal study could lead to new instruments for determining either the amount of pyroclasts inside the volcanic jets and their exit velocity on the basis of the recorded acoustic signals.

Predicting the onset, style and duration of explosive volcanic eruptions remains a great challenge. While the fundamental underlying processes are thought to be known, a clear correlation between eruptive features observable above Earth’s surface and conditions and properties in the immediate subsurface is far from complete. Furthermore, the highly dynamic nature and inaccessibility of explosive events means that progress in the field investigation of such events remains slow. Scaled experimental investigations represent an opportunity to study individual volcanic processes separately and, despite their highly dynamic nature, to quantify them systematically. Here, impulsively generated vertical gas-particle jets were generated using rapid decompression shock-tube experiments. The angular deviation from the vertical, defined as the “spreading angle”, has been quantified for gas and particles on both sides of the jets at different time steps using high-speed video analysis. The experimental variables investigated are 1) vent geometry, 2) tube length, 3) particle load, 4) particle size, and 5) temperature. Immediately prior to the first above-vent observations, gas expansion accommodates the initial gas overpressure. All experimental jets inevitably start with a particle-free gas phase (gas-only), which is typically clearly visible due to expansion-induced cooling and condensation. We record that the gas spreading angle is directly influenced by 1) vent geometry and 2) the duration of the initial gas-only phase. After some delay, whose length depends on the experimental conditions, the jet incorporates particles becoming a gas-particle jet. Below we quantify how our experimental conditions affect the temporal evolution of these two phases (gas-only and gas-particle) of each jet. As expected, the gas spreading angle is always at least as large as the particle spreading angle. The latter is positively correlated with particle load and negatively correlated with particle size. Such empirical experimentally derived relationships between the observable features of the gas-particle jets and known initial conditions can serve as input for the parameterisation of equivalent observations at active volcanoes, alleviating the circumstances where an a priori knowledge of magma textures and ascent rate, temperature and gas overpressure and/or the geometry of the shallow plumbing system is typically chronically lacking. The generation of experimental parameterisations raises the possibility that detailed field investigations on gas-particle jets at frequently erupting volcanoes might be used for elucidating subsurface parameters and their temporal variability, with all the implications that may have for better defining hazard assessment. Supplementary Information The online version contains supplementary material available at 10.1007/s00445-021-01473-0.

International audience; Due to their generally low eruptive melt viscosities and concomitant high diffusivities of volatiles, basaltic magmas degas relatively efficiently. This relative efficiency, combined with variations in style, extent, timing and length scales of degassing govern the range of eruptive styles observed at basaltic volcanoes. The result is a surprising complexity of degassing regimes and products in basaltic volcanism. In particular, the transition between closed- and open-system degassing at low pressure at the percolation threshold may strongly affect the type of eruption. Here we aim to better understand degassing and gas percolation processes in basaltic magmas and their implications for eruptive style. Combining new and literature data, we present a database of vesicle metrics in basaltic rocks including vesicularity, vesicle number density, vesicle size distribution (and its polydispersivity), vesicle connectivity and permeability. We combine these textural and petrophysical data with a numerical model of percolation for systems having polydisperse vesicle size distributions. Using this model, we also evaluate different definitions of vesicle connectivity inherent to different measurement techniques. Our results show that polydispersivity exerts a strong control on the percolation threshold of basaltic magmas and consequently on eruptive style. Intermediate to highly polydisperse bubble networks are more typical of Hawaiian activity and are characterized by higher values of percolation threshold. This results in delayed coalescence and an increase in magma vesicularity hindering the formation of large decoupled and buoyant bubbles, which in turn can promote magma acceleration, fragmentation by inertia below the percolation threshold and sustained fountaining activity. Bubble populations with lower polydispersivity, typical of Strombolian eruptions, promote early coalescence prior to fragmentation, which in turn may lead to the formation of large decoupled slugs or gas pockets and/or plugs at the surface via outgassing. Further, we discuss the implications of our findings for Plinian, violent Strombolian, Surtseyan, deep submarine and effusive basaltic eruptions.

Combustion temperatures (>1200 °C) in gas turbines cause softening and eventually melting of ingested volcanic ash particles. In the downstream section, lower temperatures lead to droplet deposition onto the thermal barrier coatings (TBCs) of the turbine blades. The intensity of interaction (wetting and chemical interaction) depends on the ash and TBC chemistry as well as the TBC structure. We have determined the spreading behavior of five different volcanic ash melts on four types of TBCs. The TBCs differed in their composition - yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) – and in their fabrication - air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). The spreading properties have been parameterized on the basis of four parameters, 1) the CaO-SiO2-ratio, 2) optical basicity, 3) the Rb/a ratio and 4) the viscosity of the volcanic ash melts. Infiltration efficiency of the melts into the TBCs and corrosion characteristics have been determined via electron microprobe analysis of cross sections. In this study, the highest damage potential was found following interaction with basaltic melt, as its low viscosity is favorable to extensive spreading and high chemical reactivity.

Volcanic ash (VA) from explosive eruptions contributes to aerosol loadings in the atmosphere. Aside from the negative impact of VA on air quality and aviation, these particles can alter the optical and microphysical properties of clouds by triggering ice formation, thereby influencing precipitation and climate. Depending on the volcano and eruption style, VA displays a wide range of different physical, chemical, and mineralogical properties. Here, we present a unique data set on the ice nucleation activity of 15 VA samples obtained from different volcanoes worldwide. The ice nucleation activities of these samples were studied in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud simulation chamber as well as with the Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology (INSEKT). All VA particles nucleated ice in the immersion freezing mode from 263 to 238K with ice nucleation active site (INAS) densities ranging from ∼105 to 1011 m−2, respectively. The variabilities observed among the VA samples, at any given temperature, range over 3.5 orders of magnitude. The ice‐nucleating abilities of VA samples correlate to varying degrees with their bulk pyroxene and plagioclase contents as a function of temperature. We combined our new data set with existing literature data to develop an improved ice nucleation parameterization for natural VA in the immersion freezing mode. This should be useful for modeling the impact of VA on clouds., Plain Language Summary: Volcanic ash particles, which are generated during volcanic eruptions, can initiate ice formation in clouds. The clouds formed by these volcanic ash particles can influence precipitation, and in turn, weather and climate. In our study, we investigated the ability with which volcanic ash particles form ice in clouds. We performed our study in a state‐of‐the‐art aerosol and cloud simulation chamber and on a cold‐stage instrument. The findings show that volcanic ash particles can form ice as effectively as mineral dust particles or their components. These results will help scientists to have a better understanding of the impact of volcanic ash particles on clouds., Key Points: The ice‐nucleating ability of natural volcanic ash particles in the immersion freezing mode can vary by 3.5 orders of magnitude. Ice‐nucleating properties of volcanic ash particles correlate to varying degrees with their pyroxene and plagioclase contents. The temperature‐dependent immersion freezing ability of volcanic ash is approximated with an exponential fit line., Alexander von Humboldt‐Stiftung (Humboldt‐Stiftung) http://dx.doi.org/10.13039/100005156, Marie Skłodowska‐Curie Actions, ERC 2018 ADG, Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, Helmholtz Association of German Research Centres, EUROCHAMP 2020 Infrastructure Activity

Mt. Pelée is a historically active stratovolcano, situated on the island of Martinique in the French Caribbean. It exhibits a variety of eruptive styles, from dome formation to highly violent explosivity. In 1902, a Pelean event destroyed the town of St. Pierre, killing more than 28,000 residents (Lacroix, 1904). As this town is now re-established, along with several others along the volcano’s flanks, it is of utmost importance to understand the range of eruptive activity possible such that preparedness of the local authorities and population can be improved.There remains a gap in quantitative understanding of the energy required to fragment material to produce explosive eruptions, as this process is not directly observable. Further, eruption records are incomplete (as at most volcanic islands) due to product loss to the ocean and intense tropical erosion. Here, we constrain the energies of past eruptions by performing rapid decompression experiments and comparing the resulting grain-size distributions with primary deposits and dispersal in the field.During a field campaign in March 2019, we collected ash and pumice blocks from five recent magmatic eruptions. Two of these eruptions are historic (the Pelean episodes of 1902-1905, and 1929-1932), and three are prehistoric (the Plinian eruptions of 1300 CE P1, 280 CE P2, and 79 CE P3)(Carazzo et al. 2012). We characterized ash (morphology), and constrained petrophysical (porosity, density, and permeability) and thermal properties of cylindrical samples. These cores (58-70% porosity) were subjected to rapid decompression in shock tube experiments to mimic explosive eruptions. Fragmentation efficiency results from a combination of material properties and experimental conditions (temperature and overpressure). The particulate products were evaluated for their grain-size distribution in order to calculate the fractal dimension Df and constrain eruptive conditions.Our results provide new insights into the energy required for magma fragmentation at Mt. Pelée and similar volcanoes. We hope to elucidate whether the 1902 eruption was catastrophic due to significant and measurable differences in eruption dynamics, or due to the flank topography and direction of the initial blast. References:Carazzo, G., Tait, S., Kaminski, E., Gardner, E., (2012), The recent Plinian explosive activity of Mt. Pelée volcano (Lesser Antilles): The P1 AD 1300 eruption, Bull. Volc., 74, 2187-2203, doi: 10.1007/s00445-012-0655-4Lacroix, A. (1904) La Montagne Pelée et ses éruptions. Masson, Paris

The European Astronaut Centre (EAC) is currently constructing the European Lunar Exploration Laboratory (LUNA), a large training and operations facility to be located adjacent to EAC at the DLR (German Aerospace Centre) campus in Cologne, Germany. With an estimated representative lunar testbed area of approximately 660 m2, a large volume of lunar regolith simulant material is needed for this purpose. In this study, a basanitic sandy silt from a quarry located in the Siebengebirge Volcanic Field is evaluated as a large-volume source of material. The focus of this project has been to conduct a physical and chemical characterisation of the fine-grained material to be used in LUNA; the European Astronaut Centre lunar regolith simulant 1 (EAC-1 A). The physical characterisation tests undertaken include sphericity, density measurements, cohesion and static angle of repose, with mineralogical investigations via petrographical analysis with optical microscope and SEM, XRF, XRD and DSC measurements. The results of the EAC-1A tests are compared to published data on existing widely used lunar regolith simulants, namely JSC-1A, JSC-2A, NU-LHT-3M, DNA and FJS-1.

Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.

Volcanic eruptions are associated with a wide range of electrostatic effects. Increasing evidence suggests that high-altitude discharges (lightning) in maturing plumes are driven by electrification processes that require the formation of ice (analogous to processes underpinning meteorological thunderstorms). However, electrical discharges are also common at or near the volcanic vent. A number of “ice-free” electrification mechanisms have been proposed to account for this activity: fractocharging, triboelectric charging, radioactive charging, and charging through induction. Yet, the degree to which each mechanism contributes to a jet's total electrification and how electrification in the gas-thrust region influences electrostatic processes aloft remains poorly constrained. Here, we use a shock-tube to simulate overpressured volcanic jets capable of producing spark discharges in the absence of ice. These discharges may be representative of the continual radio frequency (CRF) emissions observed at a number of eruptions. Using a suite of electrostatic sensors, we demonstrate the presence of size-dependent bipolar charging (SDBC) in a discharge-bearing flow for the first time. SDBC has been readily associated with triboelectric charging in other contexts and provides direct evidence that contact and frictional electrification play significant roles in electrostatic processes in the vent and near-vent regions of an eruption. Additionally, we find that particles leaving the region where discharges occur remain moderately electrified. This degree of electrification may be sufficient to drive near-vent lightning higher in the column. Thus, near-vent discharges may be underpinned by the same electrification mechanisms driving CRF, albeit involving greater degrees of charge separation.

Pyroclast ejection during explosive volcanic eruptions occurs under highly dynamic conditions involving great variations in flux, particle sizes, and velocities. This variability must be a direct consequence of complex interactions between physical and chemical parameters inside the volcanic plumbing system. The boundary conditions of such phenomena cannot be fully characterized via field observation and indirect measurements alone. In order to understand better eruptive processes, we conducted scaled and controlled laboratory experiments. By performing shock-tube experiments at known conditions, we defined the influence of physical boundary conditions on the dynamics of pyroclast ejection. If applied to nature, we are focusing in the near-vent processes where, independently of fragmentation mechanism, impulsively released gas-pyroclast mixtures can be observed. These conditions can be met during, e.g., Strombolian or Vulcanian eruptions, parts of Plinian eruptions, or phreatomagmatic explosions. The following parameters were varied: (1) tube length, (2) vent geometry, (3) particle load, (4) temperature, and (5) particle size distribution. Gas and particles in the experiments are not coupled (St>>1). The initial overpressure, with respect to atmosphere, was always at 15MPa. We found a positive correlation of pyroclast ejection velocity with (1) particle load, (2) diverging vent walls, and (3) temperature as well as a negative correlation with (1) tube length and (2) particle size. Additionally, we found that particle load strongly affects the temporal evolution of particle ejection velocity. These findings stress the importance of scaled and repeatable laboratory experiments for a better understanding of volcanic phenomena and therefore volcanic hazard assessment.

Shallow‐water coral reef ecosystems, particularly those already impaired by anthropogenic pressures, may be highly sensitive to disturbances from natural catastrophic events, such as volcanic eruptions. Explosive volcanic eruptions expel large quantities of silicate ash particles into the atmosphere, which can disperse across millions of square kilometres and deposit into coral reef ecosystems. Following heavy ash deposition, mass mortality of reef biota is expected, but little is known about the recovery of post‐burial reef ecosystems. Reef regeneration depends partly upon the capacity of the ash deposit to be colonised by waterborne bacterial communities and may be influenced to an unknown extent by the physiochemical properties of the ash substrate itself. To determine the potential for volcanic ash to support pioneer bacterial colonisation, we exposed five well‐characterised volcanic and coral reef substrates to a marine aquarium under low light conditions for 3 months: volcanic ash, synthetic volcanic glass, carbonate reef sand, calcite sand and quartz sand. Multivariate statistical analysis of Automated Ribosomal Intergenic Spacer Analysis (ARISA) fingerprinting data demonstrates clear segregation of volcanic substrates from the quartz and coral reef substrates over 3 months of bacterial colonisation. Overall bacterial diversity showed shared and substrate‐specific bacterial communities; however, the volcanic ash substrate supported the most diverse bacterial community. These data suggest a significant influence of substrate properties (composition, granulometry and colour) on bacterial settlement. Our findings provide first insights into physicochemical controls on pioneer bacterial colonisation of volcanic ash and highlight the potential for volcanic ash deposits to support bacterial diversity in the aftermath of reef burial, on timescales that could permit cascading effects on larval settlement.

Pore connectivity is a measure of the fraction of pore space (vesicles, voids or cracks) in a material that is interconnected on the system length scale. Pore connectivity is fundamentally related to permeability, which has been shown to control magma outgassing and the explosive potential of magma during ascent in the shallowest part of the crust. Here, we compile a database of connectivity and porosity from published sources and supplement this with additional measurements, using natural volcanic rocks produced in a broad range of eruptive styles and with a range of bulk composition. The database comprises 2715 pairs of connectivity C and porosity φ values for rocks from 35 volcanoes as well as 116 products of experimental work. For 535 volcanic rock samples, the permeability k was also measured. Data from experimental studies constrain the general features of the relationship between C and φassociated with both vesiculation and densification processes, which can then be used to interpret natural data. To a first order, we show that a suite of rocks originating from effusive eruptive behaviour can be distinguished from rocks originating from explosive eruptive behaviour using C and φ. We observe that on this basis, a particularly clear distinction can be made between scoria formed in fire-fountains and that formed in Strombolian activity. With increasing φ, the onset of connectivity occurs at the percolation threshold φcwhich in turn can be hugely variable. We demonstrate that C is an excellent metric for constraining φcin suites of porous rocks formed in a common process and discuss the range of φcvalues recorded in volcanic rocks. The percolation threshold is key to understanding the onset of permeability, outgassing and compaction in shallow magmas. We show that this threshold is dramatically different in rocks formed during densification processes than in rocks formed in vesiculating processes and propose that this value is the biggest factor in controlling the evolution of permeability at porosities above φc.