Bettina Scheu

Explosive volcanic eruptions occur when coherent bubbly magma breaks apart in a process called “fragmentation.” Accurate conceptual and numerical models of fragmentation are a pre‐requisite for prediction of eruption explosivity. However, existing models, which neglect bubble‐scale magma physics, agree poorly with new experimental evidence that we present. Here, we derive and validate a mathematical model for magma fragmentation, based on viscoelastic failure of the liquid around rapidly growing bubbles in magma. The model, which explicitly captures processes at the bubble scale, shows excellent agreement with experiments, accurately predicting the conditions under which magma does and does not fragment. We apply the new numerical model to predict the critical rate of magma decompression during ascent required for explosive eruption across different magma types. The predicted rates are consistent with chemical and textural proxies for the decompression rate, providing further validation of the model under natural eruption conditions.

Hydrothermal alteration exerts strong control on shallow permeability and degassing dynamics in geothermal systems. Here, we investigate how soil alteration influences near-surface gas flow by combining in situ petrophysical measurements with horizontal and vertical subsurface gas-concentration profiles at the Rotokawa geothermal field, New Zealand. These data are compared against unoccupied aerial system (UAS) thermal surveys of collapse structures across the field. Soil permeability at Rotokawa ranges from 8.7 × 10 ^–14 to > 6.5 × 10 ^–13 m ² , highlighting strong meter- to decimeter-scale heterogeneity in shallow soil properties. Pumice-rich horizons are the main conduits for CO ₂ and CH ₄ flow, whereas clay-rich horizons locally act as semiconfining layers that promote CO ₂ accumulation and lateral diversion (leading to concentrations of > 4 × 10 ⁴ ppm). Since CO ₂ does not condense under near-surface conditions, the clay layers promote horizontal flow along permeable beds until gas encounters a high-permeability escape route or collapse-related discontinuity. Collapse structures locally disrupt and reorganize flow by acting as conduits or sinks that focus, capture, or redistribute gases near their margins. The gas profiles reveal patterns consistent with a shallow gas–steam decoupling zone in which steam condensation may contribute to sealing processes, as previously observed in steam-heated geothermal systems worldwide. These coupled effects of soil type and structural and alteration controls explain the spatial heterogeneity of surficial degassing at Rotokawa and provide a framework for interpreting evolving degassing patterns in similar steam-heated geothermal systems. In the context of the reported decrease in diffuse CO ₂ emissions at Rotokawa, progressive shallow sealing and gas refocusing may have contributed to apparent changes in emission patterns, alongside reservoir processes, recharge variability, environmental effects, and methodological uncertainty.

Phosphorus is essential for extant and nascent life. However, phosphate’s low terrestrial abundance and the poor aqueous solubility of phosphorus-bearing minerals pose major hurdles for prebiotic chemistry. Here, we show that silicate-phosphate immiscibility in volcanic melts could have provided a widely available mechanism for the local enrichment of phosphate. Starting with a phosphorus-enriched Archean melt, we observe that rapid cooling results in the formation of immiscible glassy droplets with up to 21 weight % phosphorus pentoxide, which can be delivered to the surface by volcanic processes. In aqueous leaching, we detect millimolar phosphate concentrations, which enable the synthesis of phosphorylating agents such as polyphosphates and imidazole phosphate with up to 34% yield. These results demonstrate how volcanic processes can enrich phosphate to fuel prebiotic chemistry.

Large-scale hydrothermal eruptions, although rare, pose significant hazards in geothermal regions. While aquifer lithology is known to influence fluid dynamics and eruption initiation for smaller-scale events, systematic studies linking lithological properties to large-scale eruptions remain limited. Here, we investigate the role of lithological variability in priming and eruption mechanisms at Rotokawa, New Zealand. We measured petrophysical properties and conducted rapid decompression experiments on disrupted aquifer lithologies, including consolidated lacustrine silts, clay-altered vitric tuffs, silicified volcaniclastic sandstones, and silicic-argillic altered paleo-breccia. Lithologies cover a broad range of properties and fragmentation characteristics, namely, density (0.9–1.9 g/cm ³ for bulk, 2.5–3.2 g/cm ³ for skeletal), porosity (3.7–63.5%), permeability (1.2 × 10 ⁻¹⁷ –1.2 × 10 ¹¹ m ² ), compressional wave velocity (0.9–5.2 m/s), fragmentation threshold (2–22.5 MPa), energy density (1.3–12.8 MJ/m ³ ), and fragmentation velocity (1–160 m/s). We evaluate contrasting priming scenarios for large hydrothermal eruptions, from rapid lake drainage to influx of deep hot fluids with either localized or widespread mixing. Results indicate that widespread and efficient mixing of hot and cold fluids in clay-altered aquifer lithologies is essential to reach energy thresholds for large-scale eruptions. Lithologies that have undergone argillic alteration are mechanically weakened and promote fragmentation, whereas silicification increases fragmentation thresholds by a factor of 2–3. We propose a conceptual model for the Rotokawa eruption in which initiation and progression are governed by lithology-dependent fragmentation energies, ranging from 1.6 to 8.5 MJ m ³ in lacustrine silts and sands, 1.3–6 MJ m ³ in altered vitric tuffs, 1.6–6.5 MJ m ³ in silicified sandstones, and 1.3–12.8 MJ m ³ in paleo-breccias. These findings refine estimates of the energy required for hydrothermal eruptions and define conditions favoring large-scale events. By improving understanding of priming conditions, eruption mechanisms, and lithological susceptibility, this study enhances forecasting and hazard mitigation strategies in geothermal systems.

Explosive volcanic eruptions generate electrical discharges, a phenomenon termed volcanic lightning (VL). VL is increasingly well-investigated and monitored for modern eruptions, however volcanism has been active since Earth’s origin. Thus, investigating VL under different atmospheric conditions is relevant for studies of early atmospheric chemistry and potential prebiotic reactions. We developed an experimental setup to investigate VL in varying atmospheres. We present the first experiments of laboratory discharges in particle-laden jets in varying atmospheric conditions. The new experimental setup is a mobile fragmentation bomb erupting into a gas-tight particle collector tank. This setup enables the testing of different atmospheric conditions, changes in the carrier gas of the jet, changes in the pressure within the tank, monitoring of the jet behaviour, and sampling of the atmosphere together with the decompressed solid materials. We find that the number and magnitude of near-vent electrical discharge events are similar in CO₂-CO and air atmospheres.

We present seismic and (infra)acoustic observations of the 15 January 2022 Hunga–Tonga–Hunga–Ha’apai (HTHH) eruption from Switzerland and central Europe at epicentral distances around 153°. Our observations indicate that the efficient seismoacoustic coupling of locally confined atmospheric oscillations (at the volcanic plume) and long-reaching (global) atmospheric waves at frequencies around 4 mHz is responsible for the global seismic reach of the HTHH eruption at low frequencies. At high frequencies (HFs), we recorded many short impulsive seismoacoustic signals with frequencies well above 20 Hz. They coincided with the passage of the HF part of the rather low-frequency atmospheric wave and triggered numerous false local-earthquake detections on the Swiss monitoring system. Simultaneous reliable ear witness reports of thunderlike sounds from southwest Germany suggest that signals related to the eruption were audible in central Europe at a distance of more than 17,000 km from HTHH. We postulate that the source of these HF seismoacoustic signals was the vigorous volcanic lightning activity observed at HTHH.

Self‐ignition during the explosive eruption of mud volcanoes can create flames that in some cases reach heights that exceed hundreds of meters. To study the controls on electrical discharge in natural mud, we performed laboratory experiments using a shock‐tube apparatus to simulate explosive eruptions of mud. We vary the water content of the mud and proportions of fine particles. We measure electric discharge within a Faraday cage and we use a high‐speed video camera to image the eruption of mud and some of the electric discharge events. We find that (a) decreasing the proportion of fine particles and (b) increasing water content each suppress the number and magnitude of electric discharge events. Experimentally observed mud volcano lightning occurs where particles exit from the vent and within the jet of erupting particles.

Over the last decades, remote observation tools and models have been developed to improve the forecasting of ash‐rich volcanic plumes. One challenge in these forecasts is knowing the properties at the vent, including the mass eruption rate and grain size distribution (GSD). Volcanic lightning is a common feature of explosive eruptions with high mass eruption rates of fine particles. The GSD is expected to play a major role in generating lightning in the gas thrust region via triboelectrification. Here, we experimentally investigate the electrical discharges of volcanic ash as a function of varying GSD. We employ two natural materials, a phonolitic pumice and a tholeiitic basalt (TB), and one synthetic material (soda‐lime glass beads [GB]). For each of the three materials, coarse and fine grain size fractions with known GSDs are mixed, and the particle mixture is subjected to rapid decompression. The experiments are observed using a high‐speed camera to track particle‐gas dispersion dynamics during the experiments. A Faraday cage is used to count the number and measure the magnitude of electrical discharge events. Although quite different in chemical composition, TB and GB show similar vent dynamics and lightning properties. The phonolitic pumice displays significantly different ejection dynamics and a significant reduction in lightning generation. We conclude that particle‐gas coupling during an eruption, which in turn depends on the GSD and bulk density, plays a major role in defining the generation of lightning. The presence of fines, a broad GSD, and dense particles all promote lightning.