Julia E. Gestrich

Explosive volcanic eruptions produce vast amounts of acoustic energy, generally in the infrasound range. For sustained eruptions, infrasound is often dominated by jet noise, a form of noise similar to the sound from human‐made jet engines. Jet noise largely results from the combination of two sound‐producing components: fine‐scale turbulence (FST) and large‐scale turbulence (LST), with distinctive frequency spectra. FST and LST spectra successfully reproduce single‐phase jet noise data (gas‐only), but little research has been directed toward multi‐phase jets, like volcanic jets (gas with solid particles). Here, we report novel experiments investigating the noise produced by particle‐laden jets. We produced both pure‐air and particle‐laden jets (nozzle diameters: 3.5–8 mm) and recorded the resulting sound at observation angles 42–159° (where 180° is the jet's propagation direction). Fitting FST and LST spectra to the particle‐laden noise spectra was less accurate than in the pure‐air case. The most prominent difference is the absence of LST‐dominated noise at high observation angles. We hypothesize that the particles break down large‐scale structures in the jet, thus favoring FST noise generation. Slower jet exit velocities due to the presence of particles may also explain some of the discrepancies observed and will require careful constraining in the future. The effect of particles on jet noise was most easily measured at high observation angles, which are also the most challenging to monitor at active volcanoes. The experiments presented here mark a first step toward integrating experimental, numerical, and observational approaches, to unravel the acoustic signatures of volcanic eruptions.

Geyser eruptions generate transient water fountains that modify the atmospheric electric field under repeatable and well-constrained conditions. Yet, the mechanisms controlling the observed signals remain unclear. We present the first systematic investigation of short-term electric field fluctuations during geyser eruptions, combining electrical measurements with high-speed video observations at Strokkur geyser, Iceland. Using a Biral Thunderstorm Detector and an Electric Field Mill, we recorded electrical signals during eruption events in October 2025 and relate them directly to the evolving fountain geometry. We observe that the polarity and amplitude of electrical signals correlate strongly with the ambient atmospheric potential gradient. We develop a finite-element electrostatic model (FEMM) to quantify the partial electrostatic shielding effect of a grounded water fountain on the ambient electric field. We derive empirical equations relating fountain height, radius, and potential gradient to induced electrical currents, allowing for a direct comparison between modeled and observed signals. By inverting the electrical signal, we reconstruct time-varying fountain radii that agree with qualitative observations from independent high-speed videos. The close agreement between reconstructed and observed fountain geometry suggests that electrical signals can be explained by changes in the ambient atmospheric electric field by the conductive water column. These measurements provide a new, non-intrusive approach for quantifying geyser fountain dynamics.

Geysers provide natural laboratories for studying eruptive dynamics, analogous to those observed at volcanoes, offering a safe and accessible setting in which processes can be observed at high spatial and temporal resolution. However, despite their easy accessibility and reliable activity, there is a lack of research regarding the electrical signals they generate. In this study, we investigate the source of electrical signals recorded by a Biral Thunderstorm Detector (BTD) in close proximity to Strokkur Geyser in Iceland. We focus on the effect known as shielding, where a moving conductive object connected to the ground distorts the electric field lines, inducing a current in a conductor, in our case, the BTD antenna. To test whether this effect is the source of the recorded signals, Finite Element Method Magnetics (FEMM) models are used to model the rising fountain. The results show that the induced charge and current are dependent on the fountain height, radius, and atmospheric potential gradient. We determine the atmospheric potential gradient using an electric field mill, colocated with the BTD, and measure the fountain height using video recordings. After deriving an empirical equation from the FEMM results, we can reproduce the measured BTD signal with the model by inverting for the fountain radius. Due to the high coherence between the signals and the good agreement between observed and calculated fountain radius, we conclude that the shielding effect is mostly responsible for the electric signals measured close to a geyser. This result is a significant contribution to understanding electric signals from various natural phenomena, including lava fountain activity and discharge generation in volcanic plumes.

Geysers are natural geothermal features that episodically erupt hot water and steam due to the buildup and release of subsurface vapor bubbles. At Strokkur geyser, Iceland, eruptions begin with the growth of a surface bulge caused by rising bubble clusters, followed by gradual rupture and disintegration into a water fountain. In this study, we investigate the high-resolution acoustic and visual signatures of this initial phase of the eruption, providing detailed insights into the fluid dynamics that govern bulge formation and rupture. While similar eruptive behavior is sometimes observed in low-viscosity volcanic systems, Strokkur offers a uniquely transparent medium in which processes like bubble rise and clustering can be directly observed, providing analogies to otherwise obscured dynamics in lava or mud-dominated settings. We combine low-frequency infrasound and high-frequency audio recordings with high-speed video, using synchronized data to track the evolution of the bulge. The results demonstrate that infrasound effectively detects bulge growth, while the onset of rupture is marked by a rise in audio-frequency amplitude. A monopole model is used to simulate pressure variations during bulge growth. The observed decompression signal is associated with the downward water motion during bulge disintegration. These findings improve our understanding of geyser eruption dynamics and suggest how acoustic monitoring can provide valuable information about subsurface processes in both geysers and volcanoes, such as dome inflation or gas bubble accumulation beneath magma surfaces.

Since 2019, the frequency of major explosive eruptions at Stromboli volcano (Italy) has increased, heightening the exposure of population and scientists to the hazards posed by ejecta. Morphological changes can directly alter the hazard potential associated with these phenomena. Here, we present a quantitative morphological analysis of changes of the crater terrace area linked to the 13 May 2022 major explosive event. High resolution (2.5 cm pixel-1) aerial imagery was acquired by unoccupied aircraft systems 2 hours before and 19 hours after the event. The 13 May 2022 major explosive event consisted of a minimum of seven explosions from four vents located in the south-central crater area. The opportune timing of this campaign enabled the quantification of morphological changes at Stromboli related to a single major explosive event at high temporal and spatial resolution. A total of 12.7 × 103 m3 was excavated and 5.5 × 103~m3 deposited. Via the mapping and classification of bomb distributions we observe that angular blocks make up the largest fraction of ballistics >0.2 m, from which we infer a strong interaction with wall rock and/or fragmentation of solidified plugs in the shallow plumbing system. The morphological changes observed provide valuable constraints on how much material is displaced, and the shift in location and the number of active vents during major explosive events at Stromboli.

Infrasound (low frequency sound waves) can be used to monitor and characterize volcanic eruptions. However, infrasound sensors are usually placed on the ground, thus providing a limited sampling of the acoustic radiation pattern that can bias source size estimates. We present observations of explosive eruptions from a novel uncrewed aircraft system (UAS)‐based infrasound sensor platform that was strategically hovered near the active vents of Stromboli volcano, Italy. We captured eruption infrasound from short‐duration explosions and jetting events. While potential vertical directionality was inconclusive for the short‐duration explosion, we find that jetting events exhibit vertical sound directionality that was observed with a UAS close to vertical. This directionality would not have been observed using only traditional deployments of ground‐based infrasound sensors, but is consistent with jet noise theory. This proof‐of‐concept study provides unique information that can improve our ability to characterize and quantify the directionality of volcanic eruptions and their associated hazards.

Real-time monitoring is crucial to assess hazards and mitigate risks of sustained volcanic eruptions that last hours to months or more. Sustained eruptions have been shown to produce a low frequency (infrasonic) form of jet noise. We analyze the lava fountaining at fissure 8 during the 2018 Lower East Rift Zone eruption of Kīlauea volcano, Hawaii, and connect changes in fountain properties with recorded infrasound signals from an array about 500 m from the fountain using jet noise scaling laws and visual imagery. Video footage from the eruption reveals a change in lava fountain dynamics from a tall, distinct fountain at the beginning of June to a low fountain with a turbulent, out-pouring lava pond surrounded by a tephra cone by mid-June. During mid-June, the sound pressure level reaches a maximum, and peak frequency drops. We develop a model that uses jet noise scaling relationships to estimate changes in volcanic jet diameter and jet velocity from infrasound sound pressure levels and peak frequencies. The results of this model indicate a decrease in velocity in mid-June which coincides with the decrease in fountain height. Furthermore, the model results suggest an increase in jet diameter, which can be explained by the larger width of the fountain that resembles a turbulent lava pond compared to the distinct fountain at the beginning of June. The agreement between the infrasound-derived and visually observed changes in fountain dynamics suggests that jet noise scaling relationships can be used to monitor lava fountain dynamics using infrasound recordings.

Over the past two decades (2000–2020), volcano infrasound (acoustic waves with frequencies less than 20 Hz propagating in the atmosphere) has evolved from an area of academic research to a useful monitoring tool. As a result, infrasound is routinely used by volcano observatories around the world to detect, locate, and characterize volcanic activity. It is particularly useful in confirming subaerial activity and monitoring remote eruptions, and it has shown promise in forecasting paroxysmal activity at open-vent systems. Fundamental research on volcano infrasound is providing substantial new insights on eruption dynamics and volcanic processes and will continue to do so over the next decade. The increased availability of infrasound sensors will expand observations of varied eruption styles, and the associated increase in data volume will make machine learning workflows more feasible. More sophisticated modeling will be applied to examine infrasound source and propagation effects from local to global distances, leading to improved infrasound-derived estimates of eruption properties. Future work will use infrasound to detect, locate, and characterize moving flows, such as pyroclastic density currents, lahars, rockfalls, lava flows, and avalanches. Infrasound observations will be further integrated with other data streams, such as seismic, ground- and satellite-based thermal and visual imagery, geodetic, lightning, and gas data. The volcano infrasound community should continue efforts to make data and codes accessible and to improve diversity, equity, and inclusion in the field. In summary, the next decade of volcano infrasound research will continue to advance our understanding of complex volcano processes through increased data availability, sensor technologies, enhanced modeling capabilities, and novel data analysis methods that will improve hazard detection and mitigation.

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.