Volcanic lightning-a near ubiquitous feature of explosive volcanic eruptions-possesses great potential for the analysis of volcanic plume dynamics. To date, the lack of quantitative knowledge on the relationships between plume characteristics hinders efficient data analysis and application of the resulting parameterizations. We use a shock-tube apparatus for rapid decompression experiments to produce particleladen jets. We have systematically and independently varied the water content (0-27 wt%) and the temperature (25-320°C) of the particle-gas mixture. The addition of a few weight percent of water is sufficient to reduce the observed electrification by an order of magnitude. With increasing temperature, a larger number of smaller discharges are observed, with the overall amount of electrification staying similar. Changes in jet dynamics are proposed as the cause of the temperature-dependence, while multiple factors (including the higher conductivity of wet ash) can be seen responsible for the decreased electrification in wet experiments.Plain Language Summary Volcanic explosive eruptions are accompanied by lightning strikes generated from the volcanic dust cloud. Here we have experimentally studied the effects of atmospheric water and plume temperature on the frequency and intensity of the lightning strikes. The results will feed a model for the use of volcanic lightning strikes to estimate plume contents and intensity. Key Points:• Natural observations on the influence of temperature and water content on volcanic lightning have been modelled in laboratory experiments. • The magnitude of electrification and discharging in plumes decreases significantly with added water. • At higher temperatures a decrease in magnitude of individual discharges is observed, while the total neutralized charge stays similar.

Volcanic jet flows in explosive eruptions emit radio frequency signatures, indicative of their fluid dynamic and electrostatic conditions. The emissions originate from sparks supported by an electric field built up by the ejected charged volcanic particles. When shock-defined, low-pressure regions confine the sparks, the signatures may be limited to high-frequency content corresponding to the early components of the avalanche-streamer-leader hierarchy. Here, we image sparks and a standing shock together in a transient supersonic jet of micro-diamonds entrained in argon. Fluid dynamic and kinetic simulations of the experiment demonstrate that the observed sparks originate upstream of the standing shock. The sparks are initiated in the rarefaction region, and cut off at the shock, which would limit their radio frequency emissions to a tell-tale high-frequency regime. We show that sparks transmit an impression of the explosive flow, and open the way for novel instrumentation to diagnose currently inaccessible explosive phenomena.

Accurate predictions of volcanological phenomena, such as the trajectory of blocks accelerated by volcanic explosions, require quantitative skills training. Large outdoor experiments can be useful to convey concepts of volcanic processes to students in an exciting way. Beyond the fun aspects, these experiments provide an opportunity to engage with the physics of projectile flight and help promote mathematical learning within the Earth Sciences. We present a quantitative framework required to interpret ballistic trajectories and the outdoor experiment known commonly as "trashcano", taking a step-by-step approach to the physics of this problem, and deriving a range of mathematical solutions involving different levels of complexity. Our solutions are consistent with the predictions from established computer programs for volcanic ballistic trajectory modelling, but we additionally provide a nested set of simplified solutions, useful for a range of teaching scenarios as well as downloadable simulated datasets for use where the full experiment may not be possible.

<p>Submarine eruptions dominate volcanism on Earth, but the recent eruption of Hunga Tonga–Hunga Haʻapai volcano in January 2022 was one of the most explosive eruptions ever recorded. Many large calderas collapse during eruptions and the resulting morphology provides unvaluable information for understanding the processes during highly unpredictable eruptions.</p> <p>Here we present a detailed analyses of the post-eruption morphology of the caldera of the Hunga Tonga–Hunga Haʻapai submarine volcano. We use the first multibeam bathymetry of the caldera, acquired only 5 months after the eruption on the MV Pacific Horizon, in May 2022.</p> <p>The multibeam data shows landslides with 0.5-1 km wide scars, mainly on the southern rim, with the deposits extending to the central part of the caldera. However, the flat inner caldera suggests that most of the material was deposited simultaneously to the caldera drop following the eruption, on the order of 800 m. Sediment cores collected inside the caldera show repeated turbidity current sedimentation pointing to ongoing mass wasting, which could have potentially led to eventual breaching of the rim on the north and east side. Submarine ridges were preserved on these sites, separating the inner caldera and two erosional channels on the outer part, which point to the main debris transport paths during the eruption. More than 50 active gas plumes are observed on the eastern side, located following a straight W-E transect, and on the northern side, where the vents are covering the collapse walls close to the eastern Hunga Tonga–Hunga Haʻapai island. The presence of these vents and their distribution related to the morphology of the caldera, indicate the most energetic parts of the volcano, which can potentially still be hazardous. Our morphological analyses provide new insights of transport and depositional processes following highly energetic submarine eruptions.</p>

<p>In December 2014, eruptions began from a submarine vent between the islands of Hunga Tonga and Hunga Ha’apai, 65 km north of Tongatapu, Tonga. The “Hungas” represent small NW and NE remnants of the flanks of a larger edifice, with a ~5 km-diameter collapse caldera south of them. The 2014/15 Surtseyan explosive eruptions lasted for 5 weeks, building a 140 m-high tuff ring.</p><p>Deposits on Hunga Ha’apai and tephra fall on Tongatapu record two very large magnitude eruptions producing local pyroclastic density currents and tephra falls of >10 cm-thick >65 km away. These likely derive from the central edifice/caldera. The 2022 eruption produced slightly less tephra fall, but an extremely large explosive event, with regional tsunami indicating substantive topographic change.</p><p>Here we report the bathymetric details of the caldera as of November 2015. A multibeam sounder (WASSP) was used to mapping the shallow (<250 m) seafloor concentrating on the edges of the Hunga caldera. These results were combined with an aerial survey of the 2015 tuff cone, using a combination of drone photogrammetry and real-time kinematic GPS surveys. The bathymetry reveals that previous historical eruptions, including 1988 and 2009, and likely many other recent unknown produced a series of well-preserved cones around the rim of the caldera. Aside from the raised ground in the northern caldera produced by the 2009 and 2014/15 eruptions, the southern portion is also elevated to within a few m below sea level, with reefs present. During the 2015 visit, uplifted fresh coral showed that inflation was ongoing and that the caldera was likely in the process of resurgence.</p><p>Much of Hunga Tonga and the 2014/2015 cone was destroyed in the 2022 eruptions, with Hunga Ha’apai intact, but dropping vertically by ~10-15 m. The violence of the 2022 eruption was likely augmented by either caldera collapse or flank collapse from the upper edifice, rapidly unroofing the andesitic magma system and enabling efficient water ingress.</p><p>This data provides an essential base layer for assessing changes on the ocean floor, especially to determine any caldera or upper-flank changes. Understanding these changes is crucial for future forecasting future volcanic hazards at Hunga and other nearby large submarine volcanoes.</p>

<p>In the AD 1450s, one of the three largest climate-forcing eruptions of the largest 1000 years took place, with similar impacts as the AD 1815 event of Tambora, Indonesia, that caused the ‘year without a summer’ of AD 1816. The submarine caldera of Kuwae, Vanuatu, has long been suggested to be the source of the AD 1450s eruption, but this is still highly debated.</p> <p>Today, the 12-by-6 km large Kuwae caldera lies between the islands of Epi and Tongoa. Here, an eruption occurred in the 15<sup>th</sup> century eruption and locally caused devastation, covering the islands surrounding it with vast amounts of pyroclastic material. We present the first full stratigraphy of the event, enabling us to reconstruct the eruptive sequence. First, a small ash plume produced fine ash deposits overlying faulted soil sequences, indicating a low-energy, precursory phase. Afterwards, the eruption built a Plinian eruption column, causing lapilli fall in excess of 3 m in proximal locations, and sending volcanic particles and gases high up into the atmosphere. This column then collapsed, producing the first pyroclastic flows that devastated the islands surrounding the caldera, incorporating many trees that became charcoalised and are still fully preserved in the deposits. Following further pyroclastic flow activity, a massive lithic lag breccia contains megaclasts (>10 m breccia), recording the collapse of a pre-existent edifice. Later pumice-rich pyroclastic flows occurred bring the total thickness of the sequence to between 30 and >80 m in proximal locations.</p> <p>These results are combined with initial results from a recent bathymetric study, mapping the submarine caldera floor, addressing the question of whether the entire caldera was generated during the mid-15<sup>th</sup> century event, or whether earlier eruptions have played a part in its collapse. This is essential for estimating the total collapse volume of the mid-15<sup>th</sup> century event at Kuwae, representative of the total erupted volume.</p> <p>We will use information on a) the eruptive sequence (based on field observations), b) eruptive volume (based on bathymetry), c) total emitted sulphur (based on eruptive volume and preliminary geochemistry data), and d) the eruptive date (based on radiocarbon analysis of collected charcoal samples) to characterise the mid-15<sup>th</sup> century eruption of the Kuwae caldera and test whether it was the source of the AD 1450s volcanic event that had a global climate impact.</p>