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Understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas. Eruptions from hydrothermal centers may occur associated with magmatic phases, but also as isolated events without magmatic input, with the most recent examples being those of Te Maari (Tongariro, New Zealand) in 2012 and Ontake (Japan) in 2014.The most recent caldera of the Island of Vulcano (southern Italy) hosts in its center the La Fossa cone, active since 5.5 ka and now characterized by continuous fumarolic degassing. In historical times La Fossa cone has experienced several hydrothermal eruptions, with the most violent event being the Breccia di Commenda eruption, that occurred during the 13 th century AD. Based on analysis of 170 stratigraphic logs, we show that the Breccia di Commenda eruption occurred in three main phases.After an opening, low-intensity ash emission phase (Phase 1), the eruption energy climaxed during Phase 2, when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed, lithic-rich, blast-like pyroclastic density currents (PDCs). The tephra units emplaced during Phase 2, ranging in volume from 0.2 to 2.7  10 5 m 3 , were covered in turn by thin ash fall deposits (Phase 3). The dynamics of the most violent and intense stage of the eruption (Phase 2) was investigated by numerical simulations. A threedimensional numerical model was applied, describing the eruptive mixture as a Eulerian-Eulerian, two-phase, non-equilibrium gas-particle fluid (plus a one-way coupled Lagrangian ballistic block fraction). At the initial simulation time, a mass of about 10 9 kg, with initial overpressure above 10 MPa, and a temperature of 250°C, was suddenly ejected from a 200-m-long, eastward inclined, NNE-SSW trending fissure. The mass release formed blast-like PDCs on both sides of the fissure and launched ballistic blocks eastwards. Field investigations and numerical simulations confirm that hydrothermal explosions at La Fossa cone includes intense ballistic fallout of blocks, emission of PDCs potentially travelling beyond the La Fossa caldera, and significant ash fallout. The hazard associated with both ballistic impact and PDC ingress, as associated with hydrothermal eruption, is significantly larger with respect to that associated with Vulcanian-type events of La Fossa.

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The gravitational collapse of eruption columns generates ground-hugging pyroclastic density currents (PDCs) with highly variable temperatures, high enough to be a threat for communities surrounding volcanoes. The reasons for such great temperature variability are debated in terms of eruptive versus transport and emplacement processes. Here, using a three-dimensional multiphase model, we show that the initial temperature of PDCs linearly correlates to the percentage of collapsing mass, with a maximum temperature decrease of 45% in the case of low percentages of collapse (10%), owing to an efficient entrainment of air into the jet structure. Analyses also demonstrate that column collapse limits the dispersal capabilities of volcanic plumes, reducing their maximum height by up to 45%. Our findings provide quantitative insights into the mechanism of turbulent mixing, and suggest that temperatures of PDC deposits may serve as a marker for determining column collapse conditions, which are of primarily importance in hazard studies.

Validation and benchmarking of pyroclastic current (PC) models is required to evaluate their performance and their reliability for hazard assessment. Here we present results of a benchmarking initiative built to evaluate four models commonly used to assess concentrated PC hazard: SHALTOP, TITAN2D, VolcFlow and IMEX_SfloW2D. The benchmark focuses on the simulation of channelized flows with similar source conditions over five different synthetic channel geometries: 1) a flat incline plane, 2) a channel with a sharp 45° bend, 3) a straight channel with a break-in-slope, 4) a straight channel with an obstacle, and 5) a straight channel with a constriction. Several outputs from 60 simulations using three different initial volume fluxes were investigated to evaluate the performance of the four models when simulating valley-confined PC kinematics, including overflows induced by topographic changes. Quantification of the differences obtained between model outputs at t = 100 s allowed us to identify: 1) issues with the Voellmy-Salm implementation of TITAN2D and 2) small discrepancies between the three other codes that are either due to various curvature and velocity formulations and/or numerical frameworks. Benchmark results were also in agreement with field observations of natural PCs: a sudden change in channel geometries combined with a high-volume flux are keys to generate overflows. The synthetic benchmarks proved to be useful for evaluating model performance, needed for PCs hazard assessment. The overarching goal is to provide an interpretation framework for volcanic mass flow hazard assessment studies to the geoscience community.