Physical modelling and numerical simulation of a shallow magmatic-hydrothermal explosion: pyroclastic density currents generation and ballistic ejection during the 13th century “Breccia di Commenda” eruption (Vulcano island, Italy)

We investigate, by numerical simulation, the initial stages and eruptive mechanism of the 13th century “Breccia di Commenda” eruption at the Vulcano island (Italy). Complementary to this study, recent field work analysis and literature review (Pistolesi et al., session III.10), allowed to accurately characterize the eruptive sequence and products and to put quantitative constraints to the physical model. To simulate the eruption we have adopted a new three-dimensional numerical model describing the eruptive mixture as a Eulerian-Eulerian two-phase non-equilibrium gas-particle fluid plus a one-way coupled Lagrangian ballistic particle phase. At the initial simulation time, a pressurized, high-temperature mixture (initially confined within the shallow vent) is allowed to decompress and expand in the atmosphere forming an eruptive cloud, while ballistic particles are rapidly accelerated by the coupling effect of drag and pressure forces. After an initial expansion stage (lasting only 2-3 seconds), the eruptive cloud collapses (while ballistic particles tend to keep their trajectories) forming lateral pyroclastic density currents (PDCs). We show that PDC features and ballistic ranges (up to 2.2 km) are consistent with an exploding body having a mass of about 109 kg, an initial overpressure above 10 MPa, a maximum temperature of about 250°C and a maximum depth less than 200m. We also show that the observed deposits are consistent with explosion from an elongated fissure oriented North20°East, with preferential ballistic ejection and pyroclastic density current generation occurring in the directions orthogonal to the fissure orientation. We finally discuss present findings in the framework of volcanic hazards posed by phreatic explosions on active volcanoes and calderas worldwide.