Dynamics of shallow hydrothermal eruptions: new insights from Vulcano’s Breccia di Commenda eruption

Understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas. Eruptions from hydrothermal centres 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 centre the La Fossa cone, active since 5.5 ka and now characterised 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 thirteenth 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 m3, 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 three-dimensional 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 include 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.