Dynamics of the 1873 CE “Breccia De Fiore” phreatic eruption at Vulcano (Aeolian Islands, Italy) through historical chronicles, physical volcanology, and numerical modelling

Phreatic events may represent precursors of magmatic eruptions, but they can also occur as single or multiple episodes punctuating the activity of volcanoes characterised by active hydrothermal systems. The Breccia De Fiore deposit represents evidence of the phreatic activity preceding by fifteen years the onset of the 1888–1890 magmatic eruption at La Fossa di Vulcano. This study integrates historical chronicles, sedimentological and physical descriptions of the deposit, and 3D numerical simulations to reconstruct eruption dynamics. Results indicate that this deposit was the product of multiple, shallow explosions of low magnitude (< 5×10 ⁴ m ³ cumulative volume), possibly occurred during a short time interval at the beginning of the phreatic sequence in 1873. The explosions produced pyroclastic density currents and ballistic fallout, affecting the slopes of the La Fossa cone. The asymmetry of the deposit, outcropping on the north-western flank of the cone, is well reproduced by 3D simulations as the product of explosions from an inclined vent driven by shallow (i.e., < 150 m deep) pressure build-up (up to 5 MPa) from accumulating magmatic fluids. Simulations of explosion dynamics and the distribution of eruptive products allows us to put some constraints on the two main controlling parameters of phreatic scenarios: erupted total mass and specific energy. This study also highlights the potential impact of such relatively small phreatic explosions on the Vulcano island and the need for quantifying their hazards, especially given the proximity to populated sites and the growing tourism in this active volcanic area.