Volcanic gas constraints on the trigger mechanisms and incubation timescales of basaltic paroxysms

Persistent open-vent activity at Etna and Stromboli volcanoes in Sicily is frequently interrupted by powerful paroxysmal events that range in style and size from sequences of lava fountaining episodes (at Etna) to short-lived but violent vulcanian blasts (at Stromboli). Interpretation of the causes the lead to such events is challenged by complex genetic processes and (at Stromboli) deep trigger. However, consensus has been reached on a central role played by the exsolved magmatic gas phase in acting as key eruption trigger. Here, we review volcanic gas time-series acquired at both Etna and Stromboli using permanent instrumental volcanic gas networks to demonstrate distinct paroxysm’s trigger mechanisms and incubation timescales at the two volcanoes. On Mt. Etna, we use volcanic SO2 flux observations derived from a permanent UV camera system to show that the paroxysmal sequences taking place at its South-East Crater (SEC) are preceded by clear acceleration in SO2 degassing occurring over periods of months to weeks [Lo Bue Trisciuzzi et al., 2022, this volume]. From this, we propose that initiation of a paroxysmal sequence is triggered by progressive pressure build-up in a shallow (<3-5 km deep) magma storage zone below the summit region, caused by escalating supply of fresh volatile-rich magma. No obvious change in the SO2 flux is instead detectable prior to Stromboli’s paroxysmal and major explosions, implicating different (and deeper) causal mechanisms. We show that these sudden and violent explosions are anticipated by systematic increases in the plume CO2/SO2 ratio, typically occurring over timescales of weeks. We interpret these precursory CO2/SO2 ratio increases as reflecting accumulation of CO2-rich foam in a deep (>>3 km) reservoir, as caused by early (deep) degassing of a Low Porphiricity (LP) basaltic magma [Aiuppa et al., 2021, Sci. Adv.]. This accumulating foam would initially leak passively (for days/weeks) until a over-pressure threshold is reached, at which the LP magma+gas mixture is suddenly (in hours) erupted in a major explosion/paroxysm.