The February–April 2021 sequence of lava fountain paroxysms on MtEtna: source mechanism deciphered from ground‑based and satellitesurvey of volcanic gas emissions

Between 16 February and 1 April 2021, the Southeast Crater (SEC) of Mt Etna produced a spectacular sequence of 17 lava fountain paroxysms, separated by 2.5 days on average, which generated ≥ 10 km high eruptive columns and heavy tephra fallout over populated surroundings. We examine the magmatic processes responsible for these events based on pre- and syn-eruptive data for the mass flux and chemistry of Etna gas emissions, surveyed both from the ground (scanning DOAS, OP-FTIR spectroscopy) and from space (TROPOMI, SEVIRI), and comparing with the seismic tremor. Bulk plume SO2 emission rates determined from the ground and from space are consistent with one another. We show that after several months of background summit activity, sustained since June 2020 by open-system degassing of ~ 29 × 106 m3 (DRE) of magma through the central volcano conduits, an influx of deeply derived primitive magma led to a pressure build-up phase from early December 2020 to 13 February 2021, marked by a rapid increase in the SO2 flux and tremor (the former interpreted to represent an ~ 3 times higher magma degassing rate) and decreasing SO2/HCl plume ratio. A series of 17 lava fountains began immediately after a shallow seismic cluster and a sharp drop in the SO2 emission rate from the summit craters, reflecting the lateral transfer of pressurized primitive magma to beneath the SEC. The fountain paroxysms were characterized by sharp increases in tremor amplitude, intense SO2 release, and higher volcanic gas SO2/HCl ratios. The magnitude of SO2 emission rate correlates with the proportion of primitive magma in co-erupted products during the first half of the sequence. The estimated total gas discharge, compared to the co-erupted tephra mass, suggests a large excess gas release for most events, which is proportional to the length of the repose interval. Combining these observations with models of S and Cl degassing from Etna trachybasalt, we infer that the February–April 2021 lava fountain series resulted from the recurrent accumulation of H2O-CO2-rich bubble foams at ~ 2–3 km depth beneath SEC, whose periodic collapse promoted fast magma ascent and fragmentation associated with essentially syn-eruptive degassing of S and Cl. Our study thus provides further insight into the complexity of magmatic processes determining lava fountain paroxysms at Mt Etna and, possibly, other basaltic volcanoes.