Gas buffering of magma chamber contraction during persistent explosive activity at Mt. Etna volcano

A sequence of more than 20 short-lasting explosive eruptions took place at Mt. Etna volcano, during a 2-month period in 2021. Here we perform a joint analysis of the gravity decrease and ground deflation that accompanied the sequence of eruptions. Results from this joint analysis are cross-checked against the output of a numerical code providing independent geochemical insight on how the density of the magmatic liquid/gas mixture in the source reservoir varies as a function of the pressure. This cross-analysis provides a framework to explain why (i) the bulk volume reduction sensed by the ground deformation data is much lower than the volume of the erupted products and (ii) the observed gravity changes point to a strong mass decrease, incompatible with a pure mechanism of magma withdrawal. We conclude that pressure-driven gas exsolution and expansion compensated the withdrawal of magma, thus buffering the contraction of the source reservoir and leading to the inferred mass decrease. Volatile degassing in magma storage reservoirs can buffer magma withdrawal and reduce deformation signals, according to a multi-parameter analysis combining gravity and ground deformation measurements from Mt. Etna with geochemical modeling.