Multiparameter insights into the months-long evolution of Mt. Etna discharge system prior to the December 2018 eruption

The 24–27 December 2018 Mt. Etna's flank eruption was one of the most important from this volcano in the last 20 years, especially in terms of the associated pre-, syn - and post-eruptive dynamics, as testified by the impressive ground deformation and release of seismic energy. Here, we build on existing literature and integrate it with previously unpublished findings, retrieved from the analysis of gravity time series, to gain unprecedented insight into the deep processes that determined the conditions for the breakout of the December 2018 eruption. Pre-eruptive emplacement of magma at shallow depth (∼2 km b.s.l.) did not occur progressively over the months preceding the eruption, but, rather, within a one-month timeframe (early October to early November 2018), as revealed by a marked gravity increase. This magma transfer was favored by enhanced permeability of the central discharge system, which also led to a decrease in the peripheral CO 2 flux. Ground deformation data point to a volume change at deeper depth, suggesting that the shallower magma influx was mostly accommodated (and “hid”) by compression of pre-existing magma. Since early November, further overpressurization could no more be buffered by the compressibility of the magma at 2 km depth, due to the decrease in the volume fraction of exsolved gas, and was more effectively transmitted to the host rock, which enhanced transfer of gas and magma through the uppermost part of the discharge system, accompanied by growing unrest. In our view, the important amount of magma which accumulated at relatively shallow depth during early October to early November pushed the plumbing system of Mt. Etna under disequilibrium conditions, ultimately favoring the emplacement of the dike which fed the December 2018 eruption. Besides establishing a long-term framework to consistently explain the geophysical and geochemical observations related to the December 2018 eruption, we come to the more general conclusion that multiparameter evaluation is essential to properly understand the behavior of a complex volcano, like Mt. Etna. We also highlight the important role played by magma compressibility in controlling pre-eruptive processes and shaping the characteristics of certain observables used to monitor and study active volcanoes.