Forecasting Etnean eruptions by real-time observations of volcanic gas composition.

It is generally accepted but not experimentally proven that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data. By discussing the results of two years of real-time observation of H2O, CO2 and SO2 in volcanic gases from Mt. Etna volcano, we unambiguously demonstrate that increasing CO2/SO2 ratios can allow detecting the pre-eruptive degassing of uprising magmas. Quantitative modeling by the use of a saturation model allows us to relate the pre-eruptive increases of the CO2/SO2 ratio to the refilling of Etna’s shallow conduits with CO2-rich deep-reservoir magmas, leading to pressurization and eruption triggering. The advent of real-time observations of H2O, CO2 and SO2, combined with well-constrained models of degassing, represents a step forward in eruption forecasting.Keywords: Mount Etna, volcanoes, degassing, volatiles, gas plumes, volcanogenic processes.