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In September 2021, the La Fossa crater at Vulcano, in Italy, entered a new phase of unrest. We discuss a set of monitoring parameters included in the INGV surveillance network, which closely tracked the sequence of effects related to the crisis. The low-frequency local seismicity sharply increased, while the GPS and tiltmeter networks recorded the inflation of the cone, as an effect of fluid expansion in the hydrothermal system. Gravity variations were probably the effects of fast processes within shallow sources. The anomalies in soil CO2 flux, fumarole temperature, and in plume SO2 flux marked the strong increase in the vapor output from crater fumaroles. The signs of the impending crisis had been evident in the chemical and isotopic composition of fumarole gases since July 2021. These geochemical anomalies were clearly indicative of the enhanced input of gases from a magmatic source. In October, the massive degassing also influenced the areas at the base of the cone. In some areas, soil CO2 degassing and the thermal aquifer recorded strong anomalies. By early November, the crisis reached its acme. Afterward, the monitored parameters started a slow and discontinuous decreasing trend although remaining, some of them, sensibly above the background for several months. The multidisciplinary approach proved decisive for the interpretation of the underlying processes acting in the different phases of the unrest, thus allowing a consistent evaluation of the multiple hazards.

Diffuse degassing has been attracting considerable interest in volcanology over the past century since it was discovered that CO 2 is the main component of magmatic gases behind H 2 O. After its formation in the mantle, magma rising at shallower levels releases CO 2 through decompression, which flows into the atmosphere through summit crater vents, fumaroles, and mofetes in peripheral zones. Diffuse degassing has received attention

This study assessed the use of an H 2 fuel cell as an H 2 -selective sensor for volcano monitoring. The resolution, repeatability, and cross-sensitivity of the sensor were investigated and evaluated under known laboratory conditions. A tailor-made device was developed and used for continuously monitoring H 2 and CO 2 at Mt Etna throughout 2009 and 2010. The temporal variations of both parameters were strongly correlated with the evolution of the volcanic activity during the monitoring period. In particular, the CO 2 flux exhibited long-term variations, while H 2 exhibited pulses immediately before the explosive activity that occurred at Mt Etna during 2010.