Tracking precursors and degassing by radon monitoring during major eruptions at Stromboli Volcano (Aeolian Islands, Italy)

We monitored radon anomalies before, during and after the recent paroxysmal eruptions of Stromboli. Major eruptive events are preceded by relative minima in radon emissions at stations located at the base of the cone, whereas three of the summit stations reached peak values (~20,000 Bq/m3 or higher) 12 to 14 days before major eruptions, and are nearly coeval with earth tides. Radon emissions at/or above such high threshold values are proposed to result from the dynamic response of the fracture network due to degassing processes associated with magma ascent. Relative minima recorded at the base of the cone are related to fractures' self-sealing within the hydrothermal system, eventually coupled with the atmospheric stack-effect. The coexistence of the above anomalies is interpreted as the first precursory signals to major eruptive events.