ALBERT

Description: Understanding the mechanisms that cause effusive eruptions is the key to mitigating their associated hazard. Here, we combine results from permanent ultra-violet (UV) cameras, and from other geophysical observations (seismic very long period, thermal, and infrasonic activity), to characterize volcanic SO 2 flux regime in the period prior, during, and after Stromboli's August-November 2014 effusive eruption. We show that, in the two months prior to effusion onset, the SO 2 flux levels are two times average level. We explain this anomalously high SO 2 regime as primarily determined by venting of rapidly rising, pressurized SO 2 -rich gas pockets, produced by strombolian explosions being more frequent and intense than usual. We develop a procedure to track (and count), in the UV camera record, the SO 2 flux pulses produced by individual explosions and puffing activity (active degassing). We find that these SO 2 pulses are far more numerous (67 ± 47 events/hour) before the effusion onset than during normal activity (20 ± 15 events/hour). This observation, combined with geophysical evidence, demonstrates an elevated gas bubble supply to the shallow conduits, causing elevated explosive and puffing activity. This increase (≥0.1 m 3 s -1 ) in magma transport rate in the north-east feeding conduits finally triggers effusion onset. Active degassing remains elevated also during the effusive phase, supporting the persistence of explosive and puffing activity during the effusive eruption, deep in the volcanic conduit. Our results demonstrate that permanent UV cameras can valuably contribute to monitoring at high sampling frequency gas dynamics and fluxes, thus opening the way to direct comparison with more established geophysical observations.