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We used a one-year long SO2 flux record, which was obtained using a novel algorithm for real-time automatic processing of ultraviolet (UV) camera data, to characterize changes in degassing dynamics at the Mt. Etna volcano in 2016. These SO2 flux records, when combined with independent thermal and seismic evidence, allowed for capturing switches in activity from paroxysmal explosive eruptions to quiescent degassing. We found SO2 fluxes 1.5−2 times higher than the 2016 average (1588 tons/day) during the Etna’s May 16−25 eruptive paroxysmal activity, and mild but detectable SO2 flux increases more than one month before its onset. The SO2 flux typically peaked during a lava fountain. Here, the average SO2 degassing rate was ~158 kg/s, the peak emission was ~260 kg/s, and the total released SO2 mass was ~1700 tons (in 3 h on 18 May, 2016). Comparison between our data and prior (2014−2015) results revealed systematic SO2 emission patterns prior to, during, and after an Etna’s paroxysmal phases, which allows us to tentatively identify thresholds between pre-eruptive, syn-eruptive, and post-eruptive degassing regimes.

Stromboli is a model volcano for studying eruptions driven by degassing. The current paradigm posits that Strombolian eruptions represent the bursting of gas slugs ascending through melt-filled conduits, but petrological observations show that magma at shallow depth is crystalline enough to form a three-phase plug consisting of crystals, bubbles, and melt. We combine a 1-D model of gas flushing a crystalline mush with a 3-D stress model. Our results suggest that localized gas segregation establishes hot conduits of mobile magma within a stagnant plug. The plug is prone to tensile failure controlled by gas overpressure and tectonic stress, with failure most likely beneath the observed vent locations. We hence argue that Strombolian eruptions are related to plug failure rather than flow. Our proposed three-phase model of the shallow plumbing system may provide a promising framework for integrating geophysical, petrological, and morphological observations at Stromboli and in open-system volcanism more generally.

18 páginas, 13 figuras, 1 tabla., An ephemeral (and proximal) outcrop at the quaternary basaltic Puig de la Garrinada cinder volcano was studied in order to decipher eruptive mechanism variations in the course of a single eruption. Once the volcanostratigraphy was established (and recorded by photographs), a very detailed sampling of underlying lava flows, bombs and a large number of fine-sized lapilli and ash beds was carried out. An ad hoc description card was elaborated for study under stereo microscope in order to identify the different types of materials that form the volcanic cone. Ten material types were recognised: sedimentary accidental clasts, massive lava accidental clasts, bombs, xenoliths, xenocrysts and phenocrysts, oxidised scoria, scoria with oxidised film, fluidal pyroclasts, non-fluidal pyroclasts, and coatings. Subsequent studies under SEM + EDS and basic chemical characterization of the samples (whole rock XRF for major and trace elements) and EMPA were conducted. All these studies make possible the reconstruction of the eruptive history of the La Garrinada volcano, which shows a transition from initial strombolian activity to gradually incorporated phreatomagmatic activity. This permits the calculation of magma eruption rates, magma column oscillations and aquifer interactions with magma column. Since our general conclusions agree well with the discrimination of eruptive episodes obtained from the stereo microscope description card, macroscopic volcanostratigraphy and chemical results, it is proposed that a similar study protocol (quick volcanostratigraphy logging, photographic record, detailed sampling, description following this card) might be useful in the reconstruction of eruptive activity of other cinder cones in the Garrotxa region and elsewhere, and therefore can provide a good and quick methodological tool, especially when rapid field description and sampling is required (ephemeral outcrops, risky sampling during active eruptive episodes, etc.). Moreover, the conducted sampling allows for subsequent studies on the same set of pyroclasts., This work has benefited funding by research group SGR 2005 795 (AGAUR, DURSI de la Generalitat de Catalunya) and Spanish research project CGL2007-63727/BTE (MEC).