RESEARCH ARTICLE On the transition from strombolian to fountaining activity: a thermal energy-based driver Maxime Bombrun 1,2,3,4 & Letizia Spampinato 5 & Andrew Harris 1,2 & Vincent Barra 3,4 & Tommaso Caltabiano 5 Received: 29 July 2015 /Accepted: 9 February 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Since 1999, Mount Etna’ s (Italy) South-East crater system has been characterised by episodic lava fountaining. Each episode is characterised by initial strombolian activity followed by transition to sustained fountaining to feed high- effusion rate lava flow. Here, we use thermal infrared data recorded by a permanent radiometer station to characterise the transition to sustained fountaining fed by the New South-East crater that developed on the eastern flank of the South-East crater starting from January 2011. We cover eight fountaining episodes that occurred between 2012 and 2013. We first developed a routine to characterise event waveforms apparent in the precursory, strombolian phase. This allowed extraction of a database for thermal energy and waveform shape for 1934 events. We detected between 66 and 650 events per episode, with event durations being between 4 and 55 s. In total, 1508 (78 %) of the events had short waxing phases and dominant waning phases. Event frequency in- creased as climax was approached. Events had energies of between 3.0 × 10 6 and 5.8 × 10 9 J, with rank order analysis indicating the highest possible event energy of 8.1 × 10 9 J. To visualise the temporal evolution of retrieved parameters during the precursory phase, we applied a dimensionality re- duction technique. Results show that weaker events occur during an onset period that forms a low-energy Bsink^. The transition towards fountaining occurs at 10 7 J, where subse- quent events have a temporal trend towards the highest ener- gies, and where sustained fountaining occurs when energies exceed 10 9 J. Such an energy-based framework allows re- searchers to track the evolution of fountaining episodes and to predict the time at which sustained fountaining will begin. Keywords Mount Etna . Strombolian events . Lava fountaining . Explosive regime transition . Radiometry Introduction Lava fountains, erupted as part of BHawaiian style^ explo- sions, are a relatively weak form of volcanic explosive activity during which sustained jets of molten lava and gas are ejected at mass eruption rates of 10 5 –10 6 kg/s over time periods of a few hours (e.g. Mercalli 1907; Newhall and Self 1982; Houghton and Gonnermann 2008). However, they are fre- quently occurring explosions which characterise activity at basaltic centres such as Mt. Etna and Hawaii (e.g. Heliker and Mattox 2003; Behncke et al. 2006; Stovall et al. 2010). They are capable of feeding tephra plumes to altitudes of up to several thousand metres (e.g. Swanson et al. 1979; Vergniolle and Mangan 2000; Wolff and Sumner 2000; Andronico et al. 2008), causing local air fall damage and presenting a hazard to Editorial responsibility: J. Taddeucci Key points • Strombolian activity parameterised through the transition to fountaining • A database of 1934 events is provided • Our data allow a better understanding of the strombolian-fountaining transition * Maxime Bombrun bombrun@isima.fr 1 Clermont-Université, Université Blaise Pascal, LMV, BP 10448, F-63000 Clermont-Ferrand, France 2 CNRS, UMR 6524, LMV, F-63173 Aubiere, France 3 Clermont-Université, Université Blaise Pascal, LIMOS, BP 10448, F-63000 Clermont-Ferrand, France 4 CNRS, UMR 6158, LIMOS, F-63173 Aubiere, France 5 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Piazza Roma 2, 95125 Catania, Italy Bull Volcanol (2016) 78:15 DOI 10.1007/s00445-016-1009-4