## 2 The December 2015 Activity  
Before the 2015 events, eruptive activity at Etna consisted of a series of paroxysmal episodes at NSEC, which produced 50 lava fountains from 2011 to 2014. Conversely, after several years of mainly degassing activity, starting on 3 December 2015, the VOR crater underwent an extraordinary sequence of four very energetic paroxysms until December 5. Such a rapid sequence of such powerful events was never observed before on Etna, especially if considering that, in the short period of time between 2 and 18 December 2015, all the four summit craters produced explosive activity. The most powerful event was the first one, while the following three events showed a decreasing energy. All showed the same pattern starting from strombian activity and rapidly evolving to lava fountains in a few minutes, producing very high lava fountains, launching incandescent clasts to more than 3 km above the crater rim and producing an eruptive column reaching 14 km a.s.l., then decreasing again to strombian activity before ending [2]. In detail, on 3 December, at about 02:00 UTC the explosive activity at the Voragine crater was characterized by a very fast increase in the initial strombian activity, which at about 2:40 UTC culminated in a lava fountain (I episode) lasting 90 min, reaching a maximum height of the lava jet thrust of 4100 m above the vent [3], and then gradually passed again to strombian activity. The same sequence was recorded twice on December 4; the lava fountains occurred at 9:15 UTC (II episode ca. 80 min) and at 20:30 UTC (III episode with a duration of 90 min). Finally, in the early afternoon of December 5 from 13:30 UTC, a slower increase in explosive activity was observed at the VOR crater, switching to fountain activity at about 15:00 UTC for ca. one hour and a half (IV episode). The entire sequence at VOR ended with the last episode in the afternoon of December 5, lasting only 38 h, followed by weak strombian activity and short lava flows from NSEC until December 8. Authors in [2] estimated a volume for pyroclastic deposits of about 7.1 \(\times\) 10\({}^{6}\) m\({}^{3}\), resulting in an average of about 1.8 \(\times\) 10\({}^{6}\) m\({}^{3}\) erupted during each paroxysm.  
Such a violent eruptive phase, characterized by a sequence of paroxysms ranging from lava fountains to sub-plinian explosions, are not uncommon. In the first half of 2000, Mt. Etna generated 66 paroxysmal episodes [4] from the SEC, while the VOR crater produced less but very powerful activity, a sub-plinian eruption in July 1998 [5] and another very strong paroxysm one year later, in September 1999 [6]. Between 2011 and 2015, Mt. Etna  
Figure 2: Mt. Etna summit craters: Voragine (VOR), Northeast Crater (NEC), Bocca Nuova (BN), Southeast Crater (SEC) and New Southeastern Crater (NSEC). The 3D surface map is a combination of very-high resolution optical images acquired by Pleiades satellites on 20160603 and an SRTM DEM.  
saw more than 50 paroxysmal episodes from its summit vents. Furthermore, these kinds of eruptions have also been observed on other basaltic volcanoes, from the first observations and studies of Peleean eruptions at Manam (Papua New Guinea) in the 1960s to the more recent sequences at Kilawea (Hawaii) in the 1980s or at Fuego (Guatemala) in 2018.