Rhythmic ejections of ash from the New Southeast Crater at about 16:40 GMT on 28 September 2011, seen from Trecastagni. Photo taken by Boris Behncke, INGV-Catania

Four moments in the evolution of the 28 September 2011 paroxysmal episode, from Strombolian activity and a small lava overflow to full lava fountaining, and advance of the main lava flow after the cessation of the main paroxysmal phase. These observations were made from the "Belvedere" site, about 900 m southeast of the New Southeast Crater. Photos taken by Stefano Branca, INGV-Catania

Fifteenth eruptive episode from Etna's new Southeast Crater

On the evening of 28 September 2011, the New Southeast Crater (New SEC) of Etna produced its 15th paroxysmal eruptive episode since the beginning of this year, 9-and-a-half days after the preceding episode, which had taken place during the forenoon of 19 September. This event was of brief duration but of violently explosive character, with emission of lava fountains and lava flows from vents within the crater, but also on the southeastern and northern flanks of its cone. The event produced two main lava flows, which, however, were less extensive than the flows emitted by previous episodes. An ash plume was blown southwestward by the wind, but light ash falls have been reported also from the southern flank of the volcano, including Nicolosi and Catania.

The "prelude" to this event was rather short. On the morning of 28 September, bangs were heard to originate from the New SEC, although no visibile phenomena were observed; bad weather conditions did in fact hamper visual observation throughout the day. di questo evento è stato breve. At about 14:00 GMT (= local time -2), the volcanic tremor amplitude started to increase, and rhythmic ash emission started from a vent located within the crater, at times accompanied by small Strombolian explosions. As during the previous paroxysmal episodes, the volcanic tremor source was observed to shift from its common location below the nothern part of Etna's summit area toward the New SEC, and to rise toward the surface.

From 17:30 GMT onward, the Strombolian activity progressively increased both in the intensity and frequency of explosion, and eventually became continuous, with some bombs and scoriae being launched well beyond the crater rim. About 19:15 GMT, a small lava overflow was noted at the notch cutting into the southeastern flank of the volcano; shortly thereafter, small explosions started from a vent located within the same notch. The Strombolian activity then nearly ceased for about 3 minutes starting at 19:28 GMT, before there was a very rapid and violent upsurge in the activity at 19:31 GMT, marked by the onset of sustained lava fountaining. Nearly instantaneously, the most vigorous fountain, which rose from a vent within the New SEC, reached heights of 600-800 m. At the same time, a vent located on the southeast flank of the cone reactivated; at 19:33 and 19:34 GMT two powerful explosions occurred from a further vent on the eastern rim of the crater, which created visible shock waves displacing the clouds hovering around the crater, and launched large bombs to hundreds of meters of distance. At 19:36 GMT, lava fountaining to about 100-150 m started from a vent located at the northern base of the New SEC cone, which soon started also to emit a small lava flow.

The paroxysmal phase, with high lava fountains from all active vents, went on for about 20 minutes and then, about 19:55 GMT, the activity started to show a marked diminution. Between 20:05 and 20:10 GMT, all explosive activity ceased, whereas the lava flows - fed from the vents on the southeast flank of the cone and the vent at its northern base - continued to advance until 21:30 due to the emptying of flow channels. The lava flow emitted from the southeast flank of the cone reached the lower portion of the western slope of the Valle del Bove, somewhat southwest of Monte Centenari, following the same path as the lava flows emitted during the most recent three paroxysms (29 August, and 8 and 19 September).

Volcanic ash emitted during this paroxysmal episode was mainly blown southwestward by the wind; however, light ash falls also occurred on the south flank of Etna, including the towns of Nicolosi and Catania.

With this episode, the tendency of progressively longer repose intervals between paroxysms observed since the end  of July has been broken; this event came little more than 9.5 days after its predecessor. Furthermore, it has to be remarked that:  (1) the "prelude" phase was rather brief; (2) there was a short interval, only a few minutes long, of nearly zero activity before the most intense phase of activity; (3) the height of the lava fountains in this event was well superior to the fountain heights during the previous paroxysms; (4) the lava volume was comparatively small; (5) the paroxysmal phase was rather brief  (~30 minutes); (6) the fracture systems extending from the New SEC toward southeast and north reactivated.