Image captured from video recorded by the thermal monitoring camera of the INGV-Osservatorio Etneo on the Montagnola, about 3.5 km south of the summit craters of Etna, at 19:05 GMT on 23 October 2011. The lava fountain seen here is about 300 m tall

Lava flow descending the western slope of the Valle del Bove, seen from Dagala del Re, on the east flank of Etna, at 20:00 GMT on 23 October 2011. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

On the evening of 23 October 2011, the 17th paroxysmal eruptive episode since the beginning of this year occurred at the New Southeast Crater (New SEC) of Etna, 15 days after the previous episode that had taken place on 8 October.This event was characterized by a rather short "prelude", whereas the phase of lava fountaining lasted longer than 90 minutes, and was followed by a slow diminution in the activity, differently from the latest paroxysms that had been shorter and ended rather abruptly. The cloud of gas and tephra produced during this episode was carried eastward by the wind, leading to ash and lapilli falls in a sector from Zafferana Etnea and Milo to Giarre and Riposto.

At 17:13 GMT (= local time -2) the visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Schiena dell'Asino started recording weak explosive activity; around 17:35 GMT small anomalies also appeared in images recorded by the thermal camera on the Montagnola. At the same time the volcanic tremor amplitude started to rise, and the source of the tremor moved toward the New SEC and became shallower.

From 17:40 GMT onward, the explosive activity rapidly intensified, and by 18:07 the crater was completely filled with lava, which then started to overflow through the breach in the eastern crater rim, where during recent paroxysms a short eruptive fissure has been active. The lava formed a flow that slowly expanded toward the Valle del Bove, while the explosive activity continued to show a rapid augmentation, and at 18:26 GMT, the Strombolian explosions passed into continuous lava fountaining to a few tens of meters above the crater rim. At 18:36 GMT, a vent opened on the southeastern flank of the cone, producing a second lava fountain, and leading to a significant increase in the lava effusion rate.

The height of the lava fountains increased significantly after 19:00 GMT, reaching about 300 m above the crater. Around 19:30 a third eruptive vent became active within the New SEC, possibly in a position near its northern rim. The products of the explosive activity began to fall abundantly on the eastern flank of the cone and formed a dense curtain, while large incandescent blocks rolled to the base of the cone on more gently sloping terrain. Around 20:29 GMT two lightning flashes occurred near the erupting crater, possibilmente generated by the fragmentation of the magma. After 20:30 both effusive and explosive activity showed a marked diminution, passing again into Strombolian activity around 21:00, and ceasing altogether at 21:15 GMT.

The lava flow, which continued to advance toward the floor of the Valle del Bove until about 22:40 GMT, followed the same path of those emitted during the previous paroxysms, stagnating a bit upslope of Monte Centenari. The sector most heavily affected by the tephra (ash and small scoriaceous lapilli) fall was the eastern flank of Etna, including the northern portion of the town of Zafferana, Milo and Fornazzo, and downslope across Santa Venerina and Dagala del Re to Giarre and Riposto as well as nearby villages.