Etna update, 13 January 2011

Eruption column illuminated by the glow from the lava fountain and lava flows descending into the Valle del Bove, seen from the village of S. Tecla at 00:17 (local time = GMT+1) on 13 January 2011. Photo taken by Mauro Coltelli, INGV-Catania

Lava fountain at Etna, 12-13 January 2011

During the night of 12-13 January 2011 the pit crater on the lower eastern flank of the Southeast Crater was the site of a spectacular paroxysm, which produced sustained lava fountains, voluminous lava flows, and an eruption column several kilometers tall, which caused ash falls on the south flank of Etna. This event, whose phenomenology resembles that of numerous episodes observed during the past few tens of years at Etna's summit craters, marked the culmination of an episode of eruptive activity that had started about 18 hours earlier, and then gradually increased in intensity. The 12-13 January 2011 paroxysm is the first event of this kind since a powerful eruptive episode on 10 May 2008, which originated at the same pit crater, just a few days before the onset of the latest flank eruption of Etna (13 May 2008 – 6 July 2009).

This event was preceded by two episodes of weak Strombolian activity from the same eruptive vent, on the afternoon of 23 December 2010, and during the night of 2-3 January 2011. Those events produced neither lava overflows nor volcanic ash.

The first small explosions within the pit crater were observed by the Etnean mountain guides during the forenoon of 11 January, whereas the video surveillance cameras (visible light and thermal) did not show any anomalies. During the evening of the same day, small anomalies appeared on the video footage recorded by the Montagnola thermal surveillance camera (EMOT); at the same time, small incandescent bursts were recorded by the visible light cameras of the Montagnola (EMOV) and Schiena dell’Asino (ESV).

Sustained lava fountaining at the culmination of the paroxysm, at  23:30 (local time = GMT+1), seen from the central square of Zafferana Etnea. Photo taken by Boris Behncke, INGV-Catania

During the night both the frequency and intensity of these phenomena, caused by weak Strombolian activity within the pit crater,  showed a gradual increase. The intensification in the eruptive activity was accompanied by a slow augmentation of the volcanic tremor amplitude, and a shift of the volcanic tremor source from a previous position to the north of the Northeast Crater toward the Southeast Crater.

In the evening of 12 January, the  increase in the intensity of the eruptive activity and of the volcanic tremor amplitude showed a marked acceleration; at the same time the pit rapidly filled with fluid lava, which around 21:40 h overflowed th low eastern rim of the crater, forming a small lava flow toward east. This lava flow initially advanced very slowly, then gained strength as it was fed more and more vigorously, and rapidly reached the western rim of the Valle del Bove.

At 22:48, the eruptive activity, which thus far had been Strombolian in character, suddenly passed into sustained lava fountaining, with a jet of incandescent lava rising 300-500 m high, and the generation of an eruptive column rich in volcanic ash that rose several kilometers high. The plume was bent southward by the wind, causing ash falls over population centers such as Nicolosi, on the south flank of the volcano, and scoriaceous up to several centimeters in diameter fell around the Rifugio Sapienza, at 1910 m elevation on the south flank.

Lava flow-field of the 12-13 January 2011 paroxysm, photographed from Monte Zoccolaro, on the southeast flank of Etna. (a) Thermal image and corresponding visible light image (b) of the lava flow-field on the western slope of the Valle del Bove. The most advanced lava flows have reached an elevation of about 1630 m. Yellow line in (b) traces the approximate outline of the lava flow-field. CSE=Southeast Crater; "cratere a pozzo"= pit crater on the east flank of the Southeast Crater cone. Photos and elaboration by Letizia Spampinato, Emanuela De Beni and Stefano Branca, INGV-Catania

Last moments of the lava fountain, and lava flows on the western wall of the Valle del Bove, at 00:40 on 13 January 2011, shortly before the cessation of the paroxysm. View from the village of Fornazzo, on the east flank of Etna. Photo taken by Boris Behncke, INGV-Catania

The lava fountain from the eruptive vent remained sustained until about 23:30, after which it became pulsating in character, reaching heights of no more than 150-200 m above the vent. At about 00:30 on 13 January, of the fountain remained a single, very narrow jet less than 100 m high, which ceased around 00:55.

The main lava flow descended, in at least three major branches, on the steep western slope of the Valle del Bove, reaching its base shortly after midnight. The most advanced flow fronts within the Valle del Bove had surrounded the northern side of Monte Centenari, reaching a maximum distance of about 4.2 km from the vent. A small lobe of rhemorphic lava was emplaced a bit further to the north, reaching a length of less than 1 km.

Aerial view of the Southeast Crater cone on the morning fo 13 January 2011, with the pit crater biting deeply into the eastern flank of the Southeast Crater cone, and, in the foreground, the main lava flow at left, and a smaller rheomorphic flow at right. A third, much smaller (also rheomorphic) lava lobe is barely distinguishable on the left side of the pit. Photo taken by Alfio Amantia, INGV-Catania