Activity of the Sicilian volcanoes, October 2011

Aerial view of the Southest Crater complex, showing the old cone at right (Old SEC), and the new one at left (New SEC), taken on 2 October 2011 looking south-southwest. The eruptive fractures and vents that opened on 8 and 28 September can be seen on the northern flank of the New SEC cone, the more recent of which has cut through the flank of the "Levantino", a subterminal cone formed during the paroxysmal eruptive episodes of 2000-2001 on the northeastern flank of the old SEC cone. In the center of the image is the lava flow emitted on the evening of 28 September, which covered a part of the upper eruptive fissure of May 2008. The Bocca Nuova (BN) lies behind the crest at upper right. Photo taken by Mauro Coltelli, INGV-Osservatorio Etneo (Catania)

ETNA

During October 2011, two further eruptive episodes have occurred at the New Southeast Crater (SEC; formerly "pit crater"), bringing the number of such episodes since the beginning of the current eruptive period (January 2011) to 17. During the whole month, the common deep-seated explosive activity within the Northeast Crater conduit has continued, accompanied by strong and often pulsating degassing and loud bangs audible from the crater rim; furthermore there have been minor emissions of ash from the Bocca Nuova on 20 October.

All times are in GMT (= local time -2 until 29 October, local time -1 from 30 October).

Volcanic ash plume produced during the paroxysmal eruptive episode of 8 October 2011, seen from the village of Monterosso, on the southeast flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

8 October 2011 paroxysmal episode Nearly 10 days after the paroxysmal eruptive episode of 28 September 2011, on the morning of 8 October, the seismic network of the INGV-Osservatorio Etneo (Catania) showed a rapid augmentation in the volcanic tremor amplitude. This was accompanied by a shift of the tremor source toward the surface, and from its "normal" location beneath the Northeast Crater toward the New SEC. At about 11:00 h the start of eruptive activity was distinctly evident in images recorded by the surveillance cameras as weak and discontinuous Strombolian explosions. Two hours later (13:30 h), lava started to overflow from the crater, as usual through the deep notch in its southeastern rim. At around 13:45 GMT, vigorous Strombolian activity was observed from numerous vents along a short eruptive fissure on the southeast flank of the cone, which had first been active during the  29 August  paroxysm. After 14:15 GMT, weather conditions deteriorated, and the passage from Strombolian activity to sustained lava fountaining and ash emission was not directly observed. However, this passage, around 14:30 GMT, was well audible, and a dense ash and vapor plume rapidly rose above the weather clouds, and then drifted east. At the same time, a lava flow descended the western slope of the Valle del Bove, following the same path as the previous lava flows.

It was probably during this phase that eruptive vents opened on the northeastern flank of the cone, approximately along the fracture that had first opened during the 8 September paroxysm, and two small lava flows were emitted. The more voluminous of these, which issued from the lower end of the fracture, extended downslope for a few hundred meters to invade the central portion of the May 2008 eruptive fissure.

The paroxysmal phase lasted little longer than 20 minutes and ended around 14:50 GMT; ash emission continued until about 17:45, when the volcanic tremor amplitude returned to levels similar to those preceding the paroxysmal episode, and the lava flow fronts appeared to stagnate. The ash cloud was blown east-northeastward by the wind, leading to ash and lapilli falls in a narrow sector that passed from the Ripe della Naca area and the village of Puntalazzo to the town of Mascali.

The 8 October paroxysm was the shortest in the series of similar episodes since January 2011 - the the culminating phase of sustained lava fountaining and ash emission lasted only about 20 minutes. Another notable feature of this event is the minor eruptive activity on the northeastern flank of the New SEC cone, which occurred along the fracture of 8 September, oriented SSW-NNE in its upslope portion and bending SW-NE in its lower portion. In contrast, the fracture that opened in this area during the 28 September paroxysm lies further to the west, cutting the flank of the "Levantino", a secondary cone formed in 2000-2001 on the north flank of the old SEC cone. The morphology of the New SEC did not undergo significant changes during this latest paroxysm, though it is likely that the accumulation of new pyroclastic material has led to some further growth in height of the cone.

Dry fracture field formed at the northern base of the New Southeast Crater cone during some of the latest paroxysmal eruptive episodes in September 2011, and observed during a field visit on 12 October 2011. In the left background is the observatory at Pizzi Deneri, and in the distance at right are the Ionian coast of northeast Sicily, the strait of Messina, and the southern tip of Calabria. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

A field visit to the summit area of Etna on 12 October 2011 revealed the presence of an extensive field of dry (non-eruptive) fractures in the area to the north of the New SEC cone, which represent the northeastward continuation of the eruptive fractures active during the 28 September and 8 October paroxysms. The fractures were grouped in three main sets and showed opening locally exceeding 0.5 m; the fracture field extended downslope for about 150-200 m, without intersecting the upper portion of the 13 May 2008 eruptive fissure.

Lava fountain, ash cloud, and lava overflow from the New Southeast Crater during the initial phase of the paroxysmal eruptive episode of 23 October 2011, seen from Trecastagni (southeast flank of Etna) at about 18:30 GMT. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

23 October 2011 paroxysmal episode After a quiescent interval of 15 days, the New SEC woke up again on the evening of 23 October 2011, to produce the 17th paroxysmal eruptive episode of the year. This event had a very short phase of "prelude", passing from the first small Strombolian explosions, at 17:13 h, into full sustained lava fountaining in less than 1.5 hours. In contrast, the main paroxysmal phase lasted more than 90 minutes. 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 h 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 h, 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.

The quiescent interval between this and the preceding episodes (15 days) was longer than any repose interval since 9 July 2011; in contrast the phase of "prelude" was shorter than those of the previous episodes. The culminating phase of this paroxysm was longer, with alternating phases of temporary diminution and intensification; yet the maximum height of lava fountains was only 300 m (those of the 28 September paroxysm were 600-800 m tall). Furthermore, during this event, the system of fractures and eruptive vents that had activated during the paroxysms of 8, 28 September and 8 October, did not show any activity.

The Southeast Crater couple (old SEC in left background, the new SEC in the foreground) seen from the air on 30 October 2011, looking northwest. The persistent eruptive fissure cutting the southeast flank of the new SEC cone is seen in the lower foreground; furthermore there is a degassing deep vent in the northern portion of the crater, well visible in the center of the image by the emission of bluish gas. Photo taken by Dario Teri and published here with kind permission of the author (Source: etnasci.it)

STROMBOLI

Stromboli volcano is in a state of persistent eruptive activity, normally with explosions of medium to low intensity occurring from a number of vents located within the summit crater. During September 2011, the activity continued at fluctuating levels, and with a number of episodes of spattering. The activity was concentrated at the two active vent areas, in the northern and southern sectors of the crater terrace; the spattering episodes occurred in the southern area and did not lead to the formation of any lava flows. At times the spattering was rather intense, as on 25 and 30 October.