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Activity of the Sicilian volcanoes during September 2011

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Two lava fountains are seen in this photograph taken during the paroxysmal eruptive episode of 19 September 2011 at the New Southeast Crater of Etna - one fountain rises from within the crater itself (at left), whereas another fountain is ejected from a vent on the southeastern flank of the pyroclastic cone surrounding the crater (in center foreground). View is from the western rim of the Valle del Bove, at a distance of 1.7 km from the crater. Photo taken by Zoltán Zajacz and published here with the kind permission of the author

ETNA

During September 2011, the episodic paroxysmal activity from the New Southeast Crater (SEC; formerly "pit crater") continued with three episodes, which occurred on 8, 19 and 28 September. The pyroclastic cone growing around the New SEC has undergone further morphological changes, and new eruptive vents opened during the paroxysmal episodes on the southeastern and northern flanks of the cone. 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.

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Eruption column formed during the 8 September 2011 paroxysmal eruptive episode, seen from the village of Pisano (near Zafferana Etnea) on the southeastern flank of Etna. Photo taken by Boris Behncke, INGV-Catania

8 September 2011 paroxysmal episode

The first paroxysmal eruptive episode of September, the 13th of the year 2000, took place during the forenoon of 8 September, ten days after the previous episode, which had occurred on 29 August 2011.

The first signs of reawakening were observed on 6 September, when a few ash emissions were observed to originate from the New SEC. In the night of 7-8 September, weak and sporadic Strombolian activity occurred, and at daybreak on the 8th, ash emissions were seen, followed by a rapid increase in the intensity and frequency of Strombolian explosions starting at 05:30 GMT (= local time -2). As in the case of the earlier paroxysmal episode, this phase was marked by a conspicuous increase in the volcanic tremor amplitude and a shift of the tremor source from its previous position below the Northeast Crater toward the New SEC, and toward the surface. At about 06:30 GMT pulsating lava fountaining started, producing increasing amounts of volcanic ash.

It was not before 06:50 GMT that lava started overflowing the crater rim, first through the deep notch in the eastern crater rim, and then along the eruptive fracture tha had opened on the southeast side of the cone during the 29 August 2011 paroxysm. This lava overflow was accompanied by repeated collapse and rockfalls from unstable portions of the cone in that area. Video footage recorded by the network of surveillance cameras of the INGV-Catania showed that the 29 August fracture did not reactivate on the occasion of the 8 September paroxysm. The lava flow took the same path as the lava emitted from the newly opened fracture on the southeast flank of the cone on 29 August.

From 07:20 GMT onward, there were repeated emissions of brown ash mixed with white water vapor from two or three spots on the northern flank of the New SEC cone, in the area of the lava overflows from the north rim of the crater that had started shortly after the onset of the paroxysmal activity. The first of these emissions occurred from a spot just below the northern crater rim, where an eruptive fissure had opened during the paroxysmal episode of 29 August.

A second explosive event took place about 20 minutes later, from a spot further downslope; this emission lasted 1-2 minutes. A third emission, which was stronger and lasted a bit longer, started at 08:18 GMT from a spot slightly upslope from the source of the previous emission, and lasted approximately 5 minutes. Field observations carried out during the days following the 8 September paroxysm and satellite images revealed the presence of a conspicuous fracture, curving from a SSW-NNE trend in its southern (upslope) portion toward a SW-NE trend in its lower, northern portion. The fracture is well visible in an image acquired by the Advanced Land Imager system on board of the NASA's EO (Earth-Observing)-1 satellite.

The paroxysmal activity started to diminish in intensity between 08:25 and 08:30 GMT, and totally ceased around 08:45 GMT; it was followed by a series of ash emissions that had a more and more passive character. In the meantime, the lava flow descending on the western slope of the Valle del Bove had taken the same path as that emitted during the 29 August paroxysmal episode from the newly opened fissure on the southeast flank of the cone; expansion of the most advanced lava fronts continued for some time after feeding of the lava had ceased, mostly due to gravitational flow. Small active lava flows were observed for many hours after the cessation of the paroxysmal activity, remaining confined to the immediate vicinity of the crater; all eruptive activity ceased during the night of 8-9 September.

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Southeastern flank of the New Southeast Crater cone and the peculiar rock "spine" formed during the paroxysmal eruptive episode of 8 September 2011, and the lava flow that remained active for many hours after the cessation of the proper paroxysm. Mosaic composed of two photographs taken on 12 September 2011 by Boris Behncke, INGV-Catania

Once more, the pyroclastic cone that has grown around the New SEC during the current series of paroxysmal eruptive episodes has undergone significant morphological changes: the southern and northern crater rims have further increased in height, whereas degradation and mass wasting on its southeastern flank has become more conspicuous. A large slab of rock composed of stratified and partly welded scoriae and bombs, constituting a part of the crater rim, was entrained by flowing lava, and then rotated and uplifted into a vertical position, forming a steep-sided "spine" about 20-30 m tall, with locally vertical and sub-vertical flanks. During the following days, this rock pinnacle suffered significant losses in height due to frequent collapse; however it remained a conspiuous landmark visible even from tens of kilometers away, until it was nearly completely obliterated during the next paroxysmal episode, on 19 September.

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Ash plume produced during the paroxysmal eruptive episode on the forenoon of 19 September 2011, seen from Trecastagni on the southeast flank of Etna. Photo taken by Catherine Lemercier

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Eastern flank of the New Southeast Crater, before (top) and after (bottom) the eruptive paroxysms of 19 September 2011. In the upper image, the rock spine dominating the profile of the crater since the end of the 8 September 2011 paroxysm is still visible. This spine was eroded by the eruptive activity of 19 September, even though a low, sub-rounded stump remained of it (lower image), as indicated by the black arrows. Photos taken by Marco Neri, INGV-Catania

19 September 2011 paroxysmal episode

Following 8 days of calm, at 10:27 GMT on 16 September 2011, the New SEC once more showed the signs of an imminent eruptive episode, with an explosion that launched ash and meter-sized blocks to the eastern base of the cone, far beyond the rocky "spine" formed during the 8 September paroxysm. Further explosions occurred at 20:20 GMT on 16 September, at 16:14 on the 17th, and at 06:09 GMT on 18 September. Strombolian activity started at 02:00 GMT on 19 September, and gradually increased during the following hours; at 06:30 GMT lava started to overflow the eastern rim of the crater.

The lava overflow continued for the whole morning, showing a gradual increase in the effusion rate, and generating a rather broad flow front tens of meters wide, which advanced eastward, toward the western slope of the Valle del Bove.

At 12:20 GMT the eruptive activity reached an energetic peak, giving lava fountains about 100 m tall from two sites aligned along a WNW-ESE trending fracture. A strong wind blowing eastt bent the plume of gas, ash and lapilli down into the Valle del Bove and then dispersed the more fine-grained products over the eastern and northeastern sectors of the volcano. As has happened during many of the previous eruptive episodes of this year, the paroxysmal activity came to a rather abrupt halt shortly after 13:00 GMT, accompanied by a short series of powerful detonations.

At the height of the paroxysm, the lava flow began to descend the steep western slope of the Valle del Bove, expanding over the lava flows emitted during the previous paroxysms. The peculiar rock "spine" formed during the 8 September 2011 paroxysm was almost entirely destroyed by the effusive and explosive activity. Lava emission continued, at a much reduced rate, into the evening of 19 September, and was accompanied by weak spattering from a vent located near the former "spine".

This episode occurred 11 days after its predecessor, making this the longest interval between paroxysms since 9 July 2011.

After the 19 September paroxysm, the New SEC did not show any signs of activity for 9 days; during a field survey on the morning of 26 September, all was quiet. On 27 September, observations were seriously hampered by cloud cover with heavy rains and hailstorms in Etna's summit area.

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Southeast flank of the New Southeast Crater cone and the low stump remaining of the once-spectacular rocky "spine", which was largely destroyed during the 19 September 2011 paroxysm, at right. Mosaic composed of four photographs taken on 26 September 2011 by Boris Behncke, INGV-Catania

28 September 2011 paroxysmal episode

The third paroxysmal eruptive episode of September came 9.5 days after its predecessor, and was preceded by an unusually short "prelude". On the morning of 28 September, explosion sounds were heard coming from the New SEC, but were not accompanied by any visible phenomena; inclement weather precluded visual observation throughout the day. At about 14:00 GMT on 28 September, the volcanic tremor amplitude began to rise, and rhythmic ash emissions started from a vent located within the New SEC, 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.

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Lava flows emitted during the paroxysmal eruptive episode on 28 September, still advancing after the end of eruptive activity at the New Southeast Crater (at upper left). The small lava flow at right was emitted from a vent located at the northern base of the cone, whereas the lava at left and center was fed from the crater itself, and from a fracture on the southeast flank of its cone. View from Monterosso, on the southeast flank of Etna. Photo taken by Boris Behncke, INGV-Catania

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). The much smaller northern lava flow advanced toward the May 2008 eruptive fissure, stagnating at about 2900 m elevation.

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; minor amounts of fine ash fell also on the southeast and northeast slopes of the volcano.

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Panoramic view of the summit area and upper eastern flank of Etna after the 28 September 2011 paroxysm, from the Rocca Capra area in the Valle del Bove, on the eastern slope of the volcano. At left is the couple of the old and new Southeast Craters, and the lava flow emitted from a vent on the northern base of the cone is seen as a dark patch to the right. Immediately to the left of that flow, white vapor plumes and yellow sulfur deposits mark the eruptive fissure of May 2008. Photo taken on 30 September 2011 by Boris Behncke, INGV-Catania

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; furthermore a more strongly explosive event occurred on 5 September, and there was an episode of spattering that generated a small rheomorphic lava flow on 7 September. The activity was concentrated at the two active vent areas, in the northern and southern sectors of the crater terrace.

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A series of frames captured from video recorded by the thermal monitoring camera of the INGV-Catania on the Pizzo, of the explosive sequence early on 5 September 2011

The explosive sequence of 5 September started at 03:52:18 GMT from vent S2 in the southern vent area, with an explosion from two closely-spaced vents, which caused the fall of bombs and scoriae onto the crater terrace, and was followed by about 50 seconds of lava fountaning. A plume of ash and lapilli rose more than 250 m above the craters. In this phase, pumiceous lapilli were emitted, which fell abundantly on the Pizzo sopra la Fossa (as reported by the Alpine Rescue Service of the Guardia di Finanza). At 03:54:25 a twin explosion of relatively low energy occurred from vent S1, which threw coarse-grained pyroclastics less than 80 m high; this was followed, at 03:54:32, by an explosion from vent N1 that ejected coarse-grained material up to 200 m high, marking the conclusion of the explosive sequence.

Vent S1, located on top of a conspicuous cone overlooking the Sciara del Fuoco in the southern crater area, was the site of an episode of more continuous spattering on 6-7 September. The spattering activity was quite intense between 18:50 and 19:50 GMT on 6 September, and still more so between 09:00 and 16:30 GMT on 7 September, when for determined periods it became rather vigorous. During these phases of intense activity, the rapid accumulation of fluid spatter on the slopes of the cone lead to the formation of minor rheomorphic lava flows.

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