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Etna update, 8 September 2011

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Explosive activity from a spot located on the northern flank of the New Southeast Crater cone (seen in full eruption at left), generating a plume of brown ash and white vapor, at 08:18 GMT (= local time -2) on 8 September 2011. Note the blue gas rising from active lava flows near the ash and vapor emission. The yellow patches at right indicate the position of the May 2008 eruptive fissure; at top right a the souther portion of the Northeast Crater cone can be seen. Photo taken by Boris Behncke, INGV-Catania

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Explosive activity from different sources on the northern flank of the New Southeast Crater cone during the paroxysm of 8 September 2011, seen from the "Mareneve" road on the northeast flank of Etna. Photos taken by Catherine Lemercier

13th paroxysmal eruptive episode from Etna's New Southeast Crater

After a relatively quiet interval of 10 days, a new paroxysmal eruptive episode has taken place at the New Southeast Crater (New SEC) of Etna on the morning of 8 September 2011. This was the 13th event of this type since the beginning of the year; the previous episode had occurred on 29 August 2011. Like its predecessors, this event generated a tall eruption column that was blown south-southeastward by the wind, leading to ash and lapilli falls over a number of population centers such as Monterosso, Viagrande, Trecastagni, San Giovanni La Punta, and the easternmost portions of Catania. Moreover, during this paroxysm lava overflows occurred not only, as usual, through the deep breach in the eastern to southeastern sector of the pyroclastic cone of the New SEC, but also from its northern rim, and brief, intermittent emissions of ash and water vapor took place from different spots - possibly new eruptive vents - on the north flank of the cone.

The "prelude" to this episode started with a few emissions of ash from the New SEC on 6 September, followed by a rather calm day (7 September). On the late evening of 7 September, the visual surveillance system of the INGV-Catania recorded the onset of sporadic, very weak Strombolian explosions from the New SEC, which continued in a rather subdued manner through the night. At daybreak on 8 September, there was a series of ash emissions that were followed, from 05:30 GMT (= local time -2) onward, by a rapid increase both in the intensity and frequency of the Strombolian explosions. This activity generated loud detonations that were well audible in a vast sector on Etna's densely populated southeastern to eastern flanks; simultaneously the volcanic tremor amplitude showed a sharp increase, and the location of the tremor source shifted from its previous position below the Northeast Crater toward the New SEC, moving toward the surface. At about 06:30 GMT, the activity passed from Strombolian into a pulsating lava fountain, accompanied by increasing amounts of volcanic ash.

While lava fountaining and ash emission from the vents within the New SEC became more and more vigorous, lava overflow occurred not earlier than 06:50 GMT, first through the deep breach in the eastern crater rim, and then also along the fracture that 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. From the footage recorded by the visual and thermal surveillance cameras of the INGV-Catania it seems that the eruptive fissure of 29 August did not reactivate during the 8 September paroxysm.

However, 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 (the existence of this fracture was revealed only during a field visit to the summit area of Etna on 1 September 2011), as seen in the first photo at left. A second explosive event took place about 20 minutes later, from a spot further downslope (second photo at left); 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 (third photo at left). Interestingly, none of these events produced any lava fountains, the activity lasted only for very brief periods, and the relationship between the sources of the explosions and the effusive activity in the same area remains unclear.

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Eruption column of the paroxysmal eruptive episode of 8 September 2011 seen from near the Fontanarossa international airport of Catania, about 27 km south of the summit of Etna. Photo taken by Simona Scollo, INGV-Catania
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The peculiar "spine" formed during the paroxysm of 8 September 2011 on the lower southeast flank of the New Southeast Crater cone, seen on the late afternoon of the same day. Note the small active lava flow in the center of the image, indicating residual effusive activity going on for many hours after the cessation of the paroxysmal activity. Photo taken by Salvatore Gambino, INGV-Catania

These explosive episodes on the north flank of the New SEC cone stand in neat contrast with the opening of an eruptive fracture on the southeastern flank of the cone during the paroxysmal episode of 29 August 2011, where the activity continued until the cessation of the paroxysm and was characterized by the production of lava fountains and flows.

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 (see photo at left).

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 chunk of rock on the lower southeast flank was rotated and uplifted by some mechanism thus far unknown, forming a steep-sided "spine" about 20-30 m tall, with locally vertical and sub-vertical flanks (see images at left and below).

This episode occurred 10 days after its predecessor, and it was particular for the rather brief phase of transition from weak Strombolian explosions to sustained fountains and generation of a tall eruption column. Another peculiar feature of this event were the short-lived explosive events from various spots on the northern flank of the New SEC cone, possibly related to the opening of new vents; however, the character of these events was rather different from the activity typically observed during the opening of new eruptive fissures, such as on 29 August, and in many previous paroxysmal eruptive episodes, foremost during the long series of paroxysmal episodes between January and August 2000.

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One of the most spectacular effects of the 8 September 2011 paroxysm on the morphology of the New Southeast Crater is the formation of a rocky "spine" in the area of the lava overflows on the lower southeast flank of the cone. This feature, whose mode and time of formation remain largely unknown, is about 20-30 m tall and has rather steep flanks, locally sub-vertical to vertical. The upper photograph, taken by Wolfgang Lübke from the southeastern base of the cone, shows the "spine" at left, and surrounded by still hot and slowly moving lava, at about 14.25 GMT. The lower image, taken on the evening of 8 September by Gerhard Hornsteiner from the village of Presa on the northeast flank of Etna, shows the "spine" at the left base of the cone

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