Etna activity summary: 4-22 Nov 2006
4-22 November 2006: Summit eruption continues at Mount Etna
Episodic eruptive activity continued at the Southeast Crater of Etna (SEC)
through late November 2006, involving a number of explosive and effusive vents on and around the SEC cone. At the same time, constant lava effusion continued from a short fissure at 2800 m elevation on the upper ESE flank (the western headwall of the Valle del Bove), contributing to the growth of a complex lava field up to 3 km long. One particularly violent episode on 16 November culminated with the fracturing and piecemeal collapse of the SSE flank of the SEC cone, accompanied by explosions, rockfalls, steam and rock debris.
The following report, supplied by Boris Behncke and Sonia Calvari (INGV Catania), is based on daily observations by numerous individuals of the INGV staff (seewww.ct.ingv.itfor reports in Italian, updates and individual authorships).
Following one week of intermittent ash emissions and weak Strombolian activity on late 4 November, a new strong eruptive episode (the 10th since late August 2006) started at the SEC summit vent at 2004 on 5 November and continued with some fluctuations and intermittent ash emissions for the next 9.5 hours. Light ash falls occurred over populated areas to the SE. At about 21.47 on the 5th, the effusion rate increased at a vent at 3050 m altitude at the southern base of the central summit cone (herafter named "3050 m vent"), which had been continuously active since 27 October. A new lobe of lava traveled across the flat area south of Etna's summit cone complex (known as the Cratere del Piano). An apparent increase in the effusion rate was also noted at the effusive fissure at 2800 m on the ESE flank (hereafter named "2800 m vent"), with active lava lobes extending downslope. Lava effusion from the 3050 vent ended during the forenoon of 6 November, and for the following 48 hours, lava emission continued only at the 2800 vent.
The next (11th) eruptive episode at the SEC occurred on 8 November. Ash emissions from the summit of the SEC were followed by vigorous Strombolian activity that continued until approximately 2200. Around 1600, lava started to flow from a new vent located in the saddle between the SEC cone and the adjacent main summit cone, at an elevation of ~3180 m (hereafter referred to as "3180 m vent"). The lava reached the SW base of the SEC cone in a few minutes where it bifurcated into several short lobes, the largest and westernmost lobe stopping at the E margin of the lava flow field from the 3050 m vent. Lava from the 3180 m vent had ceased flowing by ~1845 whereas spattering and lava effusion continued at the 3050 m vent for some time; spattering ended at that vent around 1930 but lava continued to flow for another 24 hours.
Eruptive episode #12 from the SEC started at 2100 on 10 November, when the volcanic tremor amplitude showed a rapid increase. Bad weather hampered visual observations until 11 November, when it became evident that this episode had repeated in much detail its predecessor, with lava emission occurring from both the 3050 m and 3180 m vents. Strombolian activity from the SEC summit ceased at 1100 on 11 November. Lava emission from the 3050 m vent continued until the
following night, and the associated lava flow field grew mainly on its W side, with flow fronts descending to ~2800 m. For the next five days, lava emission continued unabated from the 2800 m vent, whereas the SEC and all other vents remained inactive.
Following a sharp increase in volcanic tremor amplitude at 0500 on 16 November, vigorous ash emissions started at the SEC summit at 0507 and were gradually replaced by intense Strombolian bursts, marking the onset of eruptive episode#13. Very early during this episode, lava issued from the 3180 m vent, forming a lobe about 100 m long before activity at this vent ceased. Lava effusion from the summit started at 0615 and triggered a series of rockfalls down the SE flank of the SEC cone, before the lava descended on the same flank. At 0626, brownish ash was emitted from a spot next to the effusive vent, and major rockfalls and avalanches started shortly thereafter. These originated at the S rim of what remained of the 2004-2005 collapse pit on the E flank of the SEC. Plumes rising from the descending avalanches contained both brownish ash and white steam. Avalanching was most intense between 0631 and 0640, after which the new lava flow rapidly descended the lower SE flank of the cone and began to extend beyond its base toward the area of the 2800 m vent. At the same time, strong emissions of black ash marked the opening of another explosive vent next to the summit, and a third explosive vent became active in the same area. For the next several hours, the vents continued to eject ash and occasionally bombs, and to produce vigorous Strombolian activity.
At 0700 emissons of white vapor occurred from the SE flank of the SEC cone; a few minutes later large rock avalanches started to descend that flank. Simultaneously a fissure began to cut from near the summit downslope on the SSE flank, triggering rockfalls and dust avalanches also on that side of the cone. This fissure initially propagated approximately 100 m downslope and then temporarily stopped, before it propagated another 150 m downslope at 0720. During the following 15 minutes, another fissure perpendicular to the earlier one cut SE-ward across the flank, generating more rockfalls and dust avalanches. The resulting fissure system had the form of an inverted Y delimiting a block that was actively pushed outward by magma intruding into the flank of the cone.
Lava began to issue from the lower end of the W branch of the fissure system at about 0810. At approximately the same time, the 3050 m vent started to emit lava. By this time, the upper portion of the fissure cutting the SSE flank of the SEC cone had significantly enlarged and become a deep trench. Dense volumes of steam were emitted from this trench at 0831 and were followed a few minutes later by another series of rockfalls and avalanches. Direct observation from ~0.7 km distance showed that the most energetic of these avalanches resulted from the collapse of low fountains of gas and tephra that occurred at the lower end of the large trench. The avalanches and rockfalls lasted approximately 15 minutes, then a voluminous surge of lava issued from the lower end of the opening trench.
Over the next few hours this sequence of events (vapor emission - rockfalls and avalanches - lava emission) was repeated several times as the trench widened and propagated further downslope. During the few moments when steam and dust clouds cleared and the interior of the trench became visible, a cascade of very fluid lava was seen in the center of the trench. Apparently, the lava issued from a source high in the head wall of the trench, and at times spurted from the vent like a firehose.
At 1100, white steam plumes, rockfalls and dust avalanches appeared high on the SE flank of the SEC cone, in the area where the summit lava flow was emitted. These phenomena marked a major collapse of the eastern wall of the trench, which eventually cut into the descending summit lava flow, diverting it from its original course into the trench. The original flow, which had descended immediately S of the 2800 m vent down to ~2600 m elevation, rapidly stopped, although lava continued to drain from the main flow channel and accumulated in a thickening lobe at the base of the cone. A series of events started at approximately 1425 with several vertical jets of black tephra that shot upward. These emissions occurred at ~150 m distance from the base of the cone and were very distinct in color from the brownish dust clouds which at the same time descended from the trench. The activity at the new site appeared to migrate rapidly both toward the SEC as dark plumes began to rise closer to the cone, while a ground-hugging plume of white vapor shot in the opposite direction. A few tens of seconds later, very dense clouds of dark brown material began to appear at the base of the surging white cloud and formed a distinct flow that rapidly overtook the front of the white cloud while speeding toward SE. At the slope break that forms the W rim of the Valle del Bove (~2800 m) both clouds disappeared from view in weather clouds, but at the site where the activity had originated, a huge plume of white vapor soared skyward. White vapor continued to rise from the area and from the path of the white and dark brown clouds for more than 15 minutes.
Another explosive emission of white steam and dark brown plumes occurred at ~1455 from a spot still farther away from the. Like the 1425 event, it generated ground-hugging clouds of steam and dark brown material, the latter again travelling faster than the earlier. During the following hours, the activity at the SEC gradually decreased, with several spectacular cascades of lava descending through the trench on the SSE side of the cone. Steam explosions and rock avalanches occurred at the lower termination of the trench at 1525. Strombolian activity ceased at 1500, but lava emission continued into the night. This lava does not seem to have extended far from the base of the SEC cone, since investigation during the following day failed to reveal any fresh lava on top of the debris deposits emplaced during the major explosive events at 1425 and 1455. A minor lava flow was also fed from a new short fissure approximately 80 m E of the 3050 m vent. During the evening a small lobe of lava was emitted from the accumulation of lava the base of the SEC cone.
Fieldwork and aerial surveys during the following two days revealed that the 1425 and 1455 explosions and related flows had left two main types of deposit. One was of lobate shape and extended a few hundred meters from the source of the explosions to the SE, covering a footpath that had been made by mountain guides to allow tourists to approach the persistently active 2800 m vent. On the ground the deposit consisted of very fine-grained reddish-brown ash made up almost exclusively of lithic fragments. Toward north the deposit gradually thickened and larger clasts were found on its surface, some of which represented fresh magmatic material. Close to the 2800 m vent, the deposit abruptly graded into a sort of debris flow rich in lithics but with up to 25% of fresh magmatic clasts. These latter showed a peculiar flattened-out morphology. Where this deposit overlay the tourist path near the 2800 vent it was 1.5-2 m thick. In one place the flow had entrained a plastic-coated sign warning tourists from leaving the path. The plastic showed no evidence of strong heating, so that it is assumed that the flow was cold in the moment when it arrived at that spot.
Volcanic tremor amplitude began to increase during the late afternoon of 18 November, and during a helicopter flight at 1800 the 2800 m vent showed vigorous spattering. Active lava from the vent travelled ~3 km to Monte Centenari. Bright incandescence was also noted within the 3180 m vent during this overflight.
At 0400 on 19 November, episode #14 started from the SEC. Strombolian activity occurred from 2 vents at the summit while lava flowed through the 16 November trench and divided into numerous braiding lobes on top of the debris deposit emplaced 3 days earlier. The longest lobe travelled along the prominent channel in the main debris flow, passing immediately to the S of the 2800 m vent and extending to ~2600 m elevation. This episode was much less violent than its predecessor and lacked the explosions, surges and flows characteristic of that event. Strombolian activity continued until the late evening, while lava effusion ended early on 20 November. As during previous episodes, lava had also briefly issued from the 3050 m and 3180 m vents. In addition, a few meters long flow started from another fissure that opened at about 3200 m, on the saddle between Bocca Nuova and SEC. This upper flow merged with the flow coming out from the 3180 m vent.
The 15th eruptive episode at the SEC started at 1200 on 21 November. Direct observations were rendered impossible by inclement weather through nightfall, but at ~1500 a black ash plume was seen rising above the cloud cover to approximately 1.5 km above the summit. Light ash falls occurred along the Ionian coast near Giarre and further north, while at Rifugio Citelli (~6 km NE of the SEC), the ash deposit was nearly continuous. After 1900, the cloud cover gradually opened, allowing direct views of the strong Strombolian explosions generating jets sometimes >300 m high. Lava once more flowed through the 16 November trench on the SSE flank of the cone toward the 2800 m vent. Likewise, the 3050 m and 3180 m vents had reactivated, although the latter had apparently ceased erupting early during the episode. Lava flowed from the trench until shortly after midnight on 22 November. Bad weather precluded further observations until the evening of that day, when all activity was again limited to the 2800 vent.
Dr. Sonia Calvari
Istituto Nazionale di Geofisica e Vulcanologia
Sezione di Catania
Piazza Roma 2
95123 Catania (Italy)
Tel. +39 095 7165800 direct: +39 095 7165862
Fax: +39 095 435801 url: www.ct.ingv.it