Etna activity summary: July - 4 Nov 2006
From: Boris Behncke and Sonia Calvari (INGV Catania)
(All times are in UTC)
The following report, supplied by Boris Behncke and Sonia Calvari, is based on the daily reports written by volcanologists of the Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania (INGV-CT), available in Italian on the INGV-CT web page (http://www.ct.ingv.it/Etna2006/Main.htm).
Following the 10-days long eruption from the base of the Southeast Crater (SEC) in mid-July 2006, eruptive activity shifted to the summit vent of that crater by 31 August and continued until early 15 September, leading to lava overflows and repeated collapse on the E side of the SEC cone. Seven further episodes of eruptive activity from the SEC occurred between 22 September and 31 October; these became shorter and more vigorous in time. In late October new effusive vents opened on the upper ESE flank and at the southern base of the central summit cone of Etna, while episodic activity continued at the SEC.
A new eruptive episode started on late 22 September from the summit of the SEC, repeating the preceding episode (31 August-15 September) except that it was much shorter. During the first two days the activity was
limited to mild Strombolian explosions, but lava began to overflow on 24 September on the SE flank of the cone, covering the lava flows of the previous episode. This activity ceased on 27 September, although the exact time could not be established due to poor weather. The next episode started on the late afternoon of 3 October with Strombolian explosions from the SEC summit, which increased in vigor during the following hours, and late that evening lava began to spill down the SE side of the SEC cone adjacent to flows of the previous two episodes. Following a sharp decline in tremor amplitude on the afternoon of 5 October, the activity ended sometime between midnight and the early morning of 6 October; again observations at this time were hampered by inclement weather.
A third eruptive episode occurred between the evening of 10 October and
the evening of the following day, with vigorous Strombolian activity and lava again descending the SE flank of the SEC cone. A sharp decrease in tremor amplitude on the afternoon of 11 October indicated the imminent cessation of eruptive activity.
A short, N90-100°E trending eruptive fissure opened at approximately 2800 m a.s.l. on the ESE flank of the volcano at the base of SEC on the late evening of 12 October. The activity was recorded by the INGV-CT monitoring cameras starting about 2228h UTC. No explosive activity was observed at the summit of the SEC. For the first few days, lava was emitted quietly spreading in the upper Valle del Bove, without any explosive activity, and advanced a few hundred meters downslope. Mild spattering on 17 October led to the growth of three hornitos on the upper portion of the eruptive fissure, on the western wall of the Valle del Bove.
While effusive activity from the 2800 m fissure continued, the SEC produced another powerful eruptive episode starting at 0500h UTC on 20 October, as indicated by a rapid increase in tremor amplitude. Vigorous Strombolian activity occurred from a vent located in the central portion of the SEC summit, whereas isolated large explosions occurred every few minutes from a vent located near the E rim of the crater, in the notch created by the collapse events of early September. A new pyroclastic cone rapidly grew at the summit of the SEC. Lava once more flowed own the SE side of the cone, reaching a length of <1 km and stopping to the N of the eruptive fissure at 2800 m elevation. At the latter site, lava emission appeared reduced when compared to the previous days, but continued while activity at the SEC ceased late on 20 October.
A few isolated bursts of ash from the SEC occurred on 22 October, and on the next day at 0600 UTC, vigorous Strombolian activity and pulsating lava fountaining from two vents at the summit of the SEC marked the onset of the fifth eruptive episode from this crater since mid-September. Lava spilled down the ESE flank of the cone, to the north of the flows of the previous episodes, and the pyroclastic cone first seen three days earlier underwent further rapid growth. At the same time, the rate of lava emission from the fissure at 2800 m on the upper ESE flank increased, leading to several overflows from the area of the hornitos formed on 17 October. Strombolian activity and fountaining at the SEC diminished on the afternoon of 23 October and were followed by strong ash emissions started around 1600 UTC, producing a plume that drifted ESE. Pulsating ash emissions and occasional bursts of incandescent tephra were still continuing when, at ~1650 UTC, the S flank of the SEC cone fractured and lava was emitted from the lower end of the fracture. Much of the lava travelled toward SE upon reaching the base of the cone, but a small lobe took a more westerly direction, stagnating before having reached the base of the cone. The supply of lava from the new fissure diminished during the early morning of 24 October and came to a complete halt around noon, the most advanced flow fronts having travelled approximately 1 km. At the same time, effusive activity continued without significant variations at the ESE flank vents at 2800 m elevation on the west wall of the Valle del Bove. The farthest flow fronts had reached an elevation of approximately 2000 m, to the NW of Monte Centenari, and roughly 2.5 km from their source.
Field observations made on 24 October revealed that part of the new cone that had grown at the summit of the SEC had subsided and a new collapse pit, approximately 50 m wide, had opened on the SE flank of the SEC cone, roughly in the center of the largely obliterated 2004-2005 collapse pit.
A marked increase in volcanic tremor amplitude and ash emissions from the summit of the SEC on the late afternoon of 25 October heralded the next episode of activity from this crater, which produced only weak Strombolian activity. Tremor amplitude and Strombolian activity both decreased on the late evening of 25 October, but at 2354 UTC, lava was emitted from a new fissure on the SSE flank of the SEC cone. This fissure was active only for a few hours and produced a very small lava flow. Another effusive vent opened at 0131 UTC on 26 October at ~3050 m elevation at the S base of the central summit cone of Etna, below the Bocca Nuova, and approximately 700 m to the W of the fissure that had erupted 2 hours earlier. At the same time, lava from the fissure at 2800 m elevation on the ESE flank continued to discharge. Fieldwork carried out on 26 October revealed that the new effusive vent at 3050 m elevation had formed at the southern end of a fracture field extending across the SE flank of the central summit cone to the W flank of the SEC cone. Lava emisson from this vent continued at a decreasing rate before ending completely on the evening of 26 October, whereas the ESE flank fissure remained active.
A new increase in volcanic tremor amplitude and ash emissions from the SEC started on the afternoon of 27 October; these were followed at 0106 UTC on the following day by the reactivation of the effusive vent at 3050 m elevation at the S base of the central summit cone. Ash emissions and Strombolian activity occurred from the SEC between 0830 and 1100, but no lava overflows were produced. On the evening of 28 October, both effusive vents at 3050 m on the S side of the central summit cone and at 2800 m on the ESE flank were active. Ash emissions from the SEC, probably caused by collapse within its conduit, were observed on the afternoon of 29 October, and became more vigorous during the early morning of the 30th, when fine ash fell over inhabited areas to the S, including Catania (27 km from the SEC). Intermittent bursts of incandescent tephra were recorded by surveillance cameras of the INGV-CT, although analysis of the ash revealed that most of it was lithic. Ash emissions gradually diminished and ceased at around 0700 UTC. Later that day, the effusion rate from the vent at 3050 m elevation on the S side of the Bocca Nuova was estimated at 1-5 m3 s-1, and lava had descended toward SW to approximately 2400 m elevation. Simultaneously, lava continued to flow from the fissure at 2800 m elevation on the ESE flank, but the lava flow front showed little advance since 24 October.
Lava continued to flow from both vents during the first days of November, but a marked diminution in the effusion rate became evident on the 3rd when active flows had retreated upslope from the most advanced fronts in both lava fields. A helicopter overflight on the morning of 5 November revealed actively flowing lava only in the uppermost portions of the lava flow fields.
Ash was again emitted from the SEC shortly before noon (UTC) on 31 October, and in minor quantities at least once per day through 5 November. No incandescent ejections occurred from this crater after 28 October until the evening of 4 November (1730-1905 UTC), when weak Strombolian explosions were recorded by the INGV-CT surveillance cameras.
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