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Summit eruption continues at Mount Etna
Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania
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From: Boris Behncke and Sonia Calvari
(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 down
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.
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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
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