Fig.
1. Map prepared by the Laboratorio di Cartrografia of the INGV-OE of
Etna's summit craters, showing the position of the eruptive vents active
during the past 10 days. NEC=Northeast Crater; VOR=Voragine; BN=Bocca
Nuova, SEC=Southeast Crater (not active since 2007); NSEC=New Southeast
Crater; "Sudestino"=an eruptive vent that built a low cone at the
southern base of the SEC cone in the spring of 2000; 2001 hornito=first
eruptive vents to open during the 2001 flank eruption; 2850 vent and
2800 vent=eruptive vents that opened near the Belvedere monitoring
station on 20-21 February 2013; TDF=Torre del Filosofo. Note that the
vent of the NEC (shown in orange color) is open and contains degassing
magma at depth since many years, but without any surface activity. Light
brown area is the zone of the "saddle" between the two cones of the
SEC, which collapsed during the 28 February 2013 paroxysm. Orange lines
are eruptive fissures in the "saddle" and on the southeastern flank of
the NSEC cone, which have been repeatedly active since the early morning
paroxysm on 20 February 2013.
The month of February 2013 at Etna ended
with spectacular eruptive episodes at the Bocca Nuova (BN) and at the
New Southeast Crater (NSEC), and with the resumption of eruptive
activity at the Voragine (VOR), which had remained inactive since
October 1999. The location of the summit craters of Etna and the various
eruptive vents that participated in the recent activity are shown in
Fig. 1. At Stromboli there have been renewed intermittent lava overflows
from the crater terrace.
Fig.
2. Eruptive episode at the Bocca Nuova on 27 February 2013. Besides
intense Strombolian activity and low lava fountaining, there were also
emissions of brown ash, probably caused by collapse or sliding of
material within the crater. Photo taken from Adrano, on the lower
southwestern flank of Etna, by Roberto Schillaci and published here with
kind permission of the author (original photo at Flickr)
27 February 2013 eruptive episode at the Bocca Nuova. On
the morning of 27 February 2013, an episode of intense Strombolian
activity and low lava fountaining started within the BN, the largest of
Etna's summit craters (Fig. 2). This new episode occurred
three-and-a-half days after the latest paroxysmal eruptive episode at
the nearby NSEC, and five days after an episode of mild Strombolian
activity at the BN. There was a rather novel element in this eruptive
episode: the reactivation of the VOR, often also referred to as
"Central Crater" (said correctly, the BN and VOR together constitute the
Central Crater of Etna), or "The Chasm". The last magmatic activity
observed at the VOR was in early October 1999.
Already during the night of 26-27
February, there was a gradual increase in the seismic activity and of
the volcanic tremor amplitude, especially at the seismic station at
Cratere del Piano (ECPN), which suggested a resumption of Strombolian
activity at the BN. Copious, puffing emission of dense vapor from the BN
was seen at sunrise; over the next few hours these emissions slowly
grew more energetic. Between 09:30 and 09:45 GMT (=local time -1), the
volcanic tremor amplitude showed a rapid rise; during the same interval,
the visuala and thermal surveillance systems of the INGV-OE showed the
formation of an eruption column largely composed of vapor, and emission
of hot material in the thermal images (Fig. 3a).
Fig. 3. Left:
thermal image recorded by the thermal monitoring camera of the INGV-OE
on the Montagnola (EMOT), showing an eruption column that contains hot
pyroclastic material (yellow-red-white hues), at 10:58 GMT (=local time
-1). Right: ash emission at 12:15 GMT, recorded by the visible-light
monitoring camera on the Montagnola (EMOV).
In some intervals, the column of vapor
emitted from the BN contained moderate amounts of volcanic ash of
reddish-brown color, mostly at 11:15-11:20 GMT and 12:14-12:16 GMT (Fig.
3b), which led to the fall of small quantities of very fine ash in the
area between Zafferana and Santa Venerina, on the southeast flank of
Etna. These ash emission likely resulted from collapse or sliding of
unstable material on the steep inner crater wall; the eruptive vent,
which lies in the southeastern part of the crater, is leaning against
the wall and the rapid accumulation of pyroclastic deposits in that area
might have facilitated the sliding of unstable material.
During the phase of most intense
eruptive activity, INGV staff carrying out fieldwork in the summit area,
noted that volcanic bombs fell outside the crater rim onto the
southwestern flank of the central cone (Fig. 4). Moreover, intense
explosive activity was also observed within the VOR, which since early
October 1999 had not shown any magmatic activity. The activity began to
diminish around 12:20 GMT, as was also evident from a reduction in the
volcanic tremor amplitude; at 13:30 GMT, the episode was essentially
over, even though strong degassing continued at both the BN and the VOR.
Fig. 4. Explosive activity within the Bocca
Nuova, with volcanic bombs flying well beyond the crater rim (in the
right part of the photo), and ash emission (at left). Photo taken from
the southwestern slope of Etna's central cone
(which hosts both the Bocca Nuova and the Voragine) on the late
forenoon of 27 February 2013 by Francesco Ciancitto, INGV-Osservatorio
Etneo
On the evening of 27 February, intense
glows were seen in the area of the Central Crater of Etna, and a number
of observers in the Bronte area (on the western flank of Etna) and at
Giarre and Riposto (on the lower east flank of Etna) clearly saw that
the main source of these glows was the VOR, where vigorous Strombolian
activity was taking place (Fig. 5a); hovever, it seems that minor and
discontinuous Strombolian activity also occurred at the BN (Fig. 5b).
This activity, which often launched incandescent volcanic bombs up to
150 m above the crater rim (Fig. 6a), continued through the night, and
was still going on early on 28 February, when also the NSEC started
showing signs of renewed activity after more than 4 days of repose.
Fig. 5. (a)
Strombolian activity a the Voragine on the evening of 27 February
2013, photographed from Giarre (on the lower eastern flank of Etna). At
left there is the NSEC cone, with several recent but dark (inactive)
lava flows extending downslope; behind the NSEC a gas plume from the BN
is illuminated possibly by weak Strombolian activity going on within
that crater. At right, the NEC is emitting a dense gas column. The dark
streak extending obliquely across the flank below the NEX, is the
pyroclastic deposit of the 23 February 2013 paroxysm at the NSEC. Photo
taken by Giovanni Puglionisi and published here with kind permission of
the author. (b) Strombolian
activity at the VOR (at left) and at the BN (at right) on the evening
of 27 February 2013, seen from Adrano (southwestern flank of Etna). Photo taken by Roberto Schillaci and published here with kind permission of the author
Fig. 6. Frames extracted from video recorded by the the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola (EMOH) on the morning of 28 February 2013. (a)
Jet of incandescent pyroclastics from the VOR at 03:22 GMT. (b)
Simultaneous Strombolian activity at the VOR (in the center) and at the
NSEC (at right) at 04:39 GMT.
28 February 2013 eruptive episode at the New Southeast Crater. During
the early morning hours of 28 February 2013, before sunrise, the visual
surveillance systems of the INGV-OE showed the onset of weak explosive
activity at the NSEC (Fig. 6b). With growing daylight, sporadic weak ash
emissions were seen at the crater. At 08:17 GMT, a small thermal
anomaly appeared in the western part of the NSEC, as recorded by the
thermal monitoring camera of the INGV-OE on the Montagnola (EMOT),
which was caused by weak Strombolian activity in the area of the former
"pittino". At 08:30 GMT, Strombolian activity was also underway at the
main vent in the center of the NSEC. Contemporaneously, the volcanic
tremore amplitude started to rise. During the following hour, the
eruptive activitya at the NSEC gradually waxed, while the volcanic
tremor amplitude increased rapidly, following the characteristic trend
of the eruptive episodes in Etna's summit area; at 09:30, at least three
vents were erupting: the former "pittino" to the west, and two vents
within the This activity generated a dense gas plume containing modest
quantities of volcanic ash.
Fig.
7. Lava fountain rising from the former "pittino", which lay in the
saddle between the old SEC cone (at left) and the NSEC cone (which in
this image is completely hidden by a dense veil of pyroclastics falling
from the lava fountains), during the acme of the paroxysmal eruptive
episode of 28 February 2013. The conspicuous cloud of white vapor and
brown ash in the foreground is generated by the collapse of the "saddle"
between the two cones. Photo taken from Monte Frumento supino, about
1.5 km south of the NSEC, by Francesco Ciancitto, INGV-OE
At 09:40 GMT, lava started to exit from
the NSEC through the deep breach cutting the southeastern crater rim; 25
minutes later, explosive activity started to increase dramatically, and
the first lava fountains appeared, rising about 100 m above the crater
rim. From 10:15 GMT on, frequent powerful explosions generated visible
shock waves ("flashing arcs"), heavily showering the entire NSEC cone
with large volcanic bombs. The activity intensified further between
10:17 and 10:22, accompanied by the appearance of a conspicuous cloud of
vapor and brown ash that issued from the area of the "saddle" between
the two cones of the SEC (Fig. 7). This cloud marked the progressive
collapse of a large portion of the "saddle", which destroyed nearly all
of the southwestern flank of the NSEC cone, and also bit deeply into
the eastern flank of the old SEC cone. The collapse was probably caused
by magma pushing into this area already structurally weakened during the
paroxysms of the previous week. A voluminous lava flow spilled from the
deep notch left by the collapse, expanding first south and then turning
southeastward, into the direction of the monitoring station of
Belvedere. Lava was also emitted from the eruptive vent(s) at 2850 m
elevation, at the base of the NSEC cone (2850 vent in Fig. 1); this
lava mixed with the lava flow emitted directly from the NSEC toward
southeast.
Fig.
8. Eruptive plume rising several kilometers high, produced by the
eruptive episode at the NSEC on 28 February 2013, seen from Naso (ME),
close to the Tyrrhenian (northern) coast of Sicily. Photo taken by
Giorgio Costa and published here with kind permission of the author (original photo at Flickr)
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Sustained lava
fountaining, with intense pyroclastic fallout also onto the old cone of
the SEC and generation of a huge cloud of gas and ash, continued at
maximum intensity for about 20 minutes. The ash cloud was pushed by
strong wind toward east (Fig. 8), leading to abundant ash and scoria
falls in the areas of Milo-Fornazzo on Etna's flank and Giarre-Riposto
nbear the Ionian coast. At 10:42 GMT, the activity started to diminish,
whereas from the collapsed "saddle" area, dense clouds of vapor and
grayish-brown ash were emitted. At about 10:50, the activity of the
former "pittino" became phreatomagmatic, with emission of vapor and ash,
and ejection of hot, wet blocks that formed spectacular vapor trails.
Shortly after 11:00 GMT, explosive activity at the NSEC ceased, whereas
lava emission continued from the collapsed "saddle" area as well as from
the southeastern flank of the NSEC cone, at a slowly diminishing rate. |
Fig.
9. These images show the state of the monitoring station of Belvedere on
the morning of 27 February 2013, after the partial invasion by lava
flows of the early morning of 20 February 2013 paroxysm, and a dike
intrusion passing beneath the site on the early morning of 21 February,
which led to the opening of an effusive vent at 2800 m (marked as "2800
vent" in Fig. 1=) and conspicuous ground fracturing in the Belvedere
area. The site was again invaded by a lava flow after the 28 February
2013 paroxysm, and the seismic station "EBEL" was destroyed. Photos
taken by Stefano Branca, INGV-OE.
During the following hours, the lava
flows continued to advance, both at the vents at 2850 m, into the
direction of the Valle del Bove, and from the collapsed "saddle" area,
advancing toward the Belvedere. On the afternoon of 28 February, lava
invaded the area of the monitoring instruments of Belvedere, which had
already been reached and partly buried by the lava of the paroxysm on
the early morning of 20 February, and subjected to spectacular ground
fracturing caused by the intrusion of a dike on the early morning of 21
February, which led to the opening of the effusive vent at 2800 m (2800
vent in Fig. 1). Fig. 9 shows the state of Belvedere on the day before
the 28 February paroxysm; the instruments in this site continued to
record and transmit data for several hours after the conclusion of the
paroxysm. Unfortunately, on the afternoon of 28 February, the seismic
station "EBEL" (Belvedere) failed, probably after being buried (and/or
heated) by lava.
Lava emission from the vents in the
"saddle" area and at 2850 m elevation continued during the night of 28
February-1 March 2013, and ceased sometime during 1 March. In contrast,
the Strombolian activity within the VOR continued without significant
variations, and was observed by INGV staff during a field visit on the
morning of 1 March (Fig. 10). On this occasion, the entire eastern flank
of the central cone of Etna was seen to be covered with a continuous
pyroclastic deposit consisting of large bombs and spatter, probably
produced during the 27 February BN and VOR eruptive episode. As this
report goes online (3 March 2013, 14:00 GMT), Strombolian activity is
still continuing at the VOR.
Fig.
10. Strombolian activity within the VOR, seen from the eastern crater
rim on the late forenoon of 1 March 2013. The opposite, western crater
rim (in the background) is covered with a fresh, black pyroclastic
deposit. At right, the south flank of the NEC cone is visible; the BN is
at extreme left. Photo taken by Francesco Ciancitto, INGV-OE
The most marked effect of the 28
February 2013 paroxysmal episode at the NSEC is the collapse of the
"saddle" between the two SEC cones, which involved a large portion of
the southwestern flank of the NSEC cone, and the eastern base of the old
SEC cone (Fig. 11). From the collapse area, lava was emitted not only
to feed the flow into the direction of Belvedere, but two small lava
lobes also extended southwestward for little more than 100 m, between
the small cones of the "Sudestino" and the 17 luglio 2001 hornito (Fig.
1).
Fig.
11. The collapsed area of the former "saddle" between the two SEC cones
(the old one at left, and the new cone at right), 24 hours after the 28
February 2013 paroxysmal eruptive episode, seen from south. The
collapse has entrained much of the southwestern flank of the NSEC cone
as well as the lower east flank of the old SEC cone. In the central-left
portion of the photo, in front of the old SEC cone, is the hornito of
2001 (see Fig. 1 for location). Photo taken by Tonino Giorgianni and published here with kind permission of the author.
Stromboli: new lava overflows from the crater terrace. After
an interval of 10 days of normal Strombolian activity, Stromboli
returned to producing small lava overflows from the crater terrace on
the afternoon of 27 February 2013. This lava overflow ceased during the
following night; a second episode of lava overflow started on the
evening of 1 March 2013 and ceased during the afternoon of the next day.
Both overflows were fed by continuous spattering (the ejection of fluid
lava fragments) from vent N2, which lies at the top of a spatter cone
(hornito) perched on the northern rim of the crater terrace.
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