Eruptive activity at Mount Etna, 20-22 February 2013

Fig. 1. Top: the cone of the New Southeast Crater seen from Tremestieri Etneo, 2 February 2013, before the series of paroxysmal eruptive episodes of 19-21 February 2013; bottom: a comparison photograph showing the same view but after the paroxysms, 22 February 2013. Photos taken by Boris Behncke, INGV-Osservatorio Etneo

After the episode of lava fountaining (paroxysm) on the morning of 19 February 2013, the New Southeast Crater (NSEC) of Etna produced three further paroxysms during the following 54 hours, which have notably changed the profile of its cone (Fig. 1). Besides the paroxysmsn themselves, a few vents that opened on the southeastern base of the NSEC cone produced lava also during the intervals of relative calm between one paroxysm and the next. Finally, on the morning of 22 February, the Bocca Nuova returned on the scene, with a brief episode of intracrater Strombolian activity. Contemporaneous with this latest episode, the effusive activity at the base of the NSEC cone diminished significantly, but slow lava emission continued on the late forenoon. All eruptive activity ceased on the early afternoon of 22 February but resumed after about 24 hours to culminate in a new paroxysmal episode on the evening of 23 February, which is described in a separate report.

Fig. 2. The first phases of the paroxysmal eruptive episode at the NSEC on the early morning of 20 February 2013, documented in this series of frames extracted from video recorded by Klaus Dorschfeldt (who generously permitted publication on this site). (a) shortly after the beginning of the first lava overflow through the deep notch in the southeastern crater rim, about 00:45 GMT (=local time -1); (b) opening of two vents on the lower southwestern flank of the NSEC cone, at 00:55 GMT (a portion of the old SEC Cone is seen in the center background); (c) explosive interaction between snow and lava emitted from the new eruptive fissure at the southwestern base of the cone and snow, a few minutes after the previous frame; (d) lava fountaining from the vents at the southwestern base of the cone, around 01:00 GMT. Times are based on video footage recorded by the surveillance cameras of the INGV-Osservatorio Etneo. These images were taken near Monte Arcimis, on the southeastern flank of Etna

First eruptive episode of 20 February 2013. After the cessation of the paroxysmal eruptive episode on the morning of 19 February, the volcanic tremor amplitude remained slightly elevated compared to the pre-paroxysm levels; shortly before 23:00 GMT (=local time -1) of the same day, a sudden increase of the volcanic tremor amplitude was accompanied by a resumption of Strombolian activity at the NSEC. During the following 90 minutes, the eruptive activity gradually increased, and lava started to exit the crater through the breach in the southeastern crater rim at 00:43 GMT (Fig. 2a). At 00:55, an eruptive fissure with at least two vents opened at the base of the "saddle" between the old and new cones of the SEC (Fig. 2b), which was the site of vigorous spatterina, and interaction of lava emitted from these vents and snow cover on the ground led to intense explosive activity (Fig. 2c), which continued for about 30 minutes. Activity at these vents gradually increased to lava fountaining to several tens of meters in height (Fig. 2d) and emission of a copious lava flow, which first expanded southward, passing to the east of the eruptive vents of 17 July 2001, and then turned southeastward, in the direction of the multiparameter monitoring station of "Belvedere". At 01:00, at least three vents - from the former "pittino" to a vent within the notch cutting the southeastern rim of the NSEC - produced lava fountains up to 100 m tall. Ten minutes later, the incandescent jets of lava reached heights of 300 m above the crater rim; however, the fountains were rather discontunuous and interspersed with powerful explosions. In this interval, a column of ash rose several kilometers above the summit of Etna, before being bent eastward by the wind. Ash and scoria falls affected more or less the same areas already hit by the tephra falls of the previous paroxysm.

Fig. 3. The culminating moments and final phase of the paroxysmal eruptive episode at the NSEC on the early morning of 20 February 2013, documented in these frames captured from video recorded by Klaus Dorschfeldt (who generously permitted publication on this site). (a) and (b) activity of an eruptive vent on the lower southeastern flank of the NSEC cone around 01:10 GMT, a few minutes after this vent became active; (c) opening of another eruptive vent at about 01:15 GMT, further to the east of the previous vent; (d) lava flows emitted from different vents at the NSEC descend the steep slope toward the Valle del Bove. These images were taken near Monte Arcimis, on the southeastern flank of Etna

Shortly after 01:00 GMT, a new eruptive vent opened on the lower southeastern flank of the NSEC cone, producing low lava fountains, and a well-fed lava flow (Fig. 3a, b), which rapidly expanded toward the western rim of the Valle del Bove. Around 01:15 GMT, a second eruptive fissure (Fig. 4c) opened a short distance fo the east of the former, and likewise started to emit a copious lava flow directed toward the Valle del Bove (Fig. 4d). At 01:24 GMT, many jets of lava rose obliquely toward west, causing heavy fallout of incandescent bombs onto the old SEC cone. After 01:30 GMT, the height of the jets had diminished to 100-150 m, and between 01:40 and 01:45, lava fountaining ceased, followed by occasional Strombolian explosions and emission of a dense ash plume. The lava flows continued to advance slowly; the lava flow emitted from the fracture at the southwestern base of the NSEC cone menacingly approached the monitoring station of Belvedere (Fig. 4). At 02:00 the eruptive was substantially over, as was also evident from the volcanic tremor amplitude, which had returned to background levels.

Fig. 4. This scene photographed after the end of the lava fountaining activity of the paroxysmal eruptive episode at the NSEC on the early morning of 20 February 2013 shows lava flows still advancing on the steep western slope of the Valle del Bove (at right) and near the "Belvedere" monitoring station (marked by a yellow "x"), at left. An eruptive vent located at the base of the NSEC cone (which is visible in the right background) has just reactivated and is emitting a new, brightly incandescent lava flow in the center of the image. Photo taken by Marco Restivo (EtnaWalk) and published here with kind permission of the author (original photo at Flickr)

Fig. 6.Panoramic view of the area to the south-southeast of the NSEC, after the early morning paroxysm on 20 February 2013. The foreground is occupied by the broad lava flow-field emitted during the night's paroxysm from the fissure that opened at the base of the saddle between the two SEC cones (in the background). This lava expanded toward the monitoring station at Belvedere (the area seen veiled by bluish fumes in the right distance). Mosaic composed of 6 photos taken by Francesco Ciancitto, INGV-Osservatorio Etneo

Second eruptive episode of 20 February 2013. During the interval between 10:40 and 11:00 GMT on 20 February 2013, degassing from the NSEC was observed to increase, and at the eruptive fissures at 2850 m, vigorous spattering resumed and rapidly became continuous. At 10:57, the main vent within the NSEC began to emit ash, followed a few minutes later by the onset of Strombolian explosions. For more than one hour, Strombolian activity continued at fluctuating intensity and accompanied by varying amounts of ash.

The same fluctuations in the intensity of the eruptive activity were also observed in the spattering activity from the vents at 2850 m elevation, at the southeastern base of the NSEC cone, where intervals of quiet lava emission alternated with vigorous spattering and low lava fountaining. At 13:20 GMT, a sequence of very strong explosions at the NSEC opened the paroxysmal phase of this episode, with tall lava fountains, copious ash emission, and lava emission also from the eruptive vents within the NSEC. The start of this phase is shown in Fig. 8, whereas Fig. 9 shows various moments of the paroxysmal phase documented by the visual and thermal mobile monitoring cameras of the INGV-OE on the Schiena dell'Asino, which had been activated shortly after the paroxysm 12 hours earlier. Also the activity at the vents at 2850 m elevation at the southeast base of the NSEC cone, consisted of lava fountains with jets as high as 50 m (Fig. 10).

Fig. 8. Beginning of the paroxysmal phase of the second eruptive episode of 20 Febraury 2013 at the NSEC, seen from south (Cisternazza area) at about 13:20 GMT. The flanks of the NSEC cone are subjected to heavy pyroclastic fallout, producing brown dust clouds, after a series of particularly powerful explosions. At the southeastern (right) base of the cone, two low, cherry-red lava fountains mark the eruptive fissures that had opened at about 2850 m elevation at the end of the previous paroxysm. Photo taken by Francesco Ciancitto, INGV-Osservatorio Etneo

Fig. 9. Images recorded during the acme of the paroxysm on the early afternoon of 20 February 2013 by the mobile visible (left: MBV) and thermal (right: MBT) monitoring cameras of the INGV-OE on the Schiena dell'Asino.

Fig. 10. During the climax of the paroxysmal eruptive episode on the early afternoon of 20 February 2013, lava fountains not only rose from the main vents within the NSEC (top), but also from vents that had opened on its lower flank and at its base, at about 2850 m to the southeast (bottom). Photos taken from the Schiena dell'Asino by Michele Mammino and published here with kind permission of the author.

During the phase of most intense activity of lava fountains, flows, and ash emission, around 13:30 GMT, also the eruptive fissure in the saddle below the former "pittino", on the southwestern flank of the NSEC cone, was reactivated, emitting a lava flow that partly covered the lava flow-field emitted from the same fissure during the previous paroxysm, bit did not reach the area of the Belvedere. Finally, a well-fed lava flow seems to have also issued from the main vent of the NSEC, following the usual path through the deep breach cutting the southeastern crater rim.

The paroxysmal activity continued at maximum intensity for little more than 20 minutes; shortly after 13:40 GMT the height of the lava fountains was seen to be diminishing, and shortly thereafter the fountains became discontinuous and at around 14:00, passed into an activity mainly characterized by ash emission, which in turn progressively diminished and completely ceased shortly after 14:35 GMT. Also at the fissures at around 2850 m elevation to the southeast of the crater, the activity diminished, without ceasing completely, however: relatively low-rate lava emission continued, feeding two lava flows directed toward the Valle del Bove and about 1.5-2 km long (Fig. 11). This activity continued through the evening of 20 February and into the next day, accompanied by sporadic, weak Strombolian explosions at the NSEC, and frequently unstable material crumbled from its outer flanks and inner crater walls, exposing incandescent rock underneath.

Fig. 11. Effusive activity from the vents at 2850 m elevation, near Belvedere, after sunset on 20 February 2013, as recorded by the mobile visual monitoring camera of the INGV-Osservatorio Etneo on the Schiena dell'Asino (MBV).

Eruptive episode of 21 February 2013. At 01:33 GMT on 21 February 2013, in a place high on the western slope of the Valle del Bove, immediataly below the monitoring station of Belvedere, a new effusive vent opened (Fig. 12a, b), from which a lava flow issued that caused the melting of snow on the ground, generating a series of lahars (Fig. 12c, d). Descending toward the Valle del Bove, these lahars ran against the hot lava flows emitted previously from the vents at 2850 m, generting voluminous vapor plumes (the thermal images recorded by the mobile camera MBT on the Schiena dell'Asino do not allow to discern whether these plumes also contained ash).

Fig. 12. Opening of a new effusive vent below the Belvedere and emission of a lava flow with generation of lahars, starting on 01:33 GMT on 21 February 2013, documented by the mobile thermal monitoring camera of the NGV-OE on the Schiena dell'Asino (MBT). Frame (a) shows the site just before the opening of the vent, which is seen in frame (b); frames (c) and (d) show the descent of lahars toward the Valle del Bove and their interaction with hot lava emitted previously.

Fig. 13. A suggestive image taken during the paroxysm on the morning of 21 February. At left the narrow lava flow fed from the new vent that opened below the Belvedere at 00:33 GMT on 21 February 2013 is visible, whereas the more brightly incandescent and voluminous lava flows at center-right come from the NSEC and the eruptive vents at its southeast base at 2850 m elevation. The crater itself is hidden behind the cloud cover whose base is seen in the upper part of the image. Photo taken by Antonio Zimbone and published here with kind permission by the author. 

During the following hours, effusive activity continued both at the vents at 2850 m elevation and from the new vent, which lay at about 2800 m elevation. Starting from 02:00 GMT, a gradual resumption of Strombolian activity at the NSEC was observed, which by 03:40 had become virtually continuous, even though the jets of incandescent lava did not rise more than 100 m above the crater rim. As usual, this was accompanied by an increase in the volcanic tremor amplitude. Observations were rendered more and more difficult by cloud cover in the summit area, which from 04:00 on entirely veiled the NSEC; yet, at 04:40 GMT, the clouds were suddenly illuminated by an intense glow, marking the start of the lava fountaining phase of this new paroxysmal episode. At 04:45, a large lava flow had started to descend below the cloud cover, taking the same path as its predecessors on the western slope of the Valle del Bove. An ash cloud, invisible to the visual observation systems and eyewitnesses, was blown northward by the wind, leading to ash falls in the area between Randazzo and Linguaglossa, at Patti on the Tyrrhenian (northern) coast of Sicily, and even as far as Lipari, more than 80 km to the north.

Fig. 14. View from Santa Venerina, on the southeastern flank of Etna, of two lava flows fed from the eruptive vents at about 2800 m (below the Belvedere, at left) and at about 2850 m (north of Belvedere, at right), on the evening of 21 February 2013. Photo taken by Boris Behncke, INGV-Osservatorio Etneo

At about 07:15 GMT, the volcanic tremor amplitude started decreasing, and the emission of vapor puffs from the Bocca Nuova became less conspicuous. Contempranelusly, the emission of lava from the two effusive vents at the southeastern base of the NSEC cone and below the Belvedere showed a diminution, and completely ceased during the afternoon of 22 February, when the images transmitted by the thermal camera of Monte Cagliato showed that the whole lava field on the western slope of the Valle del Bove was cooling and there were no thermal anomalies indicating active lava.

Fig. 16. One of the more energetic vapor puffs emitted from the Bocca Nuova during the episode of mild eruptive activity on the morning of 22 February 2013, seen from Tremestieri Etneo, 30 km south of Etna's summit. Photos taken between 06:47 and06:48 GMT on 22 February 2013 by Boris Behncke, INGV-Osservatorio Etneo