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Paroxysm at Etna's New Southeast Crater, 23 February 2013

Fig. 1. Lava fountain at the New Southeast Crater and ash cloud drifting northeastward during the paroxysmal eruptive episode on the evening of 23 February 2013. Photo taken from Santa Venerina, on the southeastern slope of Etna, at 18:54 GMT (=local time -1) by Boris Behncke, INGV-Osservatorio Etneo

On the evening of 23 February 2013, about two-and-a-half days after the previous lava fountaining episode, the New Southeast Crater (NSEC) of Etna was the site of a new paroxysm, the fifth in as many days, and one of the most violent observed in recent years (Fig. 1). The eruptive cloud containing abundant quantities of ash and scoriae was blown by strong wind toward northeast, causing heavy fallout of scoriae in the Linguaglossa area and along the Ionian coast to Messina; ash falls were also reported from Calabria.

The "prelude" to this paroxysm started in the afternoon hours of 23 February, when the volcanic tremor amplitude showed a gradual increase, which was most pronounced at the Belvedere seismic station (EBEL). About 15:25 GMT (=local timei -1), the thermal monitoring camera at Monte Cagliato, on the eastern flank of Etna, started to record small thermal anomalies within the NSEC (Fig. 2, left). At 15:40 GMT, another small thermal anomaly appeared in the area of the effusive vent located at the southeastern base of the NSEC cone at about 2850 m elevation, near the monitoring station of Belvedere, evidence for renewed, slow emission of lava from this vent (Fig. 2, right).

Fig. 2. Resumption of eruptive activity at the NSEC on the afternoon of 23 February 2013, documented in these frames extracted from video footage of the thermal monitoring camera of the INGV-Osservatorio Etneo at Monte Cagliato, on the eastern flank of Etna. The image at left shows one of the first thermal anomalies observed at the NSEC; the right frame shows the appearance of a second thermal anomaly at the effusive vent at ~2850 m elevation ("2850 vent"), close to Belvedere.

Fig. 3. Strombolian activity at the NSEC (at upper left), and emission of lava from an effusive vent at about 2850 m elevation, at the southeastern base of the cone (at right). Photo taken from Tremestieri Etneo, on the south flank of Etna, at 17:15 GMT by Boris Behncke, INGV-Osservatorio Etneo

At nightfall, the Strombolian activity and the small lava flow, which slowly expanded downslope from the vent at 2850 m, were well visible from the populated areas in the southern and eastern sectors of the volcano (Fig. 3), even though visibility was partly hampered by weather clouds in the summit area. During the time interval between 18:00 and 18:30 GMT, the activity progressively intensified, growing from Strombolian explosions to low lava fountains from several active vents within the crater. The fountains wrere initially of low energy and pulsating, rising about 100-150 m above the crater rim; a well-fed lava flow soon started to spill through the breach in the southeastern crater rim toward the western slope of the Valle del Bove. At 18:34, the lava fountaining activity started to increase rather rapidly. About five minutes later, jets of incandescent lava rose 500-600 m above the crater rim, sometimes exceeding 800 m in height.

The evolution of the lava fountain between 18:30 and 18:44 GMT is shown in Fig. 4. In that time interval, the emission of pyroclastic material increased dramatically, forming a dense plume directed northeastward (Fig. 1); the entire northeastern flank from the NSEC to Pizzi Deneri and beyond (Fig. 5), was covered with a sheet of incandescent material. Eyewitnesses also reported the fall of large clasts, some of still incandescent, in the area of Monte Baracca, at about 1700 m elevation and ~5 km northeast of the NCSE.

Fig. 4. Video footage recorded by the mobile visibile-light monitoring camera of the INGV-OE on the Schiena dell'Asino show how, in the lapse of less than 15 minutes, between 18:30 and 18:44 GMT on 23 February 2013, the eruptive activity at the NSEC passed from pulsating lava fountains 100-150 m tall into a sustained jet 500-600 m high, and occasionally exceeding 800 m in height.

Fig. 5. A dramatic view of the Pizzi Deneri area subjected to a heavy downpour of incandescent bombs at the height of the 23 February 2013 paroxysm at the NSEC, seen from the Rifugio Citelli, on Etna's northeast flank. Photo taken by Daniele Pennisi and published here with kind permission of the author.

For about 35 minutes, high lava fountaining continued without substantial variations. At 19:14 GMT, the height of the lava fountains started to diminish rapidly, and two minutes later, the activity had decreased to Strombolian explosions that launched incandescent pyroclastics up to 100 m above the crater rim, accompanied by volcanic ash emission; at 19:30 all explosive activity was essentially over. At this time, the lava flow that had exited the NSEC through the breach in the southeastern crater rim had just reached the western rim of the Valle del Bove, and started to descend rather slowly on the steep slope. Another lava flow, presumably rheomorphic (that is, generated by the abundant fall of still fluid spatter) had descended about 100 m on the northern flank of the NSEC cone (Fig. 5, left). Lava was also emitted from the eruptive fissure located in the "saddle" between the two cones of the SEC, but due to cloud cover, little detail of the activity there could be observed.

Fig. 6. Images recorded by the thermal monitoring camera of the INGV-OE at Monte Cagliato, on the east flank of Etna, after the conclusion of the 23 February 2013 paroxysmal eruptive episode (left), and on the ensuing morning (right). The image at left shows the main lava flow emitted during the paroxysm and slowly descending the western slope of the Valle del Bove; this image further shows, a bit to the right, a small rheomorphic lava flow descending the northern flank of the NSEC cone, which during the paroxysm was subjected to heavy pyroclastic fallout. Still further to the right, an elongate, low-intensity thermal anomaly marks the pyroclastic fall deposit extending northeastward. The image at right shows how the main lava flow active in the previous frame has not expanded much further downslope during the intervening 7 hours, but is still fed, probably by the eruptive vent at 2850 m elevation near Belvedere.

During the night, the lava flow emitted from the NSEC toward southeast continued its rather slow descent toward the Valle del Bove, but stagnating before it had even traveled half way down the slope; yet the images recorded by the thermal monitoring cameras at Monte Cagliato and on the Schiena dell'Asino (mobile), show that the lava flow was still being fed on the morning of 24 February, probably by one or more vents near Belvedere, at ~2850 m elevation. In Fig. 6, two scenes recorded by the thermal monitoring camera of Monte Cagliato are seen, a few hours after the conclusion of the paroxysm, and 10 hours after the episode.

The 23 February 2013 paroxysmal episode has been one of the most energetic events of this type in recent years; this is well evident from the ratio of pyroclastic material to lava emitted in this episode. The lava volume is the smallest of all episodes from the NSEC in the period 2011-2013. In contrast, the fall of pyroclastic material has been rather abundant, aided also by the strong wind, which concentrated the tephra fall in a quite narrow sector. Within this sector, the pyroclastic fall material was unusually coarse-grained; near Linguaglossa, at about 15 km from the crater, many clasts were up to 15 cm in diameter; in this area, the scoria deposit was continuous (Fig. 7).

Fig. 7. Pyroclastic fall (scoria) deposit in the area of "Sciaramanica", near Linguaglossa on the northeast flank of Etna, about 14 km from the NSEC. Photos were taken on the morning of 24 February 2013 by Enzo Sanfilippo and are published here with kind permission of the author.