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The 5-6 March 2013 paroxysm of Etna

Fig. 1. Lava fountain at the New Southeast Crater (NSEC) during the lava fountaining episode in the night of 5-6 March 2013, seen from Piano del Vescovo (6.9 km southeast of the NSEC). Note the rather small lava flow expanding on the southeastern flank of the NSEC cone. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

During the night of 5-6 March 2013, a new episode of lava fountaining (paroxysm) occurred at Etna's New Southeast Crater (NSEC), after a relatively calm interval of five-and-a-half days (Fig. 1). It was the seventh episode of this kind in little more than two weeks; furthermore, during the past few days, vigorous Strombolian activity continued within the Voragine (VOR), but with significant variations following the latest NSEC paroxysm. Many explosions of the VOR have launched incandescent pyroclastics up to 150-200 m above the crater rim (Fig. 2). During daylight on 5 March 2013, there were also small ash emissions from the VOR, producing small, dark gray puffs that were blown northeastward by the wind.

Fig. 2. Frames extracted from video recorded by the the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola (EMOH) during the interval from 28 February until 5 March 2013, showing the near-continuous Strombolian activity at the Voragine (VOR). The first frame, of 28 February, shows still incandescence at the NSEC, caused by the collapse of unstable portions of the still-hot deposit laid down during the lava fountaining episode on the morning of the same day, and glow from a small active lava flow at the southeast base of the NSEC cone.

Fig. 3. Aerial view of the lava flows on the steep western slope of the Valle del Bove, which were emitted during the series of paroxysms at the NSEC (visible at the top), of 19-28 February 2013. The crater at the bottom is Monte Centenari (produced during an eruption in 1852-1853). The lava flow farthest to the left is skirting the craggy rocks of the Serra Giannicola; the snow-covered cones forming the skyline to the left of the NSEC are those formed during the eruptions of 2001 and 2002-2003. Photo taken during a helicopter flight on the morning of 5 March 2013 by Boris Behncke, INGV-Osservatorio Etneo.

Aerial observations of 4 and 5 March 2013. On the late afternoon of 4 March and on the morning of 5 March 2013, two helicopter overflights (organized by the enterprise Scandinature Films) were made, which allowed to observe the activity of summit craters and the distribution of recent eruptive products (Fig. 3). The NSEC showed only weak gas emission (Fig. 4), mostly from fumaroles on the crater rims and from a spot in the "saddle" between the old and new cones of the SEC, whereas the eruptive vents within the crater and in the "saddle" were entirely inactive.

Fig. 4. The double cone of the Southeast Crater (SEC) seen from a helicopter looking north, on the afternoon of 4 March 2013 - at left, the old cone, whose summit vent last erupted on 6-7 May 2007, and at right, the new cone, which has entirely grown during the past two years. In the foreground are the lava flows that have issued from the "saddle" between the two cones during the paroxysm of 28 February 2013. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

The Bocca Nuova (BN) was inactive; the intracrater cone, which had been the source of numerous eruptive episodes since July 2011, was only weakly degassing, whereas the VOR produced frequent Strombolian explosions, with jets of volcanic bombs that rose up to 100-150 m above the crater rim. Finally, the Northeast Crater (NEC) emitted a dense column of gas, often with more energetic puffs. Fig. 5 shows a panoramic view of the summit craters with the BN in the foreground, and the VOR behind it, mostly hidden by a dense gas cloud, and in the background the NEC with its gas column.

Fig. 5. The Bocca Nuova (in the foreground), the VOR (largely obscured by the gas plume in the center of the image) and, in the background, the Northeast Crater, seen from a helicopter in the afternoon of 4 March 2013. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Fig. 6. (a) Resumption of Strombolian activity at the NSEC after 4.5 days of relative calm, at 17:54 GMT (=local time -1) on 5 March 2013; (b) simultaneous Strombolian activity at the VOR (center) and at the NSEC (at right) at 21:32 GMT on 5 March 2013. Frames extracted from video recorded by the the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola (EMOH).

5-6 March 2013 eruptive episode at the New Southeast Crater. At 17:54 GMT (=local time -1) on 5 March 2013, a vent in the western part of the NSEC produced an explosion, which launched incandescent bombs to several tens of meters above the crater rim (Fig. 6a). This explosion was followed by similar events, initially separated by quiescent intervals of 15-20 minutes, but then gradually augmenting in frequency. At the same time, Strombolian activity continued at the VOR (Fig. 6b). As the evening went on, the activity at the NSEC progressively intensified; likewise, the volcanic tremor amplitude started to rise. Around 23:12 GMT, lava started to flow from the "saddle" between the two SEC cones; a few minutes later, an eruptive fissure opened in the lower portion of the "saddle", with several vents that were the source of vigorous spattering and low lava fountaining. Between 23:17 and 23:18 GMT, the lava jets became continuous, forming a fountain that rose 200-300 m above the crater rim; this lava fountaining activity abruptly ceased at 23:25, but resumed, in a much more decided and energetic way, at 23:27. Various vents were active within the NSEC, in the "saddle" between the two SEC cones, and at the base of the "saddle", from where a voluminous lava flow started to expand toward south and southeast. Around midnight (GMT), this lava flow had reached the area of Belvedere, which had already been invaded by lava flows during the paroxysms of 21 February (morning) and of 28 February 2013. Another, rather scarcely fed lava flow issued through the breach in the southeastern rim of the NSEC (Fig. 1).

Fig. 7. Lava fountain and eruption column seen from the town of Linguaglossa, on the northeastern flank of Etna, during the eruptive episode of 5-6 March 2013. At the time this photo was taken, Linguaglossa was under a heavy shower of scoriae with diameters of up to several centimeters. Photo taken by Massimo Lo Giudice and published here with kind permission of the author.

For the next 30 minutes, lava fountaining continued with jets varying strongly in height, often rising 600-700 m and sometimes more than 800 m above the crater. An eruption column heavily charged with pyroclastic material rose several kilometers above the summit of Etna (Fig. 7), and was then bent by the wind toward northeast. Fallout of coarse-grained pyroclastic material, largely incandescent, was rather intense on the upper northeast flank of Etna, covering the area of the Valle del Leone, Pizzi Deneri and Serra delle Concazze with incandescent bombs and scoriae. Further downslope, at Linguaglossa, scorie with diameters of several centimeters fell; and ash and lapilli falls were also observed at Piedimonte Etneo, Fiumefreddo, Taormina and other towns along the Messinian Ionian coast. Fig. 8 shows the scoria deposit in the area of the Monti Sartorius, on the northeastern flank of Etna, about 5 km distant from the NSEC.

Fig. 8. Pyroclastic deposit (scoriae) on the northeastern flank of Etna, in the birch tree forest near the Monti Sartorius. This area, already affected by pyroclastic fallout on 23 February 2013, was buried under 5 cm of scoriae during the 5-6 March 2013 paroxysm. Individual fragments vary in size from 5 to 30 cm in this area. Photo taken on 8 March 2013 by Boris Behncke, INGV-Osservatorio Etneo.

Shortly after 00:00 GMT on 6 March, the lava fountains started to diminish in height, and within a few minutes, explosive activity nearly completely ceased, with only a few weak Strombolian bursts occurring for some time after 00:06. In the same interval, however, an eruptive vent opened on the lower east flank of the NSEC cone, which was the site of vigorous spattering, and from where a well-fed lava flow started to advance southeastward. This activity continued at a slowly decreasing rate for a few days and ceased altogether on 9 March.

Also at the VOR, Strombolian activity continued after the 5-6 March paroxysm, but now showing a rather peculiar oscillating behavior, with short episodes of intense Strombolian activity alternating with nearly totally quiet intervals. These oscillations were clearly visible also in the volcanic tremor amplitude, which during the 24 hours following the paroxysmal episode at the NSEC showed about twenty small peaks.