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Etna update, 31 January 2013

Fig. 1. Lava fountain launching incandescent bombs and scoriae up to about 120 m above the rim of the Bocca Nuova during the phase of most intense activity of the eruptive episode on 30 January 2013, generating abundant fallout onto the southern flank of the central summit cone (in the center-right part of the image). The view is from Adrano, on the southwestern flank of Etna. Photo taken by Roberto Schillaci and published here with kind permission of the author (original photo on Flickr)

Etna: 5th eruptive episode at the Bocca Nuova. On the evening of 30 January 2013, after two days of relative calm, Etna's Bocca Nuova produced its 5th episode of intense Strombolian activity within the past 3 weeks. This episode was more energetic than its precedessor, with abundant coarse-grained pyroclastic material falling beyond the crater rim (Fig. 1).

The first signs of a resumption of eruptive activity at the Bocca Nuova were observed at 17:07 GMT (=local time -1) on 30 January, when the high-sensitivity visible surveillance camera of the INGV-OV on the Montagnola (EMOH) recorded weak glow (Fig. 2a) coming from the eruptive vent lying in the southeastern portion of the crater floor. The glow rapidly became stronger and was soon visible to the naked eye from population centers around the volcano; contemporaneously, the volcanic tremor amplitude recorded in the summit area showed a rapid rise. Furthermore, the source of the volcanic tremor was seen to shift from a position below the New Southeast Crater (NSEC) toward the Bocca Nuova, a phenomeon already observed during the 28 January eruptive episode.

Fig. 2. Frames extracted from video recorded by the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola  (EMOH), during the 30 January 2013 eruptive episode at the Bocca Nuova; all times are GMT (=local time -1). (a) 17:08, weak glows mark the beginning of Strombolian activity; (b) 18:24, besides the bright glow at the Bocca Nuova (center), a small bright spot can be seen also at the New Southeast Crater (at right); (c) 18:40, incandescent bombs are thrown more than 150 m above the crater rim; (d) 19:14, copious fallout of incandescent pyroclastics onto the southwestern flank of the central cone; (e) 20:00, lava jetting about 150 m above the crater rim; (f) 05:17 (31 January), one of the last ash puffs emitted at the conclusion of the activity at the Bocca Nuova.

Fig. 3. Lava fountain inclined southwestward at 19:36 GMT on 30 January 2013. Note the fallout of incancescent pyroclastics onto the southwestern external flank (at left) of the central cone. Photo taken from Tremestieri Etneo (south flank of Etna) by Boris Behncke (INGV-Osservatorio Etneo)

Between 18:00 and 18:15 GMT, the activity showed a considerable intensification, and from about 18:20 onward, the jets of incandescent volcanic bombs and scoriae nearly continuously rose higher than the crater rim. Some pyroclastic fragments were launched up to 150 m above the crater rim (Fig. 2c). At 18:24, a rather weak glow appeared at the NSEC (Fig. 2b); however, this phenomenon lasted only for a brief time and was not repeated throughout the eruptive episode.

During the interval from 18:30 until 19:00 GMT, the activity had the character of a lava fountain, with a sustained jet rising steadily to about 100 m above the crater rim. Shortly after 19:00 GMT, the jet became strongly inclined toward southwest, leading to heavy fallout of incandescent bombs and scoriae onto the outer southwestern flank of the central summit cone down to its base (Fig. 2d e 3). At 19:16 GMT, the jet returned to a more vertical direction, and pyroclastic fallout outside the crater diminished, affecting mostly the souther flank of the central cone.

Around 19:30 GMT, the lava fountaining activity started to diminish; the incandescent jets became discontinuous and only rarely rose more than 100 m above the crater rim, except for one jet, at 20:00 GMT, which exceeded 150 m in height above the rim of the Bocca Nuova (Fig. 2e).

In the same time interval, the volcanic tremor amplitude showed a rapid decrease, and returned to normal levels on the late evening. After 21:00 GMT, the jets of incandescent pyroclastics no longer rose above the crater rim, and the glow became progressively less brilliant. During the night hours, however, weak eruptive activity continued on the crater floor, as was evident from a dull glow coming from the crater. Finally, during the early morning hours of 31 January, the glow gradually faded away, and the episode ended with a series of sporadic, small ash emissions (Fig. 2f), the last of which was seen around 05:41 GMT.

This episode was the fifth event of this type at the Bocca Nuova during the three-week interval beginning on 10 January 2013; to these have to be added the two episodes of Strombolian activity at the NSEC on 20 and 22-23 January, bringing the sum of eruptive episodes in this period to seven. Even though these events are a far cry from the powerful paroxysmal episodes at the NSEC in 2011-2012, they mark a vigorous resumption of summit activity at Etna following several months of rather low levels of activity.