Etna update, 13 July 2011

The Bocca Nuova, at right, in full Strombolian activity, and the nearly full moon at upperleft, photographed during the field visit on the evening of 13 July 2011. Photo taken by Boris Behncke, INGV-Catania

Strombolian activity within the Bocca Nuova

On the evening of 11 July 2011, vivid Strombolian activity started within the Bocca Nuova, which at night produced a glow well visible with the naked eye from various population centers in the southern sector of Etna. A glow first appeared in footage of the Schiena dell’Asino (southeast flank) monitoring camera of the INGV-Catania about 21:00 GMT on 11 July. During the night of 12-13 July, the glow was more continuous and more intense than during the preceding night. At varying intervals, incandescent volcanic bombs were seen to be thrown above the crater rim; these products fell back into the crater.

On the evening of 13 July, INGV-Catania staff (Mauro Coltelli, Luigi Lodato e Boris Behncke) carried out a field visit to the Bocca Nuova, which revealed that on the crater floor there was a single large vent, which was the source of intense and continuous Strombolian activity. The strongest explosions often occurred in series of 2-5 events, launching incandescent bombs to several tens of meters above the crater rim. However, most of the bombs fell back into the crater, whereas a few bombs flew over the rocky septum that still divides the Bocca Nuova from the Voragine, to fall into the southern portion of the latter.

The current Strombolian activity is the first magmatic eruptive manifestation at the Bocca Nuova since the summer of 2002, when a brief period of weak Strombolian activity was observed within the crater. Since the summer of 2010, the Bocca Nuova has shown various signs of an imminent reawakening, in particular since 25 August, when a series of ash emissions began, which ended on 22 December 2010. Ash emissions resumed, more frequently, on 14 June 2011. Some of these emissions, during the first few days of July, contained hot material, and an explosion at 02:03 GMT on 6 July launced incandescent bombs or blocks about 100 m above the crater rim.

Born in 1968, the Bocca Nuova was initially a small pit crater about 8 m in diameter, which progressively enlarged during the following years, to reach a diameter of more than 300 m in the mid-1990s. Since the second half of the 1970s, Strombolian activity frequently occurred on the crater floor, sometimes accompanied by small intracrater lava flows. From the end of July 1995, intense eruptive activity occurred from two main vent areas, in the northwestern and southeastern parts of the Bocca Nuova. This activity led to the growth of pyroclastic cones on the crater floor, which already in the fall of 1997 had attained notable dimensions, and simultaneous emission of lava onto the crater floor left to the filling of the crater. Still, in the summer of 1998, the crater floor lay about 150 m below the lowest point of the crater rim, on the western side of the Bocca Nuova.

After a few months of relative quiet, the Bocca Nuova reawoke in early September 1999 with a series of paroxysmal episodes, which brought about the rapid filling of the crater depression to 60 m below the low western crater rim. On 5 October 1999, intense and almost continuous explosive activity started from multiple vents, and during the following days was accompanied by voluminous lava emission. These products completely filled the crater, and on the late afternoon of 17 October 1999, lava for the first time overflowed the western rim of the Bocca Nuova, generating a lava flow on the western flank of Etna, which advanced several kilometers. During the following two weeks, frequent paroxysmal episodes produced further overflows, with lava flows that on 27 October extended more than 4 km downslope, reaching the forest above the town of Bronte, and interrupting the forest service road.

This eruption lasted until 5 November 1999, after which two new pit craters formed in the two vent areas that had been present within the Bocca Nuova since 1995, to the northwest and to the southeast. The surrounding area, where the Bocca Nuova crater had been completely filled, constituted a broad platform. During the winter of 1999-2000, several episodes of intense Strombolian activity took place in these pits, especially in the southeastern one, but with the onset of a series of paroxysms at the Southeast Crater (from 26 January 2000), activity at the Bocca Nuova was limited to gas emission, which generated numerous gas rings. In the fall of 2000, Strombolian activity once more returned to the two pits within the Bocca Nuova, and continued until April 2001.

Following the July-August 2001 flank eruption, the Bocca Nuova started producing ash emissions in the spring of 2002, and in mid-June, for a short time weak Strombolian activity was observed in the northwestern pit. This was the last magmatic activity in the Bocca Nuova until July 2011.