Figure 2. Initial phase of the lava overflow through the deep breach in the southeastern rim of the New Southeast Crater, shortly after 06:00 GMT on 18 March 2012. Photo taken from Trecastagni by Boris Behncke, INGV-Osservatorio Etneo (Catania)

Figure 3. Eruption column rising 4-5 km above the summit of Etna, seen from near the harbor of Catania, about 27 km south of the erupting New Southeast Crater. Photo taken around 10:00 GMT on 18 March 2012 by Elisabetta Ferrera, University of Catania

After a quiescent interval of exactly two weeks following the episode of lava fountaining (paroxysm) of 4 March 2012, the New Southeast Crater (New SEC) of Etna has been the site of yet another episode on the morning of 18 March 2012. This was the 22nd episode in the current series that began in January 2011. As during the previous episodes, also this one was characterized by tall lava fountains, a tephra and vapor colum several kilometers high (Figures 1 and 3) that led to ash falls in the eastern sector of the volcano, and lava flows that descended into the Valle del Bove, locally showing violent explosive interaction with thick snow on the ground.

The first indicators of an imminent reawakening of the New SEC were observed during a field survey in the summit area of Etna made by INGV-Osservatorio Etneo staff on the morning of 16 March 2012, when the crater emitted loud roaring noises caused by high-pressure degassing, but no visible phenomena were noted. These emissions occurred at intervals of a few minutes and increased in frequency during the summit visit. On the next day, the New SEC emitted repeated puffs of vapor with minor quantities of diluted ash; after sunset, weak glow emanated from within the crater. During the first hours of 18 March, the glows became more intense, evidence for Strombolian activity on the crater floor that was gradually intensifying. As in the earlier episodes, the volcanic tremor amplitude showed a rapid increase in this phase.

During the following hours, the Strombolian activity progressively intensified, and shortly before 06:00 GMT (= local time -1), lava began to slowly overflow through the deep breach in the southeastern crater rim (Figure 2). At around 07:25 GMT, the ash content in the gas plume rising from the crater became more significant, whereas from a vent on the crater floor pulsating lava fountains rose about 100 m high. Shortly before 08:00 GMT, two vents were active within the crater, whereas a third jet of fluid lava was emitted from a vent that lay within the breach in the southeastern crater rim, following the trend of a fissure that was repeatedly active during many of the paroxysms between August 2011 and January 2012 (Figure 4).

During the interval between 08:00 and 08:15 GMT, the lava fountaining activity became essentially continuous at all three vents. The intense downpour of coarse-grained pyroclastic material onto the northern and northeastern flanks of the cone generated avalanches and clouds of rock and dust, which descended to the base of the cone. The eruption column rapidly rose to a height of 4-5 km above the summit of the volcano (7-8 km above the sea-level) around 09:00 GMT, and then blown eastward by the wind (Figures 1 and 3). Ash and lapilli fell mainly in the area between the mountain villages of Zafferana Etnea and Sant'Alfio, extending toward the Ionian sea between Riposto and Pozzillo.

The main lava flow fed through the breach in the southeastern rim of the New SEC initially followed the same path as the flows emitted furing the preceding episodes, descending the steep western slope of the Valle del Bove. Several lava lobes, however, took a more northerly parcourse, invading areas covered with thick snow. Interaction between the lava and snow led to rapid melting of the latter, generating small lahars, and strong explosions that produced ground-hugging vapor and ash clouds resembling pyroclastic flows, which repeatedly descended on the floor of the Valle del Bove. The vapor and ash clouds generated by these explosions rose up to 1-1.5 km above the floor of the Valle del Bove (Figure 5). This phenomenon continued intermittently for some time after the cessation of the lava fountaining and ash emission, until about 10:30 GMT.

Figure 6. Lava flow-field still expanding on the ash-covered western slope of the Valle del Bove, after the end of the paroxysmal activity on 18 March 2012, seen from Santa Venerina, on the southeast flank of Etna. Photo taken at 10:50 GMT by Boris Behncke, INGV-Osservatorio Etneo (Catania)

Lava fountaining and strong ash emission continued without significant variations until about 09:40 GMT; after this the activity rapdily diminished in intensity, and the last puffs of ash were observed around 10:10 GMT.

As in the previous episodes, the lava flow emitted from through the breach in the southeastern crater rim remained active for several hours after the cessation of the paroxysmal activity, advancing slowly in the upper part of the Valle del Bove (Figure 6). The most advanced lava fronts reached a distance of about 4 km from the source, stagnating south of Monte Centenari. A small alva flow was also emitted from a fracture on the northern flank of the cone; this flow follwed the same path as a flow emitted from the same fracture during the 4 March paroxysm, reaching a length of a few hundred meters. Differently from the 4 March paroxysm, the fracture on the southwest flank of the cone, which had been the site of violent explosive phenomena caused by magma-snow interaction, did not reactivate on 18 March.

This latest episode occurred precisely 14 days after its predecessor, the paroxysm of 4 March 2012, making the interval between these two events the shortest in the past 5 months. Once more, the pyroclastic cone of the New SEC has grown in height, mainly on the northern rim, but to some minor degree also on its southern rim (Figure 7).