Figura 3. Heavily tephra-laden eruption column several kilometers high seen from Trecastagni (southeast flank of Etna) at 08:25 GMT on 4 March 2012. At the bottom center of the image is the lava flow descending on the western slope of the Valle del Bove. Photo taken by Catherine Lemercier

After the lava fountaining episode of 9 February 2012, Etna remained quiescent for one week. On the morning of 16 February, small ash emissions resumed from the New SEC, and for 18 days, weak, sporadic Strombolian activity continued on the crater floor. Occasionally, faint glow was observed at night; there was also a conspicuous increase in the number of sources and in the volume of fumarolic emissions along the southern rim of the crater. During the last few days of February, this activity was accompanied by an increase of the explosive activity within the conduit of the Northeast Crater, producing loud bangs, which were well audible all over Etna's summit area. The volcanic unrest during the second half of February was accompanied by more accentuated fluctuations of the volcanic tremor.

During the early morning hours of 4 March 2012, the volcanic tremor amplitude showed a rapid increase; at the same time, the Strombolian explosions within the crater became more frequent and more intense. Shortly after 06:00 GMT (local time -1), lava started to overflow through the deep breach that cuts the southeastern rim of the crater. The lava flow reached the southeastern base of the cone after about 15 minutes and from there advanced toward the western rim of the Valle del Bove. In the meantime, the explosive activity was continuously waxing, and passed into continuous lava fountaining with development of an eruption column about 07:30 GMT (Figures 2 and 3). The abundant fall of large-sized pyroclasts onto the steep flanks of the cone led to the formation of rock and dust avalanches; around 07:50 small pyroclastic flows were generated by the partial collapse of the eruption column. These flows descended mainly on the northeastern flank of the cone, and to some lesser degree on the south flank.

Also around 07:50 GMT, a lava flow was emitted from a new eruptive vent on the upper southwestern flank of the New SEC cone and started to descend in the saddle between the old and new SEC cones, interacting violently with thick snow covering the ground. This interaction provoked powerful explosions and small pyroclastic flows, the largest of which advanced rapidly across the flat terrain immediately to the east of the first eruptive fissure that opened on 17 July 2001 (Figures 4 and 5). Melting of the snow, in turn, led to the formation of a lahar, which descended toward the "Belvedere" monitoring station, on the western rim of the Valle del Bove, passing a few tens of meters to the north of the monitoring instruments.

Figure 6. Lava flow emitted from one or more vents on the northern flank of the New Southeast Crater cone, still advancing slowly after the cessation of explosive activity. This is the view from the Ripe della Naca area, on the east-northeast flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

During the phase of maximum intensity in the eruptive activity, a lava flow was also emitted from an eruptive fissure on the upper northern flank of the cone (Figure 6). This flow descended a few hundred meters toward northeast, surrounding the northern base of the cone. The main lava flow, which was fed across the breach in the southeastern rim of the crater, followed a nearly identical path to that of the lava flow emitted during the 9 February eruptive episode. After descending the steep western slope of the Valle del Bove, the flow split into several branches on the more gently sloping terrain at the base of the slope. These branches exceeded in length those of the 9 February flow, reaching a total distance of about 3.5 km from the crater (Figure 7).

The lava flow emitted from the fissure on the southwestern flank of the cone remained active for a few hours after the cessation of the paroxysmal activity, advancing slowly on the trace of the lahar that had occurred during the culminating events of 07:52 GMT.

The advance of the lava flows on thick snow cover was often accompanied by phreatic explosions, which generated violent jets of vapor and launched rock fragments to several tens of meters away; these phenomena were observed along the southern lava flow and along the main lava flow descending into the Valle del Bove.