The 9 February 2012 paroxysm of Etna

Figure 1. Lava fountain and lava flow seen from near Santa Venerina (ESE flank of Etna) during the eruptive episode at the New Southeast Crater on 9 February 2012. Photo taken at 03:33 GMT (local time -1) by Boris Behncke, INGV-Osservatorio Etneo (Catania)

The 9 February 2012 paroxysmal eruptive episode at Etna's New Southeast Crater

Following a "prelude" of Strombolian activity lasting more than 12 days, the New Southeast Crater (New SEC) of Etna has produced a new paroxysmal eruptive episode on the early morning of 9 February 2012 (the 20th of the series initiated on 12 January 2011). This episode was less intense than its predecessors but produced spectacular lava fountains and a lava flow that advanced toward the Valle del Bove (Figure 1).

Figure 2. Eruption column and umbrella cloud illuminated by the nearly full moon and by the glow of the lava fountain at the New Southeast Crater at 01:43 GMT on 9 February 2012. The upper portion of the cloud is expanding toward west (left). Photo taken near Monte Gorna, close to the village of Trecastagni by Boris Behncke, INGV-Osservatorio Etneo (Catania)

During the afternoon of 8 February 2012, the Strombolian activity that had started on 27 January, began to increase both in the frequency and intensity of the explosions; this was accompanied by a rapid augmentation in the volcanic tremor amplitude. The source of the Strombolian activity was a vent in the western portion of the crater floor; occasionally explosions occurred also from a second vent immediately to the east of the former. Shortly after 19:00 GMT, lava slowly began to spill into the deep breach in the southeastern crater rim, then descended toward the base of the cone, and by 21:00 the flow began to expand on the gently sloping terrain at the base of the cone. Over the next few hours, the Strombolian activity very slowly increased in intensity, and then gradually passed into discontinuous lava fountaining between 23:30 and 00:00 GMT. Subsequently, a pulsating lava fountain rose to 100-150 m above the crater rim until about 00:30 GMT, at which time the fountain had become continuous and was jetting to 300-400 m above the crater. The eruption column, which contained considerably inferior quantities of tephra compared to the previous paroxysmal episodes, rose about 6 km above the summit of Etna and then spread out into a broad umbrella cloud, which slowly drifted westward (Figure 2). For these reasons - the notably less voluminous tephra production and the almost complete absence of wind - pyroclastic fallout was limited mostly to the upper portions of the mountain.

Between 02:00 and 04:00 GMT, the activity continued at stable levels, with lava fountains rising from the two vents within the crater, and occasionally rising higher than 500 m. The fountain from the more easterly vent was often inclined eastward, causing heavy fallout of pyroclastic material onto the crater rim and the flank of the cone in that direction. A third vent lying in the breach cutting the southeastern crater rim was the source of sporadic, violent explosions that launched bombs up to many meters in diameter in huge concentric bursts all over the eastern portion of the cone.

The lava flow reached the western rim of the Valle del Bove shortly before 01:30 GMT and began its descent on the steep slope below, at times interacting explosively with the snow covering the terrain. Once the flow passed the slope break at the base at the base of the steep wall, it slowed its advance and spread out into three smaller branches on the gently sloping terrain. The lava followed virtually the same path as the lava flows emitted during the preceding paroxysmal episodes, stagnating at a distance of less than 3 km from the New SEC.

Figure 3. After the cessation of lava fountaining, the New Southeast Crater produced a long series of powerful explosions caused by the detonation of huge lava bubbles on top of the magma column in the conduit. These explosions went on for several hours and were accompanied by loud bangs heard in much of the Etnean area. Photo taken around 05:50 GMT from near the village of Milo by Boris Behncke, INGV-Osservatorio Etneo (Catania)

The end of this eruptive episode was rather gradual, and waning activity continued for several hours. Lava fountaining started to diminish in intensity and become discontinuous around 05:30 GMT. 15 minutes later, the activity consisted of sporadic jets, some of which, however, rose higher than 300 m. At the same time, the vent sitting in the breach in the southeastern crater rim was the source of very violent explosions produced by the detonation of magma bubbles within the conduit; each explosion produced a huge expanding sphere of countless bombs up to many meters in diameter, as seen in Figure 3. These explosions continued after the cessation of lava fountaining, generating loud bangs that were heard over much of the Etna area and small ash emissions; the crater eventually fell silent around 09:00 GMT.

During the final phase of the paroxysmal episode, frequent small emissions of diluted ash occurred from the western portion of the Bocca Nuova, until about 10:00 GMT.

This eruptive episode has taken place 35 days after the previous paroxysm, on 5 January 2012, and was considerably less violent. The quantity of tephra emitted was much less than the amounts produced during the preceding paroxysms; furthermore this time no eruptive vents opened outside the crater. In terms of duration, however, the paroxysmal phase of this episode was one of the longest in the current eruptive sequence. The pyroclastic cone surrounding the New SEC grew further in height, mostly on the northeastern crater rim. With this event, the number of eruptive episodes in the sequence initiated in January 2011 has risen to 20, a number similar to those of earlier sequences of eruptive episodes such as at the Northeast Crater in 1977-1978 (23 episodes) or at the Southeast Crater in 1998-1999 (22 episodes).