The 24 April 2012 paroxysm of Etna

Figure 1. Strombolian activity from three vents - two on the eruptive fissure on the southeast flank of the New Southeast Crater cone, and a third within the crater - and active lava flow at 18:20 GMT (= local time -2) on 23 April 2012, seen from Piano del Vescovo on the southeast flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

During the early morning hours of 24 April 2012, the new Southeast Crater (New SEC) of Etna produced yet another episode of lava fountaining (paroxysm), the 25th such event in the current eruptive sequence that started in January 2011, and the 7th in the year 2012 (Figure 1). As its predecessors, this episode was characterized by tall lava fountains, a column of tephra and vapor several kilometers high, with heavy falls of ash and lapilli in the northeastern sector of the volcano, and lava flows that descended into the Valle del Bove.

Figura 2. Emission of a small lava flow from a vent on the southeast flank of the New Southeast Crater cone on the early morning of 23 April 2012, seen from Trecastagni on the southeast flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

The first clear signs that the New SEC was reawakening after the latest paroxysm, on 12 April 2012, were observed on the morning of 21 April, when the system of surveillance cameras of the INGV-Osservatorio Etneo revealed the beginning of a series of small ash emissions. The strong wind blew the ash puffs eastward, pushing them downslope, without allowing them to rise above the crater rim. These emissions continued intermittently for the next 24 hours; on the morning of 22 April, there were also prolonged emissions of white vapor from a spot in the upper portion of the eruptive fissure that cuts the southeastern flank of the cone. After nightfall, several glowing spots were visible on the northeastern and southern crater rims, where hot gas heated the surrounding rocks to incandescence. During the first few hours of 23 April 2012, the surveillance camera of Monte Cagliato (east flank of Etna) began to show a small thermal anomaly in the area of the New SEC. Direct observations made by INGV-Osservatorio Etneo staff revealed that this anomaly was caused by a tiny lava flow (Figure 2), which issued from the same vent that had produced the prolonged vapor emissions on the previous day, on the fissure cutting the southeast flank of the cone. The lava flow slowly advanced a few hundred meters toward the Valle del Bove, but stagnated on its western rim without descending the steep western slope. Around 08:00 GMT (= local time -2), lava emission temporarily ceased, but slowly resumed during the afternoon, and from 15:30 GMT on, a new lava flow began to descend on top of the earlier flow.

Around 17:00 GMT, the effusive vent started to display vigorous spattering; shortly thereafter, a second vent became active a few tens of meters further upslope, which initially produced spattering as well, but rapidly the activity grew into frequent Strombolian explosions. This was accompanied by a slow rise in the volcanic tremor amplitude. During the following hours, sporadic explosions were also observed from a vent located within the New SEC; for a long time the activity remained more or less stationary, while the volcanic tremor amplitude fluctuated at only slightly elevated levels. Finally, at around 01:10 GMT on 24 April, the intensity of the Strombolian activity began to show a marked increase and passed into sustained lava fountaining at 01:30 GMT. Contemporaneously, an eruption column heavily charged with ash and lapilli rose a few kilometers above the summit. Its lower portion was blown by the wind toward northeast, resulting in heavy ash and lapilli falls in the area of Linguaglossa-Piedimonte-Presa. The upper portion of the eruption column, however, was blown into a more easterly direction, leading to the fall of pea-sized lapilli in the area between Fornazzo and Giarre.

Figura 3. Start of lava fountaining, shortly after 01:00 GMT on 24 April 2012, seen from Monte Fontane on the east flank of Etna. Note how the lava flow has not yet started to spill down the steep western slope of the Valle del Bove. Photo taken by Stefano Branca, INGV-Osservatorio Etneo (Catania)

The phase of most intense lava fountaining lasted approximately 25 minutes, from 01:40 until 02:05 GMT on 24 April, after which the activity rapidly decreased, passing back into Strombolian activity and ceasing completely around 02:40 GMT. During the culminating phase of the paroxysm, a small lava flow was also emitted from the northern flank of the cone; this flow reached a length of a few hundred meters and descended into the direction of the May 2008 eruptive fissure.

This latest eruptive episode occurred 11.5 after its predecessor, and thus after an interval nearly as long as that between the episodes of 1 and 12 April. The prelude to the main phase of lava fountaining was somewhat longer, and the waxing of the activity was slower; the most peculiar detail was that the first significant activity of this episode was the quiet and slow extrusion of a lava flow without being accompanied by any explosive activity on the early morning of 23 April. This resembles the paroxysmal episodes of 2000-2001 at the old Southeast Crater, most of which started with quiet lava outflow from vents on the flanks of the cone. In contrast, the main lava fountaining phase of 24 April 2012 was a repetition in virtually all details of the preceding paroxysms in the current series.

Figura 4. The phase of intense lava fountaining in the early morning hours of 24 April 2012, seen from Zafferana Etnea, on the southeast flank of the volcano. Photo taken by Marco Neri, INGV-Osservatorio Etneo (Catania)