Activity of the Sicilian volcanoes during May 2011
ETNA
During May 2011, the period of summit activity initiated at the end of December 2010 continued with a new paroxysmal episode, which took place during the early morning hours of 12 May, from the crater located on the eastern flank of the Southeast Crater (CSE) cone. Throughout the month, strong degassing took place from the Northeast Crater and from the Bocca Nuova, and the usual deep-seated explosive activity within the conduit of the Northeast Crater continued.
On the morning of 8 May, mild Strombolian activity started within the crater on the east flank of the SEC cone, continuing at low levels for the next three days and then increasing, both in the intensity and frequency of explosions, during the afternoon of 11 May.
From 17:00 h (local time = GMT+2) on 11 May, a progressive increase in the volcanic tremor amplitude was observed, which was accompanied by a progressive waxing in the intensity of the Strombolian activity. About 20:30, lava overflowed the eastern rim of the active crater, forming a narrow flow that headed toward the Valle del Bove. The activity intensified during the following hours and shortly before 04:00 on 12 May culminated with lava fountaining, which generated an ash cloud driven south-southeastward by the wind.
As early as 03:20, the lava fountain grew in height and became wider at its base, producing heavy fallout of incandescent lava fragments over a broad area on the flanks of the cone surrounding the crater, forming small rootless flows toward the base of the cone. Around 04:00 the fountain rose more than 200-300 m high, breaking up at its top and producing consistent emission of ash and lapilli, which rapidly formed an eruption column rising about 2-3 km above the summit of the volcano, before being bent south-southeastward by the wind. The culminating phase of the paroxysm lasted for about 1 hour, after which the eruption column was fed by a moderately intense lava fountain less than 300 m tall. The fountain then further diminished in height and became strongly pulsating; fountaining stopped shortly before 06:00.
During this period the lava flows fed by overflow from the crater and by agglutination of spatter deposited from the fountain reached their maximum extent. The emplacement of the lava field was well observed from the northern rim and eastern end of the Valle del Bove. The main lava flow split into two branches that travelled, at approximately identical speed, parallel to each other to the baser of the steep western slope of the Valle del Bove, where they coalesced into one single flow. The lava front reached an elevation of about 2000 m above the sea level, stopping little more than 2 km from the crater, and covering nearly exclusively lava flows emitted during the previous eruptive episodes.
Starting from 06:00, fallout of coarse- and fine-grained ash was reported from a number of locations on the middle and lower southern slope of the volcano (Nicolosi, Mascalucia, S. Giovanni Galermo, Catania), reaching the International Fontanarossa airport of Catania, and extending as far as the southern margin of the Catania Plain. The abundant fallout of ash in the area of the airport led to the interruption of air traffic and airport operations throughout the whole day of 12 May.
A field survey carried out on 13 May revealed the presence of an extensive field of impact craters surrounding the crater, formed by the downpour of meter-sized blocks during both the Strombolian and paroxysmal phases of the eruptive episode. Some of these craters were as far as 400 m away from the base of the SEC cone.
Near the source of the lava flow, on the eastern rim of the crater, several lava flow units were seen to be superimposed on each other, which surrounded the hornito formed at the end of the 2006 eruption. The scoria deposit laid down in proximity of the crater by the explosive activity continued to be hot and unstable on 13 May, leading to frequent small landslides.
A further field survey on 20 May allowed to appreciate the growth of the new cone surrounding the crater on the flank of the SEC cone, which was separated from the old SEC cone by a low saddle. Fumaroles on the crater rims continued to emit high-temperature gas.
As for a historical note, May 2011 marks the 40th birthday of the Southeast Crater (18 May 1971) and the 100th birthday of the Northeast Crater (27 May 1911).
STROMBOLI
Stromboli volcano is in a state of persistent eruptive activity, normally with explosions of medium to low intensity occurring from a number of vents located within the summit crater. During the month of April 2011, the activity continued at fluctuating levels, without, however, culminating in any major events. The activity was concentrated at the two active vent areas, in the northern and southern sectors of the crater terrace.