Activity of the Sicilian volcanoes during April 2011
ETNA
During the month of April 2011, the eruptive period initiated in late-December 2010 culminated, on 10 April, in a new paroxysmal eruptive episode from the pit crater located on the lower east flank of the Southeast Crater (SEC) 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 conduig of the Northeast Crater continued.
During the first week of the month, the small emissions of gas and ash from the interior of the pit crater on the east flank of the SEC cone continued. The plumes produced by these events rose a few tens of meters above the crater rim before being blown away by the wind. Video recorded by the Montagnola thermal monitoring camera (EMOT) showed the presence, in some of these emissions, of hot material (up to a few tens of degrees centigrade); a few of these events also produced small quantities of incandescent material.
On the morning of 8 April, weak Strombolian activity started from two small vents on the bottom of the pit crater. The erupted material exclusively fell back onto the crater floor surrounding the vents.
A gradual increase in the seismic activity produced by the Strombolian explosions during the forenoon of 9 April indicated that the intensity of the activity was waxing. From the early afternoon of that day, the activity intensified further, and lava began to issue from the flanks of the small cone that was forming around the vents, generating a lava flow which covered the crater floor before, around 18.00 h UTC (=local time -2), the first incandescent rocks began to spill over the lowest point of the crater rim, on its eastern side. A narrow lava flow slowly advanced on top of the lava of the previous paroxysm, on 18 February, into the direction of the Valle del Bove. This overflow continued, at a rather low effusion rate, throughout the night, and on the morning of 10 April, the front of the lava flow had advanced about 1.5 km, reaching an altitude of about 2500 above the sea level.
In the same interval, Strombolian activity within the pit crater grew both in intensity and explosion frequency, launching incandescent pyroclasts more than 100 m high, and often beyond the crater rims. Between 06.00 and 08.00 on 10 April, the frequency of the explosions rapidly increased, as evident from video recorded by the EMOT monitoring camera. Simultaneously, the volcanic tremor amplitude showed a strong increase. About 07.40, two vents were observed to produce explosive activity, and shortly after 08.00, the Strombolian activity blended into a continuous, though still small, lava fountain.
About one hour later, the lava fountain grew in height and width, producing heavy fallout of fluid spatter onto the flanks of the low cone building around the pit crater, forming small rheomorphic flows. 09.15 and 09.30, the fountain rose more than 200 m high, generating a sustained column of ash and vapor that rose about 2 km above the summit of the volcano before being bent southeastward by the wind.
After 11.30, the intensity of lava fountaining started to diminish, mostly decreasing in height but expanding laterally, and for prolonged periods two jets were distinctly visible. Lava fountaining continued at moderate intensity with jets still more than 300 m high, until the activity started to rapidly diminish around 13.10, and the height of the jets dropped below 100 m.
In this period (11.30-13.10), the expansion of the active lava flows was most accentuated; these flows formed both from agglutination of liquid spatter from the lava fountain and from lava overflowing the lowest (eastern) portion of the crater rim. Observation from various locations to the east and northeast clearly revealed how some of the advancing lava flows invaded, and mixed, with, thick snow on the steep western slope of the Valle del Bove, leading to frequent phreatomagmatic explosions, which in turn generated small pyroclastic flows. The main lava flow descended to about 1900 m elevation, reaching a length of little more than 2.5 km, mainly covering the lava flow of 18 February, but extending onto snow both to the north and to the south. This occurred mainly 11.40 and 12.30, when numerous phreatomagmatic explosions were observed, the most intense of which took place at 11.41 and 11.58.
After 13.10 lava fountaining rapidly diminished, and from 13.30 only discontinuous jets a few tens of meters high were observed, which ceased altogether at 14.03. During the last phase of the paroxysm, an eruptive vent opened at the base of the cone surrounding the pit crater and emitted a small lava flow, which advanced about 1 km to the south of the main lava flow. The main lava flow still showed active fronts around 17.00, three hours after the end of the activity, which were still well visible due to the detachment and movement of incandescent blocks, on the high slope of the Valle del Bove during the night of 10-11 April.
Starting on 14.00 on 10 April, fine ash and small lapilli started falling in numerous locations on the middle to lower southeast flank of Etna (Zafferana, Fleri, Viagrande, Trecastagni, Pedara, San Gregorio) and to as far as the Ionian coast (Acireale, Capomulini, Acitrezza, Acicastello). This tephra fall continued until about 16.30-17.00.
Fallout of scoriaceous lapilli and bombs affected a large portion of the Piano del Lago and the south flank of the Montagnola. Going westward from Belvedere, the deposit pinched out before Torre del Filosofo, confirming that the fallout was confined exclusively to the southeastern sector of the volcano, whereas the south, toward Nicolosi and Catania, was spared. At Belvedere, the tephra deposit was 6 cm thick but contained clasts up to 20 cm in diameter, most of which had broken into smaller fragments upon impact due to their highly vesicular and fragile nature.
No further eruptive activity was observed at the pit crater after the 10 April episode. On 12 April, the floor of the crater was seen to be flat and devoid of any volcanic activity; small fumaroles were present on the crater rims. At the end of the month, the crater had cooled sufficiently to support freshly fallen snow.
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.
As during the preceding month, the most active vent was N1, the northernmost vent in the northern crater area, which produced moderate to high-intensity jets of coarse-grained material (lapilli and bombs) occasionally mixed with ash. The height of these jets rarely exceeded 200 m, and the mean frequency of explosions varied between 2 and 4 events per hour.
In the southern crater area, two vents (S1 e S2) were active, launching mostly coarse-grained material up to 120 m high. The mean frequency of explosions varied from 1 to 6 events per hour.
VULCANO
Vulcano is currently in a state of quiescence. During the month of April 2011, temperatures measured in fumaroles on the crater rim remained stable. No anomalies were observed in the geochemical parameters of the peripheral areas (soils in the Vulcano Porto areas and thermal aquifers).