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Activity of the Sicilian volcanoes during January 2012

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Fig. 1. A spectacular view of the eruption column produced during Etna's paroxysmal eruptive episode of 5 January 2012, seen from Palagonia on the margin of the Monti Iblei, about 60 km SSW of Etna. Photo taken by Giuseppe Salerno, INGV-Osservatorio Etneo (Catania)

ETNA

During the month of January 2012, the New Southeast Crater (New SEC) has produced a new paroxysmal eruptive episode on the 5th, following a quiescent interval of 50 days. This episode produced an eruption column several kilometers high (Fig. 1) and falls of ash and lapilli in the southwestern sector of the volcano. Partial collapse of the eruption column and heavy pyroclastic fallout onto the flanks of the cone furthermore generated small pyroclastic flows, whose interaction with snow cover on the ground led to the formation of small lahars (mud flows). From 27 January on, the New SEC has been the site of renewed, sporadic explosive activity characterized by small ash emissions and weak Strombolian explosions, without, however, evolving into a new paroxysmal eruptive episode.

Throughout the month, the usual deep-seated explosions within the conduit of the Northeast Crater have continued, though with notable fluctuations in their frequency and intensity; this has been accompanied by strong degassing and rumbling sounds audible from the crater rim. There have also been strong degassing events within the conduit of the Bocca Nuova, the most significant on the evening of 2 January, but none of these produced any significant visible phenomena.

All times are in GMT (= local time -1).

5 January 2012 paroxysmal eruptive episode

On the morning of 4 January 2012, the visual surveillance systems of the INGV-Osservatorio Etneo showed the first signs of renewed explosive activity at the New SEC, which were accompanied by small infrasonic signals recorded by the station EBELO, located about 0.9 km southeast of the crater. On the late evening, intermittent glow was observed at the New SEC, a clear sign of Strombolian activity. This activity, which came from a single vent within the crater, continued without significant variations until 02:45 on 5 January, when a small lava flow started to spill slowly through the deep breach in the southeastern crater rim. After reaching the base of the New SEC cone, the flow took the same path as the flows emitted during the previous paroxysmal episodes, toward the western rim of the Valle del Bove.

Between 03:00 and 04:45, the Strombolian activity within the New SEC began to show a gradual increase both in the frequency and intensity of the explosions, and eventually passed into pulsating lava fountaining with incandescent jets rising 100-150 m high. From 04:50 onward, the ash content in the fountain increased; at the same time the flanks of the cone were subjected to heavy bombardment by countless glowing lava fragments. By 05:15, lava fountaining was continuous and sustained, and a dense eruption column charged with ash and vapor rose into the sky, reaching a height of 7000-8000 m above the sea-level at about 06:00.

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Fig. 2. Dark, steaming pyroclastic flow deposits on the northeastern flank of the New Southeast Crater cone extending toward the eruptive fissure of 13 May 2008 (marked by a broken red line). These deposits were emplaced during the culminating phase of the 5 January 2012 paroxysmal episode, between 06:20 and 06:40 GMT. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

Shortly before 06:00, two or three eruptive vents became active in the upper portion of a fracture that had already given lava fountains and small flows during some of the eruptive episodes of September-October 2011. From the lowest and northernmost vent, a small lava flow advanced across the gently sloping area fo the north of the New SEC cone. In the meantime, the abundant fallout of incandescent pyroclastic material onto the entire cone led to the formation of small pyroclastic flows. The phenomenon was partly caused by the partial collapse of the eruption column, but also by the violent interaction of the hot fallout with the thick snow cover on the flanks and at the base of the cone. The pyroclastic flows advanced for a few hundred meters, repeatedly generating small lahars (mudflows) due to rapid snowmelt, mainly in the northern and eastern sectors of the cone.

A powerful explosion from a vent on the upper southeastern flank of the New SEC cone at 06:20 marked the beginning of the most intense phase of the paroxysm. This explosion generated a dense column of ash and ripped open a large vent, destroying a part of the crater rim. During the following interval, pyroclastic flows on the northeastern flank reached lenghts of several hundred meters and nearly reached the central portion of the 13 May 2008 eruptive fissure (Fig. 2). Several small lahars were produced in this phase, which, however, did not travel much beyond the area covered by subsequent pyroclastic flows.

At the acme of the paroxysm, at 06:30, an explosive ash emission occurred at the Bocca Nuova, which produced a dark puff of ash and was followed by numerous smaller emissions of gas and dilute ash. Shortly after 06:50, the activity at the New SEC began to show signs of diminishing, although the noise level rose considerably due to violent escape of gas, as often happens toward the end of a paroxysm. After a final loud roar at 06:57, lava fountaining stopped abruptly, and was followed by passive ash emission until the total cessation of all activity at 07:30, whereas intermittent ash emissions from the Bocca Nuova continued until 08:30.

The lava flow emitted through the breach in the southeastern crater rim descended toward the Valle del Bove in several branches, immediately to the north of Serra Giannicola, reaching a length of little more than 2 km. This is one of the smallest lava volumes emitted during the series of eruptive episodes in 2011-2012. A small lava flow was furthermore emitted from the fissure on the north flank of the cone, and advanced a few hundred meters, stopping before reaching the upper portion of the 13 May 2008 eruptive fissure. For several tens of minutes after the end of the paroxysm, the entire northern flank of the New SEC cone showed slow gravitational sliding, involving the abundant freshly fallen and still plastically deformable pyroclastic material. Strong gas emission occurred from the moving material, which, however, did not evolve into a rheomorphic lava flow.

The fall of ash and lapilli affected a broad sector on the southwestern flank of Etna, including the towns of Ragalna, Belpasso, Paternò, and Biancavilla. Fine ash was blown by the wind to the south of Sicily, near the town of Vittoria.

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Fig. 3. The new Southeast Crater seen from the east flank of the old Southeast Crater cone, on 20 January 2012. The highest point of the new cone, at right, has grown in elevation during the 5 January 2012 paroxysm. Photo taken by Robin Campion and published here with kind permission by the author

During the 5 January 2012 paroxysm, the New SEC cone grew by several meters in height, mostly on its southwestern crater rim (Fig. 3). The crater is elongate along a NW-SE orientation, due to the presence of several eruptive vents on the crater floor that are aligned along a fracture extending into the fissures on the southeast and north flanks of the cone. For several days after the paroxysm, unstable portions of the unstable crater rims - most notably around the vent that was blasted open near the southeastern crater rim at 06:20 on 5 January - continued to collapse.

Between the morning of 5 January and the forenoon of 27 January, the New SEC showed only modest degassing from various fumaroles located mostly along the northern and southern crater rims. At 11:34 on 27 January, an explosion occurred from a vent in the western portion of the crater floor, which generated a puff of ash a few tens of meters high. During the following hours, further explosions occurred, which were of lesser energy than the first one, and on the evening, at 21:40, an explosion launched incandescent pyroclastics a few tens of meters above the crater rim. This activity continued in a sporadic manner on 28 and 29 January, with ash puffs visible during the day, and small Strombolian explosions visible at night. Deteriorating weather conditions early on 30 January precluded any further visual observation for the next few days. The activity was not accompanied by any significant variation in the volcanic tremor amplitude, and as of early February has not evolved into a new paroxysmal eruptive episode.

 

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 January 2012, the activity continued at fluctuating levels at the two active vent areas, in the northern and southern sectors of the crater terrace, and did not lead to the formation of any lava flows.

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