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The 5 January 2012 paroxysm of Etna

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Figure 1. Eruption column and lava fountain from the New Southeast Crater seen from an airplane passing to the northeast of Etna during the 5 January 2012 eruptive episode. Photo taken by Gloria Guglielmo and published here with the kind permission of the author (original photo on Flickr)

The 5 January 2012 paroxysmal eruptive episode at Etna's New Southeast Crater

Following a quiet interval of 50 days, the New Southeast Crater (New SEC) of Etna reactivated on the evening of 4 January 2012, and produced the first paroxysmal eruptive episode of the year (the 19th since the beginning of the series initiated on 12 January 2011) on the morning of 5 January. The photo in Figure 1 (above) shows the acme of this paroxysm, shortly after 06:00 GMT.

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Figure 2. Eruptive activity during the first few hours of the paroxysmal eruptive episode on 5 January 2011. Top: emission of a small lava flow and weak Strombolian activity, shortly before 03:00 GMT. Bottom: initiation of pulsating lava fountaining and slow downslope expansion of the lava flow toward southeast (left), at 05:10 GMT. Photos taken from the Mareneve road above the village of Fornazzo by Boris Behncke, INGV-Osservatorio Etneo (Catania)

The reawakening was preceded by various signs of unrest recorded by the observation systems of the INGV-Osservatorio Etneo (INGV-OE) during the first few days of 2012; these included strong fluctuations in the volcanic tremor amplitude, an increase in degassing from the Bocca Nuova that culminated in an explosion quake accompanied by a minor emission of vapor and ash on the evening of 2 January, and finally by the resumption of weak explosive activity within the New SEC on 4 January. About 08:20 GMT on 4 January, small explosion signals started to be recorded by the EBELO infrasonic recorder, located about 0.9 km to the southeast of the crater.

On the late evening of 4 January weak incandescence was visible in correspondence with the New SEC; however, observations were strongly hampered by inclement weather. From 22:30 GMT Strombolian activity was observed intermittently by INGV-OE staff from various sites on the southeastern and northeastern flanks of the volcano, and from 02:00 GMT on 5 January the activity was under continuous observation. Around 02:45 GMT, a small lava flow began threading its path across the deep notch curring the southeastern crater rim (Figure 2); this flow advanced very slowly following the same path of the lava flows emitted during the previous paroxysmal episodes.

During the following hours, the Strombolian activity increased in intensity and from 04:00 GMT it increased more rapidly to become virtually continuous. Between 04:45 and 05:00 GMT, the Strombolian activity passed into discontinous, pulsating fountaining generating jets 100-150 m high.

About 04:50 GMT, ash emission had become significant, and this was accompanied heavy fallout of scoriae, spatter, and bombs onto the flanks of the cone. From 05:15 GMT onward, lava fountaining was continuous, generating an eruption column of ash and vapor that rapidly rose in height, reaching an elevation of 7000-8000 m above the sea-level around 06:00 GMT (see Figure 1).

During the time interval between 05:35 and 05:45 GMT, incandescent pyroclastics completely covered the cone, which interacting with snow began to form avalanches and small pyroclastic flows extending for a few hundred meters (Figure 3). These flows repeatedly pushed far into the snow cover at the base of the cone, provoking phreatomagmatic phenomena and small lahars (mud flows), in particular on the northeastern, eastern, and southern flanks of the cone (Figure 4). The longest flows nearly reached the central portion of the eruptive fissure of 13 May 2008.

The vents on the upper northern flank of the cone emitted a small lava flow that travelled a few hundred meters stopping before reaching the upper portion of the 13 May 2008 eruptive fissure.

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Figure 3. Lava fountains from the eruptive fissure cutting through the northern rim of the New Southeast Crater, pyroclastic flow on the eastern flank of the cone, and lava flow toward southeast (left), seen from the Mareneve road near Fornazzo at 06:10 GMT on 5 January 2012. Photo taken by Boris Behncke, INGV-Osservatorio Etneo (Catania)

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Figure 4. Front of a pyroclastic flow descending the eastern flank of the New Southeast Crater cone, and impacting snow cover at the base of the cone. The white vapor column originates from two small lahars (mudflows) generated by the melting of snow mixing with the material deposited by the pyroclastic flow; in the foreground at the bottom the trace of a huge volcanic bomb rolling down the snow-covered slope can be seen. Photo taken at 06:14 GMT by Boris Behncke, INGV-Osservatorio Etneo (Catania)

Around 06:00 GMT, several eruptive vents activated along the fracture that cuts the northern rim of the New SEC cone, producing small intermittent lava fountains (Figure 3). At 06:20 GMT, a powerful explosion marked the opening of a vent on the upper southeast flank of the cone, destroying a portion of the southeastern crater rim. The cloud generated by this explosion is shown in Figure 5. Figure 6 shows lava fountaining from the same vent about 10 minutes after its opening.

Shortly after 06:30 GMT, the Bocca Nuova emitted a puff of ash, followed by weaker emissions of ash mixed with ash. At the New SEC, paroxysmal eruptive activity continued with full vigor until 06:57, and then terminated rather brusquely within the next few minutes. Only passive emission of ash continued after this until about 07:30 GMT at the New SEC, and lasted until 08:30 GMT at the Bocca Nuova.

This paroxysmal episode has occurred after one of the longest repose intervals of the current eruptive sequence initiated one year ago; only the intervals between episodes #2 (18 February 2011) and #3 (10 April 2011) and between episodes #4 (12 May 2011) and #5 (9 July 2011) were longer - 51 and 58 days, respectively. In terms of explosivity, this was one of the most violent events of the sequence, but the quantity of lava emitted was much inferior to that of previous episodes. The main lava flow toward southeast in the direction of the Valle del Bove, advanced little more than 2 km, flanking the northern side of the Serra Giannicola.

For a few tens of minutes following the cessation of the paroxysm, the entire northern flank of the New SEC cone showed a wholesale gravitational movement due to the slow sliding of the abundant pyroclastic material deposited on that side. This process was accompanied by the release of abundant bluish gas, but did not result in the formation of a rheomorphic flow.

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Figure 5. Opening of a new eruptive vent on the upper southeast flank of the new Southeast Crater cone at 06:20 GMT on 5 January 2012. This strongly explosive event generates a dense column of ash. At right, pyroclastic flows are seen descending the eastern flank of the cone. Photo taken by Salvatore Allegra and published here with kind permission of the author (original photo on Flickr)

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Lava fountains from vents within the New Southeast Crater (left of center) and from a vent that has opened on the upper southeast flank of the cone at GMT on 5 January 2012; a pyroclastic flow descending the east flank of the cone is visible at right. The dense whitish plume in the right foreground comes from the lava flow descending toward the Valle del Bove. Photo taken around 06:30 GMT from Catania by Elisabetta Ferrera, University of Catania

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Zoom on the eruptive vent formed at 06:20 GMT on 5 January 2012 on the upper southeast flank of the New Southeast Crater cone, seen from east three days after the event, on 8 January 2012. In the background, the old Southeast Crater cone rears its dormant peak, with the fumarole field on its upper east flank. Photo taken from Santa Venerina by Boris Behncke, INGV-Osservatorio Etneo (Catania)

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