Hot avalanche from Etna's New Southeast Crater, 11 February 2014
Fig. 1. Ash cloud generated by the landslide originating from the base of the New Southeast Crater (NSEC) cone, visible at upper left, as seen from the village of Sant'Alfio on the eastern flank of Etna when the hot avalanche had reached the floor of the Valle del Bove. Photo courtesy of Casa di paglia Felcerossa - Permacultura sull'Etna, and published here with kind permission of the author (original link on Facebook).
On the morning of 11 February 2014, at 06:07 GMT (=local time -1), a volume of unstable and partially hot rock detached from the lower eastern flank of Etna's New Southeast Crater (NSEC) cone, forming a landslide or avalanche that very much resembled a pyroclastic flow, which in about one minute descended the steep western slope of the Valle del Bove before stopping on the relatively flat terrain at the base of the slope (Fig. 1 e 2). The phenomenon took place in an area where since 22 January 2014 several effusive vents have been emitting lava; initially these vents were two, but since the morning of 10 February, new effusive vents had become active slightly uphill from the earlier ones, one of which had also produced small puffs of vapor and ash on the morning of 10 February. Furthermore, since the onset of the ongoing persistent activity at the NSEC, an incandescent spot had been visible just below the eastern rim of the crater, where a fumarole emitted hot gas. The presence of these various vents, plus the passage of magma and hot gas through this flank of the cone, probably rendered this area progressively unstable.
Fig. 2. These images recorded by the thermal monitoring camera at Monte Cagliato (EMCT), on the eastern flank of Etna, show the advancing flow of hot, fragmented rock material at 06:07 GMT (=local time -1) on 11 February 2014. The flow originated from the lower east flank of the NSEC cone, seen at the skyline. The video of the event, recorded by EMCT, can be watched on the YouTube channel of the NGV-Osservatorio Etneo.
The landslide was preceded by sporadic emissions of vapor and ash from the zones of the active effusive vents. At 06:06:50 GMT, a puff of reddish-brown ash rose from the same area (first frame in Fig. 3), and then rapidly expanded into a large cloud that, rather than soaring into the air, started flowing down the western slope of the Valle del Bove, forming a density current at high temperature, as clearly documented in the images (Fig. 2) of the Monte Cagliato thermal camera (EMCT), on the eastern flank of the volcano.
Fig. 3. The initial ash puff and the huge, laterally expanding cloud formed by the hot avalanche can be seen in this series of images recorded by the visual surveillance camera at the CUAD in Catania (ECV), on the southern base of Etna. The rapid descent of the current is followed, in the last frame, by the soaring of the ash cloud to a height of about 1 km above the summit elevation of the volcano. The video of the event, recorded by ECV, can be watched on the YouTube channel of the INGV-Osservatorio Etneo.
The current rapidly broadened during its descent on top of the 2008-2009 lava flow-field, which it almost entirely covered, before it reached the floor of the Valle del Bove with a front about 1 km wide. Shortly after having reached the flat terrain at the base of the steep western headwall of the Valle del Bove, the current came to a halt, at a distance of about 3.5-4 km from its source, and a dense ash cloud rose from the basal avalanche, which was then blown by the wind toward northeast (see last frame in Fig. 3).
A field visit carried out on the late forenoon by INGV-Osservatorio Etneo staff at Monte Fontane, on the eastern flank of Etna, revealed that a deep notch had formed on the lower eastern flank of the NSEC cone, whose steep walls were continuously crumbling, feeding a constant plume of reddish-brown ash. The 2008-2009 lava flow-field was seen to be almost entirely covered with a deposit of brownish color (Fig. 4), which had been lain down by the hot avalanche of 06:07 GMT. From a spot within the collapse scar, a new lava flow issued, which during the following hours advanced a few hundred meters on top of lavas erupted in the past few weeks; on the evening this flow was about to reach the base of the western slope of the Valle del Bove.
Fig. 4. Western slope of the Valle del Bove (in the center) and NSEC (upper left), seen from Monte Fontane on the late forenoon of 11 February 2014. Note the reddish-brown deposit covering much of the broad 2008-2009 lava flow-field, which was emplaced by the hot avalanche of 06:07 GMT on 11 February. Below the summit of the NSEC cone, a dense plume of reddish as is seen issuing from the collapse scar, whereas a bluish gas plume further downslope marks the new lava flow exiting from the collapse scar. Photo taken by Lucia Miraglia, INGV-Osservatorio Etneo.
During and after the landslide and hot avalanche, the NSEC continued producing its persistent Strombolian activity, at times accompanied by minor ash emission. This activity is continuing as of 12 February, without showing significant variations compared to the past few days. Likewise, the volcanic tremor amplitude is showing the same behavior as in the previous few days, maintaining a level that is slightly elevated compared to background levels.
The landslide and the resulting hot avalanche of 11 February 2014 are just the latest event in a vast variety of phenomena seen at Etna in the past few years, which often have been described as "pyroclastic flows", but which often are not generated by the classical mechanisms that produce pyroclastic flows. Whereas there have been indeed a few cases of typical pyroclastic flows generated by eruption column collapse (e.g., during the powerful 24 September 1986 paroxysm at the Northeast Crater), numerous recent pyroclastic-flow-like density currents have been generated by violent explosive interaction between lava flows and melting snow, most notably during the series of paroxysmal eruptive episodes in 2011-2013. Another "variation on the common theme" has been observed during some of the paroxysmal episodes in 2013, when new effusive vents opened on the flanks of the NSEC cone, which was accompanied by the collapse of the slope above the new effusive vents, generating hot avalanches similar to the one on 11 February 2014, though on a much smaller scale. In particular, such events occurred during the paroxysmal episodes of 3 April 2013 (read report), 12 April 2013 (read report), and 27 April 2013 (read report).