Etna and Stromboli update, 19 February 2013

Fig. 1. Etna seen from Catania at the end of the paroxysmal eruptive episode at the New Southeast Crater, at about 05:00 GMT (=local time-1). Ash emission is continuing, and the ash plume is blown by the wind toward east; lava flows continue to descend into the Valle del Bove. Note the small lava flow further to the left, which has spilled down the southern flank of the New Southeast Crater cone. Photo taken by Bruno Bonaventura and published here with kind permission of the author (original photo on Flickr)

Etna: Episode of lava fountaining from the New Southeast Crater. Nearly ten months after the last episode of lava fountaining (paroxysm), which occurred on 24 April 2012 at Etna's New Southeast Crater (NSEC), during the early morning hours of 19 February 2013, the same crater has produced a new paroxysm with emission of lava flows, pyroclastic flows, lahars, and an ash cloud extending toward east (Fig. 1). The episode had been preceded for seveal days by weak Strombolian activity within the crater, which then began to increase gradually during the night of 18-19 February.

Fig. 2. Strombolian activity at the New Southeast Crater at 05:15 GMT on 17 February 2013, seen from Tremestieri Etneo. Photo taken by Boris Behncke, INGV-Osservatorio Etneo

The first signs of a resumption of eruptive activity at the NSEC were observed on the evening of 13 February, when the high-sensitivity visible surveillance camera of the INGV-OV on the Montagnola (EMOH) recorded weak glow coming from within the crater. The phenomenon persisted, in a discontinuous manner, during the whole night and was again visible on the evening of 14 February. During the early morning hours of 15 February, incandescent bombs were seen to rise just a little higher than the crater rim, without falling onto the outer flanks of the cone. The Strombolian activity gradually intensified in the early morning hours of 17 February; at the same time, the volcanic tremor amplitude started to rise. At daybreak, small Strombolian explosions occurred with a frequency of one every 1-2 seconds, launching coarse-grained pyroclastic material up to a few tens of meters above the crater rim (Fig. 2). After reaching a peak around 06:00 GMT (=local time -1), both the volcanic tremor amplitude and the intensity of the Strombolian activity started to diminish; a few hours later, the volcanic tremor amplitude returned to background levels, and on the evening of 17 February, the surveillance cameras showed no sign of eruptive activity. However, visibility was often hampered by weather clouds in the Etnean summit area.

The poor weather conditions persisted during much of 18 February; only shortly before midnight, the clouds dissipated from the summit area of Etna, revelaing a continuous, though weak Strombolian activity within the NSEC. For several hours, the activity continued much in the same mammer, without showing evident signs of increasing, repeating essentially the activity observed on the morning of 17 February. During the interval between 00:00 and 02:00 GMT on 19 February, the volcanic tremor amplitude rose rather gradually; but shortly after 02:00 it started to increase more decidedly. Contempraneously, the eruptive activity started to intensify, and volcanic bombs began to fall onto the flanks of the NSEC cone. A further intensification of the activity was evident by 03:40 GMT; yet all activity was confined to one single vent within the center of the crater. About 10 minutes later, lava started do overflow through the deep notch in the southeastern crater rim, expanding slowly toward the steep western slope of the Valle del Bove (Fig. 3a).

At 03:57 GMT the small pit crater (informally also known as "pittino") that had formed on 27 August 2012 on the southwestern rim of the NSEC started to emit ash, and during the following minutes, there were repeated collapses on its rims, generating small landslides on the southern flank of the cone (Fig. 3b). At the same time, the activity of the main vent within the crater rapidly increased; at 04:03 a lava fountain rose about 200 m above the summit of the cone. During the interval from 04:03 until 04:07 GMT, several vents became active along a fracture cutting across the crater from west to east, from the "pittino" to the notch in the southeastern crater rim (Fig. 3c). A dense cloud of ash started to rise and was then blown by the wind toward east. Immediately, the entire NSEC cone was subjected heavy fallout of coarse-grained pyroclastic material (Fig. 3d). Besides the main lava flow, which was advancing southeastward, a small lava flow developed also on the flank below the "pittino", following the trace of the fracture that had opened in the saddle between the old and new SEC cones on 4 March 2012.

Fig. 3. Frames extracted from video recorded by the visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola (EMOV) during the early phases of the 19 February 2013 paroxysm. Note how in an interval of little more than 15 minutes, the eruptive activity passed from intense Strombolian explosions from one single vent to sustained high lava fountaining from 3 or 4 eruptive vents aligned along a fracture trending roughly east-west.

At 04:15 GMT, the lava fountains rose 300-500 m above the crater rim; the abundant fall of bombs and fluid spatter covered the entire southern flank of the cone with incandescent material. Soon this material began to slide, forming small avalanches of incandescent material, which in turn generated ash clouds. During the interval between 04:16 and 04:18 GMT, the south flank of the cone was veiled by a dense curtain of incandescent fallout from the lava fountains (Fig. 4a). At the same time, dense vapor clouds rose from the upper eastern flank of Etna, which were generated by the copious fallout of incandescent pyroclastic material onto the snow covering the ground.

At 04:19 GMT, a more substantial avalanche of material accumulated during the preceding minutes detached from the southern flank of the cone, generating a small pyroclastic flow that expanded first south and then eastward to a distance of a few hundred meters. During the first about 20 seconds of advance, the front of the cloud overriding the pyroclastic flow was incandescenct (Fig. 4b and 4c); still after 30 seconds in some zones of the cloud there were zones of dull incandescence (Fig. 4d). The high temperatures of the pyroclastic flow were also evident in images recorded by the thermal monitoring camera of the INGV-OE at Monte Cagliato, on the eastern flank of Etna (Fig. 5a).

Fig. 4. Development of a small pyroclastic flow (with a glowing cloud) during the phase of maximum intensity of the paroxysm at the New Southeast Crater on 19 February 2013, documented in these still frames captured from video recorded by Klaus Dorschfeldt (who kindly permitted publication of the images on this site). Times assigned to each frame on the base of the timed footage recorded by the surveillance cameras of the INGV-Osservatorio Etneo. The original video, recorded by Klaus Dorschfeldt from Catania, can be viewed at YouTube.

The main lava flow, which had started to descend into the direction of the Valle del Bove, initially expanded only slowly and arrived at the top of the steep western headwall of the Valle at about 04:35 GMT. At 04:36, the thermal monitoring camera at Monte Cagliato (on Etna's east flank) of the INGV-OE recorded the descent of a lahar (mudflow) from near the Belvedere area (Fig. 5b and 5c), which was followed by a broad lava flow that virtually plunged down the steep slope on top of the traces of the lahar and reached the base of the slope after less than 20 minutes. During its descent, the lava continued to melt snow on the ground, producing numerous small lahars (Fig. 5d). At 04:50 GMT, a second lava flow started to spill down the slope to the north of the first flow (at right in Fig. 6), whose interaction with the snow cover likewise generated lahars. Finally, about 05:00 GMT, a third lava flow - which also generated a lahar - started to descend the western slope of the Valle del Bove to the south of the first flow (at left in Fig. 6).

Fig. 5. Frames captured from video recorded by the thermal monitoring camera of the INGV-Osservatorio Etneo at Monte Cagliato, on the east flank of Etna (EMCT), showing the emplacement of the pyroclastic flow at 04:19 GMT (a), and the subsequent formation of lahars due to the melting of snow att the contact with lava flows descending the steep western slope of the Valle del Bove.

Fig. 6. Lava flows spilling down the steep western headwall of the Valle del Bove, following the same paths as the lahars (seen as dark, ramificating flows) generated by the interaction of the same lava flows with the snow cover on the ground. The longest and largest flow, in the center, was emplaced first, followed by the flow at right (north) and finally, the one at left (south). The New Southeast Crater, source of these lava flows, is out of the field of view at the top. Photo taken from Giarre, on the lower eastern flank of Etna, by Turi Caggegi and published here with kind permission of the author (original photo on Flickr)

At a time difficult to specify, but most probably before 05:00 GMT, a small lava flow was also emitted from the northern flank of the NSEC cone, expanding downslope for a few hundred meters into the direction of the Valle del Leone (Fig. 7). It is not clear whether this flow came from a vent or fracture outside the crater, or whether it resulted from an overflow from the crater.

Fig. 7. Panoramic view of the eastern flank of Etna with the Valle del Bove at the end of the paroxysmal episode of 19 February 2013. The dark traces of the lahars which preceded the lava flows, are well distinguishable; the lava flows continue to expand downslope. At right, the snow on the slope of the mountain is covered by a layer of pyroclastic material forming a narrow black streak. Note also the small lava flow at upper right, which had issued from the northern side of the New Southeast Crater. Photo taken by Turi Caggegi and published here with kind permission of the author (original photo on Flickr)

Lava fountaining started to show signs of diminishing already at 04:25 GMT, when the height of the fountains suffered a notable reduction; around 04:35 fountaining from the "pittino" passed into ash emission alternating with brief jets of incandescent lava. At 04:45 GMT, one single vent, in the central portion of the NSEC continued to produce lava fountains about 200 m high. A few minutes later, however, a new, well-fed lava flow started to descend the lower southeast flank of the cone, possibly after the opening of a new eruptive vent in the lower part of the notch cutting that sector of the cone. The images of the surveillance cameras also show a brief interval of lava fountaining at that site, before all lava fountaining ceased shortly before 05:00 GMT.

During the interval between 05:00 and  05:15 GMT, the activity consisted essentially of the continuous emission of a dense ash plume with frequent jets of lava and powerful explosions, which launched large glowing bombs beyond the summit of the old Southeast Crater cone. After 05:15 GMT, only ash emission persisted. At 05:22, a puff of ash was emitted also from the Bocca Nuova; shortly thereafter, ash emission from the NSEC diminished notably and became discontinuous and the last, weak puffs of ash were observed around 06:10 GMT. Slow lava emission continued for a few more hours from the lowermost vent , which had opened shortly after 04:47 GMT on the southeastern flank of the cone.

During the late afternoon of 19 February 2013, small ash puffs were again emitted from the Bocca Nuova.

Fig. 8. Field images taken during a visit to the New Southeast Crater area by INGV-Osservatorio Etneo staff a few hours after the 19 February 2013 paroxysm. The photo at left shows the "saddle" between the old (left) and new (right) cones of the Southeast Crater, along which a small lava flow descended that was fed from the small pit crater ("pittino") formed on 27 August 2012 on the southwestern rim of the New Southeast Crater. Note the short fracture that has opened in the upper part of the "saddle", possibly due to collapse of the rim of the "pittino". The small cone at left is the "Sudestino", formed in the spring of 2000 during a series of paroxysmal eruptive episodes from the (old) Southeast Crater. In the photo at right is the front of the lava flow that had issued from the "pittino", which, after traveling around the southern base of the New Southeast Crater stopped a short distance from the "Belvedere" monitoring station. Note the lahar deposits (the smooth-looking material under the feet of the person) in the foreground, and the reddish-brown deposit of the pyroclastic flow of 04:19 GMT. Photos taken by Francesco Ciancitto, INGV-Osservatorio Etneo.

A field survey carried out by INGV-OE staff a few hours after the cessation of the activity revealed that the lava emitted from the "pittino" onto the southwestern flank of the cone traveled around the southern base of the cone, stopping short of reaching the area of Belvedere, site of a multiparameter monitoring station and two webcams not operated by the INGV. The two photos in Fig. 8 show the zone where the flow originated (left), and the lava flow front (right), which overlies the deposits of small lahars and of the pyroclastic flow emplaced at 04:19 GMT.

The tephra fall deposit (ash and scoriae) extends from the NSEC toward east in a rather narrow sector, passing across the populated areas of Milo and Fornazzo down to Giarre and Riposto on the Ionian coast.

The 19 February 2013 paroxysmal episode was the first such event since the episode of 24 April 2012, and in its main characteristics it was rather similar to the lava fountaining episodes of 2011-2012. The phase of lava fountaining lasted approximately 50 minutes, which is also typical of this type of activity. Besides the magmatic activity, a variety of secondary phenomena were observed, such as the pyroclastic flow of 04:19, a number of flows of ash and vapor generated by the explosive interaction of lava with snow, and lahars. The pyroclastic flow originated from the sliding of unstable hot material that had rapidly accumulated on the steep flank of the cone, a mechanism thus far little acknowledged in the scientific literature.

At the moment this report is posted (19 February 2013, 23:00 GMT), the NSEC is once more producing Strombolian activity, which is accompanied by an increase in the volcanic tremor amplitude.