The 27 April 2013 paroxysm of Etna's New Southeast Crater
Fig. 1. Eruption column of the lava fountaining episode (paroxysm) of 27 April 2013 at Etna's New Southeast Crater (NSEC) and lava flow descending toward the Valle del Bove, seen from Monte Cagliato, on the east flank of the volcano. Photo taken by Stefano Branca, INGV-Osservatorio Etneo.
One week after the violent paroxysm of 20 April 2013, the New Southeast Crater (NSEC) of Etna was the site of a new episode of lava fountaining on the evening of 27 April 2013, the 13th such episode since the beginning of the sequence of paroxysms in January 2011. The paroxysmal phase of this episode occurred after six days of Strombolian activity and ash emissions, and was less intense than that of 20 April, generating a cloud charged with pyroclastic material (Fig. 1), which was blown by the wind northeastward causing ash and lapilli falls in Linguaglossa, and fall of finer particles in areas around Taormina and as far as Messina. At the beginning of the paroxysmal phase, lava pushing from within the crater caused the collapse of a section of the eastern flank of the cone, leading to a landslide and pyroclastic flows. Lava was emitted from four sources, on the southwest, southeast, northeast, and north of the cone.
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Fig. 2. Strombolian activity and ash emission from the NSEC at daybreak on 26 April 2013. Photo taken from Tremestieri Etneo, 20 km south of the NSEC, by Boris Behncke, INGV-Osservatorio Etneo. |
The "prelude" to this latest episode started about 24 hours after the 20 April paroxysm, on the afternoon of the 21st, with Strombolian explosions and ash emission from a vent sitting in the center of the NSEC. This activity continued, with only slight fluctuations, for five days, and often generated conspicuous plumes of dark ash that were well visible to great distances due to the nearly complete absence of wind in the summit area (Fig. 2). On the afternoon of 26 April, the activity started to show a gradual intensification, passing into vigorous Strombolian activity with only very little ash emission; contemporaneously there was a progressive increase in the volcanic tremor amplitude. During the night between 26 and 27 April, rather bad weather conditions precluded any observation by means of the visual and thermal surveillance systems; however, the volcanic tremor amplitude first reached a minor peak during the night and then dropped slightly before rising again on the morning of 27 April. Durante la giornata del 27, si sono uditi frequenti boati, e sul versante nord-orientale del vulcano sono cadute piccole quantità di cenere fine. Finally, on the late afternoon of 27 April, the eruptive activity showed a rapid intensification. Direct observation and visual and thermal monitoring were still hampered by inclement weather conditions and the persistence of cloud cover in the summit area; only at about 16:55 GMT (=local time -2) visibility started to improve. |
By this time, it was possible to note that a lava flow emission from the southeastern rim of the NSEC had just started, and a small active lava flow was also seen to issue from the area of the "saddle" between the two cones of the Southeast Crater (SEC). Strombolian activity was characterized by nearly continuous explosions that generated loud detonations, and progessively increased to take on the character of a pulsating lava fountain, 150-200 m hight, which rose from an eruptive vent located in the eastern part of the crater, generating jets that were strongly inclined eastward. A second vent, located a short distance to the west within the NSEC, continued to produce strong explosions at intervals of a few seconds. At 17:14 and 17:50 GMT, major collapse occurred on the eastern flank of the cone, generating pyroclastic flows that expanded several hundred meters toward the western headwall of the Valle del Bove (Fig. 3 and 4).
Fig. 3. Small pyroclastic flow descending from the NSEC at 17:50-17:51 GMT on 27 April 2013, seen as a prominent anomaly in the images of the thermal surveillance cameras of the INGV-Osservatorio Etneo on Schiena dell'Asino (mobile, MBT; a) and at Monte Cagliato (EMCT; b).
Fig. 4. Pyroclastic flow at 17:50-17:51 GMT caused by partial collapse of the east flank of the NSEC cone. The sharp peak seen at left, behind the NSEC cone, is the old SEC cone. The view is from Schiena dell'Asino, a few kilometers southeast of Etna's summit. Photo taken by Carmelo Bagiante and published here with kind permission of the author.
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Fig. 5. Lava fountain and lava flows during the culminating phase of the 27 April 2013 paroxysm at the NSEC, seen from east. The main lava flow (L1) issues from the base of the collapse scar (C) on the east flank of the cone; a second lava flow (L2) is fed from a vent just right of the earlier, and a third lava flow (L3), farther to the right, issues from a new eruptive vent at the north base of the NSEC cone. Photo taken by Salvo Virzì and published here with kind permission of the author (original photo on Flickr).
Fig. 6. Pyroclastic density current (PDC) generated by explosive interaction between lava and snow on the ground on the western flank of the Valle del Bove, captured by the thermal camera of the INGV-Osservatorio Etneo on Monte Cagliato (EMCT) at 19:25 GMT on 27 April 2013. |
The phase of lava fountaining lasted about two hours, with strong variations in the intensity of the eruptive activity. Repeatedly observers had the impression that the paroxysm was about to end, when the lava fountaining activity passed into violent explosions accompanied by loud detonations, but then resumed with jets 200-300 m high. During the interval between 19:30 and 19:45 GMT, the main lava fountain was strongly inclined toward northeast. From the start of the paroxysmal phase lava emission had been observed to occur from the area of the "saddle" between the two cones of the SEC, feeding a flow that was directed south and southeast, and from the southeastern rim of the NSEC. After the collapse of a portion of the eastern flank of the cone, lava emission continued from the bottom of the collapse scar (C in Fig. 5), feeding a major lava flow (L1 in Fig. 5) that initially travelled east-northeast for less than 1 km and then turned southeast, running parallel to the lava flow-field formed during the preceding paroxysms. During its descent on the steep western slope of the Valle del Bove, this lava flow repeatedly interacted with patches of snow on the ground, generating small lahars. At 19:25 GMT, explosive interaction between lava and snow in the lower portion of the steep western slope of the Valle del Bove generated a small pyroclastic flow that had relatively high temperatures as seen in images of the Monte Cagliato thermal surveillance camera (Fig. 6), and which extended for a distance of about 1 km down to the base of the slope. Besides the explosive interaction between lava and snow there might also have been a collapse of the lava flow front, which contributed hot material to the pyroclastic flow. Another, much smaller lava flow was emitted from an effusive vent located at the northeastern base of the NSEC cone (L2 in Fig. 5), more or less in the same area where lava flows had been emitted during some of the paroxysms between 3 and 18 April 2013. Finally, yet another effusive vent opened at the northern base of the cone, and emitted a lava flow (L3 in Fig. 5), that flowed for about 1 km and then coalesced with the main lava flow. Lava fountaining activity continued - as noted before, with significant fluctuations - until about 20:00 GMT and then passed into Strombolian explosions and ash emissions; the last explosions were observed at around 21:20. The main lava flow remained active for many hours after this, and lava emission came to a definitive end only in the late forenoon of 28 April. During the day of 28 April 2013, frequent collapse, rockfalls and small landslides occurred within the deep scar created by the collapse of the east flank of the NSEC cone on the previous evening; one particularly large landslide occurred at 10:13 GMT, creating a cloud of reddish brown dust. Apart from these collapse events, no eruptive activity was observed at the NSEC until the afternoon of 1 May 2013, when weak explosive activity (Strombolian explosions and ash emissions) resumed, with the typical characteristics of the activity of the "prelude" for a new paroxysmal eruptive episode. |
The effects of the 27 April 2013 paroxysm on the NSEC cone are well visible in the photos of Fig. 7 and Fig. 8, taken on the morning of 29 April during excellent visibility. The most conspicuous effect is the opening of a deep depression, a collapse scar, on the eastern flank of the cone, with a width of about 150 m and a length of about 300 m. The mechanism that led to the collapse might have been an intrusion of magma through the lower flank of the cone, similar to what has been observed at the northeastern base of the cone during several of the paroxysmal episodes since 3 April 2013. The material involved in the collapse was deposited at the base of the cone, forming a prominent ridge about 500 m long and oriented east-northeast to northeast, and effectively creating a sort of barrier to the lava that exited from the bottom of the collapse depression which was thus diverted northeastward (Fig. 7 and 8). The opening of an effusive vent at the northern base of the cone, from which the lava flow L3 of Fig. 5 and Fig. 8 issued, was accompanied by the formation of a linear fracture that cuts the northern flank of the cone from the sumit to the base. In spite of these destructive processes, the cone did grow in height during this latest paroxysm, mostly on the northern rim of the crater.
Fig. 7. View from east (near Macchia di Giarre) of the NSEC (seen at left) and the Northeast Crater (emitting puffs of white vapor at right), and the lava flows emitted from the NSEC during the 27 April 2013 paroxysm. The collapse scar on the east flank of the NSEC cone is well visible. Photo taken on the morning of 29 April 2013 by Boris Behncke, INGV-Osservatorio Etneo.
Fig. 7. The NSEC cone seen from the area of Ripe della Naca, on the east-northeast flank of Etna, on the morning of 29 April 2013. The dotted yellow line marks the collapse scar on the eastern flank of the cone. The collapse deposit (Av) forms a conspicuous ridge, or rampart, at the base of the cone, preventing the main lava flow (L1) of 27 April from following the path of the previous lava flows, and diverting it northeastward. Effusive vents are indicated by red-yellow dots; L2 and L3 are smaller lava flows that were emitted from vents at the northeastern and northern base of the cone. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.
The 27 April 2013 paroxysm was much less violent than its predecessor, on 20 April, as was manifest in the lower lava fountains (300-500 m on 27 April compared to 800-1000 m on 20 April), a weaker eruption column, and a significantly smaller quantity of pyroclastic fall material. Tephra falls in downwind areas, to the northeast, thus deposited only ash and small lapilli in the area of Linguaglossa and fine ash in more distal areas such as Taormina and Messina. Also the volume of lava emitted during the 27 April episode was significantly smaller than that of the previous two paroxysms. Finally it is notable that this paroxysm was preceded by a rather long "prelud" phase, which lasted 6 days and started less than 24 hours after the conclusion of the previous paroxysm.