The 12 April 2013 paroxysm at Etna's New Southeast Crater
Fig. 1. The New Southeast Crater (NSEC) during the phase of sustained lava fountaining on the late forenoon of 12 April 2013, seen from about 1.5 km to the south. Photo taken by Filippo Murè, INGV-Osservatorio Etneo.
The 10th eruptive epsiode of the year 2013 at Etna's New Southeast Crater (NSEC) took place between 8 and 14 April 2013, with a long phase of "prelude" and culminating finally on 12 April with a paroxysmal phase characterized by sustained lava fountaining and tephra (ash and lapilli) emission, but less violent than the previous paroxysms (Fig. 1). The paroxysmal phase was preceded by a rather long phase of powerful Strombolian explosions producing audible detonations heard tens of kilometers away. Weak and discontinuous effusive activity continued for more than two days after the concludion of the paroxysmal phase, until the late evening of 14 April. The quantity of airfall tephra was notably inferior to those produced during the paroxysmal episodes of March and of 3 April 2013.
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Fig. 2. A particularly powerful explosion occurred on the evening of 8 April 2013 at the NSEC, launching coarse-grained, incandescent material beyond the old SEC cone. Frame recorded by the high-sensitivity monitoring camera of the INGV-OE on the Montagnola (EMOH).
Fig. 3. Explosive ash emission from the NSEC (at right) forming a plume that is blown dowslope toward east (left) by the strong wind, at 06:47 GMT on 10 April 2013. Photo taken from Taormina, about 28 km northeast of the NSEC, by Boris Behncke, INGV-Osservatorio Etneo. |
The first eruptive activity at the NSEC after the 3 April 2013 paroxysm occurred on the afternoon of 8 April, with sporadic ash emissions some of which were accompanied by ejection of incandescent material. Some of these events were rather violent, like an explosion at 20:52 GMT (=local time -2) on 8 April (Fig. 2), which produced a loud detonation heard up to 15 km away. The ash emissions, which often had an explosive onset (Fig. 3) but at times were rather passive, continued for the next about 48 hours, at progressively shorter intervals; on the late afternoon of 10 April, the activity took on a typical Strombolian character with minor ash emissions during some of the explosions. During the night of 10-11 April, the volcanic tremor amplitude began to show a slow increase, and at daybreak on 11 Aprio, Strombolian explosions occurred about every 2-5 seconds, launching incandescent pyroclastics several tens of meters above the crater rim. Throughout the day, the Strombolian activity continued, increasing slowly in intensity and frequency of explosions; contemporaneously the volcanic tremor amplitude continued to show a gradual rise. On the late afternoon of 11 April, the activity was characterized by frequent and very powerful Strombolian explosions, which occurred at a frequency of one every 1-2 seconds, and nearly continuously produced detonations that were widely audible in the populated areas around the volcano. The jets of incandescent pyroclastics often rose 200 m high above the crater rim and containued varying, though generally minor amounts of volcanic ash. Around 16:40 GMT, a small lava flow started to issue from a source within the deep breach in the southeastern rim of the crater, expanding slowly to the base the the cone and then spreading out laterally (Fig. 4). However, this lava flow was scarcely fed, and on the late evening it seemed to have stopped and showed evidence of cooling. Around 23:10 GMT on 11 April, another small lava flow started to exit from the area of the "saddle" between the two cones of the Southeast Crater (SEC), expanding first southward and then bending southeastward (Fig. 5), following the same path as the lava flows emitted from the "saddle" area during the previous paroxysms. Explosive continued throughout the night, with powerful explosions and loud detonations alternating with intermittent, low lava fountains, while the volcanic tremor amplitude had stabilized at a high level. At daybreak on 12 April, the eruptive activity was seen to feed a dense eruption plume, which containued relatively minor amounts of pyroclastic material and was blown by the wind toward east-southeast. |
Fig. 4. Strombolian activity and emission of a small lava flow on the southeastern flank of the NSEC cone on the evening of 11 April 2013. Photo taken from Piano del Vescovo, 6.9 km southeast of the NSEC by Boris Behncke, INGV-Osservatorio Etneo.
Fig. 5. The NSEC in strong Strombolian activity with emission of small lava flows toward southeast (at right) and toward south, from the "saddle" between the NSEC and the old SEC cone (at left), at 01:42 GMT on 12 April 2013. Photo taken from Tremestieri Etneo, 20 km south of the NSEC, by Boris Behncke, INGV-Osservatorio Etneo.
Until about 08:25 GMT on 12 April, all eruptive activity occurred exclusively at one or two closely spaced vents within the NSEC, but shortly after 08:25 GMT the thermal monitoring camera of the INGV-Osservatorio Etneo (INGV-OE) at Monte Cagliato (EMCT), on the east flank of Etna, showed the start of lava emission from two vents located at the northeastern base of the NSEC cone (Fig. 6a). At 08:39 GMT, a landslide or rock avalanche (AV in Fig. 6b) originated from the same area; the EMCT images showed that the material of this avalanche had a low temperature.
Fig. 6. This series of frames captured from video footage recorded by the thermal cameras on Monte Cagliato (EMCT), on the Schiena dell'Asino (mobile; MBT) and the visible-light camera at Milo (EMV) show (a) the lava flows emitted from the NSEC toward southeast (LF1), from the "saddle" (LF2) and from two vents at the northeast base of the cone (LF3) at 08:29 GMT on 12 April 2013; (b) a rock avalanche (AV) descending from the northeastern flank of the cone at 08:39 GMT; (c) and (d) the formation of a hot avalanche or a pyroclastic flow (PDC) at 08:54-08:55 GMT.
A flow of hot pyroclastic material originated from the same area at 08:54 GMT (PDC in Fig. 6c and 6d), which travelled about 2 km toward northeast in less than 1 minute; this flow was also observed and photographed by INGV-OE staff in the field in a spot about 1.5 km south of the NSEC (Fig. 7). In this case, the phenomenon was rather similar to the two pyroclastic flows emplaced at the end of the 3 April 2013 paroxysmal eruptive episode (read the report on the 3 April 2013 eruptive episode). The flow probably originated from the collapse and sliding of hot material from the steep northeast flank of the cone. The deposit left by the flow remained hot for at least 30 minutes, producing a strong anomaly in the thermal images of the Monte Cagliato monitoring camera (EMCT).
Fig. 7. Ash cloud produced during the descent of a kind of pyroclastic flow, or hot avalanche, from the northeastern flank of the NSEC cone at 08:54 GMT on 12 April 2013, photographed from a location about 1.5 km south of the NSEC. In the background are the northeasternmost tip of Siciliy and still further in the distance, the southern tip of Calabria. Photo taken by Filippo Murè, INGV-Osservatorio Etneo.
In the meantime, eruptive activity continued at the NSEC with frequent, powerful Strombolian explosions and emission of modest quantities of volcanic ash, which was rapidly dispersed in the atmosphere. Lava emission from the southeastern rim of the NSEC continued, feeding a small lava flow (LF1 in Fig. 6a and 6b), which advanced very slowly into the direction of the Valle del Bove, and the lava flow emitted from the "saddle" was approaching the former "Belvedere" (LF2 in Fig. 6a and 6b), whereas the lava emitted from the northeastern base of the cone slowly expanded on top of the hot avalanche deposit emplaced at 08:54-08:55 GMT.
Shortly before 10:00 GMT, the eruptive activity at the NSEC showed a marked reduction, changing from low lava fountains to Strombolian explosions and intermittent ash emissions; the activity reached a rather low level during the interval from 10:10 until 10:14 GMT. During the same interval, also the volcanic tremor amplitude was dropping sharply. Yet, at 10:14 GMT, vigorous ash emissions resumed both from the main vent of the NSEC and from the vent located on the "saddle" between the two SEC cones, which had not shown any eruptive activity during the ongoing eruptive episode. The activity soon concentrated entirely at this latter vent, with jets of ash and large blocks and bombs (Fig. 8).
Fig. 8. A jet of ash and coarse-grained pyroclastic material is seen rising from the eruptive vent located in the "saddle" between the two SEC cones (the old cone is out of the view to the left), at 10:17 GMT on 12 April 2013. Note the rather minor activity at the NSEC proper. Photo taken from Monte Frumento Supino, about 1.7 km southwest of the NSEC, by Francesco Ciancitto, INGV-Osservatorio Etneo.
Expulsion of blocks, bombs and ash from the "saddle" vent continued until 10:34 GMT, when the main vent of the NSEC reactivated, and for the next nearly 20 minutes both vents were the source of simultaneous, intense ash emissions. Finally, the activity shifted back entirely to the NSEC proper, entering into the true paroxysmal phase of this episode at 10:50 with sustained lava fountaining, accompanied by a return to high levels of the volcanic tremor amplitude. During the following 10 minutes, there was a considerably increase in the quantity of pyroclastic material in the eruptive plume, which was was bent east-southeastward by the wind (Fig. 9). Tephra falls (consisting of ash and small lapilli) affected the population centers of Fleri, Zafferana Etnea, Milo and S. Maria la Scala, although the quantity of fallout tephra was much smaller compared to those of the previous paroxysms.
Fig. 9. Eruptive plume emitted during the paroxysmal phase at the NSEC on 12 April 2013, photographed at 11:49 GMT from the Villa Bellini public garden, in the center of Catania, about 27 km south of the crater. Note the relatively minor amounts of dark pyroclastic material in the plume except for the lowermost portion up to 1 km above the summit. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.
Sustained lava fountaining continued for about one hour, showing the first signs of weakening shortly before 12:00 GMT; a few minutes after 12:00 the activity switched to sporadic Strombolian explosions and ash emissions, which gradually diminished in vigor. During the interval between 12:00 and 12:10, repeated collapse occurred on the southeast flank of the NSEC cone, probably provoked by the opening of new effusive vents at the base of the cone, which started to emit a new lava flow toward the Valle del Bove. The collapses generated avalanches and clouds of reddish-brown ash or dust (Fig. 10), which descended beyond the base of the cone. During the following hours, the overall explosive activity progressively diminished and completely ceased around 15:00 GMT.
Fig. 10. Strombolian activity at the main vent of the NSEC (at left) and reddish-brown cloud (at right) caused by collapse and avalanching on the southeast flank of the cone, during the opening of new effusive vents in that sector, around 12:15 GMT on 12 April 2013. The photo was taken from about 1.5 km south of the NSEC by Filippo Murè, INGV-Osservatorio Etneo.
Field observations made by INGV-OE staff shortly after the cessation of the paroxysmal activity revealed that an oval-shaped depression had formed on the lower southeast flank of the NSEC cone, and an effusive vent lay at the bottom of this depression (Fig. 11), whereas a second effusive vent lay further downslope. The two vents continued to emit lava at a rather low rate, feeding several branches that did not extend as far as the western rim of the Valle del Bove.
Fig. 11. Southeastern flank of the NSEC cone after the end of the paroxysmal eruptive episode on 12 April 2013, showing a new collapse depression that had during the activation of new effusive vents around 12:00 GMT (see Fig. 10). Note the steaming, still moving lava in the lower right half of the view. Photo taken by Daniele Andronico, INGV-Osservatorio Etneo.
Fig. 12. A panoramic view of the "saddle" area between the two SEC cones (the old cone at left, and the new one at right; "SV" indicates the position of the eruptive vent in the "saddle"), which emitted the lava flow seen in the foreground. The main branch of this flow expanded initially southward and then turned southeastward (toward right), whereas a smaller lobe (seen in the left half of the image) passed between the largest of the 17 July 2001 hornitos ("hornito 2001") and the small 2000 cone ("Sudestino"). Photo taken by Daniele Andronico, INGV-Osservatorio Etneo.
The field observations also showed that the lava flow emitted from the "saddle" between the two SEC cones had essentially expanded southeastward, into the direction of what was once the "Belvedere" monitoring station, on top of the lavas emitted from the same source during the previous paroxysms. A smaller lobe of lava, however, had passed around the largest of the hornitos formed during the early days of the July-August 2001 eruption on its western side, bypassing the small cone formed during the spring of 2000 that is known as "Sudestino" (Fig. 12).
On the evening of 12 April 2013, the lava flows emitted from the "saddle" and from the various vents on the flanks of the cone still showed incandescence and were locally slowly moving; during the night, the surveillance cameras showed the cessation of all lava emission and the cooling of the flows. However, on the morning of 13 April 2013, slow lava effusion resumed from the lower of the vents that had opened at the end of the paroxysm at the southeast base of the NSEC cone, and a small lava flow advanced a few hundred meters into the direction of the Valle del Bove. This flow was still active on the evening of 13 April (Fig. 13), ceasing in the early morning hours of the following day. Once more, on the evening of 14 April, there was a rather weak resumption of effusive activity from this vent, which ceased after a few hours.
Fig. 13. Emission of a small lava flow from the lower of the two effusive vents that had opened at the end of the paroxysmal episode of 12 April 2013 at the southeastern base of the NSEC cone (in the center), seen from Zafferana Etnea on the evening of 13 April 2013. Note the two small red dots on the upper northeastern flank of the cone, which correspond to the sites of hot gas emission. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.
The overall duration of this eruptive episode was more than 6 days, from the afternoon of 8 April when the first ash emissions occurred, until the late evening of 14 April 2013, when all effusive activity definitely came to an end. The episode was characterized by an unusually long phase of vigorous Strombolian activity with only minor, discontinuous lava fountaining, before it finally entered into the true paroxysmal phase on the late forenoon of 12 April. During the previous eruptive episodes in the series of paroxysms initiated on 12 January 2011, until the 3 April 2013 episode, the buildup from intense Strombolian activity to sustained lava fountaining has been more rapid; however, in the past there have been paroxysmal episodes that were similar to the 12 April episode. One particular example is the 4-5 September 2007 episode, whose main paroxysmal phase was preceded by more than one week of slowly increasing Strombolian activity. Also during the exceptional series of paroxysms in the year 2000, a few events - such as on 16 April and on the morning of 1 April 2000 - seemed to "hesitate" for quite long periods before finally entering into the phase of sustained fountaining and tephra emission. In terms of the intensity of lava fountaining, the 12 April 2013 episode was definitely one of the least significant since the beginning of the current series of paroxysms in 2011. The volumes of the lava flows were quite small, as were the quantities of pyroclastic material emitted in this event. Yet, there have been significant changes to the morphology of the cone, mostly for the opening of the new effusive vents on the southeastern and northeastern flanks of the cone. The cone itself gained significantly in height during the long phase of strong Strombolian activity and discontinuous lava fountaining, but some of this height increase was lost during the final phase of the paroxysm.