Activity of Etna and Stromboli, 27 February - 2 March 2013

Fig. 1. Map prepared by the Laboratorio di Cartrografia of the INGV-OE of Etna's summit craters, showing the position of the eruptive vents active during the past 10 days. NEC=Northeast Crater; VOR=Voragine; BN=Bocca Nuova, SEC=Southeast Crater (not active since 2007); NSEC=New Southeast Crater; "Sudestino"=an eruptive vent that built a low cone at the southern base of the SEC cone in the spring of 2000; 2001 hornito=first eruptive vents to open during the 2001 flank eruption; 2850 vent and 2800 vent=eruptive vents that opened near the Belvedere monitoring station on 20-21 February 2013; TDF=Torre del Filosofo. Note that the vent of the NEC (shown in orange color) is open and contains degassing magma at depth since many years, but without any surface activity. Light brown area is the zone of the "saddle" between the two cones of the SEC, which collapsed during the 28 February 2013 paroxysm. Orange lines are eruptive fissures in the "saddle" and on the southeastern flank of the NSEC cone, which have been repeatedly active since the early morning paroxysm on 20 February 2013.

The month of February 2013 at Etna ended with spectacular eruptive episodes at the Bocca Nuova (BN) and at the New Southeast Crater (NSEC), and with the resumption of eruptive activity at the Voragine (VOR), which had remained inactive since October 1999. The location of the summit craters of Etna and the various eruptive vents that participated in the recent activity are shown in  Fig. 1. At Stromboli there have been renewed intermittent lava overflows from the crater terrace.

Fig. 2. Eruptive episode at the Bocca Nuova on 27 February 2013. Besides intense Strombolian activity and low lava fountaining, there were also emissions of brown ash, probably caused by collapse or sliding of material within the crater. Photo taken from Adrano, on the lower southwestern flank of Etna, by Roberto Schillaci and published here with kind permission of the author (original photo at Flickr)

27 February 2013 eruptive episode at the Bocca Nuova. On the morning of 27 February 2013, an episode of intense Strombolian activity and low lava fountaining started within the BN, the largest of Etna's summit craters (Fig. 2). This new episode occurred three-and-a-half days after the latest paroxysmal eruptive episode at the nearby NSEC, and five days after an episode of mild Strombolian activity at the BN. There was a rather novel element in this eruptive episode: the reactivation of the VOR, often also referred to as "Central Crater" (said correctly, the BN and VOR together constitute the Central Crater of Etna), or "The Chasm". The last magmatic activity observed at the VOR was in early October 1999.

Already during the night of 26-27 February, there was a gradual increase in the seismic activity and of the volcanic tremor amplitude, especially at the seismic station at Cratere del Piano (ECPN), which suggested a resumption of Strombolian activity at the BN. Copious, puffing emission of dense vapor from the BN was seen at sunrise; over the next few hours these emissions slowly grew more energetic. Between 09:30 and 09:45 GMT (=local time -1), the volcanic tremor amplitude showed a rapid rise; during the same interval, the visuala and thermal surveillance systems of the INGV-OE showed the formation of an eruption column largely composed of vapor, and emission of hot material in the thermal images (Fig. 3a).

Fig. 3. Left: thermal image recorded by the thermal monitoring camera of the INGV-OE on the Montagnola (EMOT), showing an eruption column that contains hot pyroclastic material (yellow-red-white hues), at 10:58 GMT (=local time -1). Right: ash emission at 12:15 GMT, recorded by the visible-light monitoring camera on the Montagnola (EMOV).

In some intervals, the column of vapor emitted from the BN contained moderate amounts of volcanic ash of reddish-brown color, mostly at 11:15-11:20 GMT and 12:14-12:16 GMT (Fig. 3b), which led to the fall of small quantities of very fine ash in the area between Zafferana and Santa Venerina, on the southeast flank of Etna. These ash emission likely resulted from collapse or sliding of unstable material on the steep inner crater wall; the eruptive vent, which lies in the southeastern part of the crater, is leaning against the wall and the rapid accumulation of pyroclastic deposits in that area might have facilitated the sliding of unstable material.

During the phase of most intense eruptive activity, INGV staff carrying out fieldwork in the summit area, noted that volcanic bombs fell outside the crater rim onto the southwestern flank of the central cone (Fig. 4). Moreover, intense explosive activity was also observed within the VOR, which since early October 1999 had not shown any magmatic activity. The activity began to diminish around 12:20 GMT, as was also evident from a reduction in the volcanic tremor amplitude; at 13:30 GMT, the episode was essentially over, even though strong degassing continued at both the BN and the VOR.

Fig. 4. Explosive activity within the Bocca Nuova, with volcanic bombs flying well beyond the crater rim (in the right part of the photo), and ash emission (at left). Photo taken from the southwestern slope of Etna's central cone (which hosts both the Bocca Nuova and the Voragine) on the late forenoon of 27 February 2013 by Francesco Ciancitto, INGV-Osservatorio Etneo

On the evening of 27 February, intense glows were seen in the area of the Central Crater of Etna, and a number of observers in the Bronte area (on the western flank of Etna) and at Giarre and Riposto (on the lower east flank of Etna) clearly saw that the main source of these glows was the VOR, where vigorous Strombolian activity was taking place (Fig. 5a); hovever, it seems that minor and discontinuous Strombolian activity also occurred at the BN (Fig. 5b). This activity, which often launched incandescent volcanic bombs up to 150 m above the crater rim (Fig. 6a), continued through the night, and was still going on early on 28 February, when also the NSEC started showing signs of renewed activity after more than 4 days of repose.

Fig. 5. (a) Strombolian activity a the Voragine on the evening of 27 February 2013, photographed from Giarre (on the lower eastern flank of Etna). At left there is the NSEC cone, with several recent but dark (inactive) lava flows extending downslope; behind the NSEC a gas plume from the BN is illuminated possibly by weak Strombolian activity going on within that crater. At right, the NEC is emitting a dense gas column. The dark streak extending obliquely across the flank below the NEX, is the pyroclastic deposit of the 23 February 2013 paroxysm at the NSEC. Photo taken by Giovanni Puglionisi and published here with kind permission of the author. (b) Strombolian activity at the VOR (at left) and at the BN (at right) on the evening of 27 February 2013, seen from Adrano (southwestern flank of Etna). Photo taken by Roberto Schillaci and published here with kind permission of the author

Fig. 6. Frames extracted from video recorded by the the high-sensitivity visual surveillance camera of the INGV-Osservatorio Etneo (Catania) on the Montagnola  (EMOH) on the morning of 28 February 2013. (a) Jet of incandescent pyroclastics from the VOR at 03:22 GMT. (b) Simultaneous Strombolian activity at the VOR (in the center) and at the NSEC (at right) at 04:39 GMT.

28 February 2013 eruptive episode at the New Southeast Crater. During the early morning hours of 28 February 2013, before sunrise, the visual surveillance systems of the INGV-OE showed the onset of weak explosive activity at the NSEC (Fig. 6b). With growing daylight, sporadic weak ash emissions were seen at the crater. At 08:17 GMT, a small thermal anomaly appeared in the western part of the NSEC, as recorded by the thermal monitoring camera of the INGV-OE on the Montagnola (EMOT), which was caused by weak Strombolian activity in the area of the former "pittino". At 08:30 GMT, Strombolian activity was also underway at the main vent in the center of the NSEC. Contemporaneously, the volcanic tremore amplitude started to rise. During the following hour, the eruptive activitya at the NSEC gradually waxed, while the volcanic tremor amplitude increased rapidly, following the characteristic trend of the eruptive episodes in Etna's summit area; at 09:30, at least three vents were erupting: the former "pittino" to the west, and two vents within the This activity generated a dense gas plume containing modest quantities of volcanic ash.

Fig. 7. Lava fountain rising from the former "pittino", which lay in the saddle between the old SEC cone (at left) and the NSEC cone (which in this image is completely hidden by a dense veil of pyroclastics falling from the lava fountains), during the acme of the paroxysmal eruptive episode of 28 February 2013. The conspicuous cloud of white vapor and brown ash in the foreground is generated by the collapse of the "saddle" between the two cones. Photo taken from Monte Frumento supino, about 1.5 km south of the NSEC, by Francesco Ciancitto, INGV-OE

At 09:40 GMT, lava started to exit from the NSEC through the deep breach cutting the southeastern crater rim; 25 minutes later, explosive activity started to increase dramatically, and the first lava fountains appeared, rising about 100 m above the crater rim. From 10:15 GMT on, frequent powerful explosions generated visible shock waves  ("flashing arcs"), heavily showering the entire NSEC cone with large volcanic bombs. The activity intensified further between 10:17 and 10:22, accompanied by the appearance of a conspicuous cloud of vapor and brown ash that issued from the area of the "saddle" between the two cones of the SEC (Fig. 7). This cloud marked the progressive collapse of a large portion of  the "saddle", which destroyed nearly all of the southwestern flank of the NSEC cone, and also bit deeply into the eastern flank of the old SEC cone.

Fig. 8. Eruptive plume rising several kilometers high, produced by the eruptive episode at the NSEC on 28 February 2013, seen from Naso (ME), close to the Tyrrhenian (northern) coast of Sicily. Photo taken by Giorgio Costa and published here with kind permission of the author (original photo at Flickr)

The collapse was probably caused by magma pushing into this area already structurally weakened during the paroxysms of the previous week. A voluminous lava flow spilled from the deep notch left by the collapse, expanding first south and then turning southeastward, into the direction of the monitoring station of Belvedere. Lava was also emitted from the eruptive vent(s) at 2850 m elevation, at the base of the NSEC cone (2850 vent in Fig. 1); this lava mixed with the lava flow emitted directly from the NSEC toward southeast.

Sustained lava fountaining, with intense pyroclastic fallout also onto the old cone of the SEC and generation of a huge cloud of gas and ash, continued at maximum intensity for about 20 minutes. The ash cloud was pushed by strong wind toward east (Fig. 8), leading to abundant ash and scoria falls in the areas of Milo-Fornazzo on Etna's flank and Giarre-Riposto nbear the Ionian coast. At 10:42 GMT, the activity started to diminish, whereas from the collapsed "saddle" area, dense clouds of vapor and grayish-brown ash were emitted. At about 10:50, the activity of the former "pittino" became phreatomagmatic, with emission of vapor and ash, and ejection of hot, wet blocks that formed spectacular vapor trails. Shortly after 11:00 GMT, explosive activity at the NSEC ceased, whereas lava emission continued from the collapsed "saddle" area as well as from the southeastern flank of the NSEC cone, at a slowly diminishing rate.

Fig. 9. These images show the state of the monitoring station of Belvedere on the morning of 27 February 2013, after the partial invasion by lava flows of the early morning of 20 February 2013 paroxysm, and a dike intrusion passing beneath the site on the early morning of 21 February, which led to the opening of an effusive vent at 2800 m (marked as "2800 vent" in Fig. 1=) and conspicuous ground fracturing in the Belvedere area. The site was again invaded by a lava flow after the 28 February 2013 paroxysm, and the seismic station "EBEL" was destroyed. Photos taken by Stefano Branca, INGV-OE.

During the following hours, the lava flows continued to advance, both at the vents at 2850 m, into the direction of the Valle del Bove, and from the collapsed "saddle" area, advancing toward the Belvedere. On the afternoon of 28 February, lava invaded the area of the monitoring instruments of Belvedere, which had already been reached and partly buried by the lava of the paroxysm on the early morning of 20 February, and subjected to spectacular ground fracturing caused by the intrusion of a dike on the early morning of 21 February, which led to the opening of the effusive vent at 2800 m (2800 vent in Fig. 1). Fig. 9 shows the state of Belvedere on the day before the 28 February paroxysm; the instruments in this site continued to record and transmit data for several hours after the conclusion of the paroxysm. Unfortunately, on the afternoon of 28 February, the seismic station "EBEL" (Belvedere) failed, probably after being buried (and/or heated) by lava.

Lava emission from the vents in the "saddle" area and at 2850 m elevation continued during the night of 28 February-1 March 2013, and ceased sometime during 1 March. In contrast, the Strombolian activity within the VOR continued without significant variations, and was observed by INGV staff during a field visit on the morning of 1 March (Fig. 10). On this occasion, the entire eastern flank of the central cone of Etna was seen to be covered with a continuous pyroclastic deposit consisting of large bombs and spatter, probably produced during the 27 February BN and VOR eruptive episode. As this report goes online (3 March 2013, 14:00 GMT), Strombolian activity is still continuing at the VOR.

Fig. 10. Strombolian activity within the VOR, seen from the eastern crater rim on the late forenoon of 1 March 2013. The opposite, western crater rim (in the background) is covered with a fresh, black pyroclastic deposit. At right, the south flank of the NEC cone is visible; the BN is at extreme left. Photo taken by Francesco Ciancitto, INGV-OE

The most marked effect of the 28 February 2013 paroxysmal episode at the NSEC is the collapse of the "saddle" between the two SEC cones, which involved a large portion of the southwestern flank of the NSEC cone, and the eastern base of the old SEC cone (Fig. 11). From the collapse area, lava was emitted not only to feed the flow into the direction of Belvedere, but two small lava lobes also extended southwestward for little more than 100 m, between the small cones of the "Sudestino" and the 17 luglio 2001 hornito (Fig. 1).

Fig. 11. The collapsed area of the former "saddle" between the two SEC cones (the old one at left, and the new cone at right), 24 hours after the 28 February 2013 paroxysmal eruptive episode, seen from south. The collapse has entrained much of the southwestern flank of the NSEC cone as well as the lower east flank of the old SEC cone. In the central-left portion of the photo, in front of the old SEC cone, is the hornito of 2001 (see Fig. 1 for location). Photo taken by Tonino Giorgianni and published here with kind permission of the author.

Stromboli: new lava overflows from the crater terrace. After an interval of 10 days of normal Strombolian activity, Stromboli returned to producing small lava overflows from the crater terrace on the afternoon of 27 February 2013. This lava overflow ceased during the following night; a second episode of lava overflow started on the evening of 1 March 2013 and ceased during the afternoon of the next day. Both overflows were fed by continuous spattering (the ejection of fluid lava fragments) from vent N2, which lies at the top of a spatter cone (hornito) perched on the northern rim of the crater terrace.

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