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The 20 April 2013 paroxysm of Etna's New Southeast Crater

Fig. 1. Eruption column of the lava fountaining episode (paroxysm) of 20 April 2013, seen from near Giarre, on the lower east flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Little more than two days after the lava fountaining episode of 18 April 2013, Etna's New Southeast Crater (NSEC) has produced a new paroxysm on the afternoon of 20 April 2013, the 12th episode of this year, and the 37th since the beginning of the current series of paroxysms in January 2011. This has been one of the most violent events of the series, with lava fountains that rose to heights of 800-1000 m, and with the formation of a tall column of gas and pyroclastic material, which was blown eastward by the wind (Fig. 1), as well as emission of a lava flow toward the Valle del Bove.

Fig. 2. Gas and ash plume seen at sunset on 19 April 2013 from Tremestieri Etneo, 20 km south of the NSEC. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

After the cessation of the eruptive activity at the end of the 18 April paroxysm, the volcano remained in a quiescent state for little more than 12 hours; already on the early afternoon of 19 April, discontinuous explosive activity resumed at the NSEC, with emission of ash puffs and occasional jets of hot pyroclastic material. This activity continued until the late evening, creating a conspicuous trailing plume of dark ash at dusk (Fig. 2). At night, intermittent glows were visible at the NSEC, and the surveillance cameras of the INGV-Osservatorio Etneo (INGV-OE) recorded sporadic Strombolian explosions.

This explosive activity ceased on the late evening, but shortly thereafter, slow effusive activity started from the lower of the two effusive vents that had formed at the end of the 12 April 2013 episode at the base of the NSEC cone. A small lava flow began to expand, initially very slowly, toward the Valle del Bove; during the following hours, the effusion rate gradually increased. At the same time, there was a slow rise in the volcanic tremor amplitude. However, explosive activity was nearly completely absent, and the volcano continued in this manner also during the morning and until the early afternoon of 20 April.

At around 14:00 GMT (=local time -2) on 20 April, the lava flow had traveled approximately 1.5 km from its source, advancing slowly on the steep western slope fo the Valle del Bove.

At approximately 14:30 GMT, the increase in the volcanic tremor amplitude accelerated; still there were practically no signs of explosive activity, but shortly after 15:00 the visual and thermal surveillance cameras of the INGV-OE recorded ash puffs rising from the NSEC (Fig. 3 top left), followed at 15:13 by the ejection of hot pyroclastics (Fig. 4 top left and Fig. 5 top left). Within a few minutes, these ejections became continuous, passing into a continuous lava fountain accompanied by the formation of a tall eruption column (Fig. 1 and 3-7), which was bent eastward by the wind, resulting in ash and lapilli falls over a broad area in the eastern sector of the volcano. The areas most heavily stricken by the tephra falls (ash and lapilli) extends from the southern margin of Guardia-Mangano to Fiumefreddo along the Ionian coast, including the towns of Giarrre, Riposto and Mascali, and higher upslope, encompasses the towns of Santa Venerina, Zafferana, Milo and Sant'Alfio.

Fig. 3. Evolution of the 20 April 2013 paroxysm at the NSEC, as documented by the visual surveillance camera of the INGV-Osservatorio Etneo on the Montagnola (EMOV). The first frame, at top left, shows the first signs of the onset of a new paroxysm: a puff of ash. This first ash emission was followed by others in rapid succession and increasingly intense, and 10 minutes later, by the emission of incandescent pyroclastics. The huge lava fountain is visible in the frames at top right and bottom left; the last frame at bottom right shows the last ash emissions at the conclusion of the paroxysm. 

Fig. 4. Four frames extracted from video recorded by the thermal surveillance camera of the INGV-Osservatorio Etneo on Monte Cagliato (EMCT) during the 20 April 2013 paroxysm at the NSEC. The rapid evolution of the explosive phenomena is well evident, from the first small thermal anomaly at the crater in the top left frame (besides the much more conspicuous thermal anomaly corresponding to the lava flow already active since the previous night) to a high lava fountain only fifteen minutes later (top right); heavy fallout of hot pyroclastic material onto the cone (bottom left); and finally, the end of the paroxysm (bottom right).

Fig. 5. Four scenes of the 20 April 2013 paroxysm at the NSEC, captured by the thermal surveillance camera of the INGV-Osservatorio Etneo on the Montagnola (EMOT). Top left frame shows the first thermal anomaly indicating strong explosive activity recorded at 15:13 GMT; top right shows the crater in full lava fountaining activity and the fall of large hot pyroclastics (in the white circle) as far as the area of the Bocca Nuova; bottom left frame shows lava fountain during the phase of maximum intensity of the eruptive activity; bottom right frame, recorded after the end of the paroxysm, shows the NSEC cone and also part of the old SEC cone ("old SEC") covered with hot material.

Fig. 6. The phase of maximum eruptive intensity during the 20 April 2013 paroxysm at the NSEC, documented in these images of the mobile visible (MBV; top frames) and thermal (MTB; bottom frames) surveillance cameras on the Schiena dell'Asino. Note how the sustained jet of lava extends upward well beyond the upper margin of the images.

Fig. 7. Eruption column rising to about 7 km above the summit of Etna, produced during the lava fountaining episode on 20 April 2013, seen from Adrano, on the southwestern flank of Etna. Photo taken by Laura Musarra and published here with kind permission of the author (original photo on Flickr).

Fig. 8. A flow of vapor and ash, accompanied by a lahar (marked by white vapor in foreground), formed at the contact between lava and snow on the steep western slope of the Valle del Bove during the final phase of the paroxysmal episode of 20 April 2013, seen from the area of Miscarello, on the eastern flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Fig. 9. Ash emission at the end of the paroxysmal eruptive episode of 20 April 2013, seen from the Miscarello area on the east flank of Etna, with the road signs endemic in the areas frequently affected by tephra (ash and lapilli) falls. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

During the lava fountain, the effusion rate increased notably at the eruptive vent at the base of the NSEC cone, which had been active since the late evening of 19 April, and the lava descended in several branches on the western slope of the Valle del Bove. During its descent, the lava locally interacted explosively with accumulations of show that were still present on the ground, generating lahars and vapor-and-ash flows (Fig. 8) like those already seen during numerous earlier eruptive episodes in 2011-2013. Differently from the preceding paroxysms, there was neither any lava emission from the NSEC proper, nor was there any effusive activity at the "saddle" between the two SEC cones or through the breach in the southeastern crater rim. In contrast, all explosive activity was concentrated within the NSEC, though it was impossible to ascertain whether one or more vents were active within the crater.

Lava fountaining activity continued strongly for about one hour and then showed the first signs of weakening around 16:15 GMT; at 16:25, the activity was reduced to powerful explosions and ash emissions (Fig. 9), and at 16:40 GMT the paroxysm was essentially over. Also the volcanic tremor amplitude showed a rapid diminution, without, however, completely returning to background levels. On the evening, the lava flow issuing from the effusive vent at the southeast base of the NSEC cone was still well-fed (Fig. 10). During the day of 21 April 2013, very bad weather precluded any visual observation until the evening, when the visual surveillance cameras showed sporadic Strombolian explosions accompanied by small ash puffs from the NSEC, and the emission of a small lava flow from the effusive vent at the southeast base of the cone. During the early hours of 22 April, weather conditions deteriorated once more, and Etna remained invisible as of 10:00 GMT on 22 April 2013.

Fig. 10. Lava flow issuing from the effusive vent located at the southeastern base of the NSEC cone, still active on the evening of 20 April 2013, seen from Tremestieri Etneo, on the south flank of Etna. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.

Placed into the context of the recent activity of Etna, and in particular the activity at the NSEC since 2011, the 20 April lava fountaining episode has been one of the most violent in the current eruptive sequence, and marks a notable change in the dynamics of the NSEC activity compared to the paroxysms of 3 and 12 April. This is manifest foremost in the shortening of the interval between one paroxysm and the next, from 18 days (between the 16 March and 3 April episodes) to two days between the latest two paroxysms. During the last three episodes, the duration of the "prelude" phase has progressively become shorter; most notably the time from the first ash puffs to sustained lava fountaining was extremely short (about 15 minutes). This episode is furthermore distinct for the relative simplicity of the eruptive theater, with all explosive activity concentrated at the NSEC (except for a few small ash emissions that seem to have come from the "saddle" area), and all effusive activity limited to the single eruptive vent lying at the southeastern base of the cone. Finally, after the end of the 20 April 2013 paroxysm, the monitored parameters have not returned to background levels (i.e., quiescence), indicating that the volcano is currently experiencing a phase of more vivid dynamics compared to the previous weeks.

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