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The 3 April 2013 paroxysm of Etna

Fig. 1. Eruption column produced during the culminating phase of the 3 April 2013 eruptive episode at Etna's New Southeast Crater (NSEC), seen from Catania, about 25 km to the south of the crater. Note the brownish plume descending the eastern (right) flank of the volcano, generated by the collapse of material and pyroclastic flows from the northeastern flank of the NSEC cone. Photo taken by Simona Scollo, INGV-Osservatorio Etneo.

After almost 18 days of relative quiet, the New Southeast Crater (NSEC) of Etna produced its 9th episode of lava fouintaining (paroxysm) of this year on the afternoon of 3 April 2013 (Fig. 1). This event is distinct from its predecessors for the short duration of lava fountaining (less than 30 minutes) and for the continuous, powerful explosions, which generated loud detonations audible up to tens of kilometers away. Moreover, new eruptive vents opened in the western and northeastern sectors of the cone. As during the 16 March 2013 paroxysm, the cloud of pyroclastic material was blown eastward by the wind, and the fall of lapilli and ash affected essentially the same areas that had suffered damage due to the heavy lapilli shower on 16 March.

Fig. 2. Evolution of eruptive activity at the NSEC during the early phase of the 3 April 2013 eruptive episode. The eruptive vent in the "saddle" between the NSEC and the old SEC cone is marked with "SV". Photos taken from Monte Vetore, 6.4 km southwest of the NSEC by Boris Behncke, INGV-Osservatorio Etneo.

The first signs of renewed activity were observed on the morning of 2 April 2013, when repeated ash emissions occurred from the NSEC, which produced small puffs of grayish brown color, without initially generating anomalies in the images recorded by the thermal surveillance cameras of the INGV-Osservatorio Etneo (INGV-OE), on the Montagnola and on Monte Cagliato. From the late forenoon of the same day, cloud cover prevented further visual observations; these conditions continued also during the following night. However, glow was sporadically observed during the night at the NSEC, clear evidence for Strombolian activity underway.

On the morning of 3 April, the volcanic tremor amplitude started to show a slow increase, accompanied by numerous signals produced by the Strombolian explosions. Visual observations by INGV-OE staff in the field initiated at 11:30 GMT (=local time -2) revealed that vigorous Strombolian activity was taking place from one or two vents within the NSEC (Fig. 2 top), with jets of pyroclastic materials rising up to a few tens of meters above the crater rim. In this phase, no activity was observed at the vent that lies in the "saddle" between the NSEC and the old cone of the Southeast Crater (SEC); the first image in Fig. 2 shows the small cone formed at the "saddle" vent (SV) during the previous paroxysms still covered with snow.

Between 12:00 and 12:30 GMT, the activity of the NSEC progressively intensified, with frequent, powerful Strombolian explosions, which often launched great quantities of incandescent bombs with diameters of many meters onto the flanks of the cone and generated loud bangs (Fig. 2 top). Shortly after 12:35 GMT, ash emission started from SV (Fig. 2 center), followed a few minutes later by Strombolian explosions from the same vent. At 12:50, a continuous jet of incandescent lava started to rise from this vent, forming a thin lava fountain up to 80-100 m tall, whereas the vents within the NSEC continued to produce powerful and loud explosions, but no lava fountains (Fig. 2 bottom).

For a protracted period, the eruptive activity went on in a rather regular way, even though with notable fluctuations in terms of intensity and frequency of the explosions from the vents within the NSEC, alternating between vertical and oblique jets of pyroclastic material, and the explosion of large blisters of lava. At around 13:05 GMT, a lava flow started to move through the deep breach in the southeastern rim of the NSEC, and then descended toward the western rim of the Valle del Bove before spilling down the steep western slope of the Valle. During the same time interval, lava emission started also from SV feeding a flow toward south.

Already since 12:30 GMT, the eruptive plume, which was bent southeastward by the strong wind, contained modest amounts of volcanic ash, but around 13:40 ash emission started to increase progressively. Contemporaneously, there was also an increase in the intensity of the eruptive activity, and the volcanic tremor amplitude showed a rapid rise.

Fig. 3. Evolution of the eruptive activity of the NSEC on the afternoon of 3 April 2013, passing from strong Strombolian activity into lava fountaining. "SV" marks the vent lying in the "saddle" between the old and new Southeast Crater cones. Photos taken from Monte Vetore, 6.4 km southwest of the NSEC by Boris Behncke, INGV-Osservatorio Etneo.

During the time interval between 13:40 and 14:15 GMT low lava fountaining continued from SV, whereas the vents winthin the NSEC emitted intermittent, pulsating lava fountains, which alternated with powerful detonations caused by exploding lava blisters (see Fig. 3). The incandescent jets from the vents within the NSEC rose up to 400 m above the crater rim. At 14:15 GMT, the lava fountaining activity at SV showed a rapid intensification, with jets rising 400-500 m high. This activity was accompanied by a deep, continuous roar, the characteristic noise accompanyhing sustained lava fountaining; at the same time, the explosions from the vents within the NSEC continued, producing loud detonations every 1-2 seconds.

The lava fountaining activity showed a significant diminution between 14:25 and 14:28 GMT, but soon gained new strength (Fig. 4); in this interval, a new eruptive vent (NV) opened a few tens of meters to the west of SV, on the eastern slope of the old SEC cone; this vent emitted grayish-brown ash. In some moments, ash emission from NV was more forceful and pulsating, whereas it was rather passive at other times; the ash plume was driven past the south flank of the NSEC by the wind. In the meantime, a dense cloud of pyroclastic material emitted by the NSEC vents and SV rose about 2 km high before being bent southeastward by the strong wind (Fig. 5).

Fig. 4. Lava fountaining at the NSEC and formation of a dense cloud of pyroclastic material during the culminating phase of the 3 April 2013 eruptive episode. The position of the "saddle" vent is marked with "SV, whereas "NV" denotes a new eruptive vent, located on the eastern flank of the old SEC cone, which is seen at left in the 14:35 GMT image. Photos taken from Monte Vetore, 6.4 km southwest of the NSEC by Boris Behncke, INGV-Osservatorio Etneo.

Fig. 5. Eruptive plume produced during the culminating phase of the paroxysmal episode at the NSEC on 3 April 2013, seen from Monte Vetore, 6.4 km southeast of the crater. Note the dark curtain of falling pyroclastic material below the eruptive plume, extending across the southeastern flank of the volcano. The Ionian sea is seen in the distance at right. Mosaic composed of three photos taken by Boris Behncke, INGV-Osservatorio Etneo.

Fallout of pyroclastic material affected almost the same area that had already been subjected to the heavy shower of lapilli on 16 March 2013, with the population centers of Zafferana Etnea and Santa Venerina on Etna's southeast flank, and the northern part of Acireale plus a number of smaller villages to the north up to the southern margin of Giarre, in the Ionian area. However, the deposit was thinner than that of 16 March, and the dimensions of the lapilli were notably smaller. More detailed reports (in Italian) describe the characteristics of the 3 April 2013 pyroclastic fall deposit and of the 16 March 2013 pyroclastic fall deposit.

Between 14:30 and 14:40 GMT, the eruptive activity reached a new peak in terms of explosive intensity with sustained lava fountains from SV and powerful explosions from the vents within the NSEC. At 14:37 the thermal surveillance camera of the INGV-OE on Monte Cagliato, on the eastern flank of Etna, recorded the formation of a pyroclastic flow (PDC in Fig. 6), which originated from the northeastern flank of the NSEC cone. Immediately after the passage of this pyroclastic flow, a lava flow was seen to emerge from the same area (LF in Fig. 6d), which started to descend toward the Valle del Bove. Shortly thereafter, a second lobe of lava appeared a bit further to the south, running parallel to the first flow.

Fig. 6. Development of a pyroclastic flow followed by the emission of a lava flow from the northeastern flank of the NSEC cone, at 14:37-14:39 GMT on 3 April 2013, documented in this series of frames captured from video recorded by the thermal surveillance camera of the INGV-OE at Monte Cagliato (EMCT). "LF" in frame (a) indicates the fronts of the lava flow emitted through the breach in the southeastern rim of the NSEC; "PDC" indicates the pyroclastic flow; "LF" in frame (d) denotes a new lava flow emerging from the same zone where the pyroclastic flow had originated. 

Fig. 7. Second pyroclastic flow descending from the northeastern flank of the NSEC cone, at 14:57 GMT on 3 April 2013, documented in this series of frames captured from video recorded by the thermal surveillance camera of the INGV-OE at Monte Cagliato (EMCT). "LF" indicates lava flows; "PDC" denotes the pyroclastic flow.

After 14:40 GMT, lava fountaining activity showed a rapid diminution and passed into a long series of rather violent explosions, which generated detonations heard many tens of kilometers away, vibrating doors and windows in population centers particularly on the eastern flank. During this phase of powerful explosions, a second pyroclastic flow descended from the northeastern flank of the NSEC cone (Fig. 7), reaching a length of about 1.5 km. The ash cloud rising from this flow was of brown color (Fig. 8), indicating a high proportion in its content of material derived from a collapse on the cone's flank, rather than juvenile material.

Fig. 8. This photo sequence shows the descent of the second pyroclastic flow, at 14:57 GMT on 3 April 2013, seen from the Schiena dell'Asino on the southern rim of the Valle del Bove. Note the difference in color between the gray cloud of pyroclastic material emitted from the NSEC, and the brownish hue of the pyroclastic flow, which was spawned by collapse on the northeastern flank of the NSEC cone, related to the opening of new eruptive vents on that flank. The photos were taken by Paola Garofalo and are published here with kind permission of the author.

The series of strong explosions at the conclusion of the paroxysm lasted until about 15:05 GMT; subsequently, brown ash continued to be emitted from the vents SV and NV, and was eventually followed by the emission of dense volutes of white vapor from the same area. This phase of diminishing activity was accompanied by the usual rapid return of the volcanic tremor amplitude to normal levels. After nightfall, the lava flows emitted during the paroxysm were still brightly incandescent (Fig. 9), and slow lava emission continued from several vents on the southern and northeastern sides of the cone. The volume of lava emitted during this paroxysm was notably greater than those of the lava flows of the previous paroxysms. All lava emission ceased during 4 April 2013.

Fig. 9. Lava flows emitted during the paroxysmal episode on 3 April 2013, still incandescent and locally active, on the evening of the same day. In the left part of the image is the main lava flow field, generated by lava overflow through the breach in the southeast rim of the NSEC; at upper left is a smaller flow fed by the "saddle" area between the two cones of the SEC. In the center, there are two still active lava flows, fed from eruptive vents on the lower northeast flank of the NSEC cone, in the same area where the two pyroclastic flows of 14:37 e 14:57 GMT originated. Photo taken by Giovanni Puglionisi and published here with kind permission by the author.

The 3 April 2013 paroxysm is distinct from its predecessors (33 paroxysms between 12 January 2011 and 16 March 2013) for a number of reasons. The intensity of lava fountaining activity was notably minor compared to that of the previous paroxysms, and it lasted for a relatively short time (less than 30 minutes), whereas discrete, and unusually loud explosions were predominant during this episode, even during the phase of sustained lava fountaining. Consequently, the fallout of pyroclastic material, even though affecting the same general area that had been stricken by the 16 March fallout, was less abundant and the sizes of lapilli were smaller, whereas the entire cone of the NSEC and surrounding areas were subjected to a heavy and constant bombardment of meter-sized bombs. Another noteworthy detail is the activation of new eruptive vents in the western and northeastern sectors of the cone. The westernmost vent, "NV", lies on the eastern slope of the old SEC cone, and was the site of mostly lithic ash emission, without any evidence for magmatic activity. On the contrary, the activity on the northeastern flank of the cone was clearly magmatic, with emission of several lava flows, which are conspicuous in the two photos of Fig. 10. These images also show the presence of two depressions, with diameters of about 100 and 50 m, on the northeastern flank of the cone (Fig. 10 bottom); these are probably two explosive vents from which originated the two pyroclastic flows of 14:37 and 14:57 GMT. Alternatively, the depression might be the detachment scars of slides of hot material that had accumulated rapidly and abundantly on a steep slope; such slides might also have given origin to the pyroclastic flows.

Fig. 10. The scars of the 3 April 2013 paroxysm on the northeastern flank of the NSEC cone, caused by the opening of new vents and collapse of unstable, hot material, which led to the generation of the pyroclastic flows of Fig. 6-8. Top image, photographed on 4 April 2013, shows the entire NSEC cone with the main lava flow field to the southeast at left, and the lava flows emitted from vents at the northeastern base of the cone at right, as well as the deposit of the pyroclastic flows in the lower central portion of the view. The second image, below, of 6 April 2013, shows what is probably a fracture with two eruptive vents (the depressions marked by a reddish-brown color in the center of the view)m and from which the two pyroclastic flows originated. The view in both images is from Giarre, looking west. Photos taken by Turi Caggegi and published here with kind permission by the author.

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