The 18 April 2013 paroxysm of Etna's New Southeast Crater
Fig. 1. Aerial view of Etna during the paroxysmal phase of the eruptive episode on 18 April 2013, from east-northeast. The basal portion of the eruptive cloud is seen heavily charged with pyroclastic material, whereas at greater distance, the cloud, which is drifting south-southwestward, consists largely of vapor and other volcanic gas. Photo taken by Mauro Coltelli, INGV-Osservatorio Etneo.
On the early afternoon of 18 April 2013, the 11th episode of lava fountaining of this year occurred at Etna's New Southeast Crater (NSEC) dell'Etna, with emission of a cloud of gas and pyroclastic material, which was blown by the wind toward south-southwest (Fig. 1), and a voluminous lava flow directed toward the Valle del Bove and two much smaller lava flows, which expanded from the "saddle" between the two cones of the Southeast Crater (SEC) toward south and north.
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Fig. 2. Ash emission from the NSEC on the late afternoon of 17 April 2013. Photo taken from Tremestieri Etneo, 20 km south of the NSEC, by Boris Behncke, INGV-Osservatorio Etneo.
Fig. 3. Strombolian activity at the NSEC at daybreak on 18 April 2013, seen from Tremestieri Etneo. Photo taken by Boris Behncke, INGV-Osservatorio Etneo. |
The "prelude" to this new episode started on the evening of 16 April, with small puffs of ash and the ejection of hot pyroclastic material from a vent within the NSEC, which produced anomalies in the images of the thermal surveillance cameras of the INGV-Osservatorio Etneo (INGV-OE); during the night, sporadic glows were also visible, which were produced by weak Strombolian activity within the crater. The activity continued without significant variations throughout 17 April (Fig. 2) and during the following night. On the morning of 18 April, a gradual increase was observed in both the frequency and the intensity of the Strombolian explosions (Fig. 3). The seismic monitoring systems also showed a slow increase of the volcanic tremor amplitude accompanied by the usual shift of the volcanic tremor source from a position below the central part of the summit area toward the NSEC and toward shallower depth. During the late forenoon, the eruptive activity as well as the volcanic tremor amplitude showed a progressive increase, and around 11:00 GMT (=local time -2), the Strombolian activity was nearly continuous. During the next two hours, the activity went through its culminating phase, with tall lava fountains (Fig. 4) and a dense eruptive plume, whose lower portion was heavily charged with pyroclastic material. This plume was driven by the wind toward south-southwest (Fig. 5). Tephra (ash and lapilli) falls affected an area including the population centers of Ragalna, Belpasso and Paternò, and also the area of tourist facilities "Etna Sud" (which includes the Rifugio Sapienza and the Etna cable-car); in this latter location, the deposit of lapilli was several centimeters thick and contained clasts up to 5 cm in diameter. During the phase of lava fountaining, three lava flows were emitted, the largest of which exited through the deep breach in the southeastern rim of the NSEC, forming a copious flow toward the Valle del Bove. Locally, the lava caused the melting of residual patches of snow on the western headwall of the Valle, which led to the formation of small lahars. In the late afternoon, the lava fronts were still advancing, passing to the north of Monte Centenari, almost 4 km from their souce. Two other lava flows of much smaller dimensions were emitted from the "saddle" that separates the two cones of the SEC: one toward south, into the direction of the hornito formed at 2950 m elevation in 2001 (see the 12 April 2013 report), whereas the other flow advanced northward into the direction of the upper part of the Valle del Leone. These two minor lava flows expanded to distances of only a few hundred meters. No activity, neither explosive nor effusive, was observed at any of the vents that had opened during the preceding paroxysmal episodes on the lower southeastern and northeastern flanks of the cone. |
Fig. 4. Lava fountain at the NSEC during the paroxysmal eruptive episode on the early afternoon of 18 April 2013, seen from the northeast flank of Etna. Heavy fallout of large pyroclasts onto the flank of the cone creates a veil hiding the cone from view at left; the bluish fumes in the central of the photo, at the base of the old SEC done, rise from a small lava flow that interacts with snow on the ground at right. Photo taken by Boris Behncke, INGV-Osservatorio Etneo.
Fig. 5. Eruptive cloud produced during the paroxysmal eruptive episode at the NSEC of Etna on the early afternoon of 18 April 2013, seen from Catania, about 25 km south. The cloud, which consists mostly of vapor and other gas, is drifting south-southwest (left). Photo taken by Simona Scollo, INGV-Osservatorio Etneo.
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Fig. 6. Lava flow-field on the western slope of the Valle del Bove after the end of the 18 April 2013 paroxysm, in an image of the thermal surveillance camera of the INGV-OE on Monte Cagliato, about 8.8 km east of the NSEC. |
The main lava flow-field, on the western flank of the Valle del Bove, is well visible in Fig. 6. After the conclusion of the phase of lava fountaining, the activity passed into a long sequence of rather powerful explosions, which produced detonations that were heard in many nearby population centers especially on the southwestern flank of the volcano, and as far as Catania. These explosions continued until the late afternoon; a few sporadic explosions occurred in the evening. After a relatively calm night, explosive activity resumed on the late forenoon of 19 April with small ash puffs and ejection of hot pyroclastic material; this activity was still continuing as of 14:00 GMT on 19 April 2013. In its general outlines, the 18 April 2013 paroxysmal eruptive episode was more "typical" than the previous three paroxysms, with a "prelude" phase that lasted about 36 hours and a phase of rather determined and thus rapid acceleration, which passed from intense Strombolian activity into lava fountaining and ash emission in quite short time. The quantity of fallout pyroclastic material seems to have been relatively modest, whereas the volume of lava is more significant: the main lava flow is indeed one of the longest emitted during the entire series of paroxysms initiated in January 2011. The cone grew predominantly on its southern side, which had suffered significant loss in height (by a few tens of meters) during the first eruptive episodes of this year, especially on 19 February. |
Fig. 7. The morphological changes at the NSEC cone induced by the 11 episodes of lava fountaining between 19 February and 18 April 2013 are evident in these two comparison photographs taken from Tremestieri Etneo (20 km south of the crater), on the morning of 2 February (top) and on the evening of 18 April (bottom). Note the growth of the cone especially in correspondence with the northern rim of the crater. Photos by Boris Behncke, INGV-Osservatorio Etneo.
The growth of the NSEC cone during the latest sequence of paroxysmal episodes (11 events between 19 February and 18 April) is evident in the comparison photographs shown in Fig. 7, taken from Tremestieri Etneo, on the south flank of Etna, before the resumption of lava fountaining (Fig. 7 top) and after the conclusion of the 18 April paroxysm (Fig. 7 bottom). In the more recent of these images, the rise of the northern rim of the crater is particularly notable, whereas the southern rim has experienced little growth in height since 2 February; this portion of the cone had been drastically reduced in height during the first paroxysm of this year on the early morning of 19 February.