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Etna update, 13 August 2012

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Figure 1. Strombolian activity and intracrater lava flow within the Bocca Nuova on the evening of 30 July 2012, seen from the western rim of the crater. Note the pyroclastic cone growing around the eruptive vent, at the base of the southeastern wall of the Bocca Nuova. Lava emission is occurring from vents located on the flank of the cone, in this case on the southern flank. Photo taken by Francesco Ciancitto, INGV-Osservatorio Etneo

Eruptive activity within the Bocca Nuova, the largest of Etna's summit craters, is continuing (Figure 1), but in the past 3 weeks has assumed a markedly episodic character. This new phase of activity started on the evening of 2 July 2012, and was virtually continuous for the first three weeks, until 24 July, when the activity showed a rapid diminution. Subsequently, there have been 5 episodes of intense Strombolian activity with intracrater lava emission, separated by low-level activity with sporadic explosive emissions of ash and blocks. These variations in the intensity of the eruptive activity were reflected in strong fluctuations in the volcanic tremor amplitude recorded by the seismic surveillance network of the INGV-Osservatorio Etneo (INGV-OE).

Continuous activity, 2-24 July 2012

During this three-week interval, Strombolian activity within the Bocca Nuova went on without interruptions but at varying intensity. For the first 10 days, explosions occurred in series separated by quiet intervals lasting several minutes; later the explosive activity became more continuous. In the moments of strongest activity, some lava bombs were thrown beyond the crater rim, landing on the gently sloping area to the east of the crater. Lava emission started on 4 July and remained continuous until 24 July, but occurred from frequently shifting effusive vents on the flanks of the pyroclastic cone that had begun to grow around the explosive vent. In a few rare cases, lava was emitted from the main explosive vent. The lava progressively covered the crater floor next to the pyroclastic cone, where there were three depressions (subsidence pits); the first of these was completely filled by 15 July, and subsequently lava began to fill the second, central pit (Figure 2).

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Figure 2. Weak Strombolian activity and intracrater lava flow within the Bocca Nuova on the evening of 21 July 2012, seen from the western crater rim. Note the relatively modest quantity of newly erupted (black) material that has accumulated during the first 9 days of activity on the crater floor; lava has filled a subsidence pit next to the growing pyroclastic cone, and has begun to spill into a second, larger pit more to the west. A third pit, visible in the foreground, has not yet been reached by new lava. Photo taken by Pierre Briole, Laboratoire de Géologie de l’Ecole normale supérieure, Paris (Francia)

In its general outlines, the activity of this period was rather typical for the eruptive activity of the Bocca Nuova of the past few decades. Differently from the activity of the 1990s and until the spring of 2001, this time only one single vent is active, which is located at the base of the southeastern inner crater wall. The pyroclastic cone growing around this vent had reached a height of about 30-40 m before the activity abruptly ceased on 24 July. The next day, frequent ash emissions were observed, which were mostly due to deep-seated explosions, and possibly also minor collapse of the summit area of the new cone. Contemporaneously with the diminution in the eruptive activity, the volcanic tremor amplitude dropped to background levels.

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Figure 3. Eruptive episode of 26-27 July 2012 at the Bocca Nuova. Strombolian activity is taking place from a vent a few meters wide that lies within a larger depression truncating the new pyroclastic cone, which possibly formed by subsidence during the interval of relative quiescence on 24-25 July. The view is from the western crater rim. Photo taken on 26 July by Francesco Ciancitto, INGV-Osservatorio Etneo

Episodic activity, 26 July 2012 - ?

First episode, 26-27 July 2012. At daybreak on 26 July, a strong glow marked the resumption of eruptive activity within the Bocca Nuova, with the same characteristics as the previous activity, with frequent Strombolian explosions (Figure 3) accompanied by the emission of a lava flow from an effusive vent located on the western flank of the cone. The next day, the lava had almost completely filled the central subsidence pit on the floor of the Bocca Nuova, and the cone had grown in height, but the activity showed a clear diminution, and essentially ceased by the evening. As during the previous quiescent interval, on 28 July there were intermittent ash emissions, while the volcanic tremor amplitude was at background levels.

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Figure 4. Eruptive episode of 29 July - 1 August 2012 at the Bocca Nuova. Strombolian activity from the vent at the top of the new pyroclastic cone, and lava flow on the southern flank of the cone. The view is from the southern crater rim. Photo taken on the evening of 29 July by Francesco Ciancitto, INGV-Osservatorio Etneo

Second episode, 29 July - 1 August 2012. During the day of 29 July, eruptive activity resumed within the Bocca Nuova in a rather gradual manner, with small Strombolian explosions and lava emission from a vent located on the southern flank of the pyroclastic cone (Figure 4). During the following days, the eruptive activity continued with fluctuations, and the volcanic tremore amplitude showed strong oscillations, without ever reaching the same peak levels associated with the 26-27 July episode. On the afternoon of 1 August, the volcanic tremor amplitude dropped sharply, marking the cessation of Strombolian and effusive activity; on 2 August, a few ash emissions were observed.

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Figure 5. Explosive ash emission from the pyroclastic cone formed witin the Bocca Nuova during the current eruptive period, on 9 August 2012. These explosions occur during the intervals of relative quiet that separate the episodes of more intense activity. Photo taken from the southern crater rim by Marco Neri, INGV-Osservatorio Etneo (Catania)

Third episode, 3-4 August 2012. On the evening of 3 August, a rapid increase in the volcanic tremor amplitude and a strong glow over the Bocca Nuova signaled the beginning of a new episode of Strombolian activity, once more accompanied by the emission of lava onto the crater floor. After reaching a peak in its intensity around midnight, the activity began to diminish on the morning of 4 August, and ceased on the evening of the same day. Also this episode was followed by weak residual activity within the Bocca Nuova, which generated small ash puffs.

Fourth episode, 6-7 August 2012. This episode began on the afternoon of 6 August, and showed essentially the same characteristics as the preceding episode. The activity peaked shortly before midnight, and then diminished to cease before noon on 7 August. As in the previous cases, the following interval of relatively quiescence was charaterized by sporadic emissions of ash, which were clearly caused by deep-seated explosions (Figure 5).

A field visit by INGV-Osservatorio Etneo staff on 9 August revealed that after the cessation of the previous eruptive episode, the summit portion of the new intracrater pyroclastic cone had partially collapsed (Figure 6), most probably due to the subsidence of the magmatic column within the conduit.

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Figure 6. The pyroclastic cone formed during the recent activity within the Bocca Nuova, seen during an interval of relative quiet on 9 August 2012, from the southern crater rim. Note the wide collapse depression that occupies the summit of the cone. Photo taken by Marco Neri, INGV-Osservatorio Etneo

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Figure 7. Intense glow caused by vigorous intracrater activity within the Bocca Nuova during the eruptive episode in the night of 10-11 August 2012, seen from southwest. Photo taken by "Basalto Vulcanico" and published here with kind permission of the author

Fifth episode, 10-11 August 2012. After more than three days of relative quiet, a new eruptive episode started within the Bocca Nuova on the late afternoon of 10 August. Once again, the beginning of the episode was marked by a rapid increase in the volcanic tremor amplitude; at nightfall, a bright glow was illuminating the gas plume rising from the crater, which was visible even tens of kilometers away (Figures 7 and 8). Some of the explosions launched incandescent pyroclastic material well above the crater rim. The activity started to diminish shortly after midnight, and came to an end during the late forenoon of 11 August. Subsequently, ash emissions resumed, which were particularly frequent on 13 August, and which produced small puffs of fine-grained material of brownish-gray color. On the evening of the same day, some of the ash emissions were accompanied by the expulsion of incandescent material; however, the volcanic tremor amplitude remained at background levels.

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Figure 8. Eruptive episode of 10-11 August 2012 at the Bocca Nuova. Strombolian activity and intracrater lava emission generate a strong glow visible at night, even at distances of several tens of kilometers. This image shows the view from Piano Vetore, on the southwestern flank of Etna; the conspicuous cone seen to the right of the tree is the Montagnola. Photo taken by Piero Giuffrida and published here with kind permission of the author
 
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