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Activity of the Sicilian volcanoes during July 2011

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An emblematic image illustrating the impact of the eruptive activity of Etna on the populated areas affected by ash falls during the month of July 2011: a scene captured at the entrance of the village of Milo, on the eastern flank of the volcano, after two ash falls from the paroxysms of 19 and 25 July. 36 hours later, this village and other population centers already affected by the previous ash falls, would receive another shower of pyroclastic material on the evening of 30 July 2011. Photo taken during the forenoon of 29 July 2011 by Boris Behncke, INGV-Catania

ETNA

July 2011 was marked by a notable intensification of the eruptive activity of Etna, with four paroxysmal eruptive episodes from the crater located on the eastern flank of the old Southeast Crater (SEC) cone, and a brief phase of intracrater Strombolian and effusive activity within the Bocca Nuova (BN), the first significant magmatic activity at this crater since 10 years. During the whole month, strong degassing was observed at the Northeast Crater (NEC), where the common deep-seated explosive activity continued.

Southeast Crater

During the first few days of July, the usual fumarolic activity occurred from an extensive area on the upper eastern slope of the old SEC cone, which last produced a paroxysmal eruptive episode on 6-7 May 2007. On 5 July, weak Strombolian activity started within the new crater (formerly known as "pit crater), which is located about halfway up the east flank of the old cone; this activity continued during the following days and then started to increase during the night of 7-8 July. This intensification was accompanied by a fast increase in the volcanic tremor amplitude; however after 0800 GMT (= local time -2), both the volcanic tremor amplitude and the Strombolian activity showed a marked  diminution, followed by the complete cessation of eruptive activity during the forenoon of 8 July.

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Eruption column rising several kilometers above the active crater located on the eastern flank of the old Southeast Crater cone (just visible to the left of the base of the eruption column) during the culminating phase of the paroxysmal episode of 9 July 2011. View from near the Astrophysical Observatory of the University of Catania, on the southwestern flank of Etna. Photo taken by Simona Scollo, INGV-Catania

9 July 2011 paroxysm

On the morning of the following day (9 July) about 0600 GMT, Strombolian activity resumed, and rapidly intensified during the following hours, accompanied by a significant increase in the volcanic tremor amplitude; at the same time the source of the volcanic tremor shifted from its former position below the NEC toward the SEC. Starting from 1115 GMT, lava spilled over the eastern rim of the crater, while Strombolian explosions launched pyroclastics that fell onto the flanks of the cone surrounding the crater. At about 1350 GMT the Strombolian activity passed into lava fountaining with the formation of an eruption column, which caused a rain of pyroclastic material over the southern flank of Etna, affecting the city of Catania and numerous smaller population centers such as Nicolosi, Pedara, Trecastagni, Mascalucia, Gravina and other nearby villages. The ash fall also led to the closure of the Fontanarossa airport of Catania, where operations resumed on the morning of 10 July.

Contemporaneously, the lava flow divided into several lobes descending the western slope of the Valle del Bove; the most advanced front reached an elevation of about 1900-2000 m and stopped just above M. Centenari. The eruptive activity started to diminish from about 1515 GMT when the activity returned to Strombolian explosions, and completely ended around 1740 GMT. A field survey carried out by INGV-Catania staff during the paroxysm allowed to sample the eruptive products to an elevation of about 2800 m, at the Belvedere site, where bombs with maximum dimensions of 30-40 cm were found.

The paroxysm of 9 July was the fifth episode of this type at the active crater on the east flank of the old SEC cone in this year, and had been preceded by a quiescent interval of 58 days, after the previous paroxysm early on 12 May 2011. This event was preceded by a very rapid augmentation in the intensity of the activity (from modest Strombolian activity to full sustained lava fountaining and eruption column), much more so than during the previous paroxysmal episodes.

19 July 2011 paroxysm

One week after the paroxysm of 9 July, on the morning of the 16th, a series of ash emissions from the active crater on the east flank of the old SEC cone heralded the resumption of Strombolian activity, which generated loud bangs; the explosions diminished during the afternoon of the same day, but weak Strombolian activity continued on the next day. On the evening of 18 July, the Strombolian activity started to increase in vigor and frequency of the explosions.

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Lava flow emitted onto the western slope of the Valle del Bove during the 19 July 2011 paroxysm, seen from Monte Zoccolaro shortly after the cessation of lava fountaining from the active vent on the east flank of the old Southeast Crater cone (which is seen at the upper left to the lava flow). A small rheomorphic lava flow is seen descending the south flank of the pyroclastic cone surrounding the active crater, this is the tiny lobe seen just below the peak of the old Southeast Crater cone. Photo taken by Salvatore Allegra and published here with kind permission of the author. (Source: Salvatore Allegra on Flickr)

At about 0000 GMT on 19 July, contemporaneously with an abrupt increase in the explosive activity, lava started to overflow the eastern rim of the crater, and a few minutes later a pulsating lava fountain began to produce jets of lava that were often strongly inclined toward south, causing heavy fallout of fluid spatter on that side of the cone surrounding the active crater. The lava flowed eastward, toward the steep western slope of the Valle del Bove, splitting into various lobes, and reaching approximately the same distance as the lava flow of 9 July, stopping just above M. Centenari.

At the same time, the heavy showering of the southern flank of the cone with fluid spatter generated a few rheomorphic flows, the largest of which reached the base of the cone and then turned southeastward, reaching a length of a few hundred meters. A dense cloud of gas and ash rose approximately 1.5-2 km above the summit of the volcano, and was bent eastward by the wind. This led to tephra falls in the eastern sector of the volcano, affecting population centers such as Fornazzo, Milo and down to the Ionian coast near Giarre and Riposto. The paroxysmal activity continued without major variations until 0230 GMT, after which the lava fountain became intermittent and was finally followed by a long series of powerful explosions accompanied by loud detonations that were heard over a vast sector of the volcano. The activity completely ceased around 0300 GMT.

25 July 2011 paroxysm

On the evening of 24 July, shortly after sunset, vigorous Strombolian activity started from the active crater on the east flank of the old SEC cone. This activity progressively increased through the night, contemporaneously with an augmentation of the volcanic tremor amplitude, and toward 0130 GMT on 25 July, lava started to overflow the east rim of the crater, forming a lava flow that followed the same path as the lavas emitted during the previous paroxysms. Between 0230 and 0300 GMT, the Strombolian activity gradually passed into a pulsating lava fountain, which was accompanied by increasingly voluminous ash emission.

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Lava fountain and ash plume, with abundant pyroclastic fallout in the areas immediately downslope from the active crater on the east flank of the old Southeast Crater cone (in the left half of the image), seen from Sant'Alfio on the east flank of Etna on the morning of 25 July 2011. Note the dense weather cloud covering the summit area, which is associated with high relative air humidity and strong wind. The northern crest (Serra delle Concazze) of the Valle del Bove is visible in the lower right portion of the image. Photo taken by Andrea Ercolani and published with kind permission of the author (Source: Siciltrek of Andrea Ercolani)

In the meantime, the lava flow descended the steep western slope of the Valle del Bove, forming several more or less parallel branches, stopping at the base of the slope near M. Centenari. The maximum height of the lava fountain, between 0400 and 500 GMT, was around 250-300 m, with some jets reaching 350 m; the plume of gas and ash was blown by the strong wind toward east. Ash falls were reported from numerous population centers from Fornazzo and Milo down to the Ionian coast near Riposto. As in the case of the previous paroxysm (19 July), the final phase of this episode consisted of a prolonged series of violent explosions, which generated loud detonations audible in the whole populated sector on the southeast and east flanks of the volcano.

30 July 2011 paroxysm

Mild Strombolian activity started from the same active crater on the east flank of the old SEC cone on the afternoon of 28 July 2011. This activity consisted of sporadic explosions launching incandescent pyroclastics a few tens of meters above the crater rim. During the following night, this activity ceased, and for the whole day of 29 July, no activity was observed at the SEC.

During the early morning hours of 30 July, a weak and intermittent glow appeared at the active crater, which showed a progessive intensification in the following hours, and around 0800 GMT, intense Strombolian activity was under way, with loud detonations and expulsion of lava bombs to several tens of meters above the crater rim. Most of this material fell back into the crater, but some explosions deposited large pyroclastics also on the external flanks of the cone surrounding the crater. Small quantities of fine-grained ash were blown eastward by the wind. Furthermore, a very small overflow of lava occurred within the deep notch cutting the east rim of the crater, reaching a length of about 100 m.

This eruptive activity, which was marked also by a conspicuous augmentation in the mean volcanic tremor amplitude, remained essentially constant until the early afternoon, after which it showed a marked diminution both in terms of geophysical and volcanic activity.

Around 1700 GMT, the mean level of the volcanic tremor started to increase again, and so did the intensity of the Strombolian activity. About 30 minutes later, a plume of gas and ash was becoming evident, which was blown eastward by the wind. The Strombolian activity progressively increased, more rapidly than it had done during the morning, and by 1930 GMT it produced continuous jets of incandescent lava, passing rather abruptly into sustained lava fountaining within less than 5 minutes. The overflow of lava toward east also became more vigorous, forming a flow that during the following hours spread out in a fan-shape manner, with numerous lobes descending the western slope of the Valle del Bove, reaching its base (at about 2000 m elevation) around 2100 GMT. The plume of ash and lapilli was blown eastward by the wind, leading to pyroclastic fallout essentially in the same sector already stricken by the tephra falls of the 19 and 25 July paroxysms.

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Lava flow formed on the western slope of the Valle del Bove during the 30 July 2011 paroxysm, photographed during the early morning hours of 31 July from Monte Zoccolaro. Note the numerous branches of the lava flow, the longest of which is advancing on the relatively gently sloping terrain to the north and northeast of Monte Centari (in the lower center of the image), reaching a greater distance from the source than the previous paroxysms (except the one of 12-13 January 2011). Photo taken by Andrea Salemi and published here with kind permission of the author (Source: Andrea Salemi on Flickr)

During the phase of maximum intensity, incandescent lava fragments were violently thrown to heights of 450-500 m above the crater rim, causing abundant pyroclastic fallout onto the external flanks of the cone to a distance of 200-300 m. The jets of incandescent lava emerged from at least two vents within the crater and near the notch in the eastern crater rim, which showed a west-northwest - east-southeast alignment.

From 2130 GMT onward, the activity showed a progressive diminution in intensity, until its total cessation shortly after 2200 GMT. The lava flows on the east flank continued to be mobile and incandescent for a while due to gravitational pull, even though cooling set in rapidly due to the cessation of feeding from the active crater.

In general, this eighth paroxysmal episode of the year lasted less than one day, with an initial phase characterized by weak Strombolian activity lasting about 10-12 hours, and a phase of lava fountaining concentrated in 2-3 hours. This episode occurred 5.5 days after its predecessor (of 25 July), thus marking a further shortening of the quiet interval separating the paroxysmal episodes. Since the beginning of the year, these repose intervals have been 36, 51, 32, 58, 10, 6, and 5.5 days, respectively.

This was also the most intense episode since the first one during the night of 12-13 January, with sustained lava fountains, differently from the preceding episodes with rather pulsating fountains, which rarely rose higher than 250-300 m. The lava flow emitted during the 30 July paroxysm was longer than those emitted between February and 25 July, possibly reaching the same length (or nearly so) of the 12-13 January lava flow, with its most advanced fronts to the northeast and east of M. Centenari.

Bocca Nuova
During the first ten days of July 2011, frequent emissions of ash from the Bocca Nuova (BN) continued, pursuing an activity initiated on 14 June. Some of the emissions in this period contained hot material, and one explosion at 0203 GMT on 6 July launched incandescent blocks or bombs up to 100 m above the crater rim.

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Volcanologist Mauro Coltelli of the INGV-Catania on the southern rim of the Bocca Nuova in full Strombolian activity on the evening of 13 July 2011. Photo taken by Boris Behncke, INGV-Catania

On the evening of 11 July 2011, vigorous Strombolian activity started within the BN, which at night produced a glow that was visible with the naked eye from various population centers in the southern sector of Etna. The glow was first recorded by the Schiena dell'Asino monitoring camera of the INGV-Catania, on the southeastern flank of Etna, around 2100 GMT on 11 July. In the night of 12-13 July, the glow was more intense and continuous than during the previous night. At variable intervals, incandescent bombs were seen to be thrown above the crater rim; these bombs fell back into the crater.

On the evening of 13 July, INGV-Catania staff carried out a field visit to the Bocca Nuova, which revealed that on the crater floor there was a single large vent, which was the source of intense and continuous Strombolian activity. The strongest explosions often occurred in series of 2-5 events, launching incandescent bombs to several tens of meters above the crater rim. However, most of the bombs fell back into the crater, whereas a few bombs flew over the rocky septum that still divides the Bocca Nuova from the Voragine, to fall into the southern portion of the latter.

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Strombolian activity from a vent located in the southeastern portion of the crater floor of the Bocca Nuova (right) and lava flow (left) cascading toward a deeper portion of the crater floor, on the evening of 15 July 2011. Photo taken by Boris Behncke, INGV-Catania

During another summit visit, carried out by INGV staff on the evening of 15 July, the Strombolian activity appeared slightly diminished compared to that observed two evenings before, but within one-and-a-half hours, the activity increased to levels superior to those observed on 13 July. Furthermore, immediately to the west of the explosive vent, a lava flow was observed, which was oozing from underneath a sheet of pyroclastics that were constantly deposited by the nearby Strombolian activity. The flow then cascaded into a deeper depression in the central-western portion of the crater floor. Dense gas clouds and the intense explosive activity precluded more detailed observation of this lava flow.

During the following days, the activity within the BN progressively diminished; the last observations of Strombolian activity were made on the evening of 17 July, by which time lava effusion had already ceased. After the SEC paroxysm of 19 July, ash emissions resumed at the BN; a field visit on 22 July confirmed continued ash emissions from the vent that had been the site of the Strombolian activity the week before. Recordings made with a thermal camera of the interior of the BN revealed the presence of a small patch of cooling lava, which had partly covered the creater floor during the effusive activity of 14-16 July.

After the 25 July paroxysm from the SEC, the BN produced only a few sporadic emissions of ash, most notably at 0429 GMT on 28 July, and at 0531 on the 29th, producing small quantities of hot material as recorded by the thermal monitoring camera of the INGV-Cataniaon the Montagnola (at about 2600 m elevation on the south flank).

 

STROMBOLI

Stromboli volcano is in a state of persistent eruptive activity, normally with explosions of medium to low intensity occurring from a number of vents located within the summit crater. During July 2011, the activity continued at fluctuating levels; this was punctuated by a few stronger explosive episodes and the emission of small intracrater lava flows on the crater terrace. The activity was concentrated at the two active vent areas, in the northern and southern sectors of the crater terrace.

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Explosive sequence from vent S3 of Stromboli on 5 July 2011, as recorded by the thermal monitoring camera of the INGV-Catania on the Pizzo sopra la Fossa

On 5 July  at 0245 GMT, a strong explosion lasting at least 20 seconds occurred at vent S3 in the southern portion of the crater terrace, with an initial jet of incandescent bombs and lapilli followed by an ash emission that rose more than 300 m above the crater terrace. The ash plume was then dispersed by the wind toward the eastern sector of the island, while bombs abundantly fell on the Pizzo sopra la Fossa, as recorded by the monitoring camera located at that site. The sequence continued with a second explosion from vent S1 at 0246 GMT, which lasted 12 seconds, producing incandescent fallout only on the crater terrace itself.

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Explosive sequence from vent N1 of Stromboli on 17 July 2011 as recorded by the thermal monitoring camera of the INGV-Catania on the Pizzo sopra la Fossa

At 2045 GMT on 17 July, a powerful explosion lasting about 15 seconds occurred at vent N1, in the northern portion of the crater terrace, punctuating a phase of spattering from vents N1 and N2. Ejection of incandescent bombs was followed by the emission of ash and lapilli, forming a small column that rose more than 300 m above the crater terrace. The ejecta fell over the northern part of the crater terrace, without reaching the Pizzo, and onto the northeastern slope of the Sciara del Fuoco, rolling down the whole slope.

Finally, between 1200 and 1300 GMT on 28 July, vigorous spattering started at the small cone that had formed during a series of episodes of small lava fountains and intracrater lava flows in late-January 2011, on the southwestern rim of the crater terrace. An accurate analysis of the video footage recorded by the thermal camera on the Pizzo revealed that what initially appeared to be a small lava flow was an alignment of tiny vents on the southern flank of the conelet. The spattering ceased during the late afternoon of 28 July, but a new, similar episode occurred on the morning of 29 July, during bad weather that severely hampered observations by means of the monitoring cameras of the INGV-Catania. This episode produced a very small lava flow, which descended on the eastern side of the conelet.

These episodes heralded a major lava outflow, from the northernmost vent (N1) on the crater terrace, which started on the late evening of 1 August 2011.

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