Activity of the Sicilian volcanoes during August 2011
Eruption column rising 5-6 km above the summit of Etna, seen from Calabria on the morning of 20 August 2011; in the middle ground, at lower right, the promontory of Taormina is visible. Photo taken by Rosanna Barbaro and published here with kind permission of the author.
ETNA
During August 2011, the episodic paroxysmal activity from the New Southeast Crater (SEC; formerly "pit crater") continued with four episodes, which took place on 5, 12, 20, and 29 August. Besides the rapid growth of the pyroclastic cone around the crater during each of these events, there has been evidence for growing structural instability and partial destruction of the cone has occurred, most conspicuously during the latest two episodes. Some of the SEC paroxysms were preceded, accompanied or followed by ash emissions from the Bocca Nuova; furthermore during the whole month, the common deep-seated explosive activity within the Northeast Crater conduit has continued, accompanied by strong and often pulsating degassing.
5 August 2011 paroxysmal episode
The first paroxysmal eruptive episode of August - the 9th of the year 2011 - occurred on the late evening of 5 August, six days after its predecessor (30 July 2011). Differently from the previous 8 episodes of the year, this time the paroxysmal phase of lava fountaining was not preceded by several days of Strombolian activity; the premonitory explosive activity started and accelerated just a few hours before the paroxysm, accompanied by a decided increase in the volcanic tremor amplitude.
The resumption of Strombolian activity at the New SEC had been indicated by the volcanological mountain guides of Etna during the eafternoon, but cloud cover prevented the activity from being recorded by the network of monitoring cameras of the INGV-Catania until the late afternoon. The first explosions within the crater were documented by the thermal camera at the Montagnola around 17:00 GMT (= local time -2) on 5 August. After 20:15 GMT, lava overflowed the eastern crater rim toward the western slope of the Valle del Bove. From 20:30 GMT on, the Strombolian activity was nearly continuous, although most of the ejecta fell back into the crater.
Lava flow fronts on the western slope of the Valle del Bove, somewhat uphill of Monte Centenari, in the night of 5-6 August 2011, shortly after the end of the ninth paroxysmal eruptive episode from the New Southeast Crater. Photo taken by Marco Restivo published here with kind permission of the author. (Source: Marco Restivo on Flickr)
Shortly before 21:00 GMT, the explosive activity took on the character of nearly continuous lava fountaining, with jets reaching heights of around 100 m above the crater rim. Lava fountaining significantly increased shortly after 21:30 GMT; the incandescent jets gradually rose in height to reach approximately 400-500 m around 22:00 GMT. After 22:15 the explosive activity, while still remaining strong, showed a gradual decrease in intensity until, at 23:00 GMT, the lava jets started to diminish notably in height. All explosive activity had ceased by 00:15 GMT on 6 August.
Seen from Monte Fontane, the lava flow in the meantime had expanded significantly downslope; the main lobe in descent on the western flank of the Valle del Bove had split into several sedondary branches, whose fronts showed a rather lobate pattern (see the photograph above). Around 24:00 GMT the lava flow showed signs of cooling, and only the main fronts were still advancing due to gravitational pull, covering a further few tens of meters.
The eruption plume that formed above the volcano was dispersed southeastward by the predominant winds blowing at that altitutde, causing tephra (ash and lapilli) falls in a sector of the volcano that lies between Zafferana Etnea, S. Venerina and Guardia Mangano to the north, and Fleri, Viagrande and Acitrezza to the south. The main axis of the deposit passed across the area between Fleri and Zafferana Etnea on the slope of Etna, and Acireale in the coastal area.
Lava fountains and lower portion of the eruption column during the culminating phase of the paroxysmal eruptive episode of 12 August 2011, photographed from about 1.2 km to the south. Two major incandescent fountains can be distinguished, one in the center of the crater (at left), and the other close to the eastern crater rim (at right). Photo taken by Stefano Branca, INGV-Catania |
12 August 2011 paroxysmal episode 5 days after the end of the previous paroxysmal episode, the new SEC showed the first signs of an imminent new episode - the 10th of the year 2011. At sunrise on 11 August, sporadic ash emissions from the new SEC were noted, which produced small puffs of grayish-brown color. The visible-light and thermal surveillance cameras of the INGV-Catania did not show any hot material in these emissions until late afternoon, when, after a short hiatus, the emissions resumed producing thermal anomalies. After sunset, small Strombolian explosions were observed at intervals of a few tens of minutes; this activity continued without variations during the night. At about 05:30 GMT on 12 August, the Strombolian activity began to intensity, accompanied by an increase in the volcanic tremor amplitude, and the shift of the location of the volcanic tremor source from its previous position below the central summit area toward the Southeast Crater, migrating from greater to shallower depth. After 07:30 GMT, the Strombolian explosions produced dark ash puffs, and at 07:50 GMT, lava started to overflow the eastern rim of the crater, through a deep breach left open by the lava overflows of the previous paroxysms. During the following 30 minutes or so, the Strombolian activity rapidly intensified, passing, at about 08:30 GMT, into a pulsating fountain about 100 m tall. Fifteen minutes later, a dense column of ash rose above the lava fountain, while the entire cone surrounding the New Southeast Crater was subjected to heavy fallout of large bombs and blocks. Sustained lava fountaining and formation of an eruption column were most intense during the interval between 08:45 and 10:00 GMT, when three vents were active within the crater, two in its central portion and one close to the breach in its eastern rim. Shortly after 10:00 GMT, the two vents in the center of the crater emitted only ash, while the more easterly vent continued to launch jets of incandescent lava until 10:25. This was followed by a long phase of ash emission, which was often passive but at times more forceful; this activity completely ceased around 11:00 GMT. |
The double Southeast Crater cone - in the background, the old cone partially covered with sulfur deposits, and the new one in the foreground, showing weak emission of bluish gas - seen from an airplane on the morning of 17 August 2011, looking west. The crater is seen to be elongate, with the two main vents in the central portion of the crater and close to the deep notch in the eastern (near) crater rim, through which lava flows have exited during all paroxysmal episodes this year. Photo taken by Marco Restivo and published with kind permission by the author. (Source: Marco Restivo on Flickr)
20 August 2011 paroxysmal episode
The 11th paroxysmal eruptive episode from the new SEC occurred 8 days after its predecessor, culminating in a brief but violently explosive phase. The resumption of the activity was recorded by the INGV-Catania surveillance cameras on the morning of 19 August, when two explosions at 03:36 and 03:43 GMT produced small amounts of ash. During the night of 19-20 August, weak, sporadic Strombolian activity was observed, which starting from 02:30 GMT on 20 August showed an increase in intensity and frequency. Lava began to overflow the east rim of the crater at 02:55 GMT.
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Paroxysmal phase of the eruptive episode of 20 August 2011 seen from east. In the foreground, white gas plumes are seen rising from the various branches of lava advancing down the western slope of the Valle del Bove. In the background, the northern slope of the new Southeast Crater cone is seen being subjected to heavy fallout of large pyroclasts. A dense eruption column is visible in the upper portion of the photograph. Photo taken by Salvo Virzì and published with kind permission of the author |
For the next four hours, the Strombolian activity continued with fluctuations in the strength and frequency of the explosions, while the lava flow slowly advanced toward the western slope of the Valle del Bove. Finally, shortly before 07:00 GMT the activity showed a notable intensification, and in a few minutes passed into lava fountaining, which generated heavy fallout of large pyroclastics onto the flanks of the cone. A few minutes later, light brown dust clouds appeared in an area on the lower east flank of the cone, where a small depression had formed a few hours after the 12 August paroxysmal episode, due to the sliding of a part of the still-hot deposit of that episode. Shortly thereafter, the continuous, intense projection of pyroclastics onto the flanks of the cone generated avalanches resembling pyroclastic flows, which descended a few hundred meters beyond the base of the cone, mainly toward south. Right from the start of lava fountaining, a dense column of tephra and gas rose vertically from the crater, reaching a height of about 5-6 km above the summit of Etna in a few minutes, and assuming a mushroom shape, as shown in the first photograph. The ash cloud then moved southwest and led to ash and lapilli falls in various population centers, such as Paternò, Ragalna, and Biancavilla. Closer to the crater, in the Torre del Filosofo area to the south, clasts up to several tens of centimeters in diameter hit the ground.
In the meantime, the lower portion of the east flank of the cone, on the southern side of the large breach and lava overflow channel in the eastern crater rim, began to slide and collapse under the push of lava from within the channel. Eventually a new lava flow issued from the collapsed area, taking a more southerly path than the lava emitted until then. The lava divided into numerous branches, most of which took the same parcourse as the lavas emitted during the preceding paroxysmal episodes. Lava fountaining began to show signs of diminishing shortly after 07:30 and totally ceased at about 07:50. For a few more minutes, ash was emitted from the crater, before all returned calm. At 11:59 GMT, a series of explosive ash emission started, which lasted until 12:04. 29 August 2011 paroxysmal episode The 12th paroxysmal eruptive episode of the year 2011 was heralded by a single explosion at 00:52 GMT on 28 August; further isolated explosions occurred on the same day at 15:17 and 15:59 GMT, followed by a number of minor events that produced small amounts of ash. At nightfall, weak and sporadic Strombolian activity was observed, which during the night showed a gradual intensification. At 03:15 GMT on 29 August, a first lava overflow occurred through the deep breach in the eastern crater rim, feeding a flow into the direction of the Valle del Bove. |
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A row of lava fountains rises from a series of eruptive vents, both within the New Southeast Crater (at left) and along a new eruptive fissure that has cut the southeastern slope of the cone (in the center and at right), at the height of the paroxysmal eruptive episode of 29 August 2011. Photo taken from about 1 km to the southeast at 04:36 GMT by Giuseppe Salerno, INGV-Catania |
From 03:40 GMT onward, the Strombolian activity progressively intensified, and about 04.05, two vents within the crater emitted pulsating lava fountains up to 100 m high. During the following 10 minutes, these fountains increased significantly in height, and a dense eruption column rose a few kilometers above the summit of Etna before drifting toward south-southeast. At the same time, a small lava flow began to issue from the area affected by the collapse of a portion of the cone's east-southeast flank during the 20 August paroxysm.
At 04:.20 GMT, the southeastern flank of the cone fractured with the opening of a line of new eruptive vents down to the base of the cone, from which a row of lava fountains rose (see the image at top); soon thereafter a new, broad lava flow appeared, which was fed from the new vents, and which descended into the direction of the Valle del Bove, somewhat to the south of the lava flow emitted earlier during this episode. For the next 20 minutes, lava fountaining continued at all vents, both within the crater and along the new fracture, then the activity started to weaken from 04:30 GMT on. The lava fountaining from the vents within the crater passed into ash emission shortly after 04:40, whereas the lowest vent on the new eruptive fracture continued to produce incandescent jets until 04:50 GMT. Ash emission continued at a diminishing rate until about 05:15 GMT. |
Lower portion of the eruptive fissure that opened during the paroxysm of 29 August 2011 on the southeast flank of the New Southeast Crater cone, seen during a field visit on 30 August. Photo taken by Marco Neri, INGV-Catania
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 August 2011, the activity continued at fluctuating levels; there were furthermore several stronger explosive events, and episodes of spattering accompanied by the emission of small lava flows onto the floor of the crater terrace and onto the upper slope of the Sciara del Fuoco. The activity was concentrated at the two active vent areas, in the northern and southern sectors of the crater terrace.
Aerial view of the summit area of Stromboli, taken on 19 August 2011. The two main vent areas, in the north (N) and south (S) of the crater terrace are highlighted. The lava flows emitted onto the Sciara del Fuoco are indicated by different colors and numbers: 1 (yellow) = 11-12 December 2010; 2 (pink) = 1-2 August 2011; 3 (red) = 18 August 2011. Photo taken by Mauro Coltelli, INGV-Catania
On 1 August 2011 at 20:56 GMT the northernmost vent on the crater terrace of Stromboli produced a lava flow that descended northeastward in the direction of the "Pianoro", which lies at about 650 m elevation; from there, early on the next morning, the lava expanded further downslope along the northern scarp of the Sciara del Fuoco (this lava flow is outlined in pink color and indicated by the number 2 in the image above). The phenomenon was clearly observed in the thermal images recorded by the SQT monitoring camera, located on the eastern flank of the Sciara del Fuoco at 400 m above sea-level (see images below), until at least 05:20 GMT on 2 August, when the northward lava flow descended below the lower margin of the SQT frame and its further development could not be directly followed. In order to investigate the full area covered by the lava flow, a field visit was carried out by INGV-Catania staff later on 2 August.
Frames captured from video recorded by the thermal camera (SQT) of the INGV-Catania located on the east flank of the Sciara del Fuoco, at 400 m elevation, on 1-2 August 2011. The reference temperature scale is shown at right in each frame; the time at the bottom is GMT
During the lava flow on the steep slope of the Sciara del Fuoco, the monitoring cameras repeatedly showed landslides, which generated dust clouds obscuring the view also of the thermal camera. These were triggered both by the rolling of blocks launched onto the slope by explosive activity, and blocks that detached from the lava flow front, and rolled down the slope to the sea. Furthermore, during the emplacement of the lava flow, the explosive activity at the summit vents continued without significant variations in the intensity and style of the activity.
The field visit was carried out between 18:00 and 20:00 GMT at Labronzo, and from the site of the INGV total geodetic station at about 195 m above sea-level on the eastern rim of the Sciara del Fuoco. The survey, aided by the application of a thermal camera, revealed that the lava flow had ceased and was cooling, and the most advanced lava fronts were at about 500 m elevation. However, the exact elevation of the lava fronts was difficult to determine due to the inclined vision from the survey site, and due to the fact that the lava had expanded into a depressed area. During the survey, summit explosive activity remained quite vigorous and intense, with frequent ejections of pyroclastics beyond the crater rim and down onto the Sciara del Fuoco.
Subsequent analysis of the thermal images recorded from the thermal monitoring camera SQT at 400 m elevation allowed to state that effusive activity was already declining by 14:00 GMT, and had completely ceased at the time the field survey was carried out.
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Explosive sequence of 5 August 2011 recorded by the thermal monitoring camera located on the Pizzo sopra la Fossa |
At 07:14 GMT on 5 August, a powerful explosive sequence occurred from the two northernmost vents in the southern vent area (see image at left), characterized by intense lava fountaining in several bursts that lasted for about 2 minutes and did not reach great height, but produced a significant quantity of incandescent spatter that covered the northern portion of the crater terrace, without, however, extending onto the external slopes. The explosive sequence started at 07:14:53 GMT with the emission of coarse-grained material from vent S1, which rose about 200 m above the vent and fell back toward north. This was followed, at 07:15:12 GMT, by a magmatic jet from vent S2, which rose a few tens of meters high. At 07:15:26 GMT, lava fountaining started from vent S2 and lasted about 20 seconds, producing a jet about 150 m high. At 07:15:50 GMT, lava fountaining started at vent S1, where it lasted for about 40 seconds, rising to heights of a few tens of meters. All material produced by these two small lava fountains fell back onto the area of the crater terrace. |
- from 18:30 on 11 August until 11:45 on 12 August;
- from 19:30 until 20:45 on 12 August;
- from 02:38 until 07:08 on 13 August;
- from 07:08 until about 09:30 on 19 August;
- from 01:23 until 07:20 on 20 August;
- from 21:38 on 20 August until 00:30 on 21 August;
- from 09:15 until 15:30 on 27 August;
Lava flow generated during the episode of spattering in the night of 11-12 August 2011, from the small cone sitting on the southwestern rim of the crater terrace, in the southern vent area. At lower left, at the base of the cone, a group of small glowing vents is seen, which often exploded simultaneously. Photo taken by Gijs de Reijke, Hogeschool Arnhem Nijmegen, Olanda
During some of the episodes of more intense spattering between 16 and 19 August, repeated emplacement of small rheomorphic lava flows was observed on the southeastern flank of the cone, and on 18 August, a lava flow was emitted onto the slope of the Sciara del Fuoco (communication by the COA) where it descended for about 100 m. On 20 August, from 01:23 GMT, a sudden, strong increase in the spattering activity produced abundant flowage of still-fluid lava fragments toward northeast, on the crater terrace, forming a small lava field about 80 m long (see image below), which was fed until 14:00 GMT. The phenomenon was repeated at 21:38 on the same day and continued through the forenoon of 21 August.
During the first episode of spattering on 26 August, small rheomorphic lava flows formed repeatedly on the southeastern and northeastern flanks of the conelet. On 27 August, vigorous spattering reaching maxima between 09:20 and 10:35, and between 11:35 and 13:15 GMT, produced abundant fluid spatter, whose rheomorphic flowage covered the entire flanks of the conelet.
Finally, a strong explosion from the northernmost vent of the crater terrace occurred at 05.06 GMT on 30 August, producing fallout of large pyroclasts up to the area of the Pizzo.