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A lava flow of 4 September 1999 from the Southeast Cone stopped right at the sign warning to proceed further

21 September update. Eruptive activity is continuing at Etna's summit craters, and lava continues to flow from the eruptive fracture first observed on 12 September. During the early morning hours of 20 September, an episode of lava fountaining occurred at the Bocca Nuova, but this was much less intense than the 4 September eruptive episode at the Voragine. Strombolian activity in the summit area (the precise location of this activity was not identified) was observed in the evening of 20 September from Nicolosi on the southern flank of the volcano.

Observations of the activity, 11-12 September. The following information, provided by Giuseppe Scarpinati (Italian correspondent of the French Association Volcanologique Européenne (LAVE) on 13 September, is based on a summit visit made on 11-12 September. Scarpinati did not see much of the summit craters due to bad weather, but he had the possibility to visit the area of effusive activity near the Southeast Cone repeatedly on the evening of 11 September and early the next morning.
On the late afternoon of 11 September, effusive activity near the large spatter cone formed since 27 August was limited to what Scarpinati called "residual activity", that is very little lava emission and no pyroclastic activity. However, during the evening it became evident that lava was flowing from an area about 200-250 m further downslope to the east, and there was also minor lava spattering in that area. Near the 27 August spatter cone Scarpinati heard ominous noises (similar to those heard on 12 May just before the collapse of a large lava tumulus on the western slope of the Valle del Bove), but nothing else happened.
The next morning at about 0700 h (local time=GMT+2), lava was again flowing from vents immediately below the 27 August spatter cone. One relatively large flow advanced about 70 m towards east-northeast, while two smaller flows moved only a few meters to a few tens of meters; the smaller of these two flows "was moving like toothpaste". Scarpinati then walked around the 27 August cone towards north and then turned east, descending into the direction of the Valle del Bove, and about 200-250 m east of the 27 August cone there was a new spatter cone (or rather a double spatter cone, with a smaller one standing within the vent of the larger one) that was actively ejecting small fragments of lava. Fluid lava issued (from a vent near the cone?) at a rate estimated at at least one cubic meter per second, forming a well-fed flow that moved along the margin of the flow-field formed during the past months. The intensity of pyroclastic ejections from the spatter cone increased gradually, and by 1000 h, somewhat larger blobs of lava were ejected about 5-6 m high, falling in a radius of several meters around the vent.
The new eruptive vents have become active on a terrain not covered by lavas during the last months, and it appears that this is a true new eruptive fissure. This means that the activity has migrated downslope, a process that has also been common at the Northeast Crater during its long-lived eruptions in the 1950's to 1970's, and this has nothing much to do with a flank eruption. However, the opening of new vents farther away from the Southeast Cone points to an increasing structural instability of the area.

The Voragine eruption of 4 September 1999: eyewitness accounts and field observations. On the late afternoon of 4 September 1999, the Voragine, one of Etna's four active summit craters, produced a powerful eruptive episode which may have been among the three most vigorous eruptive events at Etna during the past 100 years. As of 10 September, eruptive activity is continuing at lower levels in the Voragine, the Bocca Nuova, and at the southeastern base of the Southeast Cone. The following report consists of four sections: the first covering the activity during August and the first days of September, the second dealing with the events of 4 September, the third describing field observations made after the eruption, and the fourth summarizing the few observations of activity since 4 September.
Activity between late-July and 3 September 1999. Axel Timm from Germany visited Etna on 15 and 16 August and made the following observations. There was little activity in the Bocca Nuova on 15 August, with quiet degassing at the northeastern vent, while dilute ash clouds were emitted at intervals of several hours from the southeastern vent. In the Voragine there was only gas emission from the "diaframma vent", but minor eruptions occurred at intervals of 5-60 minutes from the central vent, allowing a descent into the crater. Rumbling noise came from deep within the central pit of the Northeast Crater, but dense gas emission prevented a view of the pit's interior. The Southeast Crater was completely quiet. Several small lava flows issued from the area of the hornitos at the upper end of the 4 February fissure at the southeastern base of the Southeast Cone.
On 16 August, occasional rumbling sounds came from the northeastern vent of the Bocca Nuova, while the southeastern vent continued to emit ash to 50-100 m above the vent at intervals of about 30 minutes without any noise. The "diaframma vent" of the Voragine was inactive, and eruptions occurred every 10-30 minutes from the central vent; these eruptions varied from noisy gas emissions to explosions that ejected bombs and scoriae far beyond the rim of the vent, mostly towards north. Activity at the Northeast Crater was similar to that observed on the day before. At the 4 February fissure, several active lava flows were visible, the largest of which was about 2 m wide and was flowing relatively fast, while all the others were much slower.
The following information about activity in the summit area was provided by e-mail on 31 August by Claude Grandpey, of the French Association Volcanologique Européenne (LAVE). Modifications to the original text are set in brackets [].
"Before [...] 20 August, there had been a strong decrease in the emission of lava [from the 4 Februar fissure] and only one flow was visible on the eruptive site. This fact was confirmed during my first visit on 24 August. The lava flow was about ten meters long and 40 cm wide and quite slow. It was located about 30 meters to the south of the February hornitos and flowing toward the SE crater. The lava was just fluid enough for the [mountain guides] to make ashtrays. It stopped in the afternoon and about one hour later, the tumulus from which it originated started cracking on the eastern side and a new flow appeared. It went along over ten meters and stopped [...]. I went back to the site the next morning (25 August) at 9 o'clock. A very small flow (about 50 cm long) was still escaping from the same tumulus, but at ten it was all over and since that time, [no further effusive activity has occurred].
"[The end of the effusive activity] corresponds with a strong increase of the activity inside the Voragine. When I arrived at Zafferana on 23 August, my friends told me that the volcano had been noisy during the night. I climbed the NE Crater on 24 August in order to have a general and safe view of the activity. The explosions were very irregular and the materials were ejected from 2 small vents situated on the northern rim of the ["diaframma vent"]. The ejecta were sent flying very high in the sky and some of them were falling on the outer western slope of the Voragine and very close to the base of the NE Crater. I was told by the guides at the Torre del Filosofo that their colleagues had stopped taking the tourists to the rim of Bocca Nuova because of the obvious danger. As clouds covered the summit later in the afternoon, I stopped my observations on that day. [...] As the activity was less strong at that time [I and a group of other volcanologists including three from the universities of Southampton and Portsmouth] decided to walk into the crater and go to the edge of the terrace that is accessible from the foot of the NE Crater. We saw and heard occasional explosions but the most interesting fact was a small lava pond about 5 meters in diameter in a small vent near the ["diaframma vent"]. The lava was very fluid and spattering and a small flow even escaped from the pond for some time. The activity of the pond lasted one hour or so and then the lava began going down inside the vent until it was no longer visible. I then decided to go up to the western rim to have a better view of it but the lava was hardly visible any longer. While I was standing on the rim, I suddenly saw the central part of the Voragine inflate over a surface about 50 meters in diameter [...]. A few seconds later, there was a first explosion - not very strong - that sent about half of that [domed area] flying toward the south and the "diaframma". I then decided to leave that place and walk down toward the Bocca Nuova, [expecting] another explosion. It happened at the very moment I reached the rim of Bocca Nuova. It was very very strong and noisy - like the noise of a cannon - and sent the bombs flying as far as the inside of BN and above all all over the western slope of the Voragine. There were other similar explosions during the rest of the day and the next day. I re-visited Bocca Nuova and the Voragine on 27 August. There was a new cavity - not very deep - at the center of the Voragine and explosions were projecting materials from one of the small vents near the ["diaframma vent"]."
Grandpey observed no eruptive activity at the SE Crater during the days of his visit, but "gas, steam and powder were coming out of the NE Crater with a loud rumbling at the bottom of the vent." No morphological changes had occurred there since late July. Bocca Nuova was relatively quiet, the southeastern eruptive vent emitting clouds of gas with little noise.
The cessation of effusive activity from the 4 February fissure on 25 August was followed two days later by the opening of a new fissure, about 50 m long, and located some 40-50 m to the north of the cluster of hornitos at the upper end of the 4 February fissure. Mild Strombolian activity during the following days built a large spatter cone up to 8 m high (oral information from Giuseppe Scarpinati), and lava began to flow towards the Valle del Bove along the northern rim of the voluminous lava field emplaced between 4 February and late August. This lava flow appears to have been fairly small; Scarpinati observed a "small incandescent spot below the Southeast Cone" when watching the mountain from his home in Acireale on the evening of 1 September. Bad weather precluded observations on 2 and 3 September.
The 4 September 1999 paroxysm. Press reports indicate that volcanic tremor began to register on seismographs on the volcano during the early afternoon of 4 September, but bad weather made visual observations impossible. Nontheless Giuseppe Scarpinati and several mountain guides as well as a group of excursionists were near the Torre del Filosofo mountain hut (located at about 2900 m elevation, and about 1 km south of the Southeast Crater) during the afternoon and began to hear a hissing sound at about 1730 h (local time=GMT+2). This sound was distinctly audible in spite of the strong wind, and gradually became stronger, until it was "similar to the noise of a starting jumbo jet". Soon afterwards Scarpinati and his companions heard the crashing noise of impacting blocks and bombs at some distance, and they began to retreat towards the Torre del Filosofo mountain hut and later to the top station of the cable car (at about 2400 m elevation), where they waited to obtain a clear view of the summit area. As it got dark, the noise of the eruption became lower, and when most of the event seemed to be over (that is, between 1930 and 1945 h), the cloud cover lifted for a brief period. Scarpinati reports that before his eyes "the whole summit cone was aglow", as it was covered with still-incandescent ejecta from the Voragine. A few moments later, "the largest explosion I have ever seen" (citation from Scarpinati who has climbed Etna about 500 times during the past 35 years) occurred from the Voragine. Huge incandescent bombs were ejected in a broad fan over the summit cone, crashing onto its flanks and rolling downslope to its base.
Some time later, at about 2030, the summit vent of the Southeast Cone, inactive since 4 February, began to produce Strombolian explosions. This activity had nothing in common with the paroxysms that had occurred at the same vent between September 1998 and 4 February, but was "like the usual Strombolian activity" that had been characteristic of the period between late-1996 and July 1998. Scarpinati and his companions left the area after some time, when the activity continued in the same manner. However, later that evening, lava began to issue from the lower part of the fracture that had been active on the southeastern flank of the Southeast Cone during a late stage of the 4 February eruptive episode (see the February 1999 updates listed in the column at left), and the output from the effusive vents at the fissure of 27 August increased, with lava flowing rapidly towards the western rim of the Valle del Bove.
While the activity initiated that evening at the Southeast Cone was relatively mild, the paroxysmal eruption from the Voragine a few hours earlier had widespread effects. Soon after the beginning of the eruption in the afternoon, loud detonations were audible in villages and towns around the volcano, reminding people of an "air attack". This was followed by a fall of scoriaceous lapilli on the eastern flank, extending to the coast near the town of Giarre, more than 15 km from the summit. Many of the lapilli were walnut-sized, and some had a diameter exceeding 5 cm. Eyewitnesses reported that some of the larger fragments were still hot when falling near the villages of Milo, Fornazzo and Sant'Alfio, but not hot enough to set vegetation afire. The impact of the larger clasts broke the windshields of many cars and seriously damaged vineyards and fruit gardens. In a narrow sector extending from the Milo-Fornazzo area towards the coastal strip near Giarre, the pyroclastic deposit was several centimeters thick, and traffic was disrupted due to the layer of scoriae on roads and on the Messina-Catania highway. Press reports put the damage caused to the agriculture and to the local infrastructure at several tens of billions of Lire (several tens of millions of US $). According to the Catania-based newspaper "La Sicilia", about one million cubic meters of pyroclastics fell on the town of Giarre alone.

The 4 September 1999 eruption and its effects All photos (except photo #3) are courtesy of Werner Keller
Left: The rim of the Voragine seen from west. The rugged area in the upper central part of the photo is the area where a broad rootless lava flow began to move down the side of the crater. Note the numerous large impact craters and lithic blocks in the foreground. Right: A partial view of the Northeast Cone seen from the same place, showing that the prominent scarp on the southern flank of the cone (produced during the 22 July 1998 eruption from the Voragine) has been largely healed by the thick pyroclastic deposit of 4 September 1999.
Left: One of the smaller impact craters (containing the bomb which created the crater) in the summit area of Etna, on the western side of the main summit cone, and about 700 m from the Voragine. Werner Keller gives scale. Right: Impact crater with a bomb whose diameter is about 50 cm (scale bar in foreground is 30 cm long). The bomb pierced the dirt road circling around the western side of the main summit cone.
Left: A view of the cluster of hornitos formed during February-March 1999 at the upper end of the fissure that opened after the eruptive episode from the Southeast Cone on 4 February 1999. Dark bilobate lava flow on the slope of the Southeast Cone in the background was erupted on the late evening of 4 September 1999. Right: A panoramic view of the new site of effusive activity (initiated on 27 August). Large steaming spatter cone formed during the first days of the activity is visible at left, and shield formed by repeated overflows of lava from effusive vents is at right. Tourists gathering around the effusive vents give scale.
Left: Distal portion of a dry debris flow deposit at the southern base of the main summit cone. This debris flow was triggered during the Voragine eruption on 4 September 1999 as pyroclastics accumulated rapidly on the steep slope of the cone and began sliding down the slope. Right: Detail of the debris flow deposit showing large propertion of juvenile clasts that were slightly rounded during transport. Brownish and bright gray blocks are entrained older clasts. Person in the background gives scale.
Left: Scoriaceous lapilli on a wall about 200 m south of the margin of Milo, on the eastern flank of Etna. White scale bar is 30 cm long. Right: Continuous pyroclastic deposit on a small square in the center of Milo, thickness of the deposit is about 4 cm. Person in the background is Boris Behncke.
Left: Entrance of a villa in the center of Milo, covered with pyroclastic deposit of 4 September 1999. Thickness of deposit is about 3 cm. Right: The "Mareneve" road which leads from Fornazzo to the Rifugio Citelli (on the northeastern flank of Etna), covered with up to 5 cm of lapilli.

Field observations, 6-8 September. The following is based on field investigations made by Boris Behncke (Dipartimento di Scienze Geologiche of Catania University) and Werner Keller (co-authors of the web site "Chile's Volcanoes") in the area of Milo, Fornazzo and Giarre on 6-8 September, and during a summit visit on 7 September. Measurements were made of the thickness of the deposit in various locations before heavy rainfalls swept part of it away, and when the cleaning of roads was still in an initial stage.
During the visit to the summit area on the afternoon of 7 September, visibility was hampered by dense clouds, but the effects of the eruption near the summit craters were seen to be striking. The cones of the summit craters were hit by countless bombs up to 5 m in largest diameter and lithic blocks up to 1 m across, most of which had created spectacular impact craters. The largest of these craters were more than 5 m across. Many bombs and some blocks had fragmented upon impact, others were found outside the craters created by their impact up to 10 m away (in the direction away from the Voragine). In many instances the projectiles had obviously arrived on a fairly flat trajectory, others had pierced the ground vertically. The density of large bombs and blocks was particularly high on the southwestern flank of the main summit cone. Some of the larger bombs were still warm to the touch (but not hot) about 60 hours after their emplacement.
A peculiar phenomenon was observed on the southern flank of the main summit cone. Here the accumulation of juvenile scoriaceous pyroclastics had apparently been so rapid that the deposit began to slide down the steep flank, forming something like a dry debris flow that extended about 500 m down the slope to its base. In its distal portion the flow ends in two distinct lobes about 1 m thick. About 80 per cent of this deposit consists of juvenile clasts 10-30 cm in diameter whose edges were rounded while sliding down the slope, the other 20 per cent are older clasts (reddish scoriae and gray lithic blocks) whose diameters are slightly less than those of the juvenile clasts. This dry debris flow deposit is different from anything seen in ten years of frequent visits to Etna's summit area by Behncke, and is striking evidence of the dynamics of the 4 September event.
Brief glimpses through the clouds permitted a view on the Voragine from a place about 500 m west of the crater rim. The heavy fallout close to the crater almost healed the large scar cut into the southern flank of the adjacent Northeast Cone. On the southwestern crater rim (close to the Bocca Nuova), the rapid accumulation of fluid ejecta led to the formation of a rootless lava flow, very similar in appearance to the flow formed during the 22 July 1998 eruption from the same crater, but this new one was broader (about 300 m) and about half as long (250-300 m). A similar but smaller flow was reported by Scarpinati (who visited the summit area the morning after the eruption) on the eastern side of the Voragine; this area was not seen during the 7 September visit. Reports by mountain guides at Piano Provenzana on the northern flank of Etna indicate that yet another lava flow cascaded into the adjacent Bocca Nuova, but some reports received during the days after the 4 September eruption saying that the Bocca Nuova was largely filled by products coming from the Voragine appear to be vastly exaggerated. According to the same guides the Bocca Nuova looks "much the same as before, except for the new lava flow".
On the eastern flank the lapilli deposit extends in a narrow strip almost due east towards the coast near Giarre. Five communities (including Milo, Mascali and Giarre) suffered heavily from the fallout. Going northwards from Zafferana, on the southeastern flank, the southern margin of the fall deposit is encountered in the forests between the small village of Petrulli (about 2 km north of the center of Zafferana) and Milo, the next village to the north, where isolated scoriaceous lapilli with diameters between 1 and 3 cm occur. Closer to Milo (1.5 km further north) the number of clasts per square meter increases as well as their mean diameter, and on the southern margin of the village the deposit becomes continuous. Most of the deposit consists of lapilli-size scoriae, with little ash mostly coating leaves and grass. The largest clasts found in the southern part of Milo were 7 cm across, and many were 5 cm large. In the northern part of Milo, the thickness of the deposit exceeds 5 cm, and many leaves (mainly vine grape plants) were damaged by the fall; some show evidence of heat.
In the southern part of the village of Fornazzo, about 1.5 km from Milo, the deposit is 5-6 cm thick, and the largest clasts are up to 10 cm across. Residents reported that even larger clasts fell, but fragmented upon impact. Going further north, the deposit thins gradually and ends with a relatively sharp margin about 2 km north of Fornazzo.
Further downslope, near the town of Giarre, the area of fallout is about 5 km wide in north-south extension, and up to 5 cm thick in its central portion. Most of the deposit here is composed of fragments with diameters between a few millimeters and 3 cm. The northern and southern margins of the deposit are strikingly sharp, it seems that very little and only fine ash fell beyond the margins of the lapilli deposit.
Comparison with the (relatively poor) descriptions of the fall deposit produced by a violent eruption from the Voragine on 17 July 1960 allows the conclusion that the 4 September 1999 eruption was of similar proportions, and therefore it was among the three largest explosive eruptions at Etna's summit craters during the past 100 years. The 1960 eruption produced about 10 million cubic meters of pyroclastics; the pyroclastic deposit then extended northeastwards in a somewhat broader sector, and clasts more than 5 cm in diameter were reported at that time. Different from the 1960 eruption, the 4 September 1999 event did not cause fires in the forests on the flanks of the volcano, but this may be only due to the fact that it was raining at the time of the eruption or shortly before. In any case this new event was significantly larger than the 22 July 1998 eruption from the Voragine.
The activity of the Southeast Cone on the evening of 4 September had much minor effects. The most impressive change since the last visit by Behncke on 28 July was the presence of a small lava flow that had issued from the lower part of the fracture active during the 4 February eruptive episode on the southeastern flank of the cone. The flow extended to the hornitos at the upper end of the fissure which emitted lava between 4 February and 25 August, with a small lobe extending eastwards from the main lobe. The main lobe filled a small depression near the hornitos and stopped at a sign saying "It is forbidden to overpass this limit".
Eruptive activity since 4 September. During the past 6 days, eruptive activity has continued at the summit craters, but visual observations have been seriously hampered by bad weather. Intense explosive activity has occurred on each day at the Bocca Nuova, and at times bombs were ejected onto the outer slopes of the main summit cone, mainly towards west. The Voragine has remained active as well, and another episode of vigorous activity occurred shortly after midnight on 9 September. There are no eyewitness accounts of that event, but intense seismicity indicates that the most intense activity occurred between 0100 and 0400 h on 9 September; this may have consisted of lava fountaining. This event, however, was significantly smaller than the 4 September eruption, and there were no reports of new tephra falls in inhabited areas around the volcano. During their summit visit on 7 September, Behncke and Keller reached the area of effusive activity and saw two small lava flows issuing from effusive vents located some 15 m below the spatter cone formed during the days after 27 August. The flows have remained active since, and are extending onto the western slope of the Valle del Bove. Mild Strombolian activity is occurring from a new cluster of hornitos located near the effusive vents. This activity is similar to the mainly effusive activity between 4 February and 25 August.

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