The information for this report was compiled by Boris Behncke at the Dipartimento di Scienze Geologiche, University of Catania (DSGUC), and posted on his internet web site. The compilation was based on personal visits to the summit, observations from Catania, and other sources cited in the text. Additional information was provided by Jean-Claude Tanguy (DSGUC), mostly about the activity during September.

Mild eruptive activity resumed at Etna's summit craters (figure 80) in early June, and gradually increased through late August before culminating with a powerful eruptive episode from the Voragine on 4 September. During the same period, lava continued to flow from fissures at the base of Southeast Crater (SEC), and occasional phases of mild lava spattering built hornitos and spatter cones at the eruptive vents.

Activity during June 1999. During early June, lava emission from the 4 February fissure on the SE base of SEC continued at a low rate. Lava issued from ephemeral vents and flowed for a few hundred meters towards the W face of the Valle del Bove (VdB).

The following information regarding activity from 30 May to 2 June was provided by John Guest (University College London, UK) and Angus Duncan (University of Luton, UK). Several explosions were heard from the summit craters on 30 May. On 1 June a brief bright red glow was seen over Bocca Nuova (BN). The active lava pile in the vent area at the foot of SEC on 2 June had increased in thickness since 30 May. Fresh lava now partly buried the 'old' tumulus of altered lava blocks, but a new tumulus had formed a few meters downflow. On 3 June Sandro Privitera (IGGUC) observed three emissions of reddish gray ash to more than 500 m above the crater.

On 4 June the two main sites of activity were generally the same as on 19 May (BGVN 24:05): an effusive vent ~25 m below the hornitos at the upper end of the fissure that became active on 4 February, and a cluster of vents at about 2,600 m elevation on the W slope of the VdB. The upper site had shifted ~30-40 m upslope. During the 16 days between the two visits, the site of lava emission had shifted frequently, sending lava flows in various directions. By 4 June lava flows had covered most traces of the tumulus collapse depression formed on 12 May. It appeared that the effusion rate had remained nearly constant for about 2 months (at ~1 m³/s). About 25-30 x 10⁶ m³ of lava had accumulated since 4 February on the western VdB rim and the slope below.

A brief visit on 10 June by Behncke and Francesca Ghisetti (DSGUC) revealed that the output of lava from the 4 February fissure had increased. The active vents were ~10-20 m below the hornitos at the upper end of the fissure. One vigorous vent was on the fissure, but lava also issued from within and on the margins of recent flows on the SW side of the lava field. A flow down the N side of the lava field appeared to have spilled over the rim of VdB. One vent continued to emit lava on the western VdB slope.

Weak explosive activity at the 4 February fissure resumed in mid-June, accompanied by an increase in the lava output. According to Giuseppe Scarpinati (L'Association Européenne Volcanologique, LAVE), an intermittent glow in the eruption area was visible from Acireale (SE of Etna) on the evening of 16 June. This glow was also clearly visible from Catania on the following evenings, and lava was seen extending from the glow area.

By the afternoon of 19 June one large and several smaller hornitos had grown on a large lava shield, ~50-80 m below the cluster of hornitos built during February-March 1999. Two lava rivers extended a few hundred meters in the direction of the VdB. The effusion rate had increased to 2-3 m³/s (it had been less than or equal to 1 m³/s during the previous month), and the volume of lava emitted since 4 February exceeded 30 x 10⁶ m³.

Between 19 and 23 June there was a notable decrease in activity at the eruptive fissure. After a visit on 26-27 June, Scarpinati reported that variable emission of lava from the 4 February fissure continued. Scarpinati also noted that the Voragine produced explosions, but made no direct observations.

On the evening of 29 June Behncke noted that the 4 February fissure had one eruptive site that produced mild lava spattering and two lava flows. Spattering from three closely spaced vents threw blobs of lava up to 3 m away. A partially drained lava tube containing incandescent but stagnant lava was seen 50 m downslope from the vents. The output was estimated at 1-3 m³/s. The SW ("diaframma") vent in the Voragine produced loud explosions every 2-10 minutes that ejected incandescent bombs above the vent.

Activity during July 1999. Another summit visit by Behncke on 1 July benefitted from perfect viewing conditions and very little wind. The generally flat floor of NEC had changed little since 5 October 1998, but now contained a large pit emitting a high-pressure gas plume charged with SO₂. There were periods lasting a few minutes when the noise level increased notably, and the plume became much denser; one time it contained brownish ash. BN had its usual two large eruptive centers, one in its NW part and the other at the base of its SE rim. While the latter periodically emitted plumes of grayish-brown ash, the former was the site of alternating ash emission and magmatic degassing.

The Voragine, according to a guide, had intensified its activity on 26 May. On the morning of 1 July explosions occurred at the SW vent every 1-10 minutes. Explosions at the SW vent started with a noise followed by large bombs that rose tens of meters above the vent, and sometimes even tens of meters above the crater rim itself, and then by a brownish ash plume. A few fresh vesicular bombs were found on the outer SW slope of the Voragine.

Claude Grandpey (LAVE) visited the eruptive fissure on 2 July and observed vigorous lava emission. The next day, lava emission had decreased. Activity was intense at the SW vent of the Voragine, with explosions ejecting bombs outside the crater on the northern side. Many bombs also fell into BN. The central vent in the Voragine had periodic gas and ash emissions. In the BN, noisy activity occurred in the SE vents (which during the 1 July visit only emitted ash), while the NW vent was relatively quiet.

The summit area was visited on 6 and 7 July by Behncke, Peter Ippach, and Eduard Harms (German Volcano Museum, Mayen, Germany). During the first of these two visits there was strong gas emission from the central pit of the NEC, and every 10-45 minutes there were explosive ejections of rocks and ash emissions. In the Voragine, explosive activity at the SW vent had decreased, and only one explosion was observed during two hours. However, the central vent was the site of Strombolian eruptions every 1-10 minutes. Incandescent bombs were ejected but only in one case rose as high as the rim of the vent, which was estimated to be at least 35-40 m deep and had a pit about 5 m wide in its floor. Recently ejected bombs up to 1.5 m long littered most of the Voragine floor.

The 7 July visit to the fissure disclosed continuing activity from two major effusive vents, one located in the area of the hornitos that formed in the past few weeks, while the other lay ~100 m downslope at the end of a lava tube. During four hours of observations, explosion sounds coming from the Voragine (and maybe also from BN) were heard every 5 to 45 minutes.

On 9 and 10 July, Behncke, Ippach, and Harms visited the summit area again, and additional information about the activity on 10-11 July was provided by Scarpinati and Charles Rivière (of Tremblay-en-France, France). Observations were restricted to the area of the 4 February fissure, but Rivière visited the summit craters early on 10 July. At the fissure, three vents were active at the tumulus ~150 m downslope from the uppermost February-March hornitos. Several lava flows were active during 9-10 July, and incandescent lava was seen in many places on the lava field. Lava also issued from several vents along the N margin of the flow-field.

Rivière, who visited the summit craters during the forenoon of 10 July, reported continuous pyroclastic activity deep within the pit of the NEC. In the Voragine, Strombolian activity occurred from the central andSW vents, with bombs at times rising high above the crater rim; Rivière noted that explosions occurred about every two minutes.

Scarpinati and Alain Catté (LAVE) observed the activity from the late afternoon of 10 July through the next morning. Shortly after 1800 on the 10th, the tumulus where the main vent had been emitting lava was seen to "inflate rapidly, and then lava came down on all its sides, forming three lava rivers." On the next morning, none of the vents on the tumulus were active, but a new vent had formed 30 m SE, burying the tourist path to the vent area; lava effusion diminished later that morning. Between 13 and 24 July lava continued to flow from the 4 February fissure, but the amount was relatively small, and short-lived flows extended only a few hundred meters downslope.

On 16 July Grandpey noted clouds of brownish ash from NEC. The Voragine was quiet, but Grandpey learned that the SW vent was active earlier during the week (around 12 or 13 July) with explosions, while lava was visible at the bottom. The NW vent inside BN was quiet, and parts of it had collapsed. Strong explosions heard every few minutes in the SE vent had been audible throughout the night.

Activity was particularly intense in the Voragine on 18 July when Rivière filmed the SW vent. Lava had again risen to ~20 m below the rim, and a small, dome-shaped mound of lava produced numerous small explosions. The mound was partly incandescent and was blown to pieces in some of the larger explosions, then rose again. During the days preceding 24 July, however, Rivière observed a diminution of activity in the Voragine, but there was explosive activity within BN.

The summit craters were visited again on 28 July by Behncke, Carmelo Monaco and Angelita Rigano (DSGUC), and others. Deep within the central pit of the NEC there were near-continuous detonations. Within the BN, explosive activity occurred deep within the two main vents. The SE vent produced near-continuous emissions of brownish ash. The Voragine central vent produced powerful explosions and at times prolonged fountains of incandescent bombs, some of them up to 1 m across. Some of the explosions ejected bombs to ~100 m above the crater rim. Many eruptions were accompanied by high-pitched roaring noises indicating high-pressure gas emission from the top of the magma column in the vent, which had risen by tens of meters since last observed directly by Behncke and others on 6 July. At the 4 February fissure, lava emission continued at a low rate. One area of effusive activity lay on the NE side of a large tumulus ~100 m downslope from the upper hornito cluster. The effusion rate was ~1-2 m³/s, and the volume of lava emitted since 4 February was estimated to exceed 35 x 10⁶ m³.

1 August-3 September 1999. Axel Timm from Germany visited on 15 and 16 August and made the following observations. There was little activity in the BN on 15 August, with quiet degassing at the NW vent, while dilute ash clouds were emitted at intervals of several hours from the SE vent. In the Voragine there was only gas emission from the SW vent, but minor eruptions occurred at intervals of 5-60 minutes from the central vent. Rumbling noises and dense gas emissions came from deep within NEC. Several small lava flows issued from the hornito area at the upper end of the 4 February fissure.

On 16 August the SE vent continued to quietly emit ash to 50-100 m above the vent at intervals of about 30 minutes. Voragine eruptions every 10-30 minutes from the central vent varied from noisy gas emissions to explosions that ejected bombs and scoriae far beyond the rim of the vent.

Grandpey reported that lava effusion from the 4 February fissure decreased notably around 20 August. Activity ceased on 25 August, and no effusive activity occurred thereafter for two days. Grandpey noted that the end of the effusive activity corresponded to a increased activity inside the Voragine. On 24 August he saw explosions from two small vents on the N rim of the SW vent. On 26 August Grandpey observed the central part of the Voragine inflate over a surface ~50 m in diameter, followed by an explosion that disrupted about half of that area, ejecting large pyroclasts. A few minutes later a much stronger explosion sent bombs as far as the center of BN and all over the W slope of the Voragine. Similar explosions followed through the next day. When Grandpey returned on 27 August, a new "cavity" had formed at the center of the Voragine and explosions were occurring near the SW vent.

The cessation of activity from the 4 February fissure on 25 August was followed two days later by the opening of a ~50 m long fissure located 40-50 m N of the hornitos. Mild Strombolian activity occurred during the following days and a small lava flow moved along the rim of the lava field.

4 September 1999 eruption from the Voragine and SEC activity. Scarpinati was observing the effusive activity at the new vents at the SE base of the SEC cone at around 1700 on 4 September and noted a hissing sound at around 1745, which gradually increased until it was "like a jumbo jet taking off." Guides at the Torre del Filosofo hut heard a loud detonation at about 1810, and saw intense red glow above the main summit cone ("the BN was incandescent all over"). Strong continuous incandescence between the Voragine and NEC suggested that lava was flowing down the E side of the main summit cone. At about the same time, Scarpinati saw through a gap in the clouds that gas and ash were rising from the summit area. Shortly afterwards he heard the crashing of impacting blocks and bombs, and retreated to the Piccolo Rifugio at about 2,500 m elevation. The climax of the eruption probably occurred between 1900 and 1930, judging from the audible detonations.

Bad weather during most of 4 September precluded observations, but a relatively clear view from Piano Provenzana (on the N flank, ~6 km from the Voragine) revealed the sudden uprise of a dark, ash-laden column that was bent eastwards. Observers at the Piano delle Concazze, about 2,600 m elevation on the N flank and ~2.5 km from the Voragine, enjoyed a splendid view of the eruption. By the time of their arrival, probably between 1830 and 1900, a huge lava fountain was rising hundreds of meters above the Voragine, and a pitch-black, tephra-laden eruption column rose ~2 km high before being blown E by winds. Large bombs fell onto the upper slopes of the NEC, which continuously emitted a dense brown ash plume, and onto the W side of the fountain. At the climax of the activity, the fountain roared to at least 1,500 m above the Voragine, an unprecedented height in the recent history of Etna.

At 1945 the cloud cover lifted, and the group at Piccolo Rifugio saw "an awesome spectacle of gigantic explosions" occurring at intervals of about 2 minutes, one of which was described by Scarpinati as "the biggest I have ever seen" (he has climbed Etna more than 500 times in the past 35 years), and which showered the main summit cone with meter-sized bombs. Some of this late activity may have come from the BN.

By 2045 all activity on the main summit cone had ended, but explosive activity began from the SEC summit vent consisting of dark "smoke" emissions mixed with incandescent pyroclasts. Ten minutes later the activity became purely Strombolian with 20-25 explosions per minute. Observations from the Piccolo Rifugio continued until about 2200 and were curtailed by bad weather; later that evening lava began to spill from the lower part of the fissure on the SE flank of the SEC cone. Lava supply increased at the vents that had become active on 27 August, and on early 5 September, a lava flow ~1 km long was observed by J.-C. Tanguy and local guides.

Effects of the 4 September 1999 eruptions. Soon after the beginning of the eruption, loud detonations were audible in villages and towns around the volcano. This was followed by a fall of scoriaceous lapilli on the E flank, extending to the coast near the town of Giarre, more than 15 km from the summit (figure 81). Many of the lapilli were walnut-sized, and some, in the area of Fornazzo, were up to 10 cm long (observation by J.-C. Tanguy). 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. Larger clasts broke windshields and seriously damaged vineyards and fruit gardens. In a narrow sector from the Milo-Fornazzo area towards the coastal strip near Giarre the pyroclastic deposit was several centimeters thick, and traffic was disrupted due to scoriae on roads. On the beach of the Ionian Sea between Riposto and Fondachello, scoriae 5-6 cm in diameter were not rare. Press reports put the damage to agriculture and infrastructure at several tens of billions of Lire (several tens of millions of US $). According to the Catania-based newspaper "La Sicilia," ~1 x 10⁶ m³ of pyroclasts fell on Giarre alone, while the full volume of pyroclasts was given as 5 x 10⁶ m³, a value that fits well with observations by Behncke and others.

Field investigations were made by Behncke and Werner Keller (Proyecto de Observación Villarrica/Internet) 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 rainfall swept part of it away, and when the cleaning of roads was still in an initial stage. During the afternoon of 7 September visibility was hampered by clouds, but the effects of the eruption were striking. The cones of the summit craters were hit by countless bombs up to 5 m in largest dimension and lithic blocks up to 1 m across. Many bombs and some blocks had fragmented upon impact, and others were found up to 10 m outside the craters created by their impact. Projectiles had arrived on both fairly flat and vertical trajectories. Some of the larger bombs were still warm about 60 hours after their emplacement.

On the S flank of the main summit cone the accumulation of juvenile scoriaceous pyroclasts had apparently been so rapid that the deposit began to slide down the steep flank, forming something like a dry debris flow that extended ~500 m down the slope to its base. In its distal portion the flow ended in two distinct lobes ~1 m thick. About 80% of this deposit consisted of juvenile clasts 10-30 cm in diameter whose edges were rounded while sliding down the slope, the other 20% were older, slightly smaller clasts (reddish scoriae and gray lithic blocks).

Brief glimpses through the clouds permitted a view on the Voragine from ~500 m W of the crater rim. The heavy fallout close to the crater almost healed the large scar cut into the S flank of the adjacent NEC cone during the 22 July 1998 Voragine eruption (BGVN 23:11). On the SW crater rim, the rapid accumulation of fluid ejecta formed a lava flow ~300 m wide and 250-300 m long. Two similar fountain-fed flows were emplaced on the E side of the Voragine, the longer of which traveled ~700 m towards the VdB. Guides on the N flank indicated that another fountain-fed lava flow cascaded into the Bocca Nuova.

On the lower E flank the lapilli deposit extended in a narrow strip E towards the coast near Giarre. Five communities (including Milo, Mascali, and Giarre) suffered heavy fallout. Going northwards from Zafferana, on the SE flank, the southern margin of the fall deposit was in the forests between Petrulli (~2 km N of the center of Zafferana) and Milo, where isolated scoriaceous lapilli with 1-3-cm diameters occurred. Closer to Milo (1.5 km farther N) the number of clasts per square meter increased as did their mean diameter, and on the southern margin of the village the deposit became continuous. Most of the deposit consisted of lapilli-size scoriae, with little ash mostly coating leaves and grass. The largest clasts found in the S part of Milo were 7 cm across, and many reached 5 cm. In the N part of Milo, the thickness of the deposit exceeded 5 cm, and many leaves were damaged. In the S part, ~1.5 km from Milo, the deposit was 5-6 cm thick, and the largest clasts were up to 10 cm across. Residents reported that larger clasts fragmented upon impact. Going north, the deposit thinned gradually and ended with a relatively sharp margin ~2 km N of Fornazzo. Downslope, near the town of Giarre, the area of fallout was ~5 km wide in N-S extension, and up to 5 cm thick in its central portion. Most of the deposit here was composed of fragments with diameters of a few millimeters to 3 cm. The N and S margins of the deposit were strikingly sharp, it seemed that only very little fine ash fell beyond the margins of the lapilli deposit.

Comparison with the (relatively poor) descriptions of the fall deposit produced by an eruption from the Voragine on 17 July 1960 allows the conclusion that the 4 September 1999 eruption was less voluminous but similarly violent, and therefore among the largest explosive eruptions at Etna's summit craters during the past 100 years. The 1960 eruption produced ~10 x 10⁶ m³ of pyroclasts, and clasts more than 5 cm in diameter were reported.

The activity at SEC on the evening of 4 September had many minor effects. The most impressive changes since 28 July were the presence of the new lava lobe that had issued from the lower part of the 4 February fissure, and the collapse of part of the E crater rim.

Activity after 4 September 1999. During the week following 4 September activity continued at the summit craters, but observations were hampered by bad weather. Intense explosive activity occurred each day at the BN, and at times bombs were ejected onto the outer slopes of the main summit cone. The Voragine remained active, and vigorous seismicity indicated that the most intense activity occurred between 0100 and 0400 on 9 September. During their summit visit on 7 September, Behncke and Keller reached the area of activity near SEC and saw two small lava flows issuing from vents 15 m below the spatter cone formed after 27 August that extended onto the W slope of the VdB after a few days. Mild Strombolian activity occurred from a new cluster of hornitos near the effusive vents.

During the evening of 11 September Scarpinati observed lava flowing from a vent ~200-250 m farther downslope to the E of the SEC effusive area. The next morning a new double spatter cone ~200-250 m E of the previous cone issued fluid lava, at an estimated rate of at least 1 m³/s, that moved along the margin of the flow-field. The new vents were on terrain not covered by lava during the previous months, and it appeared that this was a true new eruptive fissure.

Mild magmatic explosions were observed by guides every few minutes early on 18 September. On the next day, Rivière observed vigorous lava splashing from the NW cone of BN. Strombolian activity was relatively weak until early the next morning. At 0445, Tanguy observed the eruption from Trecastagni (on the SE flank). Continuous jets of incandescent material illuminated a gas plume rising more than 500 m above the crater rim. A bright glow in the area of the effusive vents at the ESE base of the SEC was noted, and weak incandescence was seen in the area of the Voragine. Tanguy arrived at the Piccolo Rifugio at about 0545, by which time the most energetic phase was over, although some incandescent bombs still rose up to 300 m above the crater rim. The activity had virtually ceased by 0630.

During the early morning hours of 20 September, vigorous lava fountaining occurred at the BN, mostly from the vents in the NW part of the crater where a broad cone had been the site of weak degassing for several weeks; previous reports noted that the area of this cone had remained virtually unchanged even during the 4 September Voragine eruption. The episode covered almost the entire floor of the BN with lava to thicknesses of several meters to tens of meters. A lava tongue invaded the depression that had previously hosted the SE vents, and only an irregularly shaped depression was left at the site of the NW vents. Explosive activity was again observed on the evening of 20 September, and a brief surge of activity occurred on late 21 September, after which BN became silent for about two weeks.

Effusive activity from the vents on the ESE base of the SEC was intense on the morning of 20 September when visited by Tanguy; lava issued from a vent that had opened the previous afternoon near the large spatter cone built after 27 August, and mild spattering occurred from this cone itself. A new vent had also formed at the fissure that had become active on 12 September. Vigorous effusive activity was continuing at the 12 September vents.

During the week following the 20 September eruptive episode at BN, the most persistently active summit crater was NEC, which had Strombolian activity in its central pit. A visit by Behncke on 28 September revealed that NEC cone had received heavy fallout of bombs on 4 September, and the footpath on its W side had vanished under a continuous cover of bombs, some up to 1.5 m in diameter. While the collapse scar on the SW flank of the cone had been largely healed by bomb fallout, a portion of the cone's flank farther to the ESE had collapsed, leaving a similar scar. Activity within the central pit consisted of near-continuous expulsions of dark ash. Good views obtained by Rivière on 25 September showed that the pit continued to a depth of several hundred meters with vertical walls.

On 28 September, good panoramic views of the Voragine from the S rim of NEC revealed that the former SW and central vents had merged into one large ~200-m-wide crater, but it appeared that there were still two eruptive centers. On the SW rim a wide U-shaped gap had formed in the former "diaframma" (septum) through which the floor of BN could be seen. Eruptive activity within the Voragine on 28 September consisted of frequent loud explosions.

Sub-concentric fractures were present on the outer ENE and E rim of the Voragine and on a ridge which now constitutes both the SE flank of the NEC cone and the NE rim of the Voragine. A fountain-fed lava flow that had formed during the 4 September eruption on the W side of the Voragine was up to 150 m wide in its upper part but narrowed to ~30 m in its distal portion where it formed a lobe along the N side of the 22 July 1998 flow; the new lobe, however, was shorter than its predecessor. Two fountain-fed lava flows also formed on the E side of the Voragine. The longer of these flows extended about halfway to the W rim of the Valle del Bove.

Rivière visited SEC on 24 September and reported that discontinuous effusive activity continued from the new vents (first seen by Scarpinati on the morning of 11 September) near the 4 February fissure. Lava flows extended ~1 km and spilled down the W face of the VdB.

Vigorous eruptive activity resumed in the BN on 30 September, ejecting large bombs hundreds of meters beyond the crater rim. At the same time, activity increased at the NEC. On 29-30 September, near-continuous Strombolian activity ejected bombs tens of meters above the crater rim, and larger bursts reached heights of up to 150 m, dropping bombs all over the crater floor and onto the flanks of the NEC cone.

Geological Summary. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Boris Behncke and Jean-Claude Tanguy, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy.