The following report summarizes activity observed at Etna from January through February 1999. Bocca Nuova exhibited minor explosive activity through early February, but Northeast Crater and Voragine were quiet. Southeast Crater had seven distinct eruptive episodes between 5 January and 4 February; the latest was accompanied by the opening of a new eruptive fissure at its southeastern base. The information for this report was compiled by Boris Behncke at the Istituto di Geologia e Geofisica, University of Catania (IGGUC), and posted on his internet web site. The compilation was based on personal summit visits, observations from Catania, and other sources cited in the text.

Activity at Southeast Crater (SEC) until 23 January. After one week of relative quiet, the sixteenth eruptive episode of SEC since 15 September occurred shortly before noon on 5 January; this was preceded by weak Strombolian activity that started around midnight. The paroxysmal phase was characterized by vigorous fountaining, and lava flowed towards the northeast while tephra was driven southwest by the strong wind. Loud detonations were audible in towns on the flanks of Etna.

Episode 17, during the night of 9-10 January, was preceded by mild Strombolian activity; the paroxysmal phase occurred shortly after midnight. Lava presumably flowed NE again and tephra fell NE; Fiumefreddo, ~8 km SW of Taormina, received a light showering of ash. Loud detonations during the final phase were audible over a wide area, and clear weather conditions permitted many in the Catania area to watch the spectacular display.

After the shortest repose interval observed since early in the current eruptive sequence in September, episode 18 took place on the morning of 13 January, between about 0630 and 0930. Visibiliby was hampered by clouds, but loud detonations were audible in a wide area around the volcano. Ash fell as far as Giarre, ~15 km E.

The next eruptive episode occurred on 18 January, shortly after 0800, and lasted ~ 45 minutes. Minor Strombolian and effusive activity had occurred earlier during the night. As in preceding episodes, the culminating phase was characterized by initial strong lava fountaining which gradually became more ash-rich, generating a dense eruption column. Due to calm conditions, the column rose several kilometers above the summit (3 km as estimated from Catania) and attained a spectacular mushroom shape visible in the morning sky from all around the volcano. At the SEC cone itself, the heavy fallout and rapid accumulation of pyroclastics led to frequent avalanches, especially on the steep eastern side. After 0830, dull explosion sounds were audible to as far as Catania, accompanying the rhythmic uprush of dark ash. The activity declined rapidly at 0845, but ash emissions became again more forceful after 0900 and continued sporadically for several hours, accompanied by sliding of hot pyroclastics from the steep E side of the cone. No information was available about lava flows although it is likely that they occurred, possibly on the NE side of SEC.

SEC erupted again after only two days and four hours of inactivity, shortly after noon on 20 January. Increased gas emission began at ~ 1215, and by 1240 a lava fountain appeared at the vent of the SE Crater cone. This fountain rapidly rose to a height of several hundred meters, and the column which rose above it became more and more ash-rich. Less than 15 minutes after the onset of the eruption there occurred the first slides of hot pyroclastics from the upper part of the cone, and five minutes later the whole cone and part of Etna's main summit cone were veiled by a black curtain of falling bombs and scoriae. By 1300, the vertical eruption column had risen several kilometers above Etna's summit. Ten minutes later the activity began to decline rapidly, and by 1315 the eruptive episode was essentially over, with only a few ash puffs being emitted during the following 30 minutes.

During a summit visit by Boris Behncke and Giovanni Sturiale (IGGUC) on 21 January, the crater was completely quiet, and only a few weak fumaroles played on the SW and E crater rims. The cone at SEC had grown higher than 3,250 m, about as high as the rim of the former Central Crater (filled by lavas and pyroclastics in the 1950's and 60's). While its flanks were steep and regular on most sides, obliterating any trace of the pre-1998 crater rim, a deep V-shaped notch was present in the northern crater rim through which lava had spilled onto the cone's flanks during recent eruptive episodes. These lavas had formed a fan-shaped lava field on the northeastern base of the cone, extending to the rim of Valle del Bove.

Behncke and Sturiale also investigated the pyroclastic deposits of the recent eruptive episodes which extended in relatively narrow fans from SEC in various directions. During the 18 and 20 January epidsodes, most fallout had occurred in a radius of <1 km from the cone, mainly on the SE side of the former Central Crater where 0.5-1 m of pyroclastics had accumulated since late 1998. Meter-sized bombs had fallen up to 500 m from SEC, creating spectacular impact craters. Among the most peculiar features of the recent eruptive products was a small lahar on the southwestern side of SEC which extended ~300 m from the base of its cone; this was probably produced during the 5 January episode. Records of lahars are relatively rare in the recent history of Etna, the most notable occurring in 1755.

On the morning of 23 January, SEC was the site of yet another eruptive episode that began at about 0630 and probably lasted less than one hour. Due to the absence of wind, an eruption column rose several kilometers above the summit then drifted slowly SE. In Catania, the ashfall was not dense, but people in the streets felt particles entering in the eyes; these particles were less than 1 mm in diameter and left a thin, discontinuous film on the ground. More serious effects were caused by the fallout in the upper southern parts of the mountain where skiing was rendered impossible by scoria on the snow. The repose period between this and the previous eruptive episode was two days and 18 hours.

There appears to have been no significant seismic or eruptive activity between 23 January and 4 February; the few clear views during that period revealed no morphological changes.

The January eruptive episodes continued to build the SEC cone, which has changed beyond recognition from its mid-1998 appearance. The large crater formed in 1990 at the summit of the SEC cone was completely filled, and a new, tall summit grew over it, burying any trace of the 1990 crater and much of the lava flows erupted from mid-1997 to late July 1998. After the 23 January episode the cone's new summit was at ~ 3,270 m elevation, almost 90 m higher than the highest point of the 1990 crater rim in 1997.

New eruptive fissure opens on 4 February. A new eruptive episode from SEC began at 1600, producing a spectacular eruption column visible from Catania and all around the mountain. Like previous episodes, this event was characterized by vigorous fire-fountaining, tephra emission, and lava, and was preceded by a gradual increase in gas emissions and then mild Strombolian activity. The activity began to culminate at around 1600 when a tall fountain jetted from the summit crater of the cone, and lava spilled through the breach in the N crater rim.

Sometime around 1630, the SE side of the cone fractured, and a new vent opened about halfway down the cone's flank, producing a tall lava fountain 250-350 m high and feeding a dense, ash-laden eruption column. An eruption column rose ~ 2-3 km above the summit before being driven SE, dropping fine ash on the flanks. Lava soon began to flow SE from this vent (figure 75). At about 1640, a row of incandescent spots appeared below the newly formed vent, indicating that a fissure had begun to propagate downslope from the base of the SEC cone. Vigorous lava fountaining and tephra emission from the new vent on the SE flank of SEC diminished rapidly shortly after 1700, but activity continued at the smaller vents on the fissure below that vent, at ~ 2,950 m elevation, and lava advanced rapidly towards the rim of Valle del Bove. At nightfall, both this lava flow and the lava erupted at the beginning of the episode onto the northern side of SEC were brightly incandescent and well visible from towns on the eastern side of the volcano, causing rumors of the opening of fractures on both sides of the cone. However, the northern flow soon stagnated and cooled, and no further lava emission occurred on that side for the remainder of February.

Figure 75. Sketch map showing Etna's summit craters SEC, Voragine (V), and Bocca Nuova (BN). The approximate extent of lava flows emitted during the 4 February eruption are in medium gray and those following the 4 February eruption are in black. Flows erupted from 1971 to 1993 are shown in light gray. Courtesy of Boris Behncke.

On 5 February, lava had begun to spill into Valle del Bove, forming a cascade on its steep western wall. The flow advanced very slowly, and had not yet reached the valley floor (at ~2,000 m elevation) on the next day when the new eruptive fissure was visited by Behncke and Giuseppe Scarpinati (L'Association Volcanologique Européenne, LAVE). Mild explosive activity was building several hornitos in the upper part of the ~100-m-long, SE-trending fissure at the base of the SEC cone while lava was issuing from numerous vents along the whole length of the fissure, feeding several channellized flows and some minor a`a flows. The effusion rate was estimated at 5 m³/s or more, significantly higher than during previous mainly effusive eruptions near Etna's summit craters (mainly at NE Crater in the 1970's) and similar to the effusion rates of some of Etna's flank eruptions. Pahoehoe lava was abundant around the effusive vents. The cone of SEC was found to be fractured from its summit down to its base, but only the main 4 February vent appeared to have produced significant eruptive activity while only minor spatter and scoriae were found in the part of the fracture between that vent and the still-active fissure.

On 15 February, Behncke and Scarpinati again visited the eruptive fissure and observed its activity for about 4 hours. By that day the lava spilling into the Valle del Bove had reached ~ 2,000 m elevation. There was no sign that the activity was diminishing, and the effusion rate remained perhaps as high as 5 m³/s.

Lava continued to issue from a number of effusive vents on the active fissure, forming at least two main rivers and several smaller and short-lived flows. In the course of a few hours Behncke and Scarpinati saw some of the lesser flows cease and others reactivate, forming blocky a`a while the more vigorous and long-lived flows moved in well-defined channels and showed no significant flux variations. Numerous short lava tubes, well-developed flow channels, and secondary vents had formed. Most effusive activity occurred ~50-100 m downslope from the upper end of the fissure, but several vents were also higher upslope. In the uppermost part of the fissure, numerous hornitos had formed, most of them concentrated in three clusters, and this area had countless incandescent vents producing high-pressure gas emission accompanied by a persistent hissing noise. The largest hornitos formed thin, vertical spires up to 3 m high while others were small humps a few tens of centimeters high. There was little explosive activity; only one vent in the uppermost hornito cluster rarely ejected incandescent pyroclastics.

Similar activity continued through the end of February. Lava flowed into the Valle del Bove, forming numerous lobes that moved on top or adjacent to earlier flows, and the farthest flow fronts did not extend much beyond 2,000 m elevation, remaining above the Monti Centenari, a cluster of cones formed during the 1852-53 eruption on the floor of Valle del Bove. The flow field gradually widened to ~500 m on the rim, and flows were issuing from numerous ephemeral vents on the W slope of the Valle.

Activity at Bocca Nuova (BN), Voragine, and Northeast Crater (NEC). Little significant activity occurred at these craters during January-February 1999 except for a brief resurgence of activity at BN during the week preceding the 4 February SEC events. During the 21 January visit by Behncke and Sturiale, spattering and Strombolian activity occurred deep within the large crater in the southeastern part of BN, accompanied by dense gas emission.

The cone in the northwestern part of BN produced violent noisy explosions every few minutes which ejected fountains of bombs high above the crater rim; ejecta frequently fell outside the crater, mostly to the W but in a few cases also SW and S. Between the explosions, deep-seated minor activity occurred within the 50-80-m-wide crater of the cone. No effusive activity had taken place in BN since it was invaded by lava from Voragine on 22 July 1998.

Bright crater glow was visible above BN in the first nights of February, the first time in about five months. This glow persisted during the night of 3-4 February but was much weaker on the evening of 4 February, indicating a drop of the magma level, probably related to the opening of the eruptive fissure on the SE base of SEC earlier that day. During the following week, only infrequent weak glows were visible above BN and then vanished altogether.

Very little activity except profuse steaming was observed within the Voragine during the 21 January visit by Behncke and Sturiale, who were able to descend into this crater and arrived at the "diaframma," the septum that separates the Voragine from Bocca Nuova. The floor of the crater was very flat in its eastern part, while a cluster of four craters with low cones occupied its central-western portion. The central crater, ~50 m wide and 30 m deep, was completely quiet; on its W side a much shallower, ~20-m-wide crater contained a 2-m-wide degassing hole with overhanging walls on whose floor numerous incandescent spots could be seen. A small crater with a diameter of less than 20 m, and ~ 10 m deep, lay on the SE side of the central crater. The largest crater in the Voragine was in the SW part of the Voragine and was between 70 and 100 m wide and more than 50 m deep with very steep and unstable walls, so that its floor could not be seen. Eruptive activity occurred at depth; as could be judged from the noises this was similar to the activity observed in the southeastern BN vents on the same day. A fifth vent that was active in August and early September 1998 on the crest of the "diaframma" appeared to have collapsed into the large SW vent, and only a part of its cone remained standing.

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, Istituto di Geologia e Geofisica (IGGUC), Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy.