Etna showed relatively low levels of activity during December 1999 and through 25 January 2000. In contrast, on 26 January, Southeast Crater (SEC) started a new series of strong eruptive episodes, and from then until the end of March, 46 episodes occurred at that crater (table 7). Episodic eruptive activity continued into April. The information for the following report, covering December 1999 to March 2000, was compiled by Boris Behncke at the University of Catania, with additional information from Marco Fulle, Roberto Carniel, and Jürg Alean of Stromboli On-Line. The compilation is based on personal visits to the summit, observations from Catania, and other sources cited in the text.

Table 7. Chronology of eruptive episodes from the Southeast Crater at Etna, 26 January-24 March 2000. Courtesy of Boris Behncke.

Activity at Southeast Crater, 26 January-29 February 2000. Eruptive activity resumed early on 26 January at the summit vent of SEC (figure 83), after 4.5 months of quiet, and 2.5 months after the cessation of lava emission from fissures at its SE base. Initially the activity was Strombolian, but at about 0500 the activity changed to fire-fountaining, followed by the opening of a fissure on the S flank of the cone. Fountaining continued intermittently until about noon, but lava continued to flow from the lower end of the fissure until the late evening, advancing ~2.5 km into the Valle del Bove along the S margin of the 1999 lava field. Field inspections later revealed that numerous blocks of older lava and welded scoriae from the upper part of the cone fell up to 500 m from the summit of the cone.

Figure 83. Map of the summit area of Etna showing the approximate extent of lavas erupted from the Southeast Crater (SEC) between late January and March 2000. Flows terminating in arrows indicate the flow direction rather than its length. Lavas erupted from fissures near SEC between February and November 1999 and from Bocca Nuova in October-November 1999 are shown for comparison. The broken line extending from TDF (Torre del Filosofo) to the W is the margin of the Piano caldera, presumably formed during a powerful explosive eruption in 122 BC. V is the Voragine and BN is the Bocca Nuova. Courtesy of Boris Behncke.

The second eruptive episode, on the morning of 29 January, lasted about 15 minutes. As during the preceding episode, the cone fractured on the S side, and lava flowed for a few hundred meters to the SSE and SE. After the cessation of lava fountaining, lava continued to trickle from the fissure until the evening.

Beginning on 1 February, eruptive episodes occurred at ever shorter intervals. For the next four days, these events were separated by quiet intervals of 20-24 hours. They essentially resembled the second episode, with initial mild Strombolian activity followed by lava fountaining at the summit vent. Fountaining continued for a few minutes before the S flank of the cone fractured and small fountains rose along the fracture, while lava flowed from its lower end. Activity continued for up to 10 minutes and then ceased.

During these days intermittent minor lava effusion occurred from vents on the lower N flank of the SEC cone, feeding short flows. On the evening of 4 February the lava output gradually increased. Mild Strombolian activity began sometime before 2330 from a vent high on the SSW flank. By this time lava apparently spilled over the N rim of the crater. The volume of lava running down the N flank increased, and explosive activity at the summit vent (and possibly at the SSW flank vent) became increasingly vigorous. Shortly after 0010 on 5 February the activity culminated with lava fountains and voluminous lava emission. After only ten minutes the activity began to diminish.

Between 5 and 18 February, eruptive episodes occurred at a rate of 2-3 events/day, separated by quiet intervals of 5.5-26 hours. During the first ten days of this period, lava slowly issued from the vents on the N flank of the SEC cone. Before each episode the volume of lava output gradually increased, followed by the onset of lava spattering at the same vents and increasing gas emission from the summit vent. The spattering soon graded into a continuous fountain of very fluid lava, jetting obliquely from the N-flank vents, and then the activity would extend to one or more vents at and near the SEC summit. At the height of the activity, an eruption column mainly consisting of white vapor rose several kilometers, and ash fell up to 40 km downwind. Towns at the base of the volcano, mostly in the E and SE sectors, received light rains of ash and lapilli.

During several episodes a fracture opened progressively across the N flank from the base up to the summit vent, with small fountains, while Strombolian bursts began at the summit vent. This activity rapidly culminated in high lava fountains from one or more summit vents. During an eruption late on 11 February, four fountains rose up to 400 m high, while a continuous line of smaller fountains played along the N-flank fracture. Lava flowed copiously from the lower end of that fracture, while a new vent high on the S flank fed a lava flow which reached the base of the cone. The activity became intermittent after 10 minutes of fountaining and ended shortly thereafter, while lava continued to flow at diminishing rate from the lower end of the N-flank fissure.

An eruptive episode on the afternoon of 14 February was observed from less than 1 km away by Behncke and Giuseppe Scarpinati (Italian delegate of the Association Européenne Volcanologique), who had arrived shortly before the onset at a spot SE of the cone. Like on other occasions, the activity started with increasing gas emission from the vents on the lower N flank. By 1600 a broad, dull red fountain roared to a height of ~100 m, and gas emissions increased rapidly on the upper N flank. Small rockfalls occurred on the SE flank.

Soon after 1605 a huge jet of glowing bombs from the summit formed a rapidly expanding eruption column. Shortly thereafter, a densely tephra-charged, cauliflower-shaped plume burst obliquely upwards from the S side of the summit. At the same time, incandescent pyroclastics fell far beyond the base of the cone, but none hit the area from where Behncke and Scarpinati were located. Instead, the curtain of falling ash and scoriae rapidly extended southwards, towards Torre del Filosofo, a mountain hut ~1 km S of the SEC. Scoria clasts up to 30 cm long fell around the building. Lapilli-sized scoriae fell abundantly up to 5 km to the S, and ash fell up to 50 km away. The continuous loud rumbling of the fountains mixed with the pattering noise of large clasts impacting the snow near Torre del Filosofo. Lava could be seen flowing down the S flank in a broad stream a few minutes after the onset of explosive activity. At about 1620, the activity began to wane, although lava trickled from a vent high on the S flank for several hours.

The next day, after 26 hours of quiet, the most powerful episode of the sequence was observed from Torre del Filosofo by a group including Marco Fulle of the Astronomical Observatory of Trieste, Italy. The event was heralded by increased gas emission from the summit vent shortly before 1800, followed by mild Strombolian activity. For about 10 minutes there was a gradual buildup of the activity, with Strombolian bursts from at least two vents. As the activity became more continuous, incandescent pyroclastics were thrown to ever greater distances, mainly onto the E flank. Shortly after 1800, a glowing spot appeared immediately below the S lip of the summit vent in a deep notch. A small pulsating fountain from this new vent gained rapidly in height and vigor. Eruption noises began to change from the intermittent gushing of the Strombolian activity to a continuous loud noise similar to heavy surf.

Within 1-2 minutes after the appearance of the S-flank vent, huge jets of fluid lava rose from that vent and from vents at the summit. The volume of lava from the S vent increased rapidly, at times generating surges overriding earlier flows. The upper part of the cone was soon covered by incandescent pyroclastics; most fallout occurred on the E side due to a strong wind from the W. Activity escalated when a huge incandescent jet burst obliquely from the S vent in the direction of the Torre del Filosofo, and within a few seconds it rose to a height of ~1,000 m. The noise soon became dominated by thousands of bombs crashing on the ground at rapidly growing distance from the cone. The observers fled under a side roof of the Torre del Filosofo building a few seconds before bombs began falling around and beyond the building. Some of them had diameters of tens of centimeters, and many were seen bouncing and bursting into fragments. This rain of bombs lasted about 20 seconds, after which activity stabilized with lava jetting vertically from S-flank and the summit vents.

As was evident from video filmed by British cameraman David Bryant, the largest fountain came from the summit, and from the video as well as from estimates made by other observers, including Behncke (in Catania) and Scarpinati (in Acireale), the fountain height was consistently 500-600 m with bursts reaching 800 m above the summit. The entire cone was covered with incandescent material, some of which developed secondary flowage, while a broad lava flow ran down the S flank.

About 10 minutes after the onset of violent fountaining, the fountains from the cone appeared slightly weaker, although the continuous uprush continued for some time. Then the fountains stopped abruptly, while thousands of incandescent projectiles continued to fall onto the cone. After a few seconds, new lava jets appeared that were short-lived and much weaker. At the end of the activity, dense ash from the summit vent blew E as far as Acireale. Lava continued to run out of a fracture on the S flank, and the gradual sinking of the lava level in the fracture indicated that the conduit was subsiding.

On the afternoon of 16 February, Fulle observed another eruptive episode from the Torre del Filosofo. This event included lava fountaining from a vent about halfway down the S flank of the SEC cone. The activity then extended to the summit vent, continued vigorously for about 10 minutes and ended abruptly.

Eruptive episodes began to diminish in frequency and intensity after 18 February. The near-continuous effusive activity at the N-flank vents stopped. During the last 10 days of the month, five episodes occurred. On the early morning of 23 February, an episode lasted more than 1 hour, but consisted mainly of strong Strombolian explosions. The last two episodes of February, on the 27th and 28th, involved significant activity from vents near the S base of the cone. In both events the activity lasted for several hours, and erupted more lava from the S vents than during earlier episodes. During some of the episodes lava was apparently also produced from N-flank vents.

Activity at Southeast Crater during March. Episodic eruptive activity at the SEC continued in March and became more focused at the vents at the S base of the SEC cone, where a cone began to grow. This cone was informally named "Sudestino" (little Southeast), following the example of the "Nordestino," a lava shield crowned by a large hornito that formed in 1970 at the NE base of the Northeast Crater. In late March, the main focus of activity returned to the SEC, and episodes became very similar to those of early- to mid-February, with lava emission mostly from fractures on the N and S flanks of the SEC cone.

The initial March activity began on the late afternoon of 3 March. At first the activity consisted of slow lava effusion from Sudestino. Loud detonations became audible in Acireale and other towns in the SE and E sectors around 0400 on 4 March, marking the period of strongest explosive activity. Scarpinati, who observed the activity from his home in Acireale, noted that even in the moments of strongest activity, no sustained lava fountaining occurred, but all activity consisted of discrete powerful explosions. When the activity began to diminish (at about 0430), a fountain of very fluid lava in the area of the Sudestino rose ~30 m. The episode ended at about 0500, but after 0430 most activity appears to have come from Sudestino. Minor outflow of lava continued for about two days from Sudestino. Another episode on 8 March was preceded by slow lava effusion from Sudestino. During a summit visit by Behncke and others on 11 March, the SEC and Sudestino were quiet.

Sudestino erupted again shortly after noon on 12 March. The activity began with increased gas emission, and by about 1300 a lava fountain rose to a height of several tens of meters. Ash was expelled from the crater lying a short distance further up the S flank. Later a densely ash-laden plume was emitted from the summit. Lava flowed abundantly from Sudestino, mainly to the S and SW. A lava flow passed only a few tens of meters W of the Torre del Filosofo and extended ~100 m down the steep slope, burying a section of the dirt road that leads to the building.

The Sudestino vent erupted once more on 14 March with a brightly incandescent lava fountain and emission of a voluminous lava flow that advanced to the S with a front hundreds of meters wide, reaching Torre del Filosofo sometime after 1100. Eyewitnesses reported that by about 1100 the lava front was still ~50 m from the building, but presumably the lava reached and encircled it on two sides shortly afterwards. The wooden shack next to the building, used as a souvenir shop by mountain guides during the summer, was burnt by the lava, melting the snow which had covered the shack. Lava flowed ~100 m further down the slope to the W of Torre del Filosofo. A new dirt road built on this flow two days later allowed monitoring and communication equipment to be salvaged prior to the building's destruction.

Lava fountains were visible at the Sudestino around 0130 on 19 March, by which time lava had already begun extending down the flanks. The activity continued vigorously until about 0300, and generated significant ash that fell in Catania and surrounding areas. A large volume of lava was emplaced on the plain SW of the SEC, and several flow lobes extended as far as the N base of Monte Frumento Supino, a prehistoric cinder cone (figure 83).

Sometime after 1800 on 22 March, mild Strombolian activity began at the summit vent, and by 1945 there was a pulsating lava fountain. A new vent burst open high on the SSW flank at about 2010, emitting a lava flow and producing a small fountain. The activity progressively increased at both vents until another vent opened at about 2025 near the NE base of the cone. At this vent, a lava fountain rapidly began to rise several tens of meters high, while two lava flows spilled into the adjacent Valle del Bove. The lava fountain from the summit began to diminish, and ceased shortly before 2100. Lava continued for some time from the flank vents. The southern flow rapidly reached the WSW base of the cone and turned W or WSW, in the direction of the 1971 "Observatory cone." About 500 m NW of Torre del Filosofo the flow turned SW and reached the slope near Monte Frumento Supino, where it advanced up to 100 m. On the NE side of the SEC cone, the two lava flows advanced several hundred meters into the Valle del Bove.

Lava effusion from the vents on the NE side of the SEC cone increased during the late afternoon of 24 March. Mild spattering and intermittent glow at the summit indicated the onset of Strombolian activity. This activity graded into a lava fountain shortly before the new year (2000), and a second, smaller fountain played at the effusive vent on the NE side of the cone. Lava flowed into the Valle del Bove, reaching a length of possibly more than 1 km. Sometime after the new year began, a vent opened on the upper S flank of the SEC cone, feeding a minor flow. The activity was still vigorous at around 2030, when loud rumbling noises could be heard in Catania, and windows were vibrating in Acireale and other towns nearer to the volcano. The strongest activity apparently ceased by 2100, but at 2230 there was still vigorous effusive activity.

The last significant activity of the reporting period occurred on the evening of 29 March, after five days of quiet, the longest repose period of the eruptive sequence. Lava effusion from vents on the NE side of the cone became evident after nightfall and gradually increased, accompanied by the weak Strombolian activity at the summit vent. By 2000, the Strombolian bursts had become more frequent, and soon blended into a continuous pulsating fountain. A new vent high on the S flank emitted a lava flow that rapidly spilled to the base of the cone, then was deflected to the SW by the Sudestino. During the following 30 minutes, at least three smaller vents opened at progressively lower elevations on the S flank in the direction of Sudestino. After the opening of the first S-flank vent, activity at the summit became weaker and discontinuous. Sometime around 2120 large fountains from vents on the N flank sent lava flows NE towards the Valle del Leone. Fountaining ceased at around 2200, but lava continued to flow from the N vents, feeding several lobes, the longest of which advanced ~2 km into the Valle del Leone. On the S side, lava extended ~1-1.5 km SW to the N base of Monte Frumento Supino.

Activity at Bocca Nuova, Voragine, and Northeast Crater. During December 1999-25 January 2000 Bocca Nuova produced intermittent mild Strombolian activity that at times ejected bombs outside the crater. Ash emissions were frequent in late December and early January.

Activity at Bocca Nuova during the eruptive episodes at SEC continued at relatively low levels. During a summit visit on 2 February, Behncke and Scarpinati observed frequent small explosions from the E part of the crater, but all ejecta fell back into the vent. Six days later, Behncke and Scarpinati entered the crater from the SW – this had become possible due to the filling of the crater in October-November 1999 – and approached the vent which was the source of intermittent night glow for most of February. Activity consisted of vigorous gas emission, punctuated by strong blasts of incandescent gas, but no pyroclastic ejections. When standing on the edge of the vent, Behncke and Scarpinati saw an incandescent hole ~2 m across on the floor of the funnel-shaped vent which was the source of the gas ejections. Scarpinati noted that the activity was similar to that observed during the first year of the life of the Bocca Nuova, when it was only a small vent ~8 m wide.

The same activity was observed in early March by Charles Rivière (from Tremblay-en-France, France). In late March activity was the same as in early February, consisting of jets of incandescent gas without pyroclastic ejections. A small amount of strongly altered, fine-grained lithics were sometimes contained in the gas jet. It appeared that no ejections of fresh magmatic material had occurred within the Bocca Nuova since at least early February.

The other two summit craters remained essentially quiet during the reporting period. Northeast Crater occasionally produced emissions of thick gas plumes, at times charged with a little lithic ash. No eruptive activity is known to have occurred at the Voragine. When seen by Behncke on 8 February, the crater emitted only wisps of vapor from the large pit formed during the 4 September 1999 eruption.

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, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Universitá di Catania, Corso Italia 55, 95129 Catania, Italy; Roberto Carniel, Dipartimento di Georisorse e Territorio, Universitá di Udine, Via Cotonificio 114, 33100 Udine (URL: http://www.swisseduc.ch/stromboli/); Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland; Marco Fulle, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy.