26 August 1998

Activity at Etna is continuing, but the near continuous ash emissions that were visible during the past few days have ceased today. The sources of the ongoing activity are in Bocca Nuova and Voragine while there seems to be little, if any, activity at the other two summit vents. The levels of activity are lower than during the past week when activity culminated (on 21 August) in an episode of vigorous lava fountaining from the Voragine that caused ash falls again in Catania; similar activity occurred again on Monday (24 August). None of the recent ash falls have been of dimensions like that of the 22 July eruption.

26 August 1998

During the past few days, activity at Etna's summit craters appears to have continued much in the same manner as last week. Activity is mainly occurring at the Voragine and Bocca Nuova. Strong fumarolic activity is occurring from NE Crater while SE Crater appears to be perfectly quiet. Very light ash falls occurred again on Monday (24 August), reaching as far as Catania. Today, frequent ash emissions are occurring from Bocca Nuova, the southwestern vent area being more active, and occasional larger ash emissions are produced by the central vent of the Voragine.
THE 22 JULY 1998 ERUPTION. The following provides more details on observations made during the summit visit on 20 August (see previous update). The visit was made in order to observe the ongoing activity, and to survey the effects of the 22 July 1998 eruption and to understand its evolution. Significant morphological changes occurred mainly at the Voragine, NE Crater and Bocca Nuova.
The 22 July eruption mainly originated from the Voragine, and it appears that both the central and the southwestern vents participated in the event. Both vents produced huge lava fountains and a tall tephra column. Most of the bombs ejected by the fountains fell on the northern, northwestern, southern and eastern sides of the crater. The rapid accumulation of fluid ejecta in the saddle on the northwestern rim of the Voragine led to the formation of a typical fountain-fed lava flow that advanced westwards along the valley between the NE Crater cone and the main summit cone, covering a footpath that had been frequently used in recent yearsa by visitors venturing to the Voragine and Bocca Nuova. The flow narrowed downvalley from about 100 m at the crater rim to 15-20 m at the flow front. The nose of the flow covered the dirt road connecting the northern and southern flanks of Etna (Rifugio Sapienza-Piana Provenzana) at a point close to where tourists are being taken by guides of the "STAR" (organisation of the guides on Etna's north flank).
A remarkable feature of this flow is that near its origin it eroded a deep scar into the the southern flank of the adjacent NE Crater cone, carrying away the entire slope that served until recently as the easiest path to reach the southern lip of that crater. The wall of the scar is vertical and tens of meters high, and has multiple slump blocks at its base. No trace remains of the cluster of scoria cones that formed in the summer of 1996 on the southwestern rim of NE Crater. However, the southern rim of the central pit of the crater still stands as a ledge some 30-40 m wide, but this may fail if NE Crater returns to more vigorous activity in the future.
The flow from the NW side of the Voragine appears to have been a short-lived phenomenon for continuing pyroclastic activity from the Voragine produced a thick scoria and bomb deposit that completely mantled the proximal part of the flow. Huge bombs (up to 5 m in length) fell in that area, but some 300 m further downslope to the west, the deposit consists entirely of centimeter-sized, highly inflated scoriae. A fan of this deposit extends about 1-1.5 km to the NW (along the road connecting the summit area with Piana Provenzana). On the western and southwestern base of the main summit cone virtually no fallout from the 22 July eruption is visible. Fallout from the fountain of the southwestern vent appears to have occurred mainly in the area of the "diaframma" (the thin wall between Voragine and Bocca Nuova) where another fountain-fed lava flow developed. This flow originated at the base of the "diaframma" and extended southward on the eastern side of the northwestern eruptive cone in that crater, turning southwestward near the southeastern eruptive cone and covering the southern and southwestern crater floor several meters deep. On 20 August the surface of this flow was extensely cracked and jumbled, with steam escaping in various places. It was probably this still-incandescent lava that was visible from passing airplanes the day after the paroxysm.
While abundant large bombs from the Voragine are present on the northern rim of Bocca Nuova, no ejecta have been found on the western, southwestern and southern rims of that crater, but in the eastern sector of the former Central Crater and on the eastern flank of the main summit cone a thick pyroclastic deposit has formed. On the E rim of the former Central Crater the deposit consists of agglutinated spatter. In this area several gaping north-south trending fractures have formed at whose bottom incandescence was visible for several days after the 22 July paroxysm. Neither incandescence nor gas emissions were noted at these fractures on 20 August.
The pyroclastic deposit on the southeastern side of the main summit cone decreases in thickness from about 1 m (on the western rim of SE Crater) to several tens of centimeters (at Torre del Filosofo). In towns on the eastern and southeastern flank, the tephra deposit was a few millimeters to a few centimeters thick, but since 22 July much of the tephra has been cleared away.
The morphological changes caused within the Voragine itself consist mainly of a large amount of filling of the crater which was followed by some subsidence, and both eruptive vents are now located in saucer-shaped depressions tens of meters wide. No new pyroclastic cones have formed around the vents. A part of the southwestern crater rim of the Voragine has collapsed during the 22 July paroxysm, and abundant collapse has also occurred on the southeastern side of the crater rim. The crest of the "diaframma" was lowered significantly on 22 July and has now a much smoother outline.
Bocca Nuova has changed much less than expected after the initial descriptions by visitors and news media. It appears that its role in the 22 July paroxysm was neglegible. The northern scoria cone has grown only little since last observed by Behncke on 13 July, and there is no evidence of large-scale rupturing or collapse of the cone in the intervening period. However, besides the summit vent (seen in eruption on 20 August), there are at least four smaller vents on the cone's flanks which have erupted recently. At the southeastern vents, some collapse has occurred, but no major morphological changes were observed. Only few relatively fresh bombs were found on the southern rim of Bocca Nuova on 20 August; these were a few days old and had possibly been ejected from the northern vent or from the Voragine.
At SE Crater, the vigorous activity of mid-late July had built the intracrater cone substantially, and now it stands some 40-50 m higher than the northwestern crater rim. Bombs from this activity, lying on top of the 22 July deposit, were scattered all over the crater area and beyond. Two post-22 July lava flows had spilled onto the southern and northeastern flanks of SE Crater, the northern flow extending a few hundred meters towards Valle del Leone. The most recent effusive activity, between 22 and 27 July, occurred from a new vent on the northeastern base of the intracrater cone. During the 20 August viosit, the crater was in a state of complete inactivity, and not even a wisp of vapor issued from the vent at the summit of the intracrater cone although some fumaroles were emitting vapor on the upper northwestern flank of the intracrater cone. Some of the lava flows on the crater floor were still hot, but no flowing or incandescent lava was seen anywhere in the crater. This state of calm appears to prevail since about 27 July and marks the longest period of inactivity at SE Crater since its reactivation in late 1996.
Although the morphological changes caused by the 22 July eruption in the area of NE Crater, Voragine and Bocca Nuova are significant, all major effects of that event are limited to the immediate vicinity of the summit craters. The eruption was surely not as overwhelming as it appeared from news media reports; nonetheless it was one of the largest events of the current eruptive cycle, being of about the same magnitude as the powerful paroxysmal episodes from NE Crater between November 1995 and June 1996. The fact that it occurred during the main tourist season and at perfect weather conditions caused widespread media interest and therefore some dramaticized reporting. As a secondary effect, still more tourists were attracted by the volcano, and manured to the scene of the activity in spite of rigorous access restrictions. Dozens of tourists are attempting to reach the rims of the summit craters even in moments of intense activity when many bombs are falling outside the craters. On 20 August, people were seen walking across areas which received heavy fallout from the Voragine.
It must be underlined that so far, most paroxysmal episodes of the current eruptive cycle (that is, since 1995) have occurred either during the night, during the winter or during bad weather, in moments when there were few or no people in the summit area. Thanks to this fact, no people have been killed at Etna since 1987. However, the 22 July paroxysm occurred without ANY clear warning and its onset appears to have been quite rapid. Had there been any people on the rim of the Voragine, the event would certainly have produced victims, and would have created drastic policy changes regarding the access to the summit craters.

20 August 1998

Carmelo Monaco and Boris Behncke of the Istituto di Geologia e Geofisica of Catania University (IGGUC) visited Etna's summit craters today. Vigorous activity occurred from both vents in the Voragine while more moderate eruptions were observed at the northern vent area in Bocca Nuova. NE Crater was degassing quietly, and virtually no activity was observed at SE Crater. The level of activity may be lower than during the preceding days, but is still very impressive. We also studied the effects of the recent activity which are notable, but some facts about the 22 July eruption and the more recent activity have evidently been exagerated in the news media.

19 August 1998

Activity at Etna's summit craters is continuing vigorously. Powerful Strombolian activity with bomb ejections and ash emission (originating most probably at the Voragine although the local press reports activity also at Bocca Nuova) caused light ash falls on the southeastern flank yesterday, reaching as far as the outskirts of Catania. Spectacular pyroclastic ejections were witnessed by geologists of the Istituto di Geologia e Geofisica of Catania University (IGGUC) from the eastern flank of Etna last night, with bombs falling down to the eastern base of the main summit cone. The height of the ejections was estimated to be 200-400 m. Near-continuous detonations were audible in the same area throughout the day, and at times as far as Catania. While no activity was observed at SE Crater by observers who spent the night of 17-18 August on the southern rim of Valle del Bove, that crater was seen again in eruption last night by Sandro Privitera of IGGUC. SE Crater had been silent since about 28 July, this being the longest period of inactivity at this vent since it began erupting in late 1996. No reports are available about the activity of NE Crater.
This morning there are near continuous explosive ash emissions from the Voragine, sending small plumes up to several hundred m above the summit which are then driven eastward by strong wind. The activity is considered stable, and there are no indicators of imminent activity elswhere on the volcano. Tomorrow scientists of IGGUC will visit the summit area in order to survey the recent morphological changes and the state of activity.

18 August 1998

A major paroxysmal eruptive event occurred at the Voragine on 22-23 July 1998. The following is based on preliminary information from scientists of the Istituto di Geologia e Geofisica of Catania University (mainly Giovanni Sturiale and Sandro Privitera) and other eyewitnesses who have visited Etna after the paroxysmal episode. More detail will be provided as research on the activity and its effects is continuing.
Activity before 22 July appears to have been quite "normal", i.e. similar to that seen during the first half of the month and described in previous updates. Small lava flows were observed to spill down the flanks of SE Crater, in particular on 20 July, and lava fountaining from the Voragine was common while Bocca Nuova had explosive activity mainly in its northwestern vent area. No significant activity was reported from NE Crater.
THE 22 JULY ERUPTION. A major explosion occurred, apparently without any premonitory phenomena, at about 1835 (local time). The explosion occurred simultaneously from the Voragine and Bocca Nuova, where all intracrater cones were blasted away. According to eyewitnesses who were on the southwestern side of the main summit cone (that is, to the southwest of Bocca Nuova), huge lava fountains rose from the Voragine, and heavy tephra falls began in the summit area. A large mushroom-shaped tephra column rose up to 10 km above the summit of the volcano; this phenomenon was clearly visible from many places in Sicily and even as far away as the Eolian Islands where residents and tourists were startled. The tephra plume was then driven south and southeastwards, and widespread ash falls occurred in that sector to more than 30 km away. Sand-sized tephra fell in Catania, leaving deposit about 1 mm thick. For the first time since 24 September 1986 (when NE Crater had a powerful explosive eruption) the Fontanarossa airport of Catania had to be closed (it was reopened after 15 hours). The tephra falls caused serious traffic problems on roads and highways in the area, but only minor accidents were reported. People in Catania were mostly annoyed by the phenomenon. In villages higher up on the slope of the volcano, the tephra deposit was several millimeters to a few centimeters thick, and at Rifugio Sapienza (at 1910 m elevation on the southern flank) the deposit was up to 1 cm thick. Closer to the summit, a thick scoria deposit had buried the dirt roads leading to the Rifugio Torre del Filosofo and around the western baser of the main summit cone. Sturiale and Privitera reported that at Torre del Filosofo the thickness of the scoria deposit was about 0.5 m, and progress to the western side of the summit cone was impossible. However, newspaper articles (based on information provided by the Istituto Internazionale di Vulcanologia in Catania) reported that a lava flow had spilled over the northwestern lip of the Voragine and reached the dirt road where it begins to descend northwards, towards the Piana Provenzana. A part of the southern flank of NE Crater was carried with the lava flow or collapsed into the adjacent Voragine. Vigorous activity occurred simultaneously at Bocca Nuova and SE Crater, indicating that this paroxysm affected much of the central conduit system at some depth (usually it is thought that at less than about 1 km depth the four summit craters have individual conduits, based on their strikingly independent behavior), possibly due to the rise of a voluminous batch of fresh, gas-rich magma.
Lava fountaining from the Voragine (and probably also from Bocca Nuova) continued intensely through the night of 22-23 July, but a marked decline was observed by about 2200 h. On the afternoon of the 23rd, Carmelo Monaco (Istituto di Geologia e Geofisica of Catania University) saw bright incandescence "all over the Bocca Nuova" even in bright daylight from an airplane due to land at Catania airport, at around 1600 h.
ACTIVITY AFTER 22 JULY 1998. During the following days, activity was limited to SE Crater where vigorous lava fountaining and lava effusion occurred. On 24 July, Sturiale and Privitera, together with others, observed vigorous Strombolian activity at SE Crater, with many bombs falling outside the crater and making approach to the crater rim dangerous. However, SE Crater activity gradually declined during the following days and virtually ceased by the end of July. As of 18 August, there has been no resumption of the SE Crater eruption. Jean Pierre Kloster (of the French Association Volcanologique Européenne L.A.V.E.) visited the summit area frequently from 29 July until 12 August and saw no activity at SE Crater; he managed to climb to the crater of the intracrater cone which he observed to be 10 m wide and about 10 m deep.
Activity resumed at Bocca Nuova on 25 July and increased notably during the following day (information from Claude Grandpey, associated with L.A.V.E. as well); activity at the northwestern eruptive area occurred from a vent initially only 2 m wide while the southeastern eruptive area was marked by a "chaotic surface" that had three vents from which gas and bombs were emitted. According to the same information source, the Voragine could be observed from NE Crater and appeared "full of materials" while there was no trace of the former intracrater cones. No activity occurred at this crater until 3 August when Kloster saw "violent and irregular explosions" from a vent in the center of the crater, ejecting bombs to as far as the Bocca Nuova. Two days later, three vents erupted in the center of the Voragine while a "small lava lake" was present at the southwestern vent; explosive activity appears to have been similar to that observed on 13 July (the bulging and bursting of huge magma blisters). On 7 August the crater was initially silent, but later "small and slow flows" occurred on the crater floor, followed by explosive activity.
Bocca Nuova was the site of the most impressive activity in late July and early August. Here, numerous vents erupted explosively at the northwestern eruptive area; subsidence of the central part of the crater floor by a few meters occurred on 1 August. The southeastern vents displayed spectacular lava cascades from one vent into the other, the lower vent being filled by this lava until an explosion blew the lava fill away. Kloster reports a lava lake in this vent area on 7 and 10 August while during the following days there was only Strombolian activity.
During the past few days (that is, 16, 17 and 18 August), activity has been particularly intense at the Voragine, with incandescent ejections being plainly visible from towns on the eastern and southeastern flanks of the volcano. Today, dark pyroclastic columns are frequently rising above the Voragine, indicating that the period of heightened activity is not yet over.