Stromboli 1994-1995: a period of dramatic changes (1)

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Stromboli volcano, Aeolian Islands (Isole Eolie), Sicily, Italy -- Eruptive history (1985-1995)

1994-1995: a period of dramatic changes (Part 1: 1994)

After more than 2.5 years, I finally returned to Stromboli in mid-March 1994. The volcano was in a "normal" state of activity and still the effects of the October 1993 explosions were very evident. The upper part of the 1985-86 lava flows and all of the 1993 flows below the NE end of Crater 1 were buried under a thick cover of explosion debris - large, reddish blocks, some several meters in diameter. The eruptive vents were at the floors of deep chasms inside the craters and barely visible from the summit. On 9 March, visual observation was rendered completely impossible by clouds moving in when I reached the summit, and the rare moments when there were breaks in the clouds only permitted the view of dense vapor and gas clouds. The next day I was back on the mountain, and there was good visibility this time. The activity was relatively low, and I decided to descend to the craters another time. I stayed about 2 hours on the rim of Crater 3, most of this after dusk when the activity of the faithful vent 1 became too fascinating to leave that place soon.

As in September 1989 and April 1990, there was a small pond of actively spattering lava at depth in this vent, and ceaselessly, bombs and spatter were ejected from this boiling pond (Figures 5-6 and 5-7).

During the visit to the craters on 12 March 1994, the most intense activity took place deep within a pit at vent location 3/1. This pit is seen here during daylight from the southern rim of Crater 1. A few bombs can be distinguished in the lower central part of the pit.

The same bocca seen from the NE rim of Crater 3 on the evening of 10 March 1994. The lava fountain visible here was about 50 m high; others rose to about 120 m height. Acid vapors had destroyed my wide-angle zoom lens during the preceding hours so I had to use a 50 mm lens, failing to capture the full fountain.

During the summer of 1994, Stromboli entered another period of highly intense activity, still more vigorous than that of autumn 1990. Strangely enough, no reports of visual observation of this activity were submitted to GVN Bulletin for the first one or two months. The activity was intense enough to alter significantly the crater landscape I had so intimately surveyed in March.

Ralf Seyfried from GEOMAR Volcanology was on Stromboli on 17 June 1994. He observed vigorous but non-continuous activity from several vents in Crater 1 and apparently from 3/2. A conelet had begun to form in the central part of Crater 1 but was "by no means as large as later in August".

On 10-11 July 1994, Richard Pichl of the University of Prague stayed overnight on Stromboli and observed three small cones ("hornitos") at the "Gemelli" site (1/3) and in the center of Crater 1 (vent area 1/2). These vents were erupting 4-8 times per hour, ejecting incandescent pyroclastics to 20-150 m height. Richard noted that the activity decreased during the night, reaching a minimum at about 0200 on 11 July. However, it was during this period of comparatively low activity that six vents within Crater 1 erupted simultaneously, ejecting bombs to 250-300 m which then fell onto the Sciara del Fuoco. During this tremendous eruption, also vents 3/1 and 3/2 ejected glowing tephra to about 200 m. This powerful event heralded the reintensification of activity at two vents in the central part of Crater 1; by daybreak none of the intracrateral cones were erupting.

The two active centers within Crater 3 had about 2 eruptions per hour (probably most of them from 3/2), with incandescent material commonly rising to about 150 m. Dense, black ash plumes formed during the eruptions.

The seismic activity monitored by the Stromboli Project (Udine) showed an increase in mid-July and reached very high levels on the 19th and 22nd while tremor energy culminated on the 27th. Reports from Stromboli guides indicated that pyroclastics fell as far as "the usual tourist zones", which is, Pizzo sopra la Fossa and the two ridges extending down from it ( GVN Bulletin vol 19 No 10 ). Seismic activity then declined, but visual activity did so only very little. The visit to Stromboli on 21-22 August 1994 has been my most spectacular so far (also because it was my first visit of the volcano together with Giada !). A detailed description of our observations has been published in the September 1994 issue of the GVN Bulletin which can be retrieved here. Compared to five months earlier, both the activity and the crater morphology had drastically changed.

Increased activity had already been visible a week earlier when passing the island with the ferry coming from Napoli. Almost continuous activity occurred from a number of vents when the ship approached Stromboli.

Numerous eruptions occurred while we climbed the volcano on the evening of 21 August. We climbed quite slowly and arrived in the area of the "stone castles" (where the craters become again visible to climbers after climbing through a zone without a view of them) at around midnight. By this time, continuous Strombolian bursts occurred from two neighboring boccas in Crater 1, one at the summit of a tall spatter cone that had not existed in March, and the other on its NE side, on a lower but much broader cone. The image shown at the top of the Stromboli title page was taken during this phase, shortly after midnight on 22 August.

Several vents erupting simultaneously in Crater 1 during the first hours of 22 August 1994, seen from Pizzo sopra la Fossa. Eruptions occur from the summit of the large central cone (vent location 1/2) and from a broader bocca on a lower cone at 1/1. Several smaller incandescent vents can be seen around the two fountaining ones. Height of fountains is about 50 m in this image.

When arriving at the summit, we found excellent observation conditions: there was little vapor, and the bright glow from numerous vents made details of the crater terrace plainly visible. The full moon (again!) helped a further bit.

The deep craters of five months before were now filled almost to their rims with pyroclastics and newly-formed conelets. Thus, the process that I had observed in 1989-1991 had been repeated, but this time in a much shorter period. And the new cones were impressively larger than their predecessors - they were even still growing.

Crater 1 had at least 6 erupting vents, three of which are seen in simultaneous eruption in Figure 5-8. As during the vigorous activity during my 7-8 November 1990 visit, Crater 1 was the main focus of activity. The largest cone, in its center (area 1/2) and about 30 m high, was the site of very frequent Strombolian eruptions, and of several periods of uninterrupted fountaining that lasted from 60 to 90 minutes. It had a small bocca on its SE flank that seemed connected and obliquely ejected spatter before the largest bursts of the adjacent main vent. Fountains rose to about 150 m during the largest bursts, and often, the cone was completely covered with incandescent spatter, forming minor rootless flows.

The second vigorous Crater 1 vent was a much larger bocca on the broad cone to the NE of the tall cone, at vent location 1/1. Its eruptive behavior was identical to that of the former but did not have the same timing. Instead, it had its periods of continuous activity independently. Only for about half an hour shortly after midnight, both vents erupted simultaneously without interruptions. The fountains of this broader vent were much broader than those of its taller neighbor and often rose higher. Also this vent had its parasitic neighbor, but this one made sympathetic oblique spatter ejections after the largest bursts of its main vent.

The "Gemelli" vents (1/3) that had made cross-shaped fountains in March had maintained the same manner of erupting but had now formed two cones somewhat smaller than the central one. These cones were of identical shape and altitude (maybe 20 m above the crater floor), and they made almost all eruptions together. As in March, the more southerly one would start ejecting a very narrow column of burning gas, immediately followed by the violent ejection of bombs. Within one or two seconds, the northern twin would join the party, making a fountain of identical shape and altitude. Accompanied by the same tremendous noise as in March, these two would do so for less than 10 seconds, and then they would remain quiet for at least half an hour following (which is why I did not get them photographed well). The highly symmetrical manner of their simultaneous eruptions was striking. If there was ever anything regular at Stromboli, it was these cones and their eruptions.

Crater 3 had, as previously, two main centers of activity, 3/1 and 3/2. The earlier was like I had seen it in November 1990 but with only one opening this time. This opening was no wider than 5 m, maybe even less, but it was one of the most active points in the entire crater area. Like the most active boccas in Crater 1, this vent had long periods of continuous lava fountaining and very frequent eruptions during the "quieter" periods. During the more intense periods, the magma level evidently approached the surface, producing lava sprays of ever increasing angles.

If the activity from all these points had already been impressive, the most powerful, and still more impressive, eruptions came from a complex feature in vent area 3/2. During the ascent, a few of its eruptions had already been visible but it had not been clear where they originated. We had to stay about 30 minutes on Pizzo sopra la Fossa to see the first of these bursts directly. Until then, the vent area had shown bright incandescence, and we were aware that there was still another active place. It formed an irregular depression in the otherwise very high floor of Crater 1.

Eruption from vent 2 in Crater 3 at dawn, 22 August 1994. Broad lava fountains rise from three closely spaced vents in the center of the soutwestern depression of Crater 3. This eruption did not reach heights like others did, in the night before. Note shallowing of the craters.

Eruptions from this vent, or vent complex, did not make as much noise as those from the other vents (very much like during all of my previous visits), but it made enormous fountains. Sometimes, only one vent erupted, ejecting fountains more than 150 m high. In most cases, there were three connected vents lying close to each other (in an area about 30 m wide) that erupted simultaneously, forming very broad fountains that rose much higher than Pizzo sopra la Fossa, to maybe 200 m above the vents. Fallout from these fountains covered virtually all of the area where I had stayed during my crater visits in April 1990, August 1991, and March 1994. During one of the largest eruptions, the heat could be distinctly felt on Pizzo sopra la Fossa. On no other occasion had I experienced this before.

Crater 1 seen from the northeast on the morning of 12 March 1994. The slope below the crater is covered with debris from the October 1993 explosions, and no traces of the May 1993 lava flows are visible. Note absence of cones within the crater.

Crater 1 seen from the northeast on the forenoon of 22 August 1994. A spectacular cluster of cones has grown since the last visit, and a small, very recent lava tongue (indicated by an arrow) extends down the N flank below the lowest point of the crater rim. New dark tephra covers the areas around the crater. Get comparison view of this and previous image (figs. 5-9 and 5-10) here . Still more detail of the lava flow is visible here .

Later investigation of comparison photographs (figures 5-9 and 5-10, taken in March and August 1994) of the NE slope of Crater 1 showed that a small lava tongue had issued from the northernmost conelet at vent area 1/1 between the two visits. The upper part of the flow was still steaming on the forenoon of 22 August 1994, indicating that it was very recent, maybe of 21 August - it might have even been erupted during the few hours that we were sleeping in our "casa sullo Stromboli". Unfortunately, we had only arrived during the night so we had not seen the place during daylight the evening before.

Scientists from the University of Udine, University of Stuttgart and Open University of Milton Keynes visited Stromboli between late September and mid-October. The activity during this period was apparently similar to that of 21-22 August, with up to 10 vents erupting and tephra rising as high as 300 m above the vents. Apparently, the largest cone in Crater 1 was no longer erupting in late September but eruptions occurred from two vents on the NE and S sides of the central conelet where I had noted no vents in August (vents 1/4 and, probably, 1/2, on the map in Oct 1994 GVN Bulletin. Note that labeling in this map is different from that used here). Still another vent closer to the SE rim of Crater 1 made oblique ejections. The "Gemelli" did not erupt together anymore. In October, only the westernmost twin erupted, still ejecting gas jets with a few bombs while its neighbor showed only degassing activity. The largest spatter cone in Crater 1 had a continuous gas flare rising 1-2 m above its vent. Eruptions occurred from vents 1/2 and 1/3, depositing pyroclastics outside the crater. Vent 1/4 threw material onto the Sciara del Fuoco.

A cluster of vents were present in late September at location 1 in Crater 3 (named 3/3, 3/4 and 3/5 on the map in GVN Bulletin of Oct 1994), and incandescent magma could be seen in one of the openings; however, there were no eruptions. Powerful eruptions still occurred from vent area 2 in Crater 3, being stronger than those from Crater 1. There were two or three vents erupting successively from this site on 5 October, including a smaller vent first observed on 1 October. The amount of ash in the emissions from Crater 3 decreased from 1 until 5 October, but the content of incandescent pyroclastics in the eruptions increased, and eruption frequency increased as well. The fountains now rose 50-300 m high.

From 5 until 8 October, eruptions from Crater 3 again became smaller and more ash-rich. Glow was visible at night above the vents in area 1 of Crater 3.

Northern part of Crater 1 seen from Pizzo sopra la Fossa, sometime in mid-October 1994. The largest vent of 1/1 ejects a broad fountain to about 80 m. To the left rises the cone at 1/2, showing a glowing bocca but not erupting. Another glowing vent is visible at its base. Photo supplied by Pietro Constantino, University of Kiel.

When visited by a student of Kiel University, Pietro Constantino, in mid-October 1994, frequent eruptions occurred from the largest bocca in the cluster of vents at 1/1, immediately adjacent to the central cone that itself showe a glowing summit vent but did not erupt. Apparently, this was the only erupting vent within Crater 1 during Pietro's brief visit. At 3/1, there were two glowing boccas that did not erupt. Most eruptions occurred from 3/2 which had notably subsided and had at least two eruptive boccas, one in its southeastern part and another to the southwest. Both erupted simultaneously, the latter producing more vigorous fountains that rose to about 150 m above the crater. The subsidence of 3/2 is the only morphologic change evident in Pietro's night photographs.

The vigorous activity initiated sometime during June or July 1994 ended during November (as several persons living on the island told us), and "normal" Strombolian activity continued through the end of February 1995.

Continue with Stromboli in 1994-1995 (Part 2)

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