Stromboli 1994-1995: a period of dramatic changes (2)

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

1994-1995: a period of dramatic changes (Part 2: 1995)

View of Stromboli's craters from Pizzo sopra la Fossa on the forenoon of 20 April 1995. Half of the cone in center of Crater 1 has been blasted away by the strong 5 March 1995 explosion, exposing its conduit.

The following is summarized in the April 1995 issue of the GVN Bulletin; this is a slightly extended version.

A powerful explosion occurred at 1740 on 5 March 1995 (see the seismicity graph of the Stromboli Project at Università di Udine). A tephra column was observed from the village of Stromboli. Visitors on the summit reported lapilli falling on Pizzo sopra la Fossa but thought that "this was normal" (until they learned that this was by no means the case). No one was injured this time.

The explosion blasted a new pit out of the northeastern part of Crater 1, also taking part of the most conspicious August 1994 cone with it (the one seen in eruption on the top of this page and on the Stromboli home page). The lower and broader northern August 1994 cone (1/1) was also largely destroyed during this event.

Detail of Crater 1 with the central August 1994 spatter cone (vent area 1/2) bisected by the 5 March 1995 explosion, the explosion pit before it and the "Gemelli" cones to the left. Photo taken 20 April 1995 from Pizzo sopra la Fossa.

Stromboli residents reported that during the remainder of March and thorugh early April, explosions could sometimes be heard in the village of Stromboli, indicating that there was a new phase of somewhat stronger activity. By mid-April, though, conditions had again changed. The day before Giada and I arrived at the island, the mayor of the town of Lipari (to which Stromboli belongs politically) officially declared a state of alarm and restriction of access of the volcano. As was told in the news that evening, there were fears of a larger eruption, lava flows, and possible "seaquakes" (tsunami? The word "maremoto", seaquake, is often used in Italy but is highly misleading since it is the earth , or its crust, below the sea that moves!). The reason for these fears was the unusually low level of eruptive and seismic activity, the news reports said.

When arriving at Stromboli on the late forenoon of 19 April, local rumours were highly contradictory. Some had it that the whole alarm thing had been some kind of error and that a failure had occurred at one of the seismic stations. However, when we made the ascent on the evening of the same day, we encountered a barrier that had been placed across the main path leading uphill, close to the "Osservatorio" pizzeria at Punta Labronzo. Numerous tourists had already built a new path bypassing the barrier. We were astounded when finding that a smaller track parallel to the main one had not been blocked at all.

The activity was unusually low indeed. Only when approaching the upper section of the ascent, we noticed that there were some eruptions. They all came from Crater 3, however, while Crater 1 did not show any activity. Due to the darkness and a gas plume, the changes at the craters were not yet visible. We arrived at Pizzo sopra la Fossa shortly after midnight on 20 April and immediately began with continuous observations.

Sizeable eruptions occurred only from vent area 3/2 at invervals ranging from 2 minutes to more than one hour. It appeared that only one bocca in the center of the vent area was active; due to subsidence of the crater floor, the vent itself was not visible. Fountains were considerably lower than in August 1994, rising less than 100 m above the crater. All material fell back into the vent and onto the northwestern crater rim. Vent area 3/1 had returned to spattering from an active lava pond at depth, extremely similar to the activity I had observed in detail in March 1994. Occasionally, there were larger ejections of large clots of fluid lava from the pond, but all fell back into the pit except in two cases when spatter fell on the outer NE side of the pit. The usual whooshing noises were audible from that site, accompanied again by the somewhat goofy "pif-paf" noises heard 13 months earlier.

At long intervals, very small gas puffs occurred from a small bocca in the central part of Crater 1. They were accompanied by brief "thudding" noises and solely ejected burning gas to less than 10 m height. People we had encountered while climbing to the summit had told us that one notable eruption had occurred from this vent; there were no real eruptions (of solid material) while we were on the summit, neither during the night nor during the day.

By daybreak, features of the crater area became more clearly visible, and after 0900, vapor and gas emission notably decreased (due to decreasing humidity), allowing a perfect panoramic view (FIgures 5-11 and 5-12). Some significant changes had occurred since August 1994, the most notable being the formation of the explosion pit of 5 March 1995 in Crater 1, and the subsidence of the active areas within Crater 3. The central cone of vent area 1/2 which had been so vigorously active eight months earlier presented a spectacular aspect, its interior exposed neatly to observers on the Pizzo sopra la Fossa. Some stratification and its now-inactive conduit were plainly visible (Figure 5-12). The "Gemelli" cones had remained intact and appeared larger than in August 1994. No trace was visible of the lower but broader cones in vent area 1/1; these features had been completely obliterated by the March explosion.

As usually during the hours after sunrise, most of the eruptions were accompanied by the emission of ash plumes, all from vent area 3/2. We observed a striking increase in the frequency of these eruptions after 0800 when, during a 25 minutes period, there were six eruptions. From 0910 until 1210, the same vent produced only five more eruptions.

The only signs of activity within Crater 1 came from a small bocca on the near base of what was left of the central August 1994 cone, consisting of very small and brief gas explosions. Occasionally, dark fumes rose from these explosions, but there were no ejections of clastic material. Crater 1 was ominously silent, and this was probably one reason for the state of alarm.

Staying on the summit until mid-afternoon, we had the chance to extensively photograph the craters and explore the Vancori region where we had not been before. We did not approach the craters but saw a person who did it. He said later that everything down there had been so quiet that, for the first time in all his Stromboli experiences, he had been able to get to the far (Sciara) side of Crater 1. The same man had been at the craters during the intense activity of July-August 1994 (!).

On our way back to the civilized world, we had a spectacular profile view of the half-destroyed central cone in Crater 1 (Figure 5-13), rising vertically above the pit created by the 5 March 1995 explosion - a symbol of the rapid changes that have occurred at Stromboli during the past two years.

Profile view of Crater 1 from northeast, afternoon of 20 April 1995. The 5 March 1995 explosion has blasted out a new pit to the left of what remains of the central August 1994 conelet. Half of the cone as well as some of the smaller features adjacent to it have been destroyed during that explosion. Compare to figures 4 and 5, and the April 1990 photo.

We observed Stromboli for a couple of hours on the evening of 21 April from Punta Labronzo. Compared with the day before, no changes were evident. About twice per hour, lava fountains with ash plumes rose from vent area 3/2. The ash plumes were slowly driven down the Sciara del Fuoco by a strong wind until arriving at sea level, a phenomenon not witnessed by us earlier. A weak, fluctuating glow in the central part of the crater area indicated continuing lava pond activity at 3/1.

Two days later, we were back on the same site, and conditions had significantly changed. A much stronger, though still fluctuating, glow was present above 3/1, and eruptions from 3/2 were the same as before. However, a new brightly glowing spot had appeared on the northern rim of Crater 1. Soon after our arrival at Punta Labronzo, a spectacular ash-free lava fountain shot up from that site to at least 150 m; it was narrow and obliquely directed to the northwest, right towards the upper Sciara del Fuoco. While a cannon-shot like bang arrived at our vantage point, abundant spatter and bombs fell onto the Sciara and cascaded down to about half its extension. For several minutes afterwards, innumerable glowing spots moved, rolled and jumped down the scree, accompanied by crackling and clattering noises. Stromboli thus manifested its return to more normal conditions; during our observation period there were four more of these eruptions, each strikingly resembling the previous, at intervals of 10-25 minutes.

In fact, from what we saw during those April days, we did not quite get the impression that something abnormal was at hand. True enough, the volcano was quieter than I had ever seen it before. However, from what I knew about Stromboli's eruptive behavior during recorded history, a period of quiet did not necessarily imply that the volcano was preparing for a major eruption. All larger or paroxysmal eruptions occurred quite randomly, some after a period of reduced activity, others after a period of increased activity, and by far the largest part was preceded by absolutely "normal" conditions. The reasons for the declaration of alarm to me were highly unclear, and have remained so until now. Indeed, the access restrictions have been somewhat modified (see latest news) since. The risk meanwhile has remained essentially the same, and it will be so once no one will talk any longer about an alarm. Stromboli is ALWAYS capable of doing bad things, and it is to be hoped that this will not have tragic consequences one day.

Small ash explosion from Crater 1 on the afternoon of 28 September 1995, seen from the NE. This is probably the most impressive photo that I took during the second 1995 visit to Stromboli (18-19 September 1995) since weather conditions were quite unfavorable, preventing good visibility. Additionally, the activity was extremely weak on that day, not representative of that period.

We got back to Stromboli on 28 September 1995. This visit was not too successful, regarding observations and photography. It did however, allow some general statements about the activity and vent conditions. This is our "official" report to the GVN Bulletin, not reflecting much our personal feelings.

Visit to Stromboli, 28-29 September 1995

Basically, the situation of Stromboli had not changed significantly since the April 1995 visit. The most notable developments were the further collapse and destruction of the central 1994 cone within Crater 1 and the formation of a new incandescent bocca on the SE base of the southern "twin" cone, vent area 3. This bocca made no solid ejections but was the site of occasional gas explosions. Andy Harris of Open University, Milton Keynes, UK, has done a study of this kind of activity (more soon).

The size of the eruptions was lower than normal on 28-29 September 1995 but generally normal during September 1995. There were eruptions from up to four vents (1 and 2 within Crater 3 and probably two vents in the northern part of Crater 1), producing beautiful lava fountains. The fountains from the northernmost vent in Crater 1 projected material onto the upper part of Sciara del Fuoco.

Stromboli is ending the decade 1985-1995 in a relatively unspectacular manner - if I may say so, given that Stromboli is spectacular even during a low level of activity. There are no indications of major changes in its activity within the foreseeable future. But is there anything foreseeable at this volcano? All of the major events described on these Stromboli-related pages have not been preceded by anything particular. We will have to go into the 1995-2005 decade to see if the improved monitoring of this volcano will give further clues about its behavior.

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