Stromboli volcano, Aeolian Islands (Isole Eolie), Sicily, Italy
Eruptive history
4. 1985-1995: a decade of Stromboli eruptive history, Part 1. General developments
Map of Stromboli's craters showing "vent areas" referred to in the text below, and features visible during the 20 April 1995 visit by Giada Giuntoli and Boris Behncke. Individual vents are not labeled by themselves but rather related to vent areas. This map is a slightly modified version of the one published in the April 1995 issure of GVN Bulletin.
The decade beginning with the most recent major effusive episode (Dec 1985-April 1986) may be one of the best documented periods ever during the history of Stromboli. It is chosen here as a period representative for Stromboli's eruptive behavior because it includes one period of significant lava emission (as well as at least two minor ones), several periods of very intense, though chiefly explosive, activity, several large phreatic or phreatomagmatic explosions, and at least one fatality.
This section is made up of two parts, a summarizing one (with general remarks on vent areas, their activity and morphologic processes), and a narrative of the eruptive behavior observed in the course of the last ten years. The latter is intended not only to document the details of what has been observed during this period, but also to illustrate what personal impressions an observer may have (who, in this case, is myself). Stromboli is, for a number of reasons, a significant component of my life, and surely this applies to many other persons as well as, in their case, to other volcanoes.
Labeling of craters and vents, and their characteristic activity
Crater and vent labeling has been difficult due to frequent changes of vent locations and crater shape ever since observations have been made. However, during the period discussed here, there were some more persistent features allowing the use of more general numbers in spite of the continuous modifications to the vent configuration.
First, there have been three principal craters during the entire period 1985-1985 (they had already been in existence since at least 1972; see Nappi 1975 ), named 1, 2 and 3 from northeast to southwest (see figure 4-1). Crater 1 has, during all of my visits, had at least three vent areas showing distinct eruptive behavior. The northeasternmost, here called vent area 1/1 (for Crater 1/vent area 1) lies at the NE end of the crater, in what seems a deep gap from Pizzo sopra la Fossa, slightly to the right of the highest point of the crater rim. Usually, this vent erupts infrequently but produces tall, narrow fountains, accompanied by a loud crashing noise. Eruptions are brief (no more than 10 sec) during periods of normal activity. During periods of increased activity, there may be more vents active in area 1/1. During the effusive events in May 1993 and August (?) 1994, lava issued from vents very close to the lowermost point in the NE rim of Crater 1.
Vent area 1/2 lies in the center of Crater 1 and is the site of most pronounced spatter cone growth during periods of intense activity; there is little eruptive activity during "normal" periods. Activity during more intense activity may occur from as many as four vents and consist of vigorous fountaining, sometimes being continuous over periods of several hours (such as in Oct-Nov 1990, May 1993 and July-Aug 1994).
Vent area 1/3 has been the site of one or two vents during the decade whose activity includes the most violent gas emissions of all vents at Stromboli. Often, eruptions from this place consist of gas jets only, but during periods of increased activity, bombs and spatter are also ejected, usually after an initial jet of burning gas. Activity here has been from two vents since at least March 1994, with eruptions often occurring from both vents together in a very similar manner. This and the growth of two very similar cones at the vents between March and August 1994 has led to their designation as "twins" (Italian: "gemelli") in the text below.
Eruptions from the "Gemelli" site are always accompanied by remendous crashing noises that begin quite abruptly and fade rapidly after about 10 seconds. Sometimes, like in March 1994, eruptions occur in series of up to four bursts, each being weaker than its predecessor, with intervening pauses of 30-60 seconds. On other occasions, violent and noisy gas emission may continue for several hours, resembling a jet airplane starting nearby. Such was the case during the night of 7-8 November 1990 when there were no discrete explosive gas and bomb ejections from the "Gemelli" area.
Crater 2 is the smallest and least active of the three. It lies somewhat behind the northeastern extension of Crater 3, separated from vent area 3/1 (see below) by a low and narrow septum. There may be one or more vents within this crater, considered here one cluster that needs no special labeling. Two incandescent vents were visible within the crater in August 1991, and their shape made us call them "cat eyes". Crater 2 has been rarely in eruption during my visits (I think, only in September 1989), but was the site of vigorous activity in the spring of 1993 and took part in the violent October 1993 explosions. Since then, it has remained largely unchanged except that a very small spatter cone grew within it between 22 August and late September 1994. The cone apparently did not change after late September and was still present in April 1995.
Crater 3 has two principal areas of activity, one 3/1) in its northeastern part and another, larger one (3/2) in its southwestern portion. The former is the site of the most persistent activity of all eruptive areas at Stromboli, usually characterized by low-level spattering from actively boiling lava in one or more vents. These are present at the floor of a roughly circular pit during periods of normal activity and not visible from Pizzo sopra la Fossa. Typical activity was observed at very close range in detail during my March 1994 visits to the craters and was essentially the same in April 1995. There were continuous bursts of spatter (at least one per second), punctuated by occasional larger fountains. During all visits, the activity of 3/1 produced a characteristic noise, resembling a somewhat hectic shoveling of sand, or slight surf at high tempo. Occasionally there was a different kind of noise resembling the "pif-paf" of toy guns. It has not been possible to correlate these strange noises to visible phenomena while the "shoveling" results from the rhythmic ejection of spatter.
During periods of increased activity (like that of Oct-Nov 1990 and of July-Aug 1994), the pit encompassing vent area 3/1 gets filled with pyroclastics, thus exposing the active vents to view from Pizzo sopra la Fossa. Spattering is then much more vigorous, producing much larger fountains (up to 150 m high) but occurring from very narrow vents (max. 5 m in diameter). The noise remains essentially as described in the previous paragraph.
Vent area 3/2 is more complex than 3/1 and subjected to more frequent changes. There may be up to four vents in this area of which one in its SW or central part is the most vigorous. In April 1990, there was one erupting vent below the SW crater rim and another, inactive one about 50 m to the NE or E. One central and several smaller vents were present in a roughly circular depression in the area in late August 1991. In March 1994, there were again two vents, one in the SW and one to the NE or E of it, both erupting this time. In August 1994, three large and connected boccas made up vent area 3/2.
Usually, eruptions from this area occur quite infrequently (1-4 times per hour), even during periods of increased activity. What does increase then is the size of the eruptions. While lava fountains usually rise 30-150 m above the active vent(s) during periods of "normal" activity, they can become significantly larger when the volcano is more vigorous. During the 22 August 1994 visit, there were eruptions from only one of the three connected boccas which were spectacular enough. About 60-70% of the eruptions that night were from all three boccas together, and broad fountains then rose more than 250 m above the crater terrace, significantly higher than the Pizzo sopra la Fossa, and heat waves arrived at the summit even though there was no fall of any tephra.
The noise of eruptions from 3/2 is always a low, dull rumbling contrasting sharply against the tremendous crashing noise of the vents in Crater 1 and the hissing noise that usually accompanies eruptions from vent area 3/1. Long-lasting to continuous lava fountaining from vent area 3/2 has not been observed during any of my visits, but some eruptions (like in April 1995) occurred in a succession of two, rarely three, pulses.
Constructive and destructive processes
Besides their strongly individual activity, the vent areas are similarly prone to certain morphologic developments, and they may have been so since long time. As at other more or less continuously active volcanoes (such as Vesuvio between 1631 and 1944, Etna, Pacaya and several other at present), there is an alternation of constructive and destructive activity. Constructive activity is characterized by filling of the craters with pyroclastics (and, rarely, lava) and growth of intracrateral spatter cones. Such periods last months to years and can be repeatedly interrupted by small-scale collapse or explosions.
Larger explosions on the other hand occur suddenly and last few minutes; they may occur, however, occur in successions over periods of several days to weeks. These explosions commonly destroy parts to all of what is inside the craters affected by the explosions. They ream out the tephra that has filled the craters and blow away the small intracrateral cones, forming steep-sided, sometimes elongated pits.
A somewhat less dramatic destructive process is the subsidence of crater floors due to the lowering of the magmatic column.
Cone growth is a characteristic feature of Crater 1 with all three vent areas being the site of cone growth during periods of vigorous activity. The largest cone seen during the decade discussed here has reached about 30 m height above its base. Smaller cones have also grown in Crater 2 and at vent area 3/1. No cone growth has been observed at vent area 3/2, but crater filling with pyroclastics is characteristic of this site.