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Volcanic hazards at Stromboli

Although Stromboli's activity is predominantly of minor scale and its effects are restricted to the immediate crater area and the Sciara del Fuoco, this volcano is capable of much larger, and potentially devastating eruptions. Eruptions like those in 1919 and 1930 are likely to occur every few tens of years and should be expected at any time in the future. Such eruptions may occur without any premonitory signs.

The worst-case scenario at Stromboli is a catastrophic sector collapse of the unstable Sciara del Fuoco flank.

The edifice of Stromboli is known to have undergone repeated sector collapse during its evolution, most recently about 5000 years ago, when the collapse scar was created which now hosts the Sciara del Fuoco and the active summit craters at its top. Such a collapse would create devastating tsunamis (sea waves generated by the entrance of large volumes of rock into the ocean, and by displacement of great rock volumes below the sea level) that might endanger the surrounding coasts of Calabria and Sicily and of the neighboring islands of the Aeolian archipelago. A minor though significant collapse occurred on the Sciara del Fuoco (in large part below the sea level) on 30 December 2002, generating tsunamis several meters high on the coasts of Stromboli island, and attaining a few tens of centimeters on the coasts of the Aeolian Islands and northern Sicily.

The largest eruptive event that might occur in the near future would be a repetition of a 1919 or 1930 size eruption.

In case of a 1930 scale event, the inhabited areas of the island would suffer serious structural damage (i.e. rupture and piercing of roofs by large blocks, breaking of all window panes, etc...). Extensive brushfires could devastate large areas of vegetation on the island; there are, however, no large cultivated areas presently. The Punta Labronzo area, with its exquisitely placed pizzeria, would be subjected to the fall of meter-sized blocks, an event that would have fatal consequences on a clear summer evening.

Areas below the main valleys are at high risk from avalanches of hot material sliding from the steep upper slopes of the volcano, not quite genuine pyroclastic flows but with similar effects. Such avalanches would in places be able to reach the coast, in particular below the "Vallonazzo" that opens towards the popular black sand beach at Piscità, a place crowded with hundreds of people on summer days. In case of more vigorous events, hot avalanches might also reach parts of the village.

In the case of a 1930 scale event, the upper part of the mountain, logically including Pizzo sopra la Fossa, would be a zone of death, with no living soul escaping. Such an event would be particularly harmful during a clear summer evening when up to hundred people would be present in the summit area. The observation platform would be covered with a continuous sheet of incandescent tephra.

A powerful explosion occurred on 5 April 2003, and probably was the most violent event at Stromboli since 1930. It fortunately occurred at a time when access to the volcano was prohibited, due to the complex eruptive activity under way since late December 2002, and the instability of the Sciara del Fuoco following the collapse event of 30 December 2002. Only eight months earlier, the summit lookout (Pizzo sopra la Fossa) was crowded with hundreds of visitors on every evening, and had the explosion occurred in August 2002, there would certainly have been numerous fatalities. The problem with such explosions is that they are virtually unpredictable since they are preceded by extremely little warning signs.

Smaller events with significant effects restricted to the immediate summit area are likely to occur about once per year to once every two years.

The most recent events of that kind occurred in March 1989, September 1992, February and October 1993, March 1995, February 1996, and June 1996. They caused light ash falls in the villages of Stromboli and Ginostra and caused brush fires on the upper slopes of the volcano. In the summit zone, however, bombs, blocks and incandescent spatter fell over a wide area, including the popular lookout of Pizzo sopra la Fossa. Fortunately, during most of these events, only few persons were in the summit area, but one or two persons were injured by incandescent pyroclastics in the October 1993 event and one person was injured when falling during flight during the March 1989 event. The most recent larger explosion, on 1 June 1996, caused four to seven injuries, but only in one case were the injuries directly due to the falling pyroclastics while other people got injured when falling during their hasty efforts to escape.

An eruption the size of the 16 October 1993 explosion would, were it to occur in an August night, at the height of the touristic season, cause many injuries and possibly fatalities. Fluid bombs that fell in the October 1993 event were up to 50 cm in diameter, projectiles that would easily squish a human being's head. Images by Jürg Alean dramatically illustrate the effects of the October 1993 explosion when at least 2 people were injured.

An eruption similar in size to the 16 October 1993 occurred shortly before midnight on 1 June 1996, during the full moon. Four people were injured, mostly when trying to escape from the summit area. More events of this kind have occurred about once or twice per year since then, and led to the tragic death of a German tourist in October 2001. Prior to the dramatic ends at the end of 2002 and in early 2003, the latest larger-than-normal explosion occurred on 24 July 2002, fortunately in the early morning when there were no people at the summit, but only 7-8 hours before the summit had been crowded with hundreds of tourists (I had been there with a group of hikers until about midnight and witnessed the explosion from the harbor of Stromboli).

Heightened activity (like in the summer of 1994 and again in the summer of 1996), at times associated with lava flow emission (such as 1985-1986 and 1993), is not likely to present a very significant hazard even in the summit area; caution is recommended anyway.

During most years, Stromboli experiences prolonged period of increased magmatic activity. Such periods were most notable in October-November 1990, during the spring of 1992, in May 1993, during July-October 1994, and most recently, in April-June 1996. They are characterized by a high frequency of individual Strombolian bursts (explosive ejection of lava fountains), often blending into sub-continuous, pulsating fountains or lava sprays and alternating with powerful isolated fountains. Fountain heights are generally much greater than those of "normal" Strombolian eruptions, i.e. reaching 300 m above the vents in contrast to the "usual" 50-100 m.

People in the summit area are basically safe during such eruptive conditions because the eruptive system is open and no sudden large explosions are likely to occur. However, large fountaining episodes might cause fall of sizeable pyroclastics up to Pizzo sopra la Fossa, especially if fountains are deflected obliquely towards the SE by material sliding into the vent from recently accumulated crater-fill.

Rarely are such intensified eruptive periods associated with lava effusion (such as in 1993 and 1994). The phenomenon of lava outflow itself is completely non-hazardous to all accessible parts of the island except Sciara del Fuoco. This area, a semicircular depression caused by the collapse of Stromboli's NW flank about 5000 years ago, is completely surrounded by the steep walls of the collapse scar, an unsurmountable obstacle to lava flows originating from the present craters.

Activity from vents other than the presently active ones, on the outer flanks of the volcano, is very unlikely but would present extremely high hazards to the villages of the island and its residents. Vents opening near the sea-level may produce devastatingly violent phreatomagmagic explosions.

During the historical period, no eruptive activity is known to have occurred from vents other than the presently active ones on the "Crater Terrace". A possible exception is the extrusion of lava at the base of the Sciara del Fuoco at sea-level in 1955. However, various authors believe that this lava extrusion occured from an ephemeral bocca at the end of a lava tube reaching down all the Sciara del Fuoco from the craters.

During its geological history, Stromboli did behave much differently than it does now. Evidence of violent eruptions in the past is visible all over the island. In coastal areas, there are spectacular outcrops of dark-colored scoria deposits interbedded with conspicuous light-colored surge beds. The latter were probably emplaced during hydromagmatic explosions as magma got in contact with ground and/or sea water, from vents in the lower or coastal areas of the volcano. Activity of this kind would render the complete evacuation of the island necessary and probably completely devastate areas up to several km away from the vents. Less violent but nonetheless dangerous magmatic activity could occur from flank vents at somewhat higher elevation. The geological map shows one major (the Timpone lava shield above Ginostra village, on the SW side of the island) and several smaller excentric eruptive centers.

The fact that no eruptions have occurred from vents on the outer flanks of Stromboli during the past millennia and the persistent open vent activity from the summit craters indicate that activity from flank vents is unlikely to occur within the near future.

Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"

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