Italy's Volcanoes: The Cradle of Volcanology

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Stromboli before 1930

Comparison views of the craters, 1889-1990

As far as I know, photographic record of the active craters was kept beginning in 1889. Above are a few of the early photographs (taken between 1889 and 1926) compared to one taken by myself in 1990 for comparison. It can be seen how the area of the active vents changed in the course of a few years, with the growth of small pyroclastic edifices alternating with their destruction (probably by larger explosive events).
The photographs of the "crater terrace" of Stromboli taken from approximately the same viewing point, looking towards NE, between 1889 and 1990. Reference point is the so-called "Torrione", the uppermost part of the Filo di Baraona which is the scarp bounding the Sciara del Fuoco on its southern side. These photos were the first to be scanned (in early May 1995) for this web site. Note that "100 years ago" written in the caption of the second (1895) photo refers to the date when the photo was scanned for the initial one-page-version of this site!
During the period covered by the first five photographs (37 years) no appreciable overall vertical growth of the crater terrace is visible, but by 1990 (last photo) the pyroclastic rims around the vents stand significantly higher than in the early photos. In fact during the late 19th century and the first half of the 20th century it was much more difficult to approach the crater rims, because one had to climb down the steep cliff to the N of the "Torrione". Much of this once-prominent rock (probably a dike) has since then been buried by the growing pyroclastic edifice around the southernmost active crater (crater 3), and the rim can be reached easily - it should be noted, however, that getting close to the craters is highly dangerous

The earliest records of activity at Stromboli date back to about 300 BC, and since then, a continuously growing number of reports about Stromboli, its geology, petrology, and eruptive history as well as ongoing activity have been, and are still, published. However, it is only since the late 19th century that the activity of Stromboli has been documented reasonably well and geologic studies were made. A large proportion of the early studies dealt with the morphologic developments at the craters (e.g., Anderson 1905 and Washington 1917), these were soon complemented by descriptions of the eruptive activity.

The first systematic studies of significant eruptive events were made by Mercalli (1891) and Riccò and Mercalli (1892), who made detailed observations of the intense eruptive activity in 1888-1889 and 1891. In those years the volcano emitted the first lava flows before the eyes of trained observers; until then Stromboli had generally been considered a volcano incapable of producing lava flows!

A series of unusually intense eruptions, or eruptive periods, during the first 30 years of the 20th century stimulated much interest among volcanologists of that time. The most important of these events took place in 1915, 1916, 1919 and 1930. The 1919 and 1930 eruptions caused serious damage in the settlements on the island and killed several people.

Activity during the 19th century

For most of the 19th century, Stromboli was visited only infrequently by scientists, and only the most violent eruptive events - such as in 1822 and 1850 - were recorded. Among the first good descriptions of the volcano and its activity is the report by Heim (1931) about his observations made in May 1872. At that time the active craters were located at about 700 m elevation (more than 200 m below the summit). Heim and his companions (Zittel and Ratzel) were able to reach a point on the outer rim of the crater complex from where they were able to look into one of the two active vents present at that time, and stated that its floor was covered with debris. Explosions, which were preceded by a deep rumbling, broke through the debris at the deepest point of the vent floor, ejecting a narrow, black column of ash mixed with blocks, most of which fell back into the vent. According to Heim these rather small explosions occured at rather irregular intervals.

Mercalli (1891) observed two active craters on the crater terrace in early September 1888, of which only the smaller, southwestern one actually erupted. The larger northeastern crater had fumarolic activity; its floor was flat and showed no trace of erosion, so that Mercalli inferred that it had until recently been erupting. Several vigorous fumaroles were furthermore seen on the uppermost part of the Sciara del Fuoco. The activity consisted of discrete explosions of varying strength, of which the strongest produced dark ash plumes through which weakly incandescent bombs rose at higher velocity. Generally the activity was at very low levels, and Mercalli noted that most explosions were accompanied by little incandescence even after nightfall.

Vigorous explosions occurred on the evening of 23 October 1888, followed by a period of strongly intensified activity. By early November three small spatter cones, or hornitos, had begun to grow in the NE part of the crater terrace, and in early January 1889 lava began to issue from a new vent that had opened at the base of the westernmost of the three hornitos. This lava flow arrived at the sea at an unspecified date prior to 15 January, and was seen in a "more evolved" state on the 19th, with three active lava rivers descending to the coast at the base of the Sciara del Fuoco.

Lava effusion continued, though at reduced levels, on 27 February when the volcano was revisited by Mercalli, and the explosive activity was at significantly higher levels than during Mercalli's previous visit in early September 1888. Explosive activity, though relatively mild, was essentially continuous at all three hornitos, and the quantity of brightly incandescent bombs and scoriae plainly visible at daylight was notably greater than during the previous visit. Lava trickled intermittently from the base of the central hornito, extending no more than 100 m down the slope of the Sciara del Fuoco. However, many incandescent blocks detached from the flow front and rolled down the scree to the coast.

For the next three weeks the activity continued at similar levels, but then increased notably on 21-23 March. The three hornitos were largely destroyed by explosive activity, and lava flowed more abundantly down the Sciara del Fuoco. During early April the three hornitos re-grew in the same locations as before, and lava now issued from a vent some 10 m below the earlier effusive vent. Mercalli (1891) reports two further episodes of explosive destruction of one of the hornitos in April, each time followed by rapid regrowth. During this period lava emission had become intermittent and effusive vents shifted from one place to another, resulting for a period in the partial filling with lava of the large crater seen in a state of quiet by Mercalli in September 1888.

For the following months the volcano experienced alternating periods of high-level and low-level activity, the former being accompanied by minor lava outflows. The date of the end of this period of heightnened activity is not known.

Stromboli was shaken by a pair of powerful explosions shortly after noon on 24 Junne 1891, accompanied by an earthquake. The detonations were audible as far as Salina, at about 40 km distance to the SW. On the island of Stromboli itself the earthquake unleashed rockfalls in the area between Punta Labronzo and the village of Stromboli, while "two powerful columns of smoke and incandescent material" rose high above the summit (Riccò and Mercalli, 1892). The upper part of the mountain was covered with incandescent pyroclastics, and small fires broke out in the higher cultivated areas on the slopes. Eyewitnesses later told Riccò and Mercalli that at least one large-sized block fell into the sea near Punta Labronzo, but most fallout consisted of lapilli-sized lava fragments. A layer of ash several millimeters thick was deposited all over the island.

It appears that the explosions were immediately followed by the outflow of lava which ran down the Sciara del Fuoco in a narrow stream which reached the sea at the base of the Sciara del Fuoco and built a small headland. The flow originated from a notch in one of the crater rims and continued to move for two days, accompanied by continued intense explosive activity. After the cessation of lava emission explosive activity continued for another day, and on 28 June the volcano calmed somewhat.

On the morning of 30 June another powerful explosion accompanied by a sensible earthquake occurred at Stromboli, but this was described as smaller than the first but larger than the second of the two 24 June explosions. An incandescent column was seen rising above the summit, and shortly thereafter lava spilled down the Sciara del Fuoco, forming two branches about half way down the scree; both branches entered the sea. The initial explosion was followed by a series of strong explosions throughout the day, and the activity continued at high levels until the evening of 2 July. When Riccò and Mercalli arrived on 3 July, the activity had returned to relatively normal levels, and a further decline was noted after 28 July.

Yet another violent explosion occurred on the morning of 31 August, again accompanied by a felt earthquake on the island. The detonation caused by the explosion was heard as far as Lipari. On the upper slopes of the volcano small bush fires were caused by the fall of incandescent scoriae, while ash and small lapilli fell over a large part of the island. Most of these fine-grained products consisted of old, fragmented rock, different from the fresh black pyroclastics emitted during the explosions of June.

Vigorous explosive activity continued on 2 September when the island was visited by the geologist Arcidiacono from Catania (whose report appears as appendix to the paper of Riccò and Mercalli, 1892). A new lava flow, most probably produced shortly after the 31 August explosion, was discovered at the Sciara del Fuoco; like its predecessors it had reached the sea and built a small promontory, but lava effusion apparently had ceased. For the next day the volcano remained vigorously active, with frequent explosions from at least 3 vents, followed by a diminuition of the activity on 4 September. From that date on the volcano continued its eruptive activity at normal levels.

Eruptive activity in 1901

Upper photo: Small pyroclastic avalanche on the upper Sciara del Fuoco on 4 March 1901. The avalanche appears to be caused by the sliding of abundantly falling bombs and ash, a rather uncommon event at Stromboli. A similar avalanche occurred at the beginning of the most recent major lava outflow in December 1985. Activity of this kind is probably driven by magma interacting with shallow water in the volcanic edifice.

Lower photo: Spectacular Strombolian explosion with ejection of large bombs on 4 March 1901. This type of activity contrasts with that illustrated in the preceding photo. Both images were taken from Brun (1909).

During this 22-year-period Stromboli was the site of frequent paroxysmal eruptive episodes, at times accompanied by lava effusion. Lava flows were emitted in August 1898, and, more significantly, over a period of several months between April and October 1900. The eruptive period of 1900 ended with strong explosive activity that may have produced hot pyroclastic avalanches. A minor pyroclastic avalanche was observed and photographed in 1901 (photo at right).

Lava flows occurred again in October 1903 and on at least three occasions in early 1905, followed in April 1905 by a powerful explosion which injured one person. Eruptive activity in January and July 1906 produced lava flows and pyroclastic avalanches; in late January 1907 a lava flow spilled down the Sciara del Fuoco to the sea.

Very strong explosive activity in April 1907 caused damage on the island, and ash fell as far as Messina, which lies about 80 km to the S.

(To be continued)


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

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