Strombolian eruptions
The Strombolian type of eruption is named after the activity on Stromboli in the Aeolian Islands, north of Sicily. Strombolian eruptions usually expel basalts and sometimes andesites that form both cinder cones and lava-flows. They are characterized by a moderate degree of lava fragmentation, moderate dispersal and moderate accumulations of both fragments and flows. The eruptions are short lived, lasting only a few months or years at the most; the explosions are an of similar intensity; and their cones rarely exceed 250m in height or their lava-flows 10 km in length. The vent largely stays open during a Strombolian eruption. As the magma rises at an increasing pace into the upper reaches of the vent, more and more volatile bubbles separate out until it becomes frothy. The bubbles burst and form an uprushing, expanding mass of steam and gas. The magma is shattered into innumerable clots and droplets and shot between 100m and 1 km into the air during brief, raucous ejections repeated at intervals of perhaps an hour or less. The cold air solidifies these fragments into ash, lapilli, spatter or cinders as they return to the ground. The fragments are solid, if still glowing, when they fall back around the vent and quickly accumulate in a cone that approaches its final size within a few months. Each explosion produces fragments of a characteristic size, determined by its force, so that individual beds are therefore usually composed of similar materials. Few fragments are thrown far from the vent, although fine ash may be blown away on the winds. At intervals of several months, lava-flows ooze from the crater, or more often from the base of the cone.
At present,
between four and six vents in the summit crater of Stromboli usually erupt every
15 or 20 minutes and lava-flows are commonly emitted several times a year and
cascade down the Sciara del Fuoco, the great "scar of fire" on the
northern flanks of the volcano. The eruptions are usually too modest
to make much impact on the media. Indeed, when Rossellini made the film
"Stromboli" in 1950, he did more for the island's fame than any number
of eruptions, although his interpretation of its eruptive style did not attain
his usual realism. Nevertheless, these eruptions are the glory of Stromboli.
What is already enthralling during the day becomes a beautiful pyrotechnic
display when seen from the balcony provided by the Pizzo Sopra la Fossa, near
the crest of the volcano. The eruptions on the piano nobile below
count amongst the most spectacular continuous natural performances on Earth. The
first sign of another eruption is a low rumbling accompanied by a slight
trembling of the ground. Then a dull red glow lights up first one and then
another of the vents as the rumbling becomes a deep echoing roar of escaping
gases. The red brightens inside the vents to pale vermilion, even yellow at
times, as the molten rock arrives. One vent hisses out a blue flame like a huge
Bunsen burner, while the roar in the other vents becomes raucous and hoarse.
Incandescent fragments are hurled 1 00m or more into the air, twisting and
twirling in a constellation of all sizes from sparks of hot ash to clots of
molten cinders, enveloped now and again by clouds of gas and steam. The
fragments finally fall back gracefully onto the cones around the vents. The
embers reveal the outlines of the cones for the first time as the glowing
vermilion deepens to red again in the vent. As the sparks die, the larger clots
slide slowly down the slopes of the pyre. The roaring in the vent calms to a
subdued moan, and the noises made by the last cooled solid cinders crashing back
down the vent echo about the crater. The silence returns as the last fragments
on the cones cool and deepen to dark red before they finally vanish into the
darkness. The whole sequence takes perhaps five minutes: there is less than half
an hour to wait for the next.
A lucky
observer may be rewarded with the eruption of a lava-flow. All the lava-flows in
the past 400 years, at least, have been channelled down the Sciara del Fuoco.
The molten stream cascades down the steep slopes and, as its surface solidifies,
blocks often break off and roll quickly into the sea, generating explosions and
billowing clouds of steam. The infrequent larger eruptions, when parts of the
island may be showered with lapilli and even volcanic bombs, usually occur after
periods of quiescence lasting perhaps several days. An abnormally vigorous
eruption, albeit of limited danger, such as those occurring in 1930, 1956, 1967
and 1975, may then be confidently expected.
This kind of activity is perhaps the most common type on land. However, although Stromboli is typical of Strombolian eruptions, its shape and size are not. It is a strato-volcano, rising 3000m from the sea-floor, which has been in continuous eruption for at least 2500 years. Such prolonged activity is exceptional, if not unique, amongst volcanoes and has created an unusually large landform as a result.
Paricutin is a well studied example of a Strombolian volcano that began erupting in a Mexican maize field on 20 February 1943 after a day of intense local earthquakes. Within a week, its cone was 167m high. When, after a year, the cone reached a height of 336m, Paricutin had already experienced most of its explosions and expelled most of its fragments. It repeatedly gave off lava-flows that changed gradually from basalts to andesites as the eruption progressed. They eventually covered 25 km2 and overwhelmed the nearby town of San Juan. The activity slowly waned and then suddenly stopped on 4 March 1952 when the cone had reached the greater than average height of 412m after an unusually long eruption. The Monti Rossi were more typical of the genre, erupting in three months on Etna in 1669. Once a Strombolian eruption stops, it often never resumes but it can, however, occur in distinct phases, forming superposed cones. Thus, in the Chain of Puys, the larger second cone of Puy de Mey completely buried the first. In continental environments, many Strombolian cones grow up along fissures, which can produce a succession of cones, erupted irregularly in time and space for several thousand years. These continental fissures thus behave differently from those in oceanic environments such as Iceland, where whole fissures seem to erupt only in one brief episode.
Apart from Stromboli itself, one of the best places to witness Strombolian eruptions in recent decades has been on Etna, especially since its contemporary phase of agitation began in 1971. It has erupted, for example, in 1974, 1978, 1981, 1984, 1985, 1986, 1989, 1990 and 1992. Etna has a collection of more than 200 Strombolian cones on its flanks as well as two large Strombolian cones, known as the North-East and South-East craters, near its summit. North-East crater started as a pit on 27 May 1911 at a height of 3100 m at the base of the summit cone. After emitting gases and lava-flows for decades, it began to construct a cinder cone in 1955. Between 1966 and 1971, as the cone continued its growth, moderately viscous basaltic lava-flows were expelled which splayed out in a fan 4km wide and 200m thick. The northern sector of the cone collapsed under the combined influence of gravity and further basaltic upwelling in October 1974 and again in January 1978, so that it is now horseshoe shaped. Nevertheless, in January 1978 the cone was 250m high and formed, and still remains, the highest point of Etna at 3345m. Upon this achievement, the activity of North-East crater waned and only brief, sporadic outbursts have since occurred in 1980, 1981 and 1986. The baton was immediately taken up by South-East crater. It, too, had begun life as a pit, exploded out in 1971, but it only began to erupt lavas and form a cone on 29 April 1978. Its initial eruptions were punctuated by periods of repose, but it began persistent activity in 1980 that still continues - including, for example, two weeks of spectacular lava-fountaining in September 1989 rising 300m above the cone. South-East crater has expelled lava-flows and built up a cone at a faster rate than its predecessor, and it may eventually take over the role of the summit cone on Etna.