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Stromboli

an active islandarc volcano



erupting Stromboli

Eruption of the Stromboli volcano

When a volcano erupt, we may read about it in the newspaper. Visiting them during their active phase is very dangerous. There are only very few volcanos which can be visited during an eruption without too much danger. One of these mountains of fire is the Stromboli in Europe. It is active since ancient times and it can still be visited. Stromboli is the most northward of the Eolian Islands, am 200 km long group of islands representing an island arc between Sicily and Italy. The name of the 924 m heigh volcano derives from "strongyle", which means cone, because of the near perfect shape of the mountain. But like an Iceberg, the biggest part of Stromboli is beneath the sea level, it´s foot is more than 2000 m under the surface of the Tyrhenian Sea.

When you arrive at Stromboli, you will find a sign, telling you not to climb the volcano without an authorized guide. Even the Stromboli may sometimes be ruthless against uncautious visitors. Although this volcano seems to be peacefull, his explosions are not predictible and comming too close to the lava fountains is very dangerous.

About evening it is the best time to start a tour to the top of the mountain. In our backpacks we have warm pullovers, because in more than 900 m above the sea level it would be real fresh,despite the mediterranean climate. The volvano itself hides behind dark clouds, but when we arrived in the morning, we saw his fiery fountains from far distances, competing with the lighthouse on Strombolicchio, a little rock northeast of the main island, the remnant of a more than 200 000 years old predecessor of our today´s volcano.

Strombolicchio

Strombolicchio

Starting at St Vincenzo, one can enjoy the beautifull view from the village to the sea. On the way up, we pass the small valley of Vallonazzo. During the great 1930´s eruption a pyroclastic flow took this way down. A pyroclastic flow is a mixture of hot gas, about 500°C, and ashes. This flow had a breadth of 10 meters and a velocity of 20 m/s (ca. 45 mph) and killed 4 people. Today nothing is left from this catastrophe, but it reminds us about the violent nature of the mountain we are climbing on.

A good place for a first stop is at the Semaforo Labronzo, just in front of the Sciara del Fuoco, the fire slide. This is a huge scar left from a sector collapse 5000 years ago at the northwest side of the volcano. Today, this scar is mostly filled with Stromboli´s younger products.
At the Semaforo Labronzo is a little pub. Here one can watch sometimes glowing lava rocks rolling down the Sciara del Fuoco to the sea while enjoying good italian wine.

The Sciara del Fuoco

But this time, we want to visit the active volcano even more close, so we start climbing to the crater. Long and winding is the path, offering breathtaking views to the Sciara del Fuoco every minute. The vegetation disappears and we find a moolike landscape with ashes and lava, not unlike the first days of our earth.
Climbing the lava fields is not too bad, but the fields of ashes are a real test for our muscles, but the volcano can still be heard at this point. The hot interior of our good old earth breaks free with a deep roar. The old lava fields belong to the Vancori, which was active from 100 000 to 13 000 years ago. Its main products rhyodacites and andesites. Rhyodacites are rhyolites rich in plagioclase, the volcanic equivalent of the plutonic granites. Andesites instead are the equivalent to the diorites, less rich in SiO2 as the rhyolites and granites.
The lava of our recent Stromboli became more basic (less rich in SiO2) during his activity. This is just the opposite way to the most other volcanoes in the world.


The crater region of Stromboli with steam of an explosion

After a last effort we reach the Cima with it´s 918 m. This is a good observation point for the crater and it´s vents, which lay about 200 m beneath the Cima. When the crater is quiet, only tge warm ground tells us about the volcanic nature of this area and when seated, hot and acid gases can affect the clothing. The air is filled with the smell of sulphur and even the fresh wind in this height is not able to remove it. This is the most suitable point to observe the activity of Stromboli.

From here also the 6 greater islands of the aeolian archipelago are visible. They resemble a almost 200 km long chain of islands between Calabria and Sicilia. Beneath these islands there is a so called Wadati-Benioff-Zone, a distinct layer of earthquake foci, dipping northwest and can be followed up to a depth of about 500 km. These Wadati-Benioff-Zones are thought to resemble a lithosphaeric plate, subducted under the island chain and dipping northwest under the Tyrrhenian Sea with 70° and from a depth of 250 km with 45° (Ellam et al.1989). Similar island arcs and earthquake zones occur on the margin of the pacific ocean, the so called "Ring of Fire". Here the pacific plate is subducted under the surrounding plates.
In the area in front of the Aeolian archipelago there is now no oceanic crust to be found, so that this subduction zone has reached an advanced stadium. Our island arc is slowly dying. And to raise the complexity of the matter, geochemists have found an additional source of magma for Stromboli from the earths mantle. These magmasources are more typical for ocean islands like the Hawaian islands and here in the Mediterranean there is the Etna the nearest volcano with such a magma.

Benioff-Zone

Direction and depth of the Wadati-Benioff-Zone beneath the Aeolian Islands, after Ellam et al (1989)

As we enjoy the view, three redeyes are glowing in the darknes 200 m beneath us like the ports of hell. With the noise of an approaching train the internal fire of our planet earth finds its way to the surface though one of the vents and a fountain of glowing molten rock raises into the dark sky. The wind removes the steam and gas clouds accompanying such an event and we have a good view of the spectacle. The scene is repeated every 10 to 15 minutes. Every time huge amounts of gas and only very little ava is produced. This kind of activity is called the strombolian activity. On average Stromboli produces only 10 to 1000 kg lava and ashes per eruption(Gilberti et al. 1992). The volcano is active in this way since almost 2000, perhaps since 5000 years without changing it´s appearance in historical times. But there are also stronger eruptions known. The biggest in the 20th century is considered the one of 1930. During this particular eruption 70 000 m3 of ashes and 10 000 m3 of lava were produced. This resembles the normal production of the volcano for the time of 5 years.

In the morning of September 11th, at 952 two heavy explosions occurred. Boulders of lava up to a weight of 30 tons were thrown as far as 3 kilometers and destroyed many buildings. Just before the explosions the whole island was lifted 1 meter and for 40 minutes glowing ashes were raining from the sky, as the explosions widened the vent. A pyroclastic flow went down through the valley of the Vallonazzo and there were streams of lava at the Sciara del Fuoco. The eruption ended after 15 hours.

Fortunately eruptions are not allways as violent as this one was. The gas bubbling in the magma forces only few lava into the air. But how comes this gas into the magma? Different gases have a different solubility in magma, depending on the hydrostatic pressure. Carbondioxide for example have only a low solubility and will be set free at a depth of about 800 m. They form bubbles that are growing by assilmilation of other gases or bubbles. Above 800 m watervapour, which the major component of gases in magma, also forms bubbles. These bubbles grow rapidly before they the surface of the magma column. The pressure of the explosions of Stromboli correspond to bubblesizes from 0,5 up to 4 meters. At Heimaey, Iceland, even bubbles with a diameter of 10 meters were observed (Blackburn et al.1976). Every explosion of Stromboli resembles the bursting of these bubbles of gas. So this volcano is just like a bottle of champagne, which is has been shaken and opened. But, unlike champagne, we prefer to observe it from a distance.

So as gases were bubbling though the magma column, bursting at the surface and give rise to fiery fountains, we also feel the strong fresh wind in 900m above the sea level. The warm pullovers were not carried in vain. The light of our flashlights unveiles another beauty of nature. The ground of the Cima is covered with crystals of the mineral amphibol, some of them twinned. They were taken as a souvenir.
As we descent through the ash fields of Rina Grande, the torches only partly illuminate a world of fine dust. Only the normal gravity reminds us, that we are stil on mother earth and not on the moon. The dust covers everything and even fills our lungs. Only by reaching the vegetation the breathing gets easier again. Our tour ends at the church of San Vincenzo, an tired as we are, we only wish to rest our feet and to remind what we have experienced.

References

BERTAGNINI, A., LANDI, P. (1996) : The Secche di Lazzaro pyroclastics of Stromboli volcano: a phreatomagmatic eruption related to the Sciara del Fuoco sector collapse. Bull. Volcanol. 58 (1996) pp. 239-245.
 

BLACKBURN, E.A., WILSON, L.;SPARKS, R.S.J. (1976) : Mechanisms and dynamics of strombolian activity. J. geol. Soc. London, Vol. 132 (1976), pp. 429-440.

ELLAM, R.M., MENZIES, M.A., HAWKESWORTH, C.J., LEEMAN, W.P., ROSI, M., SERRI, G. (1988) : The tansition from the calc-alkaline to potassic orogenic magmatism in the Aeolian Islands, Southern Italy. Bull. Volcanol. 50 (1988), pp. 386-398.

ELLAM, R.M., HAWKESWORTH, C.J., MENZIES, M.A., ROGERS, N.W. (1989) : The volcanism of Southern Italy: role of subduction and the relationship between potassic and sodic alkaline magmatism. J. Geophys. Res. 94 (1989), No. B4, pp. 4589-4601.

FERRARA, G., KELLER, J., VILLARI, L. (1974) : Evolution of eolian arc volcanism (southern Tyrrhenian Sea). EPSL 21 (1974), pp.269-276.

FRANCIS, P., OPPENHEIMER, C., STEVENSON, D. (1993) : Endogenous growth of persistently active volcanoes. Nature, Vol. 366, pp. 554-557.

GILBERTI, G., JAUPART, C., SARTORIUS, G. (1992) : Staedy state operation of Stromboli volcano, Italy: constraints on the feeding sytsem. Bull. Volcanol. 54 (1992), pp. 535-541.

KOKELAAR, P., ROMAGNOLI, C. (1995) : Sector collapse, sedimentation and clast population evolution at an active island-arc volcano: Stromboli, Italy. Bull. Volcanol. 57 (1995), pp. 240-262.

KRAFFT, M. (1984) : Führer zu den Vulkanen Europas in drei Bänden. Band 3: Italien - Griechenland. pp. 98-105, Enke Verlag, Stuttgart 1984.


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© 2001 Gunnar Ries No copying and commercial use of the pictures without my permission!


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