Italy's Volcanoes: The Cradle of Volcanology

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Is Etna changing its behavior?


There has been much discussion about that piece of news that went through the international mass media shortly after the 2001 eruption. It said: An Italo-French team of geoscientists (more precisely, geochemists) has found evidence that Etna is changing behavior. It has been a "good" volcano so far, which does not produce violent explosive eruptions, but now it is becoming a "mean", more explosive volcano. This is because until now Etna has been fed by a "hot-spot" (or mantle plume), like those of Hawaii, but now its magma is more typical of volcanoes lying at subduction zones (island arcs), such as Pinatubo or Mount St. Helens. This news, which was extensively featured in Sicilian newspapers and television, caused consternation and some alarm among the residents of towns and villages around the volcano, and some Italian scientists immediately commented on the news reports saying that the cited study was incomplete and Etna was by no means changing its character.

But who carefully read the source of all that excitation, a four-page paper published in the 30 August 2001 issue of the journal "Nature", rapidly noted that the authors were not talking about a rapid process occurring in real time. The time scale of the presumed change was 100,000 years. It is since 100,000 years that Etna is gradually assuming the character of a subduction volcano, and indeed highly explosive volcanism has occurred on various (fortunately rare) occasions in this period. When humans began to settle around the volcano, it was already in that state, and some populations have in the past suffered from cataclysmic eruptions. The essence of all this is, Etna is NOT a good volcano, even though most of its eruptions are rather benign, and if there are more explosive eruptions, that's nothing new in the life of Etna. It might be new to those who are living near to it, because such events are infrequent.

Of course, the debate was reopened when, in late 2002, Etna showed a seemingly "unusually" explosive behavior for the second time in little more than a year. Never during the past century had Catania and its surroundings received similar quantities of ash as in the weeks following the spectacular beginning of the 2002 eruption on 27 October. It is an all-too-understandable human conception that only those things are possible that we have lived in our lifetime, or, to a lesser degree, those that occurred during the life of our parents. And that's right, nearly all of the 20th century's eruptions of Etna were not very explosive, but rather delivered lava flows, even though some of them were quite voluminous and destructive. Explosive activity was restricted to short events that occurred almost exclusively in the summit area, far away from inhabited areas ...but it is true that such events had become more common in the past 25 years, and especially since 1995.

None of us were there when Etna produced the latest (pre-2001) flank eruptions with a high degree of explosivity, in 1763 for example, and again in 1879 and 1892. But many knew very well all the huge cones that dot the flanks of the volcano, products of flank eruptions in the past centuries and millennia. The Monti Silvestri, a two-peaked huge crater near the town of Nicolosi, formed in 1669. There are numerous older cones that were produced by quite explosive flank eruptions, the largest of them being Monte Minardo on the western flank of the mountain. So once again, nothing new. It's just that people at Etna are not used to such events, because the faster information spreads in these days, the faster it is forgotten, and information predating our days is equivalent to absent or worthless. In the past people believed that the Sun orbited around the Earth, and the same people witnessed explosive eruptions at Etna. That's not part of our reality.

But volcanoes have a different time scale. Many volcanoes that are still erupting began their activity many hundreds of thousands of years ago, and geological studies have revealed that even the most benign volcanoes, Kilauea on Hawaii for example, have experienced episodes of violent explosive activity during their lifetime. It is furthermore one of the fundamental geological concepts that what a (still-active) volcano has done once, it can do again, almost certainly it will, maybe in a hundred thousand years, but maybe also tomorrow.

What might seem confusing and contradictory is that we ARE currently witnessing a change in the behavior of Etna. A new magma reservoir has formed aside the ever-active central conduit system, and in 2001 and 2002 both erupted simultaneously. This is excitingly new and poses a number of questions, but it also reveals much about the incredibly complicated dynamics of this volcano. It is true that for the past century nothing similar seems to have occurred at Etna, and therefore it's a change. But it seems that in the past Etna has changed many times, only that the range of changes has always remained fairly limited. Magmas erupted between 1600 and 1669 were different from those produced since then, and the eruptions during that period were more voluminous, many occurred at lower elevation and were therefore much more destructive than more recent ones. Some scientists have attributed this to the presence of a major magma reservoir at shallow depth below the volcano, a reservoir that was drained by the large eruptions of the 17th century and has not re-formed after that. In this context the appearance of a new reservoir under the southern flank of Etna is most interesting.

But in any case whatever changes there are, they are part of the same old tale in the life of a volcano. Etna will do the same things it has done before, even if some might seem new to us. It's us that are new to the scene.

Next Question: Can the next flank eruption be forecast or predicted?

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

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