Italy's Volcanoes: The Cradle of Volcanology

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Frequently asked questions about Etna
Is there a risk that Etna might blow its top,
like Mount St. Helens?


The following is a question (from a student in the USA) regarding the types of activity observed at Etna, followed by my answer (with a few additions) to this question.

Q: Please tell me what type of eruptions Mt. Etna gives, Plinian? And explain what Plinian means, if Plinian is actually the type of eruption that Mt. Etna gives?

A: Etna's typical activity is anything else but Plinian, Plinian is the most violent type of explosive eruption and occurs, every now and then, at Vesuvius. Tthe term Plinian is derived from the Roman name Plinius (in English this has become Pliny), and this was the name of an officer of the ancient Roman empire as well as of his nephew (the former being called Pliny the Elder, the latter Pliny the Younger). Pliny the Younger is the person who left to the world the famous account of the catastrophic eruption of Vesuvius in 79 A.D. which, in fact, was violently explosive, and it is after this eyewitness report that this type of eruption is called "Plinian", because it has been so well described by Plinius! Etna does produce such eruptions very rarely, and there has been none since more than 2000 years. The activity that we commonly observe at Etna is called "Strombolian". The term is obviously derived from Stromboli where the activity consists of discrete, separate bursts of incandescent fragmental material, that is, a mild explosive type of eruption. When Etna is in a period of "persistent" - long-lasting, virtually uninterrupted - activity, like it is since 1995, this Strombolian activity occurs at the summit craters, and this has been most characteristic at the Southeast Crater between late 1996 and July 1998. Another characteristic of Etna's activity in such periods is that there is lava outflow, which is not Strombolian (the volcano Stromboli produces lava flows only very rarely, most recently in 1985-86, 1993, and - very minor - in 1994). Then there are the more vigorous eruptions like those we saw at the Southeast Crater between September 1998 and February 1999, these were no longer Strombolian because the incandescent ejections blended into a continuous jet, and thus became similar to Hawaiian style lava fountains. Eruptions like that of 22 July last year, when an eruption column rose 10 km high, are typically called "Vulcanian", but as may have been understood from my answers to other questions, at Etna it is very difficult to apply strict classification schemes because many types of eruption appear, often near simultaneously.

The following question was sent to me from the Netherlands and focuses on the destructive potential of Etna, it is followed by my answer.

Q: How destructive can the Etna be? I mean, is it possible that the Etna can come to an enormous eruption with pyroclastic storms (like Mount St. Helens, U.S.A. 1980)?

A: Explosive eruptions on Etna are rare and ususally limited to the summit area. However, at very long intervals, the volcano produces some more violent explosive eruptions, there was a large ignimbrite eruption some 15,000 to 18,000 years ago, and other major explosions occurred about 5000 years ago, and most recently in the year 122 before Christ when most roofs of Catania collapsed under the weight of tephra. To change its eruptive behavior from the normal mild Strombolian and largely effusive activity to highly explosive, something drastic must happen in the conduit system, that is, in the pathways that lead from the Upper Mantle to the craters, like a long-lived blockage that allows the magma to change chemically (differentiate, as it is called in geology) and thus become more likely to erupt explosively. This is currently not occurring, since magma rises freely and near continuously to the summit.

The third question, probably from a person in Malta, is similar, but it is reproduced here with my relative answer for completeness, and because I always find other ways to respond to similar questions... :-)

Q: What are the possibilites of a full blown eruption happening?

A: Etna does not produce very explosive eruptions, such as Mount St Helens or Pinatubo. The worst that can happen is that lava flows from a fissure very low on the flank of Etna and reaches the densely populated areas around Catania, but lava usually moves very slowly and people have time not only to save themselves, but also to carry away most of their things from their homes if necessary. The risk of such an eruption (the most recent of that kind occurred in 1669) is very low but nonetheless there is a possibility. For the moment, however, there are no indications that such an event can take place in the near future.

Next Question: Are my relatives living near Etna in danger?

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

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