Italy's Volcanoes: The Cradle of Volcanology

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Frequently asked questions about Etna
When will Etna erupt again?


In spite of the much increased knowledge and ever more sophisticate monitoring of Mount Etna, this remains one of the questions most difficult to answer. At present (early August 2003), the volcano is not erupting, and the most recent flank eruption ended on 28 January 2003, after three months of vigorous activity. So this question concerns both summit activity, and a flank eruption, which are two different pairs of shoes.

Many flank eruptions at Etna are actually preceded by eruptive activity at the summit craters (at times confined to one of the four summit craters, at others occurring at two or even more of them simultaneously), which may extend over periods of months to years. Summit activity was observed for six years prior to the 2001 flank eruption and for about three months prior to its successor in 2002-2003. From these two recent cases alone it becomes evident that there is no relation between the duration, character and volume of summit activity and any of the parameters of the ensuing flank eruption.

Unfortunately, volcanoes usually do not behave in a strictly regular manner in terms of their eruption frequencies, and they may often change the behavior manifested for a certain period (take Vesuvio (Vesuvius) which was in virtually constant eruption for three centuries until 1944, but has been completely inactive since then). At Etna, between 1971 and 1993 there were 13 well-defined flank eruptions, an average of one eruption every 1.5 years (from 1993 to 2001, that is, for 8 years, there was none). Most (but not all) of these flank eruptions were preceded by activity at the summit craters; in some cases there were real cycles of summit activity followed by flank eruption (e.g. 1980-1981, 1988-1989). When the summit craters resumed activity in 1995, it seemed that another cycle had begun, and a flank eruption would occur within a few months to a few years. However, this time the summit eruptions lasted 6 years before there was another flank eruption, and we are now realizing that eruptive cycles at Etna are more complicated.

The eight years which passed between the flank eruptions of 1991-1993 and 2001 were the longest interval without flank activity since 1971. This did not mean that the volcano was "less active", or that the magma supply had dropped. The volcano inflated vigorously, recovering more than twice the inflation lost during the large 1991-1993 eruption. Intense seismic activity in various sectors of the mountain indicated magma movements at depth, in some instances approaching quite close to the surface. Etna was recharging with magma, but it appears that its internal plumbing system was (at least until July 2001) more stable than it was from 1971 until 1993. It is likely that this state of stability has now been replaced by a state of instability.

After the summer 2001 eruption everybody wondered how long it would take to the next flank eruption. Based on the recent eruptive behavior of the volcano, a new eruption could be expected within a few years after the 2001 eruption. But looking with some attention at the period 1971-1993, one notes that during that interval flank eruptions occurred at an average rate of one every 1.5 years. Assuming that the rate of magma supply into the volcano had not dropped since the 1971-1993 period (it is rather likely that it has increased), a new flank eruption could thus occur as early as 1-2 years after the 2001 eruption. And it did. In late October 2002, the latest flank eruption began only 1 year, 3 months and 10 days after the beginning of the previous eruption. This is less than the average interval between flank eruptions during the period 1971-1993. The new eruption continued until 28 January 2003, after which the question "When will Etna erupt again" once more became a question of high priority. Although no new magma has appeared at the surface anywhere on the volcano as of early August 2003, there are many indications that the volcano continues to be unstable, probably it is less stable now than any time since 1993.

There are several possibilities how Etna will behave in the near future:

1) there will be a period of summit activity.
It is likely (but not sure) that the next activity that we will see on Etna will occur at the summit craters. This activity might extend for months to several years and build up to major events such as lava fountains and voluminous overflows of lava onto the upper flanks of the volcano, such as in 1995-2001. On the other hand, summit activity did occur between the 2001 and 2002 flank eruptions but never reached the levels of the 1995-2001 activity, possibly because the volcanic edifice was much less stable then than before 2001. With the volcano being at least as unstable as between the 2001 and 2002-2003 eruption, magma rising into the volcano will much more easily find pathways under its flanks and the next period of summit activity might be relatively brief and of modest dimensions, or there might be no summit activity at all.

2) a new eruption will occur somewhere on the flanks on the volcano.
Such an eruption will inevitably occur sooner or later, possibly after a period of summit activity. Actually it might take little time (1-2 years) until the volcano will get ready for another flank eruption. Another, not all that reassuring possibility is that the next flank eruption will be larger than those of 2001 and 2002-2003, which were rather medium-sized for Etna, although the latter of them emitted about twice as much magma as the earlier.

3) the activity will eventually die down.
Currently (early August 2003) there is no eruptive activity, so things can only go in the opposite direction. In the long therm Etna will continue to be the second most active volcano on Earth (after Kilauea, Hawaii), and currently it seems that it is getting more active rather than calming down.

So we know that a flank eruption will certainly occur in the future, but at this moment neither the time nor the location and character of that outburst can be foreseen. However, a few assumptions based on Etna's past and recent behavior can be made. The two major fracture systems (or rift zones) on the northeastern and southern to southeastern flanks are more prone to eruption than the other sectors of Etna: 18 out of the 23 flank eruptions in the 20th century as well as the 2001 and 2002-2003 eruptions have taken place in these areas. Before and during the 2001 and 2002-2003 eruptions extensive fracture systems developed from the NE to the S flank where all eruptive fissures of these eruptions became active. It may well be that these fracture systems remain unstable, so that the easiest pathways for magma can be found there, and thus the next flank eruption might affect more or less the same sectors of the volcano.

In summary, at this moment, there is no way to recognize the precise time of the next eruption of Mount Etna. However, based on the available evidence from the recent past and from the historical record, Etna seems to be in a particularly active phase, which is characterized by periods when flank eruptions occur once every few years. Such a period was initiated with the flank eruptions of 2001 and 2002-2003, and there is no reason to assume that the volcano will slow down after these events. Statistically, a new flank eruption is extremely likely within the next one or two years (from the end of the most recent eruption in late January 2003). This may or may not be preceded by summit activity, and clear geophysical evidence for renewed magma intrusion under the flanks may be available only very shortly before the event, as in October 2002, when there were only two hours of seismic warning of the imminent eruption. The sites most likely to be affected by a new flank eruption lie in the same general area of the most recent two eruptions, that is, on the upper southern and on the northeastern flanks.

One important question that remains in this context is: will there be a possibility to give more precise, timely warning of the next eruption, will it be possible to begin to react earlier than in the case of the 2002-2003 eruption? Can the next eruption be predicted or forecast?

Next Question: Is Etna changing its behavior?

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

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