Etna is studied intensively by scientists
Europe's most active volcano, Mount Etna, is getting more violent.
That is the assessment of French researchers who have studied the chemical composition of the lavas spewed out of the Sicilian mountain over the last half million years.
The scientists say their analysis of mineral samples suggests a gradual but fundamental change in the nature of Etna's volcanism - an unprecedented observation, they claim.
"The study doesn't help us predict eruptions but it does give us a clearer framework in which to understand Etna," lead researcher Pierre Schiano, at Blaise Pascal University in Clermont-Ferrand, France, told BBC News Online.
Complicated mountain
Despite being one of the most studied volcanoes on the planet, much of Etna's geology remains a mystery.
In some ways it behaves like a "hotspot"-type volcano, such as those in Hawaii, where molten rock wells up in huge plumes from deep within the Earth.
But in other respects, Etna is similar to "island-arc" volcanoes, typical of the Asia-Pacific region, where magmas are produced as a result of one tectonic plate being driven under another. Etna lies close to the point where Europe collides with Africa.
And to complicate matters still further, Etna sits atop the intersection of numerous major faults.
Professor Schiano and colleagues sought to clarify some of these issues by studying historical lavas - some were as old as 520,000 years; others were erupted as recently as 1999.
Scientists specifically looked at the trace elements caught inside large crystals of olivine, the magnesium-iron silicate mineral.
'Unique study'
"Our study of the melt inclusions shows that the composition of the primary magmas of Mount Etna has changed through time," Professor Schiano said.
"And because the composition of a primary magma is a reflection of its source, the results imply that the source beneath Etna has also changed.
"If you compare the signature of this source with what we know, it appears that Etna was initially like a hotspot volcano and is now becoming more like an island-arc volcano.
"And we know that hotspot volcanoes are effusive and not very explosive. By contrast arc volcanoes are much more explosive."
The suggestion here is that the magmatic source that has produced the volcanoes of the Aeolian Islands, such as Stromboli, Lipari and Vulcano, just to the north of Sicily, is now playing an increasingly active role under Etna.
"We can expect greater explosivity at Etna in the future," Professor Schiano said. "This is the first time for a volcano that a transition from a mantle plume source to a subduction zone source has been demonstrated. It's unique."
The research is published in the journal Nature.
-- Anonymous, August 29, 2001