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Mount Etna Morphing to More Explosive Volcano, Study Says

Bijal P. Trivedi
for National Geographic Today
August 29, 2001
 
Mount Etna's recent outbursts may be a sign that the volcano is
developing a more explosive personality.

Sicily's Mount Etna is
morphing from a "hot-spot" type of volcano, like the quietly oozing and
bubbling volcanoes of Hawaii, to an "island-arc" variety represented by
the explosive Mount St. Helens and Mount Pinatubo, according to a
controversial new study published in the August 30 issue of
Nature.











"Such a transition has never been seen before," said Graham Layne, a geochemist at the Woods Hole Oceanographic Institution in Massachusetts, who described the report as "very provocative."

"The most compelling aspect of this study is that the evolution occurred in less than a hundred thousand years," said Layne.

"That's strikingly fast," he added. "Geologists are used to dealing in periods of at least a few million years."

Study of Lava Flows

The evidence of Mount Etna's evolution comes from Pierre Schiano of Université Blaise Pascal in Clermont-Ferrand, France, and his colleagues. Schiano is lead author of the study published in Nature.

Their finding is based on a study of the lava flows spewed from Mount Etna over the last 500,000 years, which is done by analyzing specific crystals inside pieces of lava. Lava generated by early eruptions has a different geochemical "fingerprint" than more recent flows.

Recent eruptions have produced lava characteristic of explosive island-arc volcanoes. The lava has higher quantities of water and sulfur than lava from hot-spot eruptions that occurred about half a million years ago.

Island-arc volcanoes occur at subduction zones, where two oceanic plates collide and one is pushed under the other. The Marianas Trench is an example of a subduction zone caused by a collision of the Pacific and Philippine plates. The Mariana Islands are volcanic products of the magma, or molten rock, released at this subduction zone.

The melting of the rock occurs at the subduction zones as one plate is pushed under the other. The magma that's produced contains a lot of water. When this magma reaches the surface, the water expands very rapidly, which is why island-arc volcanoes are so explosive.

"It's the water and sulfur that give these volcanoes their bang," said Layne.

Debatable Findings

Schiano said the simplest explanation for Mount Etna's transition from a hot-spot to an island-arc volcano is that a subduction zone, which formed the Aeolian Islands to the north of Sicily, is moving southward underneath Mount Etna.

But James Luhr, director of the Smithsonian's Global Volcanism Program, is skeptical of the findings. He questions the authors' claim that there has been a shift from hot-spot to island-arc magma.

"There are many examples of two types of magma erupting from the same volcano," he said, noting that Costa Rica's volcano Turrialba, for example, has shown a similar phenomenon.

"The Mediterranean is an incredibly complicated part of the world that doesn't conform to simple plate tectonics," said Luhr.

"The origin of Italy's volcanoes have been hotly debated for twenty years," he said, "and there is still no consensus on the source of magma for these volcanoes—where the subduction zones are, and if there are subduction zones—and the positions of these various plates."

Schiano said that while Mount Etna appears to be moving toward a more violent future, he can't predict when and even whether a big eruption might occur. Nonetheless, he said, the new findings suggest that closer monitoring of Mount Etna may be advisable.


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