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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