10/27/99

Kathleen O'Toole, News Service (650) 725-1939; e-mail: kathleen.otoole@stanford.edu

Unusual origins of Europe's largest volcano explained

Mt. Etna, Europe's highest active volcano, has perplexed geophysicists for years because it sits alone on the east coast of Sicily and spews out lava that is chemically different from that of volcanoes caused by the clashing of Earth's tectonic plates.

Now Amos Nur of Stanford's Geophysics Department and his former student Zohar Gvirtzman of the Institute of Earth Sciences at Hebrew University of Jerusalem propose an explanation for Etna in the Oct. 21 issue of the journal Nature.

Etna's voluminous flows are the consequence of "slab rollback" where a chunk of the Tyrrhenian plate broke off, rapidly opening a narrow basin of magma that is sucked up from under the nearby African plate, they say. This magma, or pool of viscous asthenosphere, is what has erupted periodically from Etna over thousands of years. Mt. Vesuvius on the other side of the Tyrrhenian Sea from Etna may be the same sort of volcano, Nur adds, but that awaits further research.

Most of the Earth's volcanoes are situated over subduction systems ­ places where one tectonic plate is sliding under another. As the whole system converges, partial melting occurs in the wedge between the plates and is spewed through faults or cracks in the Earth's crust.

Mt. Etna sits near but not on a subduction zone where three plates of Africa and Europe are converging. Sicily was once part of Corsica and Sardinia but separated, and the Tyrrhenian Sea opened up, geologists believe. The geologic record suggests the opening of a basin between the plates occurred very fast ­ "at centimeters per year, and such basins have been a puzzle for a long time in plate tectonics," Nur says. "We asked why does something extend in the middle of convergence?"

Nur and Gvirtzman first calculated the suspected thickness of the solid upper mantle of the earth's crust, known as the lithosphere, based on the observed surface elevations of the region and what they knew about the Earth's crustal structure and buoyancy. Using a three-dimensional mechanical model of the three plates involved and a fair amount of recorded geologic data on Etna, they determined that a localized disturbance could release a narrow part of the subducted plate so that it would sink fast, creating what they call a "back-arc" basin. This basin would be shallow enough to permit a sideways flow of magma, which would be sucked out of the basin as the descending slab migrates into the Earth's mantle, leaving low pressure behind it.

"A plate with some type of topography that doesn't want to subduct could cause that type of tear" in the plate, Nur says. "The tear allows the viscous material underneath to rise from the sides, and you have a passageway to the surface."

Etna's numerous, voluminous eruptions show that it has a large underground fuel tank. Aeschylus wrote about eruptions occurring in 475 B.C. The most devastating eruptions occurred in 1169 and 1669, and the most recent in 1971. The volcano is now about 93 miles in circumference and has 260 lesser craters on its slopes.

Perhaps 15 or 20 other volcanoes in the world have similar origins, Nur says, as they seem to defy the more conventional plate tectonics model. This may be a case of "the exception proving the rule," he says. "I've never really known what that phrase meant, but I'm taking it to mean that one way to learn a lot more about plate tectonics is by understanding these exceptions."

 

CAPTION FOR FIGURE 1

The figure shows that Mt. Etna is situated at the junction of three fault zones and to the side of the South Tyrrhenian Sea subduction zone where the underlying plates of Africa and Europe are converging. Based on Etna's location, Amos Nur of Stanford and Zohar Gvirtzman if Hebrew University of Jerusalem have proposed an explanation for the unusual volume of magma that has spewed from Etna over thousands of years.

 

CAPTION FOR FIGURE 3 (to be included with release)

The cross section at left is parallel and the one at right is perpendicular to the direction of subduction underneath the South Tyrrhenian Sea. The dotted line represents the location of the top of the Ionian slab before it decoupled from the larger plate underlying southern Italy. The decoupling not only opened up the Tyrrhenian Sea but created a passageway for viscous asthenosphere to travel to the earth's surface, explaining the voluminous eruptions of Mt. Etna and also the uplifted terrain of the Calabrian Peninsula of Southern Italy, the researchers say.

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By Kathleen O'Toole