Italian lava's deep secret

October 27, 2000

Etna and the other live volcanoes of Italy have been studied for a long time. Now scientists are finding out about their undersea neighbours and submerging equipment in the deep sea to study their environment. Paul Bompard reports.

For two months, Michael Marani and his team from the Institute of Marine Geology in Bologna criss-crossed the Tyrrhenian Sea in a rented Russian research ship equipped with multi-beam sonar. In the process, they discovered two vast volcanoes that they classified as "geologically active".

"We now have a map of unprecedented detail of the bottom of the Tyrrhenian," Marani says, "and thanks to the dredging and core samples we took on a subsequent cruise, we are sure that there are active volcanoes beneath the Tyrrhenian.

"This does not mean that they are erupting, but that there is activity underneath. In geological terms, a volcano that erupted 1,000 years ago is considered active. We should now get a clear idea of how long ago it last erupted.

"The largest volcano is named Marsili, after a 17th-century Italian scientist who is considered the father of oceanography. It is 70km long, 40km wide and rises 3,000m from the seabed to 500m below the surface, making it the largest volcano in Europe. The second, Vavilov, is a little smaller."

In 1996, Marani's team hired a research ship belonging to Moscow University. "We rented it for a long period to allow the Russians to equip it with state-of-the-art multibeam sonar and computers to process the data. The only undersea exploration of comparable accuracy has been by the Americans along their continental shelf, but nothing of this kind had ever been done in the Mediterranean."

In two cruises, in October-November 1996 and February-March 1999, the ship covered 36,000km. The research area stretched from the island of Elba down to Sicily.

"We had transducers under the ship's bow sending out 40 beams in a fan of 60 degrees to each side, and one beam straight down. We sailed at 10 knots, around the clock, on parallel courses so that the outer edge of the sweeps overlapped. The deeper the water, the wider the image of the bottom and the farther apart the parallel courses. The accuracy of the readings we obtained varied from about 10m at a depth of 1,000m, to about 20m at a depth of 3,000m."

Research conducted chiefly in the 1950s and 1960s had given a rough idea of the mountains and valleys under the Tyrrhenian. "The existence of the Marsili and Vavilov volcanoes was known," Marani explains. "But we had no accurate relief map.

"Now, thanks to the accuracy of our soundings," he says, "when we dredge material and take core samples, we know exactly where the samples came from. So we can deduce much more about the volcanoes' structure."

Marani points out that the Tyrrhenian is particularly interesting compared to other parts of the Mediterranean or to the oceans. "What we are examining is known as a back-arc basin, which includes the volcanoes of Vesuvius, Etna and Stromboli. This was produced by collisions between plates of continental shelf that created an area of oceanic crust. Because this happened relatively recently in the Tyrrhenian, there is not much silt built up on the sea bottom, so we can see the structure much more clearly than in other parts of the world."

More rock samples were collected on a subsequent journey this summer. Marani says that when these are dated in the laboratory, they will allow the first real understanding of the structures of Marsili and Vavilov.

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