Vesuvius Magma Chamber Rising; May Mean Milder Eruption

Brian Handwerk
for National Geographic News
September 10, 2008

The magma reservoir beneath Mount Vesuvius has been migrating toward the surface over the past 20,000 years, according to a study published today.

The finding may be good news for some three million people living under the active volcano's shadow in Italy's Campania region, where Vesuvius's famous A.D. 79 cataclysm buried Pompeii and Herculaneum.

Magma pools at shallower depths are less volatile and less likely to produce violent eruptions, according to a research team led by Bruno Scaillet of the Université d'Orléans in France.

Magma reservoirs hold a replenishing supply of lava, which is unleashed during volcanic eruptions. A reservoir's location and characteristics offer clues to the severity of future eruptions.

"The difference in pressure depth between Pompeii and more recent eruptions [documented by] our experiments stresses that the next coming eruption may have little in common with that one which destroyed Pompeii," Scaillet said.

Rock Reveals Rising Magma

Scaillet examined the lava rock produced by four major historical eruptions and reconstructed the temperature and pressure of the magma reservoir during those events.

The data indicate that, between A.D. 79 and a later eruption in A.D. 472, the magma reservoir climbed from between 4.4 and 5 miles (7 and 8 kilometers) below the surface to between 1.8 and 2.5 miles (3 and 4 kilometers).

Scaillet said it is unknown if the reservoir has moved since the time of the last eruption or how big it may be today, but magma closer to the surface is likely to produce less destructive eruptions.

"The deeper the reservoir, the colder and the more viscous is the magma being stored," he said, explaining that such magmas "are prone to much more explosive eruptions when they reach the surface."

The stakes are high in any future Vesuvius eruption. (See photos of Vesuvius today.)

The volcano is believed to have produced major eruptions some 22,500 years ago; 17,000 years ago; 15,000 years ago; 11,400 years ago; 8,000 years ago; 3,780 years ago; and 2,000 years ago.

Still Risky

Michael Sheridan, a volcanologist at the University at Buffalo in New York, said the research is interesting.

"It presents a new perspective," said Sheridan, who was not involved in the study. "The geochemistry is good and I'm glad it's out there for discussion."

But Sheridan, who studies the impacts of eruptions in populated areas, cautioned the study isn't the last word on predicting the enormous risk factors associated with Vesuvius.

He noted that other eruption data, such as deposit distributions and tectonic factors, weren't part of the study.

"This is another model," he said. "We need to develop a scheme for weighing multiple hypotheses and somehow integrating the consequences of all those hypotheses."

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