Magma, All Cool and Bothered

Hanging on.
Fossilized mollusks embedded in pillars (inset) of a Roman market reveal clues about the area's turbulent geological past.

Christophe Morhange

Neapolitans have long feared their smoky neighbor to the east, Vesuvius, which destroyed Pompeii in 79 C.E. But a new study suggests they might want to mind the land to the west--the Phlegrean Fields--where, researchers say, an active volcano system has caused the ground to rise and fall three times--twice without eruption--since the 5th century C.E.

The Phlegrean Fields is the site of an active caldera: a large depression caused by a collapsed volcano. Historical records document only one minor volcanic eruption in 1538, yet the remains of a 2000-year-old Roman market in the port of Pozzuoli bear witness to the region's turbulent past. The market's pillars are embedded with the remains of bivalves and coral, indicating the town was once submerged by the waters of nearby Pozzuoli Bay. Now it appears that such upheaval was not uncommon. When geoarchaeologists led by Christophe Morhange and Nick Marriner of the Université Aix-Marseille in Aix-en-Provence, France, radiocarbon dated the embedded fossils, they documented three periods between the 5th and 15th centuries C.E. when the sea rushed over Pozzuoli. Magma and other fluids from the caldera periodically push the ground above sea level before eventually dissipating. Because the market was built after one of these elevations, it sunk every time the fluid retreated. Curiously, a volcanic eruption only preceded one of these events, meaning land can move dramatically without coating a region in lava, the team reports in the February issue of Geology.

There's no indication that a volcano is going to bury the area anytime soon. But residents of the Phlegrean Fields shouldn't count on the caldera remaining silent. "The earth can only sustain so much strain," says David Hill, a volcano seismologist with the U.S. Geological Survey in Menlo Park, California. Volcano geodesist Maurizio Battaglia of the University of Göttingen in Germany agrees. "Thousands of people are living in Pozzuoli again, and Naples is only about 10 miles [16 km] away." So far, he says, "we've been lucky."

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