Prediction averts volcanic disaster.

Thanks to careful planning and geologic luck, volcanologists from Papua New Guinea successfully forecast a pair of eruptions last week, allowing 30,000 people to escape the endangered town of Rabaul.

Situated on a bay along the east coast of New Britain Island, Rabaul sits within a sunken crater, or caldera, formed during a huge eruption 1,400 years ago. The current blasts emanate from two volcanoes, Vulcan and Tavurvur, on opposite sides of the bay.

Scientists at the Rabaul Volcanological Observatory called for an evacuation on Sept. 18, hours after strong quakes rocked the area and seismometers detected a distinctive tremor that precedes eruptions. Early the next day, Vulcan and Tavurvur sent plumes of ash 70,000 to 100,000 feet into the air. In the days that followed, a thick layer of ash covered much of the town, combining with rain to collapse many roofs.

The United Nations' Department of Humanitarian Affairs says that the eruption displaced 45,000 people, but it has received no reports of casualties.

Geoscientists say the work of the researchers and civil defense officials at Rabaul averted untold fatalities. "The town is pretty much nestled inside a caldera -- inside a volcano. This time, they managed to get everyone out before the eruption," says James J. Mori of the U.S. Geological Survey in Pasadena, Calif. From 1984 to 1988, Mori worked as a seismologist at the Rabaul observatory.

Rabaul is one of several active calderas -- including Long Valley, Calif., and Yellowstone National Park -- that have concerned volcanologists during the last decade. Because scientists have never witnessed such a large eruption, they remain unsure of how one starts. They suspect that several spots on the outside of the old caldera might become active, releasing magma around the entire rim of the crater. After such an outflow, the center of the volcano collapses to form a new caldera.

Scientists have worried about Rabaul because it seemed to fit this pattern of widespread unrest. Vulcan and Tavurvur both erupted strongly in 1937. In the 1970s and 1980s, the caldera produced significant quakes and other ominous signs, leading officials to draw up evacuation plans and prepare the populace.

"The eruptions in 1937 and this intense period of unrest starting in 1971 made people worry that the next event could unzipper the ring fracture and lead to a caldera-forming eruption," says volcanologist Daniel Dzurisin of the USGS' Cascades Volcano Observatory in Vancouver, Wash.

Instead, the current events seem a repeat of 1937. The blasts have weakened this week, but a USGS team is en route to set up monitoring devices in case the activity renews.

Satellite measurements indicate that the Rabaul eruption equals the 1980 Mount St. Helens blast in scale but has released far less climate-altering sulfur dioxide than the 1991 explosion of Mt. Pinatubo did.