Volcanoes under ice: recipe for a flood.

When a volcano erupted beneath Iceland's largest ice cap on Sept. 30, geoscientists there waited anxiously for the jokulhlaup, a deluge of water pouring out from beneath the ice. With records of these glacial floods going back to the 12th century, Icelanders have grown adept at anticipating jokulhlaups and avoiding any loss of life. Yet when this flood finally burst forth from the Vatnajokull ice cap, it carried some new lessons for the expectant researchers.

Instead of striking immediately after the eruption, as predicted, the jokulhlaup didn't come until the morning of Nov. 5. It then confounded forecasts by ending too quickly. While the fastest previous flood this century took 3 days to peak, this one crested in 14 hours and died within 2 days. For a few brief hours, the surge of water and icebergs flowing toward the sea formed the second largest river in the world.

"We have learned that the descriptions from older times, which we found close to unbelievable, are probably quite correct," says Oddur Sigurdsson of the National Energy Authority in Reykjavik.

Geothermally warmed rock under the Vatnajokull ice cap, which covers southeast Iceland, generates smaller jokulhlaups every few years. Giant floods take place when eruptions under the ice cap melt vast volumes of water.

The recent events started when a fissure developed between the two major volcanoes under western Vatnajokull. Within 2 days, the eruption melted through the 500-meter-thick ice cap. Water flowed beneath the ice into a large crater belonging to a hidden volcano called Grimsvotn, says Karl Gronvold of the Nordic Volcanological Institute in Reykjavik.

As the eruption subsided over the next weeks, the growing lake in Grimsvotn's crater pushed up the overlying ice cap. The lake height soon exceeded the triggering point of past jokulhlaups, but the flood did not start for another 3 weeks. This delay shows that jokulhlaups are more complex than scientists have appreciated, says Sigurdsson.

The flood volume reached an estimated 3 to 4 cubic kilometers of water, with a peak discharge rate of 45,000 cubic meters per second. It damaged or destroyed three bridges, key telephone lines, and the only road running along Iceland's southern coast. Damage totaled U.S. $15 million, a considerable burden for Iceland's 250,000 people, says Sigurdsson. On the other hand, nutrients washed into the Atlantic by the flood should improve future cod stocks.