Kilauea, the sleepy Hawaiian volcano famed for its quiet lava flows, could awaken into violent explosive eruptions at any time, geologists on the Big Island warn.

Researchers at the Hawaiian Volcano Observatory - where 5,000 mainland visitors a day come to see the lava - have uncovered detailed evidence that during three long periods over the past 2,500 years, the summit of the shaking mountain has blasted out fiery rocks, steam and carbon dioxide again and again.

It's bound to happen again, but the scientists say they can't predict when.

Donald Swanson of the U.S. Geological Survey and director of the observatory, along with his colleagues, are reporting on their research into the volcano's intermittent activity this week at the annual meeting of the American Geophysical Union in San Francisco's Moscone Center.

Quiet two centuries

"Our research shows that Kilauea is a dangerous explosive volcano for long periods of time, alternating with periods dominated by gentle lava flows," Swanson said during a briefing for reporters.

Continuous and often spectacularly scenic lava flows, interrupted only occasionally by modest explosive eruptions, have marked the volcano's activity for the past 200 years, but the 300 years before that saw one explosive eruption after another, he said.

Swanson and his colleagues have just completed a fresh look at Kilauea's worst eruption in history, which killed uncounted numbers of Hawaiian people in November 1790.

They described what's known about that eruption in a poster presentation for the meeting here.

According to sketchy accounts in local folklore, and a missionary's description from 1843, the volcano's summit exploded as a local chief named Keôua led three groups of warriors and their families across the summit on their way to battle King Kamehameha, Keôua's cousin, for supremacy over the island.

One of the three groups was annihilated, there were casualties in another, and the third suffered only a few injuries.

Remains of the bodies found afterward suggests that a dilute mixture of hot gas and volcanic ash moving at hurricane speed engulfed the victims, Swanson said.

Keôua survived, but Kamehameha later won the battle and went on to unite all the islands under his reign, according to Swanson and his colleagues at the University of Hawaii.

Eruptions and flows

The geologic record shows that Kilauea's activity has been marked by a period dominated by frequent lava flows from about 2,500 to 2,200 years ago, followed by a long period of explosive eruptions that continued for about the next 1,200 years; then another 500 years of lava flows, followed by about 300 years of eruptions, and then by more lava flows that continue today.

More than 100 radiocarbon observations, based on charcoal from the mountain's burned vegetation, have established the dates, Swanson said.

The lava flows are building up the volcano's summit now, he said, and each period of violent eruptions creates and deepens the volcano's circular crater, called the caldera. When even a small eruption is about to occur, he said, the bottom of the caldera sinks slightly. So its level is recorded daily as a possible sign that an eruption is due.

"The good news is that we are currently in a period of frequent lava flows, and the hazard of explosive eruptions is small," Swanson said. "The bad news is that we don't know when the next period of deep caldera and explosive eruptions will start. We know too little to estimate recurrence intervals."

The Geophysical Union meeting continues through Friday.

E-mail David Perlman at dperlman@sfchronicle.com.

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