Kilauea's dark side wasn't recognized earlier because the old ash and rock deposits had been buried by more recent lava flows.
The telltale materials "didn't stand out," Swanson said. "You had to go searching for them."
If such an eruption occurred today, he added, it would blanket the summit in ash and rocks the size of basketballs.
But the scientists don't see an immanent hazard of a sudden repeat.
"There is certainly no indication that it's going to happen soon," Swanson said.
In fact, he said, volcanologists think Kilauea's crater would have to collapse inward before another such eruption would occur.
"We think the caldera has to be very deep for these big explosions to take place," Swanson said, "and right now it isn't."
More Volcanic Dr. Jekylls?
Other Big Island volcanoes may also have unknown violent sides, the researchers say.
"There are very large tephra deposits on Mauna Loa, the south flanks of Kilauea, and Mauna Kea," Rose said (Hawaii map showing volcanoes).
The sources of these deposits are currently unknown. But, he said, "some are incredibly thick and quite a long distance from the nearest possible source. There must have been some very large explosive eruptions."
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