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It wasn't the first eruption to occur there, said David Tucker, a research associate at Western Washington University. And it wasn't the last either.

The newly discovered mega-eruption brings to six the tally of ancient volcanoes known to have blown in the Cascades, Tucker said.

"We know there were at least some small volcanoes [at the site]," he said.

"We don't know what kind, because they were obliterated. Now they exist merely as little bits of rock."

The blasts, he added, would have killed all life for several miles around and dumped ash over a vast area downwind.

"These are big eruptions—on the small end of what have been called supervolcanoes," Tucker said.

"If something went off like that today, a long way into British Columbia would be severely impacted." (See Washington State map.)

If it happened in the southern end of the Cascades in Oregon, "thick ash would probably fall in the Midwest [U.S.]."

Tucker's research appears in the March/April issue of the Geological Society of America Bulletin.

The first of the two eruptions that rocked the northern Cascades formed a giant crack, Tucker said.

When the crack widened, the ground on one side dropped like a trap door swinging down. The section of earth fell more than 3,000 feet (1,000 meters).

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Vast quantities of ash then filled the hole, and rivers of ash and superheated air rushed away with enough momentum to flow over surrounding mountains, incinerating everything in their paths.

(See an interactive feature on how volcanoes work.)

The second eruption occurred about a hundred thousand years later, when the upper end of the trap door dropped, forming a roughly rectangular crater measuring 5 miles by 2.5 miles (8 kilometers by 4 kilometers).

The best known ancient eruption to have occurred in the Cascades is the one that produced Oregon's Crater Lake, only 7,700 years ago. (Download a wallpaper photo of Crater Lake.)

"I suspect there are a number of others," Tucker said. "Part of my research in the future will focus on finding more."

He compared the Cascades supervolcano event to the eruption of the Indonesian island of Krakatau (or Krakatoa), which exploded in 1883, killing 36,000 people.

"Actually, Krakatoa was small compared to [the ancient Cascades eruption]."

Krakatau ejected approximately 10 cubic miles (25 cubic kilometers) of rock and ash when it blew its top.

The two eruptions described by Tucker involved a total of about 30 cubic miles (130 cubic kilometers) of magma "in a period of probably a day or two [each]," Tucker said.

Since then the site has been quiet, as volcanic activity has shifted 15 miles (25 kilometers) southwest, to the present location of 10,778-foot (3,285-meter) Mount Baker, near the city of Bellingham.

No one knows if Mount Baker, or another peak in the Cascades, might someday produce a similar mammoth eruption, Tucker said.