Blown away: since the wakeup call at Mount St. Helens, geoloogists have realized that collapsing volcanoes are far commoner than ever imagined.

Publication: Natural History

Publication Date: 01-OCT-05

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COPYRIGHT 2005 Natural History Magazine, Inc.

A priest by the name of Tsurumaki was relaxing with friends at an enclosed hot spring in central Japan on a clear morning, July 15, 1888. The hot spring was high on the flank of Mount Bandai, a volcano that had undergone a series of earthquakes the preceding week. But it is unlikely that the people on and around Bandai were concerned. The volcano was densely forested and had not erupted for nearly a century. Besides, the mountain had been steaming, nothing more, for as long as anyone could remember.

Then, at about eight o'clock that morning, the earth suddenly shook. Tsurumaki and his fellow bathers rushed outside. While they anxiously tried to find out what had taken place, a powerful explosion rocked Ko-Bandai, the youngest of an overlapping group of small volcanoes that collectively make up Bandai volcano. As the sky turned pitch-black, rocks and stones began raining down around them, and Tsurumaki and his friends fled.

What the priest did not realize was that he had had a front-row seat for one of nature's most dramatic events--the catastrophic collapse of a volcano. Ko-Bandai volcano collapsed in a massive landslide that created a horseshoe-shaped crater, 5,000 feet by 6,500 feet wide, on the northern flank of the mountain. A jagged cliff was left near where the priest had been relaxing. Almost 1.6 billion cubic yards of Ko-Bandai's former summit had collapsed, causing a high-speed avalanche that traveled seven miles, overwhelming several villages and killing 461 people. The avalanche also left massive piles of volcanic debris that covered broad mountain valleys, dammed up drainages, and eventually created five large lakes.

Geologists from the Imperial University of Tokyo immediately studied the eruption in detail. Their thorough report, however, remained relatively unknown to most volcanologists outside Japan for nearly a century. Not until another volcano exploded on the opposite side of the Pacific Rim of Fire, in Washington state, did geologists fully realize the significance of what had happened at Bandai.

The eruption and collapse of Mount St. Helens in 1980 fundamentally changed volcanologists' understanding of how volcanoes work; it led to a reassessment of the role of catastrophic collapse in shaping the Earth's volcanoes. Among Earth's...

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