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Massive rock slide scars Mount Hood

09/01/01
by RICHARD L. HILL of the Oregonian - used with permission



Darryl and Nancy Lloyd of Hood River were surprised when they first saw the dark streak down Mount Hood's east side.

The veteran hikers both spotted the mile-long rock avalanche on separate hikes during the past few weeks. Free-lance photographer Darryl Lloyd was fascinated enough to fly above the peak to take close-up pictures.

"It was impressive," said Lloyd, a former mountain-climbing guide. "There are a few house-size boulders on the surface" of the dark avalanche, which contrasts sharply with the gleaming surface of the Newton Clark Glacier that it crosses.

Richard Iverson, a landslide expert with the U.S. Geological Survey, confirmed the new avalanche Thursday after flying over the 11,240-foot volcano.

"This kind of event hasn't happened on Hood in the past couple of decades," said Iverson, a hydrologist with the Geological Survey's Cascades Volcano Observatory in Vancouver, Wash. "It's not as big as it appears, because it laps on top of a deposit from an older, larger rock avalanche that evidence indicates occurred in the 1930s."

No one knows when the avalanche occurred, but Iverson speculates it swept down the mountain in the past two weeks. He said it might have been caused by recent heavy rainfall or the rapid thawing of ice and snow that loosened the crumbly rock.

Scientists don't think an earthquake caused the avalanche, although more than a dozen small quakes -- the largest a magnitude 1.3 -- have been detected on the mountain's southeast side in the past two weeks. Such sporadic quake flurries are common beneath the mountain.

The landslide starts at about the 10,000-foot level and ends just off the edge of the glacier. It's about a quarter-mile wide at its widest point.

Iverson and Lloyd say the slide is much smaller than the massive avalanche that swept down the southeast side of Mount Adams in October 1997.

The rock avalanche isn't similar to the glacial outburst floods that swept mud and debris down Mount Rainier earlier in August. Those debris flows are caused by warm summer temperatures melting glacial ice, with the water accumulating inside or under a glacier before being suddenly released. Debris below the glacier is incorporated quickly into the water, creating a slurry dense enough to float boulders.

In September and October last year, a combination of heavy rain and warm temperatures triggered a flood from the White River and Newton Clark glaciers on Mount Hood. The water destroyed two sections of Oregon 35 east of the mountain.

The most recent event shows that rock avalanches are a hazard on Mount Hood.

"It's a reminder that there is a considerable mass of rock up there that is probably weak enough that something considerably larger could occur," Iverson said. "And these small ones are good for being a head's up for thinking about that."

You can reach Richard L. Hill at richardhill@news.oregonian.com or 503-221-8238.



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