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March 17, 1997

Research Update: Earth sciences

Not just your everyday hill of beans

By Theobolt Leung

When UCSC geologist Robert Anderson and graduate student Alex Densmore (photo) thought up their small experiment, they didn't figure it would amount to much.

Densmore took a plastic box filled with red beans and slowly lowered one wall, watching how they fell out. The two simply wanted to see how bean slides would shape a bean hill slope. But on January 17, they published the results of their work in the journal Science, changing the way geologists think about landslides and the way the slides shape river hillsides.

For the past decade, physicists have modeled landslides by dropping sand onto a plate one grain at a time until the mound slope avalanched. But the sand grains are symmetrical, unlike the jagged and often elongated chunks of bedrock that come down in real slides. Last year a team of Norwegian physicists tackled that problem by instead using rice. However, that experiment failed to address a fundamental issue: Mountain slopes aren't created by dropping grains of any sort.

So Anderson and Densmore, inspired by the rice work, devised the bean experiment. The constantly descending wall mimicked more realistically the way a river cuts into the foot of a mountain, and the red beans had dimensions more like the boulders seen in slides.

Although it was a simple experiment, said Densmore, "I was really impressed by what came out of it." As he lowered the wall one or two beans would fall at a time, leaving behind a short, steep slope, or inner gorge. A large slide would occasionally clear the slope, and the creation of the inner gorge would start again. Said Anderson, "I hadn't expected to see these inner gorges."

Inner gorges are a common sight in river canyons, but geologists had attributed them to historical bursts in the river erosion rate. The new study contests that theory by showing a way that inner gorges can form on hillsides even when the river's incision rate is constant. "We now have an alternative explanation that completely flips on its head the old interpretations," Anderson said.

Coauthors on the report were UCSC graduate student Brian McAdoo and geologist Michael Ellis of the University of Memphis.

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