Slip-sliding away

Natural History, Sept, 2004 by Robert Anderson

I'm predicting a landslide in November, but not the political kind. I'm talking about the sudden shift of hundreds or thousands of tons of rock and soil, and that kind happens nearly every day. To catch up on the latest tolls in death and destruction from major landslide events, go to the U. S. Geological Survey's (USGS) main site on the subject (landslides.usgs.gov), and click on "Recent Landslide Events."

In some places, such as the San Francisco Bay area, weather and geology seem to conspire with gravity to bring down mountainsides on a regular basis. You can get the details from a section of the USGS Web site dedicated to the causes and effects of El Nino (walrus.wr.usgs.gov/elnino/). Scroll down the page to the "Landslides" section, click on "Potential San Francisco Bay Landslides," then click on "fly-by movies." Download a cross-sectional view of a "slow-moving Failure" and watch it undermine a typical California hillside home, or take a virtual flight over Marin County or East Bay Hills to get some idea of how prevalent landslides have always been in the region. Beneath the fly by features, you'll find movies of two actual slides from the 1996-97 rainy season.

Many things can set critically unstable rock in motion. On May 18, 1980, about a mile below Mount Saint Helens, a magnitude 5.1 temblor triggered the largest landslide worldwide in the past century (see pubs.usgs.gov/ publications/msh/climactic.html and click on "Debris avalanche"). The mountain shed 0.7 cubic miles of rock, uncorking the more infamous eruption.

Human activities sometimes set the stage for catastrophic landslides. Logging is a good example. Steep slopes denuded of trees and cut with new roads don't stay put for long. The Sierra Legal Defence Fund has issued a report on the problem in British Columbia, titled "Going Downhill Fast" (www.sierralegal.org/reports/land slide_toc.html). The deadliest and most infamous landslide in Canadian history, the Frank Slide of 1903 in southwestern Alberta, may have been triggered instead by badly regulated coal mining beneath the unstable crest of Turtle Mountain. The resultant landslide brought some 90 million tons of rock down on the sleeping town of Frank, and claimed at least seventy lives (see www.canadian geographic.ca/Magazine/ma03/alacarte. asp and "The Day the Mountain Fell," at www3.sympatico.ca/goweezer/canada/ frank.htm).

The greatest potential for disaster, however, may lie offshore. Enormous blocks of volcanic islands or continental shelf can give way and travel miles underwater. As the landslide material comes to rest on the deep-sea floor, the sudden displacement of a huge vertical column of seawater can kick up deadly tsunamis across wide areas. The Monterey Bay Aquarium Research Institute provides a good explanation of how such submarine landslides have shaped the Hawaiian Islands (www.mbari.org/ volcanism/Hawaii/HR-Landslides.htm). To find out more about submarine landslides, visit the site of New Zealand's National Institute of Water and Atmospheric Research (www.niwa. co.nz/pubs/wa/09-1/avalanche.htm).

Earth, of course, is not the only planet where geologic processes combine to tear down and level the surface material. At "Geology of Mars" (www.lukew.com/marsgeo/index.html), a Web site created by Albert T. Hsui, a geologist at the University of Illinois at Urbana-Champaign, you'll learn about the "mass movements" on the Red Planet. Many of them cluster around the huge Valles Marineris, a continent-size canyon that gives gravity, some steep cliffs to work with.

ROBERT ANDERSON is a freelance science writer living in Los Angeles.