The rugged mountains of Idaho may be washing away much faster than indicated by recent records, according to a new study that estimates the average pace of erosion over many millennia. The reason for the huge gap between modern and long-term erosion rates may be catastrophic events so rare they occur only every 100 or 1000 years--suggesting that geologists have underestimated how quickly mountains can change shape.
Researchers usually calculate rates of soil erosion by such direct methods as measuring the amount of mud and gravel washed into reservoirs. But short-term measurements like these can't provide a reliable picture of how erosion's long-term effects sculpt the landscape, says earth scientist James W. Kirchner of the University of California, Berkeley. So Kirchner and others have developed a method for measuring erosion rates by tallying rare atoms, called cosmogenic radionuclides, that are produced in minerals exposed to cosmic radiation. Where erosion is slow, minerals accumulate higher concentrations of these radionuclides because they sit longer within the upper half-meter of the surface--the layer penetrated by cosmic radiation.
To see how long-term erosion rates stack up against more recent measurements, Kirchner and his team chose the steep mountains of central Idaho where other researchers had measured erosion for decades. They compiled historical records for 32 watersheds of various sizes and analyzed quartz sand from each. The radionuclide concentrations indicated that average rates of erosion during the past 5000 to 30,000 years were about 17 times greater than the past 50 years' documented rates. After ruling out factors such as glaciation or climate change, the team concluded that freak catastrophes such as devastating wildfires followed by floods most likely account for the difference in rates, they report in the July issue of Geology.
Although praising the work for breaking new ground, Ned Andrews, a hydrologist with the U.S. Geological Survey in Boulder, Colorado, has qualms about the study's conclusions. "They're talking about truly extraordinary erosional events," he points out, "and I don't know where the evidence for that is in the geologic record."