Rainfall May Affect Geological Structure of Pacific Northwest Mountain Range

Heavy rainfall causes both higher surface erosion rate and upheaval of underlying bedrock in the Washington Cascades mountain range of the Pacific Northwest, according to a study published today in the journal Nature.

Using a low temperature radioisotopic dating method that determines when and how fast erosion brings bedrock towards the surface of the Earth, Peter Reiners, assistant professor of geology and geophysics and lead author of the study, and his co-researchers, found evidence to support long-standing theories about the interplay of climate, erosion, and tectonics.

“People have long thought that the scale and pattern of rock uplift is mostly controlled by deep, plate-tectonic forces,”Reiners said. “Based on our findings, it’s not too much of a stretch to say the pattern of bedrock uplift is closely tied to climate, through erosion.”

The pattern of rainfall may cause bedrock to be pulled up towards the Earth’s surface faster in some places than others, he said.

This data is the first convincing evidence for such effects on a mountain range scale over a period of millions of years.

“Geologists usually think of erosion wearing away mountains,” said David Fountain, program director in the National Science Foundation’s division of earth sciences, which funded the research. “These results, however, show that erosion can be an important player in uplift of mountain ranges, especially in mountainous regions that receive heavy precipitation.”

The rainfall is heavy in parts of the North American Pacific Northwest because moist air moving east from the Pacific rises and cools as it encounters the ranges, dumping large amounts of rain and snow on the west side of the Cascades, where it rains about 10 times more than most places in Washington.

Co-authors include Todd Ehlers of the University of Michigan and Sara Mitchell and David Montgomery of the University of Washington.

Citation: Nature, Vol. 346: pp-pp 645-647

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