Yale Bulletin and Calendar

April 6, 2001

Volume 29, Number 25

Yale geologist Mark Brandon has been studying the growth and erosion rates of the Alps for about a decade. This photo by Brandon shows a valley gorge in the core of the Glarner Alps, north of Linthal in eastern Switzerland.

The Alps have stayed same size
for 15 million years, study finds

Steady tectonic motions are causing the European Alps to grow at the same rate that they are eroded, a study by Yale researchers concludes.

The result is that this classic mountain belt has maintained its present size for more than 15 million years.

"In geology it has been an issue for at least 100 years whether mountains are formed during short-lived events or persist over long periods of time," says Mark Brandon, associate professor of geology and geophysics and coauthor of a study recently published in the journal Geology. "Our results indicate prolonged steady growth."

The researchers arrived at their finding by determining the cooling ages of zircon, one of the minerals shed from the Alps in the last 15 million years. Zircon retains a record of the last time it cooled to temperatures below about 240šC and therefore tells of how mountains evolve with time.

"We can judge the rate of erosion by looking at the amount of time since cooling from 240šC until the rock reached the surface," Brandon says. "In each sample, we date many zircon grains with each zircon providing an estimate of the long-term erosion rate in the mountain range."

The samples came from sandstone that was shed off the Alps and deposited in nearby basins in northern Italy. The researchers also analyzed sediment from modern rivers draining the Alps.

"What we find is that the lag time (the cooling age minus the age of erosion of the zircon grains) distribution is constant over the last 15 million years," Brandon says. "This means that the average erosion rate in the Alps has been constant over that period of time. A steady erosion rate is considered strong evidence of a 'steady state' balance between uplift and erosion."

The term "steady state" means that the average volume of the mountain range has remained the same, despite the fact that huge volumes of sediment were being shed from the Alps into rivers draining north and south. Thus, the balance between uplift and decay of the mountain range was maintained.

Previous studies of exhumation in the Alps have focused on reconstructing time-temperature-depth histories for currently exposed bedrock. Brandon and his colleague John Garver of the geology department at Union College in Schenectady, New York, have spent about 10 years exploring this novel approach, which relies on using the sedimentary record to reconstruct the evolution of nearby mountains. Their approach has given new insight into how erosion and faulting work to shape and sculpt mountains.

"Exhumational steady state has been observed in some modern mountain belts, such as Taiwan, the Southern Alps of New Zealand and northern Cascadia in the northwestern United States," Brandon says. "Erosion dominates exhumation in those cases. The Alps appear to be an example where erosion and tectonic exhumation are operating together in an approximately steady-state fashion."

Other researchers on the project were Matthias Bernet of the Department of Geology and Geophysics at Yale, Massimiliano Zattin of the University of Bologna in Italy, and Joseph Vance of the Department of Geological Sciences at the University of Washington in Seattle.

-- By Jacqueline Weaver

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