Researchers Show Why Active Mountains Don’t Get Taller

"These mountains grew to 2.5 to 3 miles high over the past few million years and then they stopped increasing," says Rudy Slingerland, a geologist at Penn State University.

Mountain ranges form near the border of two tectonic plates, the large moving sheets of rock that cover the earth's surface. When one plate slides beneath the other, or subducts, a veneer of rocks on the subducted plate is scraped off and piles up to form the mountains. Even though tectonic plates subduct for tens of millions of years, mountain ranges usually stay between 2.5 and 3 miles high and about 75 to 150 miles wide.

This is because the slopes become steeper as the mountains grow in elevation and more material erodes away via landslides, river cutting and other forms of erosion. The higher and steeper the mountains, the greater the slope and the more material transported away to the oceans.

"People of many cultures inhabit the world's high mountain regions, and for these people, understanding unstable landscapes is a key for their survival and for the preservation of fragile mountain ecology," says Herman Zimmerman, director of NSF's earth sciences division, which funded the research. "But the importance of mountain landscapes goes far beyond the immediate region. These earth processes are also critical factors for the water supply of people living in cities and towns many hundred of miles from the mountains."

Adds Slingerland, "The process of river erosion redistributes the mass of the mountain and has significant influences on maintaining steady-state mountain heights and widths."

Slingerland, working with N. Hovius, a former Penn State postdoctoral fellow now at Cambridge University; K. Hartshorn, graduate student; and W. B. Dade, research scientist, also at Cambridge University, looked at the LiWu River in the East Central Range of Taiwan. The researchers monitored the site of the only water gauging station on the LiWu River. The station was established for a small, Japanese-built hydroelectric station 2.5 miles downstream.

The LiWu River originates at 11,500 feet above sea level and drains an area of about 230 square miles of mostly quartzite and schist rocks. The researchers note that the area has a high rate of tectonic uplift, about 2 to 4 miles per million years and approximately 110 million tons of sediment move through the river each year. This is about a tenth of all the sediment that goes into the sea worldwide.

"We measured the elevation of the riverbed to plus or minus two one-hundredths-of-an-inch," says Slingerland. "This really fine measurement allowed us to see how rapidly the water was eroding the riverbed."

The quartzite components of the riverbed eroded about a third-of-an-inch over two wet seasons and the schist eroded a little under a quarter-of-an-inch.

"The first season we were monitoring was quite dry, then in the second season there was a super typhoon, Supertyphoon Bilis," says Slingerland. "We found the wear rates differed between the two years."

During the typhoon year, there was some wear in the river bottom, but most of the wear was higher on the valley walls and in the corners, widening the river's course. During the non supertyphoon year, when rainfall was relatively frequent but of moderate intensity, wear occurred lower in the river valley.

"Looking at the numbers, even for only a few years, indicates that the down-cutting rate fairly closely matches the rate at which rocks move up," says the researcher. Knowing that the river cutting balances the continuous mountain up lifting answered the question of the rate of river cutting, but how that cutting takes place was another question the researchers investigated. "While violent water discharge does pluck blocks of rocks from the riverbeds, it appears to be the abrasion by suspended particles that does most of the down cutting," says Slingerland. "It is like sandblasting a stone building. The tiny particles wear away the surface."