Researchers show why active mountains don’t get taller

Active mountain ranges like the Olympic Mountains, Taiwan Central Range or the Southern Alps are still growing, but they are not getting any taller. River cutting and erosion keep the heights and widths of uplifted mountain ranges in a steady state according to an international team of geoscientists.

“These mountains grew to 2.5 to 3 miles high over the past few million years and then they stopped increasing,” says Dr. Rudy L. Slingerland, professor of geology and head of Penn State’s geosciences department. “We assumed that various erosional forces were compensating for the constant uplift of the mountains, but few observations have been available to validate this assumption.”

Mountain ranges form near the border of two tectonic plates. 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 is transported away to the oceans.

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

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. They report the results of nearly two years of monitoring in today’s (Sept. 20) issue of Science.

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.

“It just so happened that 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 Penn State 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.”