Embargoed until 2 p.m. EDT
NSF PR 02-74 - September 19, 2002
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. According
to an international team of geoscientists river cutting
and erosion keep the heights and widths of uplifted
mountain ranges in a steady state. The team reports
the results of nearly two years of monitoring in this
week's (Sept. 20) issue of Science.
"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 is 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."
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