Wearing out mountains

We often think of mountains as immoveable, static, constant – fixed in time and space. Nothing could be further from the truth according to a team of US and New Zealand geologists, who have discovered that New Zealand’s Southern Alps are eroding at a pace that sets a geologic speed record – although note that, as in all things in geologic time, it is still way slower than watching snails race.

Soils forming twice as fast as previously thought

New measurements made by the team in the rapidly uplifting steep Southern Alps indicate that the rocks are transforming into soil more than twice as fast as previous studies have suggested is possible. Using chemical tracers, they measured soil production rates at the giddying speed of up to 2.5 mm per year. The process is important, as the researchers say that rapidly eroding mountain ranges account for at least half of the total amount of the planet’s weathering and sediment production, although they occupy just a few percent of the Earth’s surface.

The global carbon cycle

This erosion process forms part of the carbon cycle (which is thought to regulate global climate over long time scales) whereby tectonic forces push up the Earth’s crust to form mountains, which are acted on by chemical and physical forces to crumble, and the sediments are washed to the sea by rivers and rainwater, eventually settling to the bottom to form new rock.

The chemical weathering process in erosion consumes CO₂, leading the authors to infer that these rapid rates have an influence over global climate. “These high weathering rates support the view that mountains play a key role in global-scale chemical weathering and thus have potentially important implications for the global carbon cycle,” the researchers write in their published paper.

In a press release from the University of Washington, lead author Dr Isaac Larsen (now a postdoctoral researcher at the California Institute of Technology in Pasadena) said, “This work takes the trend between soil production rates and chemical weathering rates and extends it to much higher values than had ever been previously observed.”

“A couple of millimetres a year sounds pretty slow to anybody but a geologist,” says co-author and University of Washington Professor David Montgomery. “Isaac measured 2 millimetres of soil production a year, so it would take just a dozen years to make an inch of soil. That’s shockingly fast for a geologist, because the conventional wisdom is it takes centuries.”

Wearing out geologists

Dr Larsen describes the Southern Alps as “an extremely rugged mountain range”, with rainfall in the wettest parts measured at just over 10 metres per year and slopes of about 35°.

To collect samples, he and co-author André Eger, then a graduate student at Lincoln University, were dropped from a helicopter onto remote mountaintops above the tree line. They would hike down to an appropriate test site and collect some 10 kilograms of soil each, and then trek the samples back up to their base camp. Sometimes, the dense bush and heavy 10 kg soil samples slowed uphill progress to less than 200 m per hour. The pair stayed at each of the mountaintop sites for about 3 days.

“I’ve worked in a lot of places,” says Dr Larsen. “This was the most challenging fieldwork I’ve done.”

Measuring the isotopes

The research team measured the amount of beryllium-10 – an isotope that forms only at the Earth’s surface by exposure to cosmic rays – in the soil samples. Those measurements showed soil production rates from the bedrock on the ridgetops ranging from 0.1 to 2.5 mm per year, with the rate decreasing exponentially with increasing soil thickness.

This means soil production and weathering rates continue to increase as the landscape gets steeper and erodes faster. This is contrary to previous studies, which suggest that soil weathering declines as erosion from uplift activity increases.

The researchers say this suggests that other very steep and wet locations such as the Himalayas and the mountains in Taiwan may also have very fast soil formation.

The role of plants

The researchers believe plant roots may be partly responsible. The mountain landscape in the Southern Alps is covered with low, dense vegetation. The roots of those plants reach into cracks in the rocks, helping break them apart and expose them to rainwater and chemical weathering.

“This opens up new questions about how soil production might happen in other locations, climates and environments,” Larsen said.

The research was published in the 7 February 2014 issue of Science magazine.