Hidden erosion: Study reveals soft mountain rock erodes three times faster than hard rock—with climate implications

AMHERST, Mass. — A University of Massachusetts Amherst-led research team has discovered that weaker rock in mountain landscapes erodes three times faster than harder rock–even on gentler slopes–and is one of the main attributes governing mountain erosion. The finding also challenges our current understanding of the relationship between erosion and the release of carbon dioxide, and suggests that geologists and atmospheric scientists may be underestimating the CO₂ contributions coming from the Earth’s rocky places.

The research, recently published in Science Advances, depends on a new technique relying on common quartz grains, the rare isotope beryllium-10 (known as ¹⁰Be) and cosmic rays from exploded stars.

Over the course of the universe’s nearly 14 billion years, countless stars have exploded, and when they do, they blast out showers of cosmic shrapnel in the form of high energy particles. These cosmic particles have been raining down on the Earth for uncountable ages in at a constant rate. When the particles penetrate rocks on the surface, ¹⁰Be is produced within quartz. For four decades, geologists have been able to harvest these quartz grains from rocks like granite and sandstone, in which the quartz grains are comparatively large, to test all sorts of hypotheses.

But in rocks where the grains are too fine, such as shale, the conventional ¹⁰Be technique hasn’t worked. And that has limited geologists’ abilities to accurately pinpoint things like erosion rates across a broad spectrum of rock types.

That is, until Marder and his colleagues developed a new method for harvesting the rare isotope from such rocks.

“When we look at landscapes, we are only seeing a snapshot in time,” says Eyal Marder, a postdoctoral researcher in Earth, geographic, and climate sciences at the University of Massachusetts Amherst and the lead author of the new paper.

But researchers want to understand the full geological movie from start to finish in order to understand the land’s lifecycle. Marder and his team studied the Colorado’s East River Watershed, which drains approximately 465 square kilometers of the southern Elk and West Elk mountains—jagged peaks whose most famous landmark, Crested Butte, is revered as one of the U.S.’s premier ski areas. The sheer alpine topography is notable for its diverse geological composition: a mix of incredibly durable igneous rock and much weaker shale, deposited by rivers eons ago.

Their new technique allowed Marder and his team to make a few surprising discoveries. As expected, weaker sedimentary rocks erode faster than harder igneous ones. What was surprising is how much faster they erode.

“Shale’s erodibility is three times that of the harder rocks in the East River Watershed,” says Marder.

It’s so fast that the contrast with the slower-eroding rocks can prolong landscape erosion over millions of years. This also means that the fastest erosion might happen not in steeper, more resistant parts of the mountains, but rather in lower, comparatively gentle, rounder, sedimentary ones.

This shift means that the best way to evaluate whether or not a landscape has reached equilibrium is not by visually gauging its pitch, but also estimating erosion rates from the amounts of ¹⁰Be in the sediment carried by the streams that drain mountainous regions.

Marder and his team have also exposed a missing piece in global climate models. Geologists recognize that weathering of different minerals on the surface can either release or absorb CO₂. But the fact that in mountainous landscape, shale, which releases CO₂ to the atmosphere, is eroding faster and at a larger magnitude than intrusive rocks, which absorb CO₂ from the atmosphere, means that geologists and atmospheric scientists may have underestimated the CO₂ contributions coming from mountains.

“Scale our work up from the Elk and West Elk mountains to all the mountainous regions of the world, and getting erosion rates and their equivalent impact on CO₂ release right has a marked impact on what we know about biogeochemical global cycles and climate change,” says Marder. “Give us the type of rock and enough quartz for our ¹⁰Be measurements, and we can estimate what effect it will have on CO₂ emissions from thousands to millions of years.”

 

Contacts: Eyal Marder, emarder@umass.edu

                 Daegan Miller, drmiller@umass.edu

 

About the University of Massachusetts Amherst 

The flagship of the commonwealth, the University of Massachusetts Amherst is a nationally ranked public land-grant research university that seeks to expand educational access, fuel innovation and creativity and share and use its knowledge for the common good. Founded in 1863, UMass Amherst sits on nearly 1,450-acres in scenic Western Massachusetts and boasts state-of-the-art facilities for teaching, research, scholarship and creative activity. The institution advances a diverse, equitable, and inclusive community where everyone feels connected and valued—and thrives, and offers a full range of undergraduate, graduate and professional degrees across 10 schools and colleges and 100 undergraduate majors.  

 

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