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Researchers study the role of rock strength in landscape evolution

At first glance, landscapes like the Great Plains and the Rockies may seem unchanging, but over geological time scales, they're dynamic systems. Plate tectonics raise mountains, while erosion—driven by glaciers, rain and wind—wears them down. But there's an often-overlooked factor in this process: the rock itself.

A team of researchers led by Lindsay Schoenbohm, a professor of Earth science at U of T Mississauga, is studying how rock strength influences erosion to better understand landscape evolution. The findings, in Science Advances in January, show that differences in rock type alone can drive a 20-fold variation in erosion rates.

"The hypothesis is that, if rock strength matters, we ought to see a correlation between rock strength and erosion rates," says Schoenbohm. "Harder rocks should equal slower erosion rates."

The team, which includes researchers from the University of Vermont and the Universidade Federal do Rio Grande do Sul in Brazil, studied a 200-kilometer stretch of land along the southeast coast of Brazil. Researchers chose this region because climate and tectonic activity are relatively stable, allowing them to home in on the effect of rock strength.

The study measured erosion rates by analyzing river sediment and the amount of beryllium-10, a radioactive isotope that forms when cosmic rays interact with Earth's atmosphere and surface. Ph.D. student Mauricio Haag likens this process to getting a tan: the longer a rock face is exposed, the more beryllium-10 accumulates in its mineral grains—just as prolonged sun exposure darkens the skin.

To measure rock strength, team members used a Schmidt hammer, a device that measures the "rebound" of a spring-loaded mass as it impacts a surface. The stronger the material, the more it rebounds—similar to how a basketball bounces higher on an asphalt driveway than on a grass lawn.

The study "really demonstrates the importance of rock type in landscape evolution," says Schoenbohm.

The researchers compiled their findings into a publicly available database of rock-strength measurements, nicknamed Thor, which was released in December.

The data could prove useful for anyone studying climate or tectonics who wants to make sure they're not overlooking the effect of lithology, Schoenbohm says.

"We created it for people like us, initially—people who are looking at erosion rates in landscapes," she says. "But I think the applications could be extremely broad."