Weak shale erodes three times faster than hard rock in Colorado mountains

Weaker rock in mountain landscapes wears away about three times faster than harder rock, even on gentler slopes, according to a University of Massachusetts Amherst-led team. Their study, published in Science Advances, suggests that rock type is one of the main controls on mountain erosion and that current estimates of carbon dioxide released from mountainous terrain may be too low.

A cosmogenic method extended to fine-grained rock

The work rests on beryllium-10 (¹⁰Be), a rare isotope that forms inside quartz when high-energy cosmic particles strike rock at the surface. For around four decades, geologists have measured ¹⁰Be in quartz from coarse-grained rocks such as granite and sandstone to calculate how quickly landscapes lower over time.

The approach has been much harder to apply to rocks with very fine grains, shale among them. That gap has restricted comparisons of erosion rates across different rock types within one landscape. Eyal Marder, a postdoctoral researcher in earth, geographic and climate sciences at UMass Amherst and lead author of the paper, and colleagues developed a new way of extracting the isotope from such rocks.

Testing it in the East River Watershed

The team applied the technique to Colorado's East River Watershed, which drains roughly 465 square kilometres of the southern Elk and West Elk mountains. The area is geologically varied, combining very durable igneous rock with much weaker shale laid down by rivers in the distant past.

Weaker sedimentary rock was expected to erode faster than the igneous rock, but the size of the difference was not. According to Marder, the erodibility of shale in the watershed is three times that of the harder rocks.

Rethinking how landscapes are read

The contrast is large enough to extend the period over which a landscape keeps adjusting, potentially for millions of years. It also implies that the fastest erosion may not occur in the steepest, most resistant terrain. Lower, rounder, gentler areas underlain by sedimentary rock can lose material more quickly.

This has a practical consequence for fieldwork. Judging whether a mountain range has reached equilibrium from slope steepness alone may mislead. The researchers argue that it is better to estimate erosion rates from the ¹⁰Be content of sediment carried by the streams draining the region.

Implications for carbon dioxide estimates

Weathering of different minerals can either release or absorb CO₂. Shale releases the gas to the atmosphere, whereas intrusive igneous rocks take it up. If shale is eroding faster and in larger volumes than the igneous rock beside it, the net CO₂ contribution from mountains is likely to be larger than has been assumed. The authors say geologists and atmospheric scientists may therefore have underestimated emissions from mountainous regions.

The source does not give quantitative CO₂ figures, and the study covers a single watershed. Extending the conclusion to global mountain systems would require similar measurements in other ranges with different lithologies, climates and tectonic settings.

What comes next

Marder says the method could be used elsewhere to estimate the effect of erosion on CO₂ emissions over timescales from thousands to millions of years, given information on rock type and enough quartz for the ¹⁰Be measurements. For researchers working on biogeochemical cycles and long-term climate models, the result points to lithology-specific erosion rates as an input that deserves closer attention. The paper is titled "Bedrock erodibility governs transient erosion and CO2 release in alpine landscapes" (DOI: 10.1126/sciadv.aei9275).

Source: Hidden erosion: Study reveals soft mountain rock erodes three times faster than hard rock—with climate implications (Phys.org, 31 October 2026).

Image: AI-generated illustration.