Weak rock in mountain landscapes wears away about three times faster than hard rock, even on gentler slopes, according to a study led by the University of Massachusetts Amherst.
The work was published in Science Advances under the title "Bedrock erodibility governs transient erosion and CO2 release in alpine landscapes". Its lead author is Eyal Marder, a postdoctoral researcher in earth, geographic and climate sciences at UMass Amherst.
The team studied the East River Watershed in Colorado, roughly 465 square kilometres draining the southern Elk and West Elk mountains, near the ski area of Crested Butte. Phys.org carried the university's own account of the findings on Friday.
"Shale's erodibility is three times that of the harder rocks in the East River Watershed," Marder said.
The real achievement is not the ratio but the measurement. Erosion rates are established by extracting beryllium-10 from rock, a technique that has worked for about four decades on coarse-grained material such as granite and sandstone, and failed on fine-grained stone such as shale. The team developed a way of extracting beryllium-10 from the fine-grained rock as well.
That gap mattered because the softest ground was also the least measurable ground. The study's implication is that the fastest erosion in a mountain range may be happening not on the steep, resistant peaks that attract attention, but in the lower, rounder sedimentary terrain below them.
It also unsettles the carbon arithmetic. Shale weathering releases carbon dioxide, while intrusive rocks absorb it, so the authors suggest that geologists and atmospheric scientists may be underestimating how much carbon dioxide the world's mountains give off.
"When we look at landscapes, we are only seeing a snapshot in time," Marder said. He has urged colleagues to "Scale our work up from the Elk and West Elk mountains to all the mountainous regions of the world".
This newspaper wishes to record its admiration for the technique, and then to restate the headline finding in full. Rock that is three times more erodible erodes three times faster.
Erodibility is the property of being eroded at a given rate. The study establishes that rock with a high value of that property has a high value of that property, and does so to within a factor of three, in Colorado, over the long term.
Our analysts regard this as the most secure result in the earth sciences this week, on the grounds that it could not have come out any other way. A landscape in which highly erodible rock eroded slowly would not have been a surprising finding. It would have been a contradiction, and would have had to be reported to a dictionary rather than to Science Advances.
The Department of Unnecessary Research has begun scaling the work up, as invited. Preliminary results from the global dataset indicate that soft things are softer than hard things in every mountain range examined so far, with no regional exceptions.
The beryllium-10 method deserves better than this, and gets it. Before the technique existed the proposition was unmeasured; it is now measured. Science does not advance by discovering surprising things. It advances by finally putting a number on the thing the word was already doing.
One matter remains unresolved, and Marder has named it himself: a landscape is only a snapshot in time. Our analysts have requested the other frames, and have been informed that they are currently eroding.