One of the Andean canyons formed by strong winds.
Noah J. Finnegan
The beauty of a ravine is mainly the artistic work of that master sculptor, the river. Of course, rivers wouldn’t exist without gravity, which also pulls down material from canyon walls. Different types of weathering – the reduction of bedrock to loose sediment – also contribute to facilitating the river’s work. But there’s another force not usually on the thanks list at the canyon awards that might deserve to be there: wind.
In dry places, wind erosion plays an important role, but its effectiveness is limited. If you come across a feature as dramatic as a canyon, you can bet it was put there by water. The contribution of wind is generally considered to be minor. Jonathan Perkins and Noah Finnegan of the University of California Santa Cruz, and Shanaka de Silva of Oregon State have found a way to put that idea to the test.
Wherever you find water, you generally find wind, so it’s a challenge to tell their effects apart. Setting up an experiment and waiting a million years for clear results is not a proposition likely to make money. But on the dry western slope of the Andes in northern Chile, the researchers found a natural experiment that began four million years ago.
After an eruption deposited a blanket of rocky, pyroclastic material that solidified, erosion began to alter the rock’s fresh surface. The northern part of the deposits, however, looks distinctly different, with a series of long, straight, parallel ravines. The southern part, on the other hand, has only a few amphitheatrical bowls at the bottom – the first nibble of what could develop into a ravine. The mouths of the northern canyons are also about 100 meters wider than the southern canyons. There is a similar drainage area to provide water to both the north and south when it rains. There is only one distinct difference between the two areas: a rock back protects the southern half from the steady local winds.
As they walked across the floors of those northern canyons, the researchers saw clear signs of wind erosion. Where boulders fall to the ground, a protected “shadow” of softer material lies on their leeward side. (The water, meanwhile, is flowing in the opposite direction.) The boulders themselves slowly disintegrate, the larger, harder chunks of volcanic rock standing out as the surrounding material is sandblasted away.
The researchers used a model to simulate surface wind speeds over the area, which confirmed that the southern canyons are significantly sheltered. There is a stretch in the middle with medium winds, and the canyons there are medium in shape as well. It’s a nice correlation.
Comparing these canyons with fast, medium and slow winds reveals quite a serious change that can seemingly be attributed to wind erosion. The fast-wind canyons have lengthened by an average of 1.7 ± 0.7 millimeters per year over the past 4 million years, while the sheltered southern canyons have only grown at 0.1 ± 0.1 millimeters per year.
The slope at the upstream end of the canyons – called the “knickpoint” – was also significantly different. The slow-wind canyons have quite steep slopes over that upstream rim, while the fast-canyon knickpoints are gradual and fairly smooth. To test the idea that wind erosion might be responsible, the researchers modeled wind shear again, this time using different knickpoint profiles.
Those simulations showed that smooth, gradual profiles were effectively streamlined, minimizing wind shear. At the edge of a steep buckling point, however, wind shear could be three times greater, focusing erosion.
All this suggests that wind erosion can be surprisingly significant in certain environments — so much so that it can modify important features like canyons.
That’s not just of interest to Earth-focused geologists. The researchers note that Martian canyons have had some three billion years of wind erosion since they last saw running water. Indeed, some of those canyons have features in common with the Andean canyons that were studied. If you want to use those canyons to draw accurate conclusions about the ancient surface water that carved them, you have to account for the subsequent wind erosion.
To do that Good will take more careful work to untangle the two erosive agents in dry places here on Earth.
Natural Geosciences2015. DOI: 10.1038/NGEO2381 (About DOIs).