Published online 20 June 2010 | Nature | doi:10.1038/news.2010.306

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Modelling Mars in a Texan torrent

Huge canyon carved out in a week may aid understanding of prehistoric megafloods.

Water raged through the upper spillway during peak discharge.Comal County, Texas

A torrential downpour that carved a seven-metre-deep canyon through the Texan landscape in just a few days is providing valuable insight into the processes that shaped the surface of Mars.

Erosion is usually a slow process, but occasionally rivers rise up in mammoth floods that cut new channels practically overnight.

Some of the biggest and best-studied of these events are thought to have occurred on Mars, and in North America at the end of the last Ice Age, when the Missoula Floods carved the 'Channeled Scablands' of Washington State. All of these occurred long before anyone was around to witness them, leaving scientists with only models of erosion to fall back on.

But in 2002, almost one metre of rain fell on the Texas hill country north of San Antonio in the course of a week. This caused a reservoir on the Guadalupe River to overflow, sending water crashing down an emergency spillway and into a narrow canyon that had previously carried only a small stream.

"The full force of the river went down this creek," says Michael Lamb, a geologist at the California Institute of Technology in Pasadena. "It was plucking out boulders on the scale of a metre."

The flow rate topped 1,450 cubic metres per second. After five days, the new river had ripped a gorge of up to 7 metres deep and up to 12 metres wide into the limestone bedrock.

Reporting in Nature Geoscience1, Lamb and his colleague Mark Fonstad of Texas State University in San Marcos now describe how they used before and after topographic images to calculate how much rock had been worn away.

Solid as a rock?

The duo found that the solid rock had eroded at the rate that would be expected for large chunks of rubble. What seems to have happened, Lamb says, is that rather than abrading away the rock a bit at a time, the current simply plucked out boulders and rolled them downstream.

Pre- and post-flood imagery shows the radically changed landscape.US Geological Survey

Although hydraulic transport of loose sediment — and even several-tonne boulders — is fairly well understood, how rocks break apart is not. "What this study suggests is maybe we don't need to know those things as well as we thought," says Lamb.

However, the boulder-plucking was facilitated by the fact that the limestone was divided by both horizontal bedding planes and vertical cracks, probably from ancient tectonic processes. "If the rock is pre-fractured, it essentially behaves like loose sediment," Lamb says.

About halfway downstream, the canyon bottom changes from limestone to loosely cemented deposits of silt and sand. Lamb found that at this point there was a corresponding change from waterfalls and ledges to streamlined, teardrop-shaped islands. These features have also been seen on a much larger scale in post-megaflood landscapes on Earth and Mars, he notes.

The study goes some way towards verifying the mathematical models used for describing the erosional rates of ancient megafloods. But those floods were much larger than that studied by Lamb and Fonstad, leading to some doubts about how generalizable the Texas findings are.

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"A megaflood is one million cubic metres per second," says Victor Baker, a planetary palaeohydrologist at the University of Arizona in Tucson. The Texan flood, he points out, was only about one thousandth of this size.

Baker says it is a "nice, natural experiment", but points out that the region's limestone easily weathers into a pitted surface that might have made it unusually easy for the current to pluck out boulders.

"It's an important study," he says. "But I think its importance is more for how rivers erode during catastrophic events in normal erosion processes on Earth, and less for Mars and big things like the Channeled Scablands." 

  • References

    1. Lamb, M. P. & Fonstad, M. A. Nature Geosci. doi:10.1038/NGEO894 (2010).

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  • #61237

    I thought evolution was a slow process and took millions of years? Or at lead it in humans I mean.

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