What goes up must come down. So why are many mountain ranges, such as the Appalachians, still standing tall long after rivers should have eaten them down? Paradoxically, the erosive forces that should wear them away might instead carve them into a stable shape.
"The longevity of ancient mountain belts is an intriguing problem, but not one that has received a great deal of attention," says Kelin Whipple at Arizona State University in Tempe.
Recent research has it that rivers only chew through bedrock if their waters carry lots of abrasive sand and grit. When mountains are being forced upwards by tectonic processes, their slopes are generally steep and liable to collapse in landslides. These provide rivers with abrasive particles, further destabilising the mountainside and in turn causing more landslides.
Now David Egholm at Aarhus University in Denmark and colleagues have modelled the fate of mountains after the tectonic uplift is over.
Rocks on the move
They found that mountain slopes soon erode to become more gentle and less likely to collapse. There are fewer landslides on mountains of that shape, so rivers carry less abrasive sediment and lose their ability to cut through the rock. Erosion no longer wears down the mountain, instead it largely grinds to a halt.
Existing models suggest that a 4-kilometre-tall mountain range would lose half of its height within 20 million years. Under Egholm's team's scenario, it would take more than 200 million years, which is closer to the age of many mountain ranges.
"I must confess I am initially surprised by the result," says Whipple, who was not involved in the analysis. His studies into the fate of mountains suggest erosion may help slow their destruction in a slightly different way.
While mountains are still rising, there is such a glut of sediment entering rivers that it can almost carpet the riverbeds. Whipple says this sediment protects the bedrock beneath the river from erosion once mountain-building ceases.
Journal reference: Nature, DOI: 10.1038/nature12218
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
Have your say
Only subscribers may leave comments on this article. Please log in.
Only personal subscribers may leave comments on this article
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
