The glaciers that form atop mountains can act like a saw or sandpaper, wearing away material as they slide and preventing the peaks from ascending too high. Until now, that’s been the consensus notion of how glaciers shape mountains. But whatever your tool shop metaphor of choice might be, neither saws nor sanders work if the glaciers don’t move. That might explain what’s happening in the far reaches of southern South America, where, scientists led by Stuart Thomson report in Nature, glaciers are not wearing down the Andes Mountains but are actually protecting them from erosion.
In the more temperate part of the range, from 38˚ to 49˚ south latitude, the glacial grinder has shaved off as much as 1000 meters from the mountains’ peaks, flattened their slopes, and smoothed their surfaces. But farther south, between 49˚ and 56˚ latitude, the mountains have been spared: The peaks are higher—some nearly 4000 meters—and the ridges are much more rugged. [ScienceNOW]
Thomson’s team bases its claims on chemical evidence locked away inside rocks, which can reveal a mountain’s history to scientists. For instance:
When uranium decays to lead, a natural process that happens at a steady rate, the split leaves a tiny track in the apatite that is visible under a microscope. But these tracks are erased when the crystal is heated above 212 degrees Fahrenheit, the temperature of rock 2.5 miles deep in the Earth. Counting the number of tracks lets geologists back-calculate the last time that the rock was that hot, and therefore how long ago it emerged from the Earth. [Wired.com]
The researchers checked their tracks with a different but similar method, checking the helium in the crystals. Both tests gave the same results: In the northern region (which is warmer, remember, because they’re in the Southern Hemisphere), the erosion of the mountains sped up about 5 to 7 million years ago, when the Patagonian glaciers were forming. But in the colder South, the rocks Thomsen’s team found were all more than 10 million years old.
He thinks that the colder climate in the south meant that the glaciers froze onto the rocks. Most glaciers have a thin layer of liquid water at their base, which lubricates them and allows them to move faster. But if it is cold enough this water freezes and the glacier gets stuck in place, where it acts as armour rather than a scouring pad. “They make a very convincing case for the buzz saw taking a break when it got too cold,” says David Egholm of the University of Aarhus, Denmark. This had been predicted by theoretical models, he says, but Thomson’s work shows that it really happens. [New Scientist]
Last summer DISCOVER covered a different study in Nature that said mountain ranges are generally taller near the equator, the logic being that warmer weather means the snowline is higher, and less of the mountain is covered by ice that would wear it away. In a way, this study complements that one: You can go far from the equator and still see tall mountains that aren’t worn down, just as long as it’s cold enough to keep the glaciers stuck in one place.
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Images: Stuart Thomsen / Nature