The Alps have grown substantially during the past 2 million years, but in some locales about two-thirds of that growth has been carved away by glacial erosion, according to a new study which will be published next month in Geology. The evidence is locked in a stalagmite pulled from a cave (main image) in western Austria. First, researchers used uranium-lead dating to determine the age of the 36-centimeter-tall cave formation (right image), which grew between 2.0 million and 2.16 million years ago. Then, they analyzed the ratios of oxygen isotopes in the stalagmite, which indicate that mineral-rich water seeping into the ancient cave fell on a landscape several hundred meters lower than that overlying the cave today. Finally, they measured the ratios of carbon isotopes in the formation's carbonate minerals, which hint that the cave, now near Earth's surface, was located about 1 kilometer underground when the stalagmite was growing. Altogether, data suggest that the mountains have, on average, grown about 7.5 centimeters each century for the last 2 million years, with erosion trimming about 5 centimeters off the top each century. Most current techniques for assessing uplift and erosion of mountains don't work if the peaks are composed of carbonate-rich rocks, so analyzing ancient cave formations offers hope to scientists seeking to estimate such trends.
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