"The top of a hill is not until the bottom is below," wrote the poet John Ciardi. "And you have to stop when you reach the top for there's no more UP to go." Fair enough, but how do you tell how high a mountain used to be, before wind and rain whittled it down? Now researchers have come up with a new method to reconstruct the heights of long since vanished peaks.
Geologists already have a few tricks for taking the measure of eroded mountain ranges. If they know when volcanoes began to spew lava or when tectonic plates collided, researchers can use computer models of mountain growth and erosion to roughly reconstruct the original mountain topography. But these methods work reasonably well only for tree-covered volcanic ranges, where researchers can estimate a peak's age by fossilized pollens or trapped volcanic gas bubbles that shrink as each successive eruption adds a new layer to the growing peak.
To develop an approach for other kinds of mountains, earth scientists Page Chamberlain and Matthew Poage of Dartmouth College in New Hampshire turned to prehistoric rain shadows. When ocean storms bump into mountains, raindrops burdened with heavy isotopes of oxygen and hydrogen are the first to precipitate. The higher the altitude, the more oxygen-18 and deuterium fall out. So the concentration of these isotopes preserved in the ancient clays in the mountain's rain shadow will depend on the mountain's height. By testing many clay samples of known ages from both sides of the mountain range and comparing their ratios of heavy isotopes, Chamberlain and Poage found that the Southern Alps of New Zealand rose abruptly by 2 kilometers 5 million years ago. California's Sierra Nevada range, on the other hand, has remained at nearly the same height for the last 16 million years, they report in the February Geology.
"This is a great idea," says Dork Sahagian, a University of New Hampshire earth scientist who pioneered the pollen count method of measuring mountain heights. Sahagian liked the idea so much that he has invited Chamberlain on an expedition next summer to the Colorado plateau to compare all three methods. And after that, Chamberlain says, "I want to measure how big the Appalachians were."