Residue from Ancient Rain May Redate Sierras

Scientists studying raindrop residue say California's Sierra Nevada mountains may date nearly to the dinosaurs' era. That conflicts with evidence suggesting the mountains are much younger.

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California Sierra Nevada Mountains may not be the youngsters that they're reputed to be. A study of ancient raindrops suggest the mountain range may have stood tall 60 million years ago, which would make it nearly as old as the dinosaurs. This provocative study is published in the latest Science magazine.

NPR's Richard Harris reports.

RICHARD HARRIS reporting:

The craggy and rugged Sierra Nevada have drawn hikers and campers were well over a century. And they've also held a deep allure, a mystery for geologists like Craig Jones.

Mr. CRAIG JONES (Geologist, University of Colorado): One of the problems that had always plagued geologists over most of the 20th century was the rock was made over 80 million years ago and yet the mountain range appeared to be young.

HARRIS: One hint is the deep and narrow canyons, which are utterly unlike what you'd find in, say, the old and worn down Appalachian Mountains. And Jones, at the University of Colorado in Boulder, says the landscape itself provides hints about how the mountain range took shape, apparently not so very long ago.

Mr. JONES: John Muir called it the Range of Light. And it's the Range of Light because so much of it is quartz and feldspar. And so, in fact, when you look at it, even in late summer, it's this bright white mountain range across the Central Valley.

HARRIS: But geologists know that the original rock here wasn't only made up of this light material. At one time, long ago, it included so much heavier and darker material.

Mr. JONES: And so the dark stuff, the heavy stuff, has to go somewhere else.

HARRIS: Jones has evidence that the dark stuff had formed a layer beneath the light colored rock. And in recent geological times, about three and a half million years ago, it broke free and fell deep into the earth's mantel. That in turn made the mountains rise. He says think of a submarine.

Mr. JONES: When you drop out lead ballast, say, and replace it with water in a submarine tank, the submarine rises because you're replacing the heavy material with the lighter material. You apparently did the same thing underneath the Sierra Nevada about three and a half million years ago.

HARRIS: So far, so good. New mountains made of old rock. But now comes a new study. Researchers at Stanford University have published some evidence that seems to contradict that story. Andreas Mulch and his colleagues decided to study ancient river sediments, clays from a Sierra river. These dated back 40 to 50 million years ago. So if the mountain range was much more recent, this river would have been running along the flats, not down a mountain.

Mr. ANDREAS MULCH (Stanford University): So basically, these clays lock in the water composition at the time they were formed.

HARRIS: The Stanford group realized the trapped water could tell them at what altitude the sediment was formed. That's because the rainwater molecules that fall high in the mountains contain a slightly different mix of atoms than rainwater molecules that fall at lower elevations.

Mr. MULCH: And that led us to conclude that the elevation at the time was very similar to what the Northern Sierras look today.

HARRIS: More than 7,000 feet, which suggests the Sierra Nevada was already a towering mountain range 40 or 50 million years ago. So Mulch says the mountains aren't that new. They actually may have grown gradually over tens of millions of years, like other mountain formations elsewhere in the world.

Mr. MULCH: The idea may be that at the time the Sierra Nevada represented more a ramp that led up to a high continental plateau similar to the one that we're observing today in the Andes or in the Tibetan Himalayan system.

HARRIS: So there are two possible explanations for the Sierra. Either they grew very gradually, like other mountains around the world, or they sprang up quite suddenly. One possible answer is both processes were at work. The Sierra were bona fide mountains long ago, with a growth spurt about three and a half million years ago. Richard Harris, NPR News.

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