Ancient raindrops fuel debate on Sierra's age

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Ancient raindrops fuel debate on Sierra's age

Post by copeg » Fri Jul 07, 2006 10:37 am

From San Fransisco Chronicle ... QM6P77.DTL
Ancient raindrops fuel debate on Sierra's age
Keay Davidson, Chronicle Science Writer
Friday, July 7, 2006

The Sierra Nevada rose at least 40 million years ago, significantly longer ago than many adult Californians learned when they were in school, Stanford scientists report in a finding that is likely to prove controversial.

The traditional estimate that the Sierra is 3 million to 5 million years old appears way too low, growing geological evidence has revealed. In a paper in today's issue of the journal Science, three Stanford geoscientists base their new age estimate on an unusual scientific technique: the analysis of prehistoric raindrops.

Many millions of years ago, those raindrops fell from clouds creeping up Sierran slopes. The raindrops were absorbed by boulders on the surface. Later, as eons passed, the boulders were buried by rising layers of sediments as the mountain eroded.

They remained buried until the mid-19th century, when the sediment was blasted away during a legendary environmental travesty of California's Gold Rush: the use of high-powered water hoses to blast gold from cliffsides.

That technique, known as hydraulic mining, devastated the local ecology and was later banned.

Although it was a horrendous practice, "it would be very difficult to find the (buried) rocks we were actually looking for" if the miners' water jets hadn't blasted away the sediments a century ago, exposing the boulders to the air, said Andreas Mulch, one of the researchers.

Mulch and fellow researchers, Page Chamberlain and Steve Graham -- all members of Stanford's Department of Geological and Environmental Sciences -- gathered the rainwater samples locked within the crumbling boulders that they found in numerous primeval, now-dry river tributaries during several camping trips in the Sierra starting in late 2004.

Their analysis led them to conclude the mountain range rose much longer ago than previously believed.

One expert said the blow to the old age estimate is so big that "people's eyes are going to pop out of their heads."

The researchers' paper "is going to generate a lot of discussion," said Walter Mooney, a geophysicist and veteran Sierra geologist at the U.S. Geological Survey in Menlo Park, who was not connected with the Science paper. "The standard party line is that uplift (of the Sierra) began subtly 10 million years ago (and) really took off 5 million years ago, but these (Stanford scientists) are saying, 'No! (It's) 40 to 50 million years ago.' "

To reach their conclusion, the researchers measured subtle compositional variations in hydrogen isotopes in primeval rainwater extracted from boulders from now-dry riverbeds in the Sierra. Those variations, like little thermometers, record the temperature of the cloud at the time the drops fell millions of years ago. Because temperature typically declines as one ascends a high mountain, the scientists used the compositional variations to estimate how high the cloud was -- and thus, how high the mountain was -- when the raindrop fell long ago.

There is no debate about the age of the rocks of which the Sierra Nevada is composed: Radioactive dating shows they're 80 million to 120 million years old. The debate, rather, has circled around the question: When did those rocks elevate into the sky, creating the mountain range?

"One might think that because the geology of the Sierra Nevada has been intensively studied for well over a century that there would be little debate about its origin. Not so," said Greg M. Stock, the park geologist at Yosemite National Park. "A flurry of scientific discovery beginning in the 1990s has challenged traditional concepts of how and when the range came to be."

Thus, the Stanford team's estimate of 40-plus million years makes theirs "an important paper in a long debate. It's definitely the first hard piece of evidence as to Sierran elevation in the last five to 10 years," said geoscientist Brian P. Wernicke, a leading expert on continental evolution at Caltech, who previously proposed the mountains elevated 80 million years ago.

A leading advocate of a "young Sierra" is Craig Jones of the University of Colorado. He called the Mulch paper "very interesting," but noted it "is at odds with more traditional geologic evidence," especially changing riverbed gradients that record primeval mountain-building.

"Where this leaves us is that something is wrong, and we are not sure what," Jones acknowledged. "There could be a weakness in the logic used by Mulch (and his colleagues), or there could be something about the ancient river channels that we have grossly misunderstood."

Grand theoretical mysteries are at stake. Robinson Cecil, a geology doctoral student at the University of Arizona who is an expert on Sierra hydrology, said by e-mail that the Stanford finding, if verified, "contradicts the general notion that high-elevation, high-relief regions are young because they haven't yet been eroded down.

"Likewise, if the Sierra is 'old,' then we have to start thinking about how it is that broad regions of high elevation can sustain themselves for so long, despite millions of years of erosional forces acting upon them."

In other words: If the Sierra is so old, then why hasn't time worn them down to dust?

Old as dirt?
Old theory

The mountain range rose 3-5 million years ago.

New theory

The range is at least 40 million years old.

E-mail Keay Davidson at

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Post by SteveB » Fri Jul 07, 2006 12:14 pm

Just posted the same article without noticing you beat me to it! :P

Good read all the same!

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Post by giantbrookie » Mon Jul 10, 2006 8:04 pm

As an author of one of the main recent papers in this debate (debunking many of the recent models that propose no uplift of the range in the last 5 million years). I think it is fair to say that this debate continues. Moreover, the debate is more complex than simply "old" (>5 m.y.) vs "young (<5 m.y. old) mountain range formation. There is ample evidence that there was a pretty high Sierra Nevada prior to 5 m.y. in the region south of Sonora Pass. Elevations appear to have reached 2500m or more (ie 8000' or more), whereas the northern part of the range had pretty low elevations (1000 m or less). There are several lines of evidence to suggest erosion rates between about 50 m.y. and 5 m.y. were very low in the Sierra, so if it was high 5 m.y. ago, then it was high 50 million years ago or more; thus the "old" elevation of the southern Sierra is 50 million plus years old. Starting sometime after 5 m.y. the entire range started to go up and the southern (high) Sierra has risen about as much as the N. Sierra in during this time. In the north this last 5 m.y. of uplift contributed 60% or more of the present elevation whereas in the High Sierra we love so much this recent episode of tectonics contributed more like 40% of the elevation. Note this is a far cry from 100%. The big difference in elevation so many of us Sierra hikers are familiar with (14000' peaks in the south, vs 7000' peaks in the north) is a product of the superposition of geologically recent uplift on an "older" southern range where more than half of the elevation is pre- 50 million years old. There are some issues with many of the studies that have concluded that the entire Sierra has stood at the same or higher elevation in times past, or that the spectacular eastern escarpment of the range is also old (rather than being less than 5 m.y. old). Much of the evidence used comes from east of ranges that are themselves east of the Sierra, such as the White Mtns., so that the tectonic history recorded is that of the Basin and Range, not the Sierra. There are a number of lines of geologic evidence (including some of those mentioned in the article by Craig Jones), that refute the notion of an old range without rejuvenation within the last 5 million years. On the other hand, as noted above, there IS evidence for pretty hefty pre-5 m.y. elevations in the southern Sierra along with fairly deep canyons.

The view of the majority of the research community today supports a young origin for much of the elevation of the Sierra. However, although my research supports the majority view, I personally like seeing an alternative viewpoint make it into the peer reviewed literature given that it keeps debate alive and focuses more scientific attention on the topographic evolution of the Sierra than we would have if everyone agreed.
Since my fishing (etc.) website is still down, you can be distracted by geology stuff at: ... ayshi.html" onclick=";return false;

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