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Swift
2012-Dec-03, 02:07 AM
From Science News (http://www.sciencenews.org/view/generic/id/346741/title/Grand_Canyon_could_be_much_older_than_thought)


Rock of ages it may be, but the Grand Canyon’s age itself is under fire. New work suggests the iconic chasm was already in place 70 million years ago — making it far older than commonly believed.

By most geologists’ definition, the Grand Canyon proper emerged between 5 million and 6 million years ago, as the Colorado River flowed across and eroded its way through layer after layer of rock. Evidence for this age comes from, among other things, great piles of washed-out gravel at the canyon’s western end that appeared around that time.

But the new study, reported online November 29 in Science, looks instead at the chemistry of rocks exposed throughout the canyon. Rocks get cooler as erosion strips away the material above them. That cooling is chemically preserved in several ways, including in helium within the mineral apatite.

“When the apatite is hot, the helium simply diffuses out of the crystal; when the apatite is cold, helium is completely retained,” says study leader Rebecca Flowers of the University of Colorado Boulder. “So by measuring the helium we are constraining when the rock went from hot to cold as it moved closer to the Earth’s surface, or as the Earth’s surface moved closer to the rock as the canyon was carved.”
Apparently this is causing a big stir in the geology community.

Hornblower
2012-Dec-03, 02:24 AM
Thanks for posting that excerpt. The Washington Post article on this topic did not adequately explain the reasoning behind the helium analysis. The explanation above is right in line with the warning we had in college physics lab about never putting helium into a Dewar flask without first chilling the flask with liquid nitrogen. The helium could diffuse through warm glass and destroy the vacuum.

grapes
2012-Dec-03, 05:59 AM
So many variables! :)

A jump from 6 million to 70 million is huge. Based on that alone, I would expect the geochemists to find an error in their reasoning.

Rhaedas
2012-Dec-03, 02:40 PM
Looking at current information at the National Park Service, they say that the canyon was formed in four different phases. Deposition, Uplift, Down cutting and Erosion. And apparently they state that the uplift was in the 75 mya period, while the down cutting point of the Colorado River began at the 6 mya date, the one in question. But I had always thought that the common understanding was that the river eroded AS the uplift occurred. There's no way I see that a river would cut through an already formed plateau, but if it cut as the uplift happened, then you would have a much older region, as well as explaining how such erosion could occur, given that large amount of time.

So I guess my question is, where did the idea that down cutting happened after uplift come from?

And just to put even more scale of time and erosion into place, the top of the canyon is not the original top. A large part of the original area is eroded away, so even at the top, we're seeing very old rocks, and the most recent deposits are long gone.

SRH
2012-Dec-03, 03:02 PM
Just looking at the GC from google earth, the cracks and fissures look like they may have formed by earthquake.

Could anyone please explain why formation by earthquake has been ruled out? (if it has)

Thanks.

Rhaedas
2012-Dec-03, 03:27 PM
Earthquakes have had some effect on the Grand Canyon, but not as you suggest, a one time thing. The major faults in the area actually run across the canyon. Found a link to a PDF that has a lot of info on faults specifically.

https://www.geology.ucdavis.edu/~shlemonc/trips/grandcanyon_12/bigpicture/Chap5_Faults.pdf

Swift
2012-Dec-03, 04:12 PM
Looking at current information at the National Park Service, they say that the canyon was formed in four different phases. Deposition, Uplift, Down cutting and Erosion.
I heard a report on NPR where they were talking to a ranger from NPS Grand Canyon park about this study. He really didn't express an opinion on the new study, but was excited about the interest and the attention.

From my experience with the NPS and other park systems, they are usually very conservative about changing formal signage and websites, and stick with the "generally accepted" explanations. On the flip side, I suspect if you did a ranger-lead hike today, they would at least touch on the controversy in their discussion (but probably not express an opinion).

geonuc
2012-Dec-03, 05:02 PM
This is exciting. I wonder if the timing of the uplift itself might be in question? That would have far-ranging implications with respect to the Farallon plate subduction.

publiusr
2012-Dec-04, 12:15 AM
The polar opposite of the great scablands debate. I would think the gradualists would welcome this. First the Earth was 4.7 billion yrs old, then 4.5. Keep it up and they'll be on the young earthers doorstep....or not. The greater age shouldn't come as that much of a surprise. That's a big gash after all.

dgavin
2012-Dec-04, 08:00 PM
I think this puts the formation of grand canyon roughly about the same time frame as the now failed rift zone that was in that genral vicinity, doesn't it? Makes a lot of sense to me.

Ara Pacis
2012-Dec-05, 09:26 AM
Can the change in rock heat and helium retention be explained by other processes, like changed in the water table?

Icefox
2012-Dec-23, 06:12 PM
I think this puts the formation of grand canyon roughly about the same time frame as the now failed rift zone that was in that genral vicinity, doesn't it? Makes a lot of sense to me.

If you mean the Rio Grande Rift, that's about 300 miles east and trending in a different direction. Also, the USGS says it's about 30 million years old; quite a bit later than the proposed 75 mya age for the Grand Canyon: http://cires.colorado.edu/science/groups/sheehan/projects/riogrande/faq/

We know the Grand Canyon is an erosional feature and not caused by earthquakes because there's no displacement in layers from one side of it to the other. It does strongly follow existing (old) faults though, because those faults create weaknesses in the rock that rivers preferentially erode out.

I read the paper introducing this idea and, as usual, it's a little more complicated than the news makes it sound. We have pretty firm dates of 0.6 million years old for a lava flow inside the canyon... but it's only 75 meters above the floor, so that only tells us where the floor was 0.6 million years ago. Above that we don't have any extremely reliable indicators. The idea the paper presents is more or less that most of the canyon was eroded over a long period of time, and the last 6 million years or whatever have simply been deepening an already-existing feature. I'm not really familiar with the ins and outs of how they're producing these ages but it's an interesting idea. However, I also suspect it might be somewhat naive of them to expect that the temperature of an area with active volcanism in it has depended only on depth for the past 75 million years.

dgavin
2012-Dec-23, 09:19 PM
That does place it a bit later. Wiki has it dated from 100-mya to 65mya ago. USGS would know better though.

I'm wondering about Wiki's accuracy on failed Rift's know, as they also list the Great Eliptical Basic as a failed rift. Which it isn't, its a dixtorial torsion feature.

Ara Pacis
2012-Dec-24, 08:42 AM
That does place it a bit later. Wiki has it dated from 100-mya to 65mya ago. USGS would know better though.

I'm wondering about Wiki's accuracy on failed Rift's know, as they also list the Great Eliptical Basic as a failed rift. Which it isn't, its a dixtorial torsion feature.

Wikipedia is only as accurate as the most recent editor.

Swift
2014-Jan-27, 08:52 PM
From NationalGeographic.com (http://news.nationalgeographic.com/news/2014/01/140126-grand-canyon-american-southwest-erosion-geology-geophysics/)


To the untrained eye, the Grand Canyon might just look like one big hole in the ground. But to some scientists, the American Southwest's iconic gorge is increasingly looking like several ancient canyons of different ages, stitched together by erosion that occurred about six million years ago, and subsequently sculpted into its modern form.

A new study published in the journal Nature Geoscience, added to more than a century's worth of fieldwork, is helping researchers decipher a geological tale that began unfolding when dinosaurs roamed the landscape.

For nearly 150 years, scientists have been debating how and when the Grand Canyon formed, says Karl Karlstrom, a geologist at the University of New Mexico in Albuquerque. In recent decades they've mostly split into two camps: those proposing a "young canyon" model in which the Colorado River alone carved much of the gorge in the past five million years or so, and those suggesting an "old canyon" model in which a series of ancient rivers carved ancestral canyons along more or less the same route. New research by Karlstrom and his colleagues bolster the notion that what actually happened lies between these two extremes.

Several older canyons more recently connected would certainly explain the different age estimates.