Seventeen Million Years Old. Deal With It

Well, golly: U-dates that get progressively older as you go higher up the canyon walls. I'd love to hear how hydrological sorting explains that.

Eamon -
Sorry, but I think you've missed a critical detail of the article, the dates are older at the top because the dates record when the river began to carve (erode) the bedrock, forming the feature we now know as the Grand Canyon. Carl exlains this well in his article - the dates come from mammarily deposits laid down in the walls of the caves. The older dates are in the oldest caves - those on top of the Canyon.

So, over 17 million years ago the landscape would have looked much different, with only a shallow depression where the river ran over the landscape. Since that time the river has carved the deep canyon out of the bed rock features.

TF

Tim: Yes, I got all that. I think you missed the sarcasm in my comment, ie: that I'd love to hear how the creationists are going to squirm out of this one (hydrological sorting being one of their stupid ideas).

Sorry Eamon-
I hadn't heard of that particular creationist claim, so thought your comment was referring to stratigraphic succession. The effort creationist's put into dismissing observable data never fails to astound me. Thanks for the clarification.
TF

In the article, you say that for 99.99% of earth's history the Grand Canyon was not there - as it is only 17M yrs old. If the earth is 4.5 billion yrs old is not the fraction:

1 - 17M/4500M ? which seems to be 0.9962222 or about 99.6% of the earth's history. Its a nit, but I thought maybe I was missing something.

Anyway just curious.

Hi, George.

Not to be snarky, but maybe Carl is taking into account that what we know as the Grand Canyon, was probably not a canyon (or if a canyon, not really all that "Grand") for some time utnil after it initially began to be "carved" out.

So, as I see it...

.9999(4500) = 4499.55

4500 - 4499.55 = .45

...So, Carl is (at most) suggesting that this particular depression in the Earth was sufficiently "Grand" enough to be considered what we now know as the Grand Canyon.

:)

Oops... hit post accidentally.

I meant to say:

...So, Carl is (at most) suggesting that this particular depression in the Earth was sufficiently "Grand" enough to be considered what we now know as the Grand Canyon after approximately 450,000 years!

Oh yeah, and I misspelled until up there.

s1mplex,

Perhaps you are right. Not even sure why it struck me such that I noticed - but I would suggest that is to complex. Perhaps it became "grand" after 4M years. But seems odd that it would take all but the last 450,000 years to become "grand".

I will say one thing. The story was Grand. Thanks Carl, for bringing this to my attention.

Carl, please write something about the latest paper on Homo Floriensis! Can it be true our hobbitses were only dwarf cretins?

Enjoyed the Wired article. Until now I was not aware that the dating problem at the Grand Canyon had been solved.

I can't agree with what I interpret as the suggestion in the Wired article that we should entrust the government, even in Park Service bookstores, to pick and choose what "truth" we hoi polloi will be permitted access to. The creationists are obviously wrong and those who are fooled by them haven't considered the question in any depth, nor, generally, with an open mind.

Far more dangerous than wrong opinion is conceding to government even the least prerogative in dictating access to information.

One question I had about the article is where this other uranium is coming from that would be distinguishable from uranium at other younger water levels. Decay from heavier elements? Obviously this cannot be all the same Uranium decaying from the original formation of the solar system or there would be no difference in trapped uranium from any two particular cave levels..?

Okay, the abstract of the study says

Samples in the western Grand Canyon yielded apparent water table decline rates of 55 to 123 meters per million years over the past 17 million years, in contrast to eastern Grand Canyon samples that yielded much faster rates (166 to 411 meters per million years).

55 meters in 1 million years is 2165 inches or a little more than 2 inches every 1,000 years. Isn't that unbelievably slow? With the the occasional flash floods that do happen in the Grand Canyon there would have to be many years where there was basically no erosion at all. Two ideas I thought of that could slow down the process are:
time spent eroding stuff that "fell into" the canyon as the walls steepened,
or stuff that was deposited in the canyon from upstream and needed to be re-eroded.

I couldn't really google a "normal" erosion rate but I did read that there was a lot of easily-erodable sandstone near the top of the canyon, so it should have been going relatively quickly at first.

The oldest of these caves may not date the onset of erosion of the canyon itself. A new article in GSA Today [download it here] suggests that the canyon formed partially because of pre-existing karst deposits in the bedrock in the area. From page 9:

"Perhaps the best solution for the integration of the Colorado River involves different styles of both headward erosion and basin spillover, with the key factor being groundwater. Surface drainage capture typically follows capture of the groundwater drainage by lower, adjacent topography (Pederson, 2001). Groundwater sapping and spring discharge then provide viable erosion mechanisms for a surface drainage to extend headward. There is also the possibility that a karst plumbing system formed by this groundwater could have collapsed, aiding in surface drainage development. The now-dissected karst system exposed in the walls of western Grand Canyon is impressive and may provide a history of Neogene groundwater lowering and canyon formation (Polyak et al., 2007)."

One question I had about the article is where this other uranium is coming from that would be distinguishable from uranium at other younger water levels.

IIRC, the uranium is carried into the caves in the water forming deposits. As the water level lowers, it no longer reaches the cave system, and thus stops depositing Uranium. What uranium is there decays into lead, while the lower caves have fresh amounts of uranium dumped in them.

Some people did another study saying the canyon might be 65 million years old. What the heck? They interviewed the author of this study and he didn't think it was crazy. The only explanation I can think of is that the karst plumbing system Randy talks about might have exposed the apatite before the canyon actually reached it.

I was questioning the 2-inches-per-1000-years erosion rate implied by the other study. This one would mean the rate was four times that slow!