Tuesday, January 28, 2014

Geologists now calling the Grand Canyon "young"?

Grand Canyon National Park.  Image source.

Note: "Grand Canyon" here refers to the actual canyon structure, not to the rocks in which it was carved.

A number of science news sites have been reporting that a new study shows that the Grand Canyon is much younger than previously thought, which may have excited some young-earth creationists.  The headlines are quite misleading, however.  Most geologists have always considered the Grand Canyon to be millions of years old, and this has not changed.  Specifically, it was widely regarded to be about 5 to 6 million years old, roughly coinciding with the time when human ancestors are thought to have started walking upright.  In 2008, a study suggested that the Grand Canyon is actually 17 million years old, pushing its origins to before the first appearance of mastodons.  Then, in November of 2012, a controversial new study claimed to have evidence that the Grand Canyon dates to at least 70 million years ago, before the last dinosaurs went extinct.  The age of the Grand Canyon was then heavily debated in the scientific community (not even including the young-earth view).  Finally, a compromise seems to have been reached this past weekend.  A recent study published in Nature claims that one segment of the Grand Canyon formed 50 to 70 million years ago, another formed 15 to 25 million years ago, and the rest of it was formed and connected 5 to 6 million years ago.  It is this study that prompted this week's headlines to declare that the Grand Canyon is now "young."

Of course, "young" and "old" are relative terms.  An age of 6 million years is "young" in the eyes of someone who believes the earth to be 4.6 billion years old, whereas it seem staggeringly old to young-earth creationists.  Even the moniker "young-earth creationist" can be a bit misleading, as a 6000-year-old earth seems quite old when compared to our own human experience.  Nevertheless, even the recent "young" 5-million-year age for the Grand Canyon does not fit the creationist model of history, which generally places the formation of the Grand Canyon just after Noah's Flood, about 4400 years ago.

The most interesting aspect of the study, in my opinion, is the method that was used to date it.  Standard radiometric dating would not be useful here, as that would give the age of the rocks, not of the canyon that formed in them.  Instead, the scientists measured the helium trapped in apatite crystals within the rocks, a technique called thermochronometry.  You may recall from part 4 of the radiometric dating series that as uranium decays, it produces helium, and that the helium leaks out of crystals.  At today's rates, helium leaks out of apatite crystals faster than it is produced by uranium as long as the temperature is above about 70°C.  Therefore, if the apatites have accumulated helium, then they have been below 70°C for some time.  The scientists measured the amount of helium in the apatite crystals at numerous depths and at several locations to determine how much helium had built up, giving them a time estimate of when those rocks cooled.  The cooling is interpreted to have been caused by the rocks getting closer to the surface because of the cutting down of the canyon.

The question from a young-earth standpoint, then, is how the scientists could get ages of millions of years if the canyon was only formed 4400 years ago.  Looking again to the series on radiometric dating, we saw that buildup of helium in zircon crystals implied an accelerated decay rate of uranium at some time in the past.  If such an acceleration did happen (say, during the Flood), then it is likely that the helium would have also built up in the apatite crystals, even at temperatures over 70°C.  To the uniformitarian (who believes rates remain unchanged), these high levels of helium would appear to indicate that the rocks have been cool for a very long time.  The differences in ages, meanwhile, may indeed have been caused by some sort of localized subterranean heating afterward.

Unfortunately I do not have full access to all of the scientists' data, nor do I have sufficient knowledge of the equations and calculations to be able to test my hypothesis (but anyone at AiG or ICR is welcome to do so!).  Regardess, the new studies of the Grand Canyon do hold potential excitement for creationists, if not for the reasons the headlines claim.

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