Friday, May 23, 2008

The Continuing Controversy Over the Age of the Grand Canyon

The Grand Canyon has been in the national spotlight recently regarding its enigmatic age. In February, three workers published a paper in Science stating that some portions of the canyon may be as old as 17 million years. (You would have thought that they had proposed that the Grand Canyon didn't actually exist from the way some geologists responded to the news). And just this month, another set of workers independently reported in the GSA Bulletin that portions of the canyon might be as old as 50 million years. The oft-repeated age of the Grand Canyon is set at 6 million years. Why do such varying ages keep cropping up?

First of all, where does the 6 million year old date come from? That is the age of the youngest part of the Hualapai Limestone Member of the Muddy Creek Formation. Exposed at the base of the Grand Wash Cliffs, this unit does not contain material derived from the modern Colorado River. This means that the river as we know it today, was not contributing (much) material to the Hualapai Limestone and that the river must be less than 6 million years. We define the modern river as one that flows out of the Grand Wash Cliffs towards the Gulf of California.

Many geologists have used this criteria to establish an age for the Grand Canyon as well, for without the river there would be no Grand Canyon. However, is it possible to have had some portion of the Grand Canyon in existence before the modern river came into being? This seems entirely possible to me since it would be quite unreasonable to assume that the canyon appeared in an instant upon the landscape. It may be a bit of a stretch then to use the same criteria for the beginning of the modern river, to that for the beginning of the canyon. Compounding the problem is that no one actually states how the beginning of the Grand Canyon should be defined. Is the beginning of the canyon the same as the beginning of the river? (If so, does that take into consideration that the modern river is an evolved descendent from some earlier river?). Or is the beginning of the canyon defined by some other criteria? Like the initial exposure of the Kaibab Limestone (rim rock) by river erosion. There are even more possibilities that could define the beginning of the Grand Canyon - an earlier canyon in the same place but into strata now completely eroded away, etc.

The modern Colorado River is believed by some to have been cobbled together from separate and distinct prior river systems. The evidence from the Hualapai Limestone Member reflects the moment in time when these separate systems were finally integrated. This integration could have been from stream capture by headward erosion, overspill from ancient lakes, or some combination of the two. In any event, these prior systems must have cut into the landscape to some extent and I believe that the recent works pick up on this idea. In this way, we could admit that parts of the Grand Canyon (both laterally and vertically) were in existence prior to 6 million years ago, while still accepting that the modern Colorado River only came into being after 6 million years ago. In this way, an older version of the canyon and a younger version of the river are both possible.

As so often is the case in any human endeavor, miscommunication, misunderstanding, and the misreading of ideas ultimately leads to the very human trait of creating conflict where in fact none may actually exist. Geologists are remiss to not have fully defined the many different aspects of this debate. And they have not given names to the prior landscape elements that may have existed. (For example, an older version of the canyon, before the modern river was in existence, could be called a proto-Grand Canyon). When specific definitions are given and explained, a clearer understanding of the possibilities emerge. My view is that portions of the canyon were in place before 6 million years ago and the new research supports this view although I still accept that the modern river is only 6 million years old.

I am reminded of an example given in "Carving Grand Canyon" (page 99, second to last paragraph):

"Deciphering the history of the Grand Canyon is similar to the story of how the three blind men describe the elephant that they can touch but cannot see. Each speaks the truth for that part of the animal that they happen to touch, but their descriptions sound as if they are describing three different animals".

This "controversy" too then, is an example of geologists describing parts of the canyon but perhaps not the whole.

Comments welcome. More later.

Wayne Ranney, Geologist