The age of the Grand Canyon of Arizona has always been controversial and more fuel has been added to the controversy with the publication in Science on November 29 of a new study by researchers Rebecca Flowers and Kenneth Farley who say they have evidence that the Grand Canyon “was largely carved out by about 70 million years ago.” Their full paper is behind a pay wall but you can read the press release here. The contentious problem with that age is that the current Colorado River has been flowing along its present course and direction for only about 6 million years. For that story, see my post, written in 2011: “Origin of the Grand Canyon.”
The Arizona Geological Survey has weighed in on this controversy in their new Fall-Winter 2012 issue of Arizona Geology magazine with an article by Wayne Ranney, a geologist who has long studied the canyon and has written a book about it. What follows are excerpts from Ranney’s article.
“The new theory involves two very complex and complicated laboratory techniques that can reveal when the canyon’s rocks were brought close to the surface. Using tiny apatite crystals collected from the basement rocks in the canyon (Vishnu Schist or Zoroaster Granite), the information yielded two different stories, one for the history of the western Grand Canyon and the other for the eastern canyon, where most tourists see the gorge. The results said that western Grand Canyon (downstream from Lava Falls) was cut to within a few hundred meters (about 1,000 feet) of its present depth by 70 Ma [million years ago]. The second story reported that the eastern area was the site of a canyon of similar proportions to the modern canyon by 55 Ma, and cut in Mesozoic rocks now completely eroded away. Incredibly, the western canyon was cut by a river that flowed exactly opposite to the modern Colorado River and the researchers call this the California River.”
Reread the paragraph above. It says that in the eastern canyon area, a canyon equal to the current one was formed, then disappeared.
“When the Cal Tech group began their study they assumed that the apatite samples would reveal that Grand Canyon’s rocks were buried in unequal amounts of overlying rock – unequal because the canyon today has 5,000 feet of relief and the lower samples should have been buried under more material than those collected from near the top.”
That concept is shown in figure 1. The red dots show the relative position where Flowers and Farley collected their samples.
“After running the laboratory technique the samples produced surprising results to the researchers. They showed that no matter from what depth the samples were collected, they all appeared to have been buried under equal amounts of overlying rock [figure 2]. When the tops of the blue arrows are connected here, they reveal a canyon-like topography in eastern Grand Canyon about 70 Ma. Below is a diagram [figure 3] that shows their interpretation of the data – a gorge of similar proportions was cut into the Mesozoic rocks that are now stripped back to the modern Echo and Vermilion Cliffs.”
Ranney opines that the laboratory technique used by Flowers and Farley “is not as evolved as one might hope for. Some assumptions are made that could result in different outcomes.”
Ranney also notes: “The evolutionary history of the Colorado River shows that its exact course through the canyon to the Gulf of California was accomplished in only the last 6 million years.” He emphasizes, however, that the age of the Colorado River is not necessarily the same as the canyon, “the age of its [the river’s] ancestors or some early incarnation of the canyon need not be so strictly confined.”
Read Ranney’s entire article here.
Check out other stories in Arizona Geology Magazine here.
For more geology stories, see my Article Index page.