How could everyone have gotten it so wrong? New research indicates that the Grand Canyon is perhaps 65 million years old, far older than previously thought--and old enough that the last surviving dinosaurs may have stomped along its rim. The study has also uncovered evidence that the canyon is not a single geological feature but possibly the result of the merger of several, different canyon systems over time.
Estimating the age and dynamics of the Grand Canyon is one of the toughest challenges in geology. It requires separating the physical process of canyon carving from the age of the ancient rocks and sediments that make it up. For many decades, geologists had assumed that the canyon was 6 million years old, based on techniques called radiometrics, which measure the rates of decay for certain radioactive isotopes contained in the canyon walls. But last month, using a variation of that method to analyze minerals in the roofs of caves in the canyon walls, a team concluded that the chasm actually began forming about 17 million years ago (Science, 7 March 2008).
Now, researchers from the California Institute of Technology (Caltech) in Pasadena have pushed back the canyon's origins even further. They developed a new technique that relies on a type of phosphate-containing mineral called apatite, which is relatively rare in the canyon but offers important clues to the age of geological formations. Apatite contains traces of uranium and thorium, naturally radioactive elements that release a form of helium when they decay. This method is similar to other radiometric techniques, but it's also tied to the temperature of the rock. Geologists can determine when the apatite, once buried deep beneath the surface, was unearthed by the erosive force of the Colorado River.
The Caltech team collected 36 apatite samples from various depths within the canyon as well as from points on the plateau above the rim. They report in an upcoming issue of the Geological Society of America's GSA Bulletin that the samples taken from the deepest parts of the canyon were close to Earth's surface at least 55 million years ago and as much as 65 million years ago, or around the time the dinosaurs became extinct. The scientists also found evidence that the canyon did not form as a unit. "Different segments and levels of the canyon appear to have evolved at different times," says geologist and team leader Rebecca Flowers, now at the University of Colorado, Boulder.
The work provides "a welcome contribution" to the resolution of the age of the canyon and surrounding structures, says geologist Richard Young of the State University of New York, Geneseo. Young, who has studied the geology of the canyon for 46 years, says the dating method is certain to improve understanding of the region's geology, particularly the events leading to the modern Colorado River system. And geologist Victor Polyak of the University of New Mexico, Albuquerque, who co-authored the study that appeared in Science last month, says the new research suggests that the canyon was forming long before even the events uncovered by his team's research. "There were more processes shaping what is now the Grand Canyon than previously thought," Polyak says.