The western Grand Canyon is so old that dinosaurs could have seen it, according to a new analysis.
The research indicates the canyon in Arizona was largely carved out by about 70 million years ago, making it more than seven times older than widely believed by scientists.
In the study, geoscientist Rebecca Flowers of the University of Colorado Boulder and colleagues used a method of dating rocks that exploits the natural decay of uranium and thorium atoms into helium atoms.
These atoms were locked in grains of a mineral called apatite as they cooled and wound up closer to the surface during the carving of the Grand Canyon, Flowers said.
Temperature variations at shallow levels underground are influenced by the Earth’s surface shape, she explained. And apatite provides a temperature history that can reveal how much time has passed since there were major changes in the canyon’s depth.
“Our research implies that the Grand Canyon was directly carved to within a few hundred meters (yards) of its modern depth by about 70 million years ago,” said Flowers. A paper on the subject by Flowers and Kenneth Farley of the California Institute of Technology was published online Nov. 29, 2012 in the research journal Science.
Scientists have debated the canyon’s age and evolution, Flowers said. A variety of data suggest that the Grand Canyon had a complicated history, and the entire modern canyon may not have been carved all at the same time. In 2008, Flowers and colleagues reported in a study that parts of the eastern canyon likely developed some 55 million years ago, although the bottom of that ancient canyon was above the height of the current canyon rim at that time before it subsequently eroded to its current depth.
The dinosaurs died out about 65 million years ago.
Over a mile (1.6 km) deep in places, Arizona’s steeply sided Grand Canyon is about 280 miles long and up to 18 miles wide in places. Visited by more than five million people annually, the iconic canyon was likely carved in large part by an ancestral waterway of the Colorado River that was flowing in the opposite direction millions of years ago, said Flowers.
“An ancient Grand Canyon has important implications for understanding the evolution of landscapes, topography, hydrology and tectonics in the western U.S. and in mountain belts more generally,” said Flowers.
Whether helium is retained or lost from the individual apatite crystals is a function of temperatures in the Earth’s crust, she said. When temperatures of the apatite grains are greater than 158 degrees Fahrenheit, they retain no helium, while at temperatures below 86 degrees F, they retain all the helium.