LOS ANGELES — The Grand Canyon was probably carved about 70 million years ago, much earlier than thought, a new study suggests — so early that dinosaurs might have roamed near this natural wonder.
Using a new dating tool, a team of scientists came up with a different age for the gorge’s western section, challenging conventional wisdom that much of the canyon was scoured by the mighty Colorado River during the last 5 million to 6 million years.
Not everyone is convinced of the latest viewpoint published online Thursday in the journal Science. Critics contend the study ignores a mountain of evidence pointing to a geologically young landscape, and they have doubts about the technique used to date it.
The notion that the Grand Canyon existed during the dinosaur era is ‘‘ludicrous,’’ said geologist Karl Karlstrom of the University of New Mexico at Albuquerque.
How the Grand Canyon became grand — with its vertical cliffs and flat plateaus — has been debated since John Wesley Powell navigated the whitewater rapids and scouted the sheer walls during his famous 1869 expedition.
Some 5 million tourists flock to Arizona each year to marvel at the 277-mile-long chasm, which plunges a mile deep in some places. It is a geologic layer cake with the most recent rock formations near the rim stacked on top of older rocks that date back 2 billion years.
Though the exposed rocks are ancient, most scientists think the Grand Canyon itself was forged in the recent geologic past, created when tectonic forces uplifted land that the Colorado River later carved through.
The new work by researchers at the University of Colorado Boulder and California Institute of Technology argued that canyon-cutting occurred long before that. They focused on the western end of the Grand Canyon occupied today by the Hualapai Reservation, owner of the Skywalk attraction, a horseshoe-shaped glass bridge that extends from the canyon’s edge.
To come up with the age, the team crushed rocks collected from the bottom of the canyon to analyze a rare type of mineral called apatite. The mineral has traces of radioactive elements that release helium during decay, allowing researchers to calculate the passage of time since the canyon eroded.
Their interpretation: The western Grand Canyon is 70 million years old and was likely shaped by an ancient river that coursed in the opposite direction of the west-flowing Colorado.
Lead researcher Rebecca Flowers of the University of Colorado Boulder realizes not everyone will accept this alternative view, which minimizes the role of the Colorado River.