Scientists decipher the geological history of the Grand Canyon

The Grand Canyon is a marvel on the landscape of North America, a breathtaking geological feature in the southwestern U.S. state of Arizona presenting dramatic contours and richly colored vistas.
Research is now providing insight into how and when the Colorado River, whose continuous might has carved out the canyon over millions of years, came to flow ​through the region.

By studying tiny grains of the mineral zircon in sandstone made up of sediments carried by the river and particles of ash ‌from volcanic eruptions long ago, the researchers managed to trace paths that the river took in the past.

The study indicated that the river, around 6.6 million years ago, began to flow into a large depression in the Earth's surface, called a basin, in northeastern Arizona, forming a wide and shallow lake east of where the Grand Canyon later took shape.

The lake water built up over time and eventually spilled over ​a low point on the lakeshore starting around 5.6 million years ago, sending it coursing through the region that became the Grand Canyon, the researchers found.

The river ​then filled and spilled through another series of basins downstream from the Grand Canyon, ultimately reaching the Gulf of California about 4.8 million ⁠years ago, emptying into the sea at a spot in northwestern Mexico, they found.

The lake, which may have reached a width of more than 90 miles (150 km), has long ​since disappeared. The researchers have informally called it Bidahochi Lake based on the name of a local geological formation. It was situated largely on what is now the Navajo Nation reservation.
"Scientists ​have long debated when the Grand Canyon was carved, and our study contributes to that conversation," said UCLA geologist John He, co-lead author of the study published this week in the journal Science, opens new tab.

"Imagine you go out to a river bank and scoop up a handful of sand. In that handful, there are hundreds of thousands of sand grains that look like any other sand grain. But within that handful there ​will be a couple of hundred or even thousands of microscopic grains of zircon crystal, each of which is a vault of information about where it comes from," He ​said.

The dating of the ash helped determine when the riverine sand beds bearing the zircons were deposited.

The Colorado River originates at La Poudre Pass in Rocky Mountain National Park in Colorado and runs about ‌1,450 miles (2,330 ⁠km).

"A longstanding question has been: where did the Colorado River go before it flowed through Grand Canyon?" said study co-lead author Ryan Crow, a research geologist at the U.S. Geological Survey in Flagstaff, Arizona.

"We have long known that the river existed in western Colorado 11 million years ago, and that it did not (run through) Grand Canyon until after 5.6 million years ago. But until now we knew almost nothing about where it was during the intervening time," Crow said.
Other geological mechanisms also may have helped shape the river's routing, Crow said.

Scientists decipher the geological history of the Grand Canyon