An abiding question among geologists is when was the Grand Canyon carved? When did the Colorado River assume its present course and become the master sculptor of the Canyon?
Recent research has supplied at least one answer. It tells of a lake that pooled up and eventually spilled over to become today’s through-flowing Colorado.
That lake formed in the Bidahochi Basin on the Navajo Nation in northeast Arizona. Key to unlocking the story are microscopic bits of zircon, which appear as small rounded particles in the sand and volcanic ash layers of the Bidahochi. Zircon is a common, long-lasting mineral that can be dated by the uranium-lead isotopes it contains. They’re like “tiny time capsules,” says one geologist.
People knew the river existed in western Colorado 11 million years ago, and first reached the Gulf of California around 5 million years ago. What happened in between—essentially when the Canyon was created—was partially revealed by the Bidahochi zircons.
With enough zircon samples, the researchers detected a fingerprint of both their age and source.
The dating indicates that about 6.6 million years ago the ancestral Colorado entered the lake. Possibly as long as a million years later, the lake spilled over and the river turned westward. It first encountered the Kaibab uplift, but by that time it was steep enough and carried enough sediment to chisel the mile-deep gorge.
That took time too, and no doubt will inspire future research. For now, another piece of the Grand Canyon puzzle has been put in place.
This Earth Note was written by Rose Houk and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
