Northern Arizona’s Grand Canyon is one of America’s most spectacular natural wonders, stretching along 277 miles of the 1,450-mile Colorado River.
Scientists have long understood that the canyon, which has over four million visitors from around the globe each year, formed due to the impact of the river between five or six million years ago – but researchers haven’t been able to agree on exactly what processes occurred and when.
Now, a new study suggests that the river may have begun to carve its path after an ancient lake overflowed around 6.6 million years ago, providing fresh evidence for a theory first introduced in the 1930s.
As the river began to drain into the vast Bidahochi basin, the basin filled up and spilled over its barrier – crossing the steep Kaibab Arch, a high point in Arizona and Utah – into what would become the Grand Canyon, the scientists said.
“In some ways, you could really think of it as the birth of the Colorado River that we know today,” UCLA geologist John He explained in a statement.
“There are rivers everywhere, but a river that carries water and sediment across the continent connects life throughout the region, and the entire ecosystem probably changed as a result of the arrival of the Colorado River into the basin,” he said.
How did they come to this conclusion? By studying the composition of sandstone collected from the Grand Canyon and the Bidahochi basin.
Both contained similar microscopic grains known as zircon crystals. These crystals form in volcanic magma as it cools and do not degrade much over time, helping scientists gain an accurate understanding of when and how they were created.
“They’re like little time vaults, and by looking at the age and geochemical signature of zircons, we can tell where a sediment that has been moved by a river originated,” He noted.
Using lasers to determine the chemical elements that made up the zircon crystals in the basin, the researchers found the signature of other sediments known to be traced to the Colorado River.
They compared the zircons they had collected with other zircons from the ancestral Colorado River, finding that the 6.6-million-year-old basin sediments matched Colorado River sediments.
And, the zircons weren’t the only evidence.
Rock layers in the area from the same time period showed signs of rippling, too. That indicated that a strong river had flowed into standing water, signaling that the Colorado River had supplied water and sediment to the basin before it spilled over the river flows through the Grand Canyon.
Of course, many questions remain about exactly what happened.
It remains unclear, for instance, if the basin’s lake led to flooding or erosion over time and the study does not rule out other potential ways the river could have flowed into the canyon.
Not everyone is convinced by the new findings, either.
Karl Karlstrom, a geologist at the University of New Mexico, told Scientific American that he’s not convinced the river formed the lake and that if an older canyon had already cut across the Kaibab the river likely wouldn’t pool to the elevations in the study.
“The key details of [the authors’] proposed lake spillover conclusion remain untested,” he said.
A coming paper further links the basin to the canyon, Matthew Heizler, a geochronologist at the New Mexico Institute of Mining and Technology, also told Science.
But the paper says that notches in the Kaibab would have allowed the river to reach the canyon without rising as high as in the new study.
Still, he and other researchers argue that a lake spillover is “perhaps the simplest mechanism to establish the course of the Colorado River through the Grand Canyon.”
“I think there is something unique and disquieting when the planet’s history is laid out before our eyes, but we cannot fully read it. We’ve always known the Grand Canyon is there, this solid towering wall of rock, but we’re learning more each day how it formed,” he said.
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