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Determining the precise geologic history of the Grand Canyon has not been an easy task. Rivers carve out canyons through erosion, a process that erases the physical evidence of their work. With no rock record left behind, scientists must look at the land formations that remain to reconstruct the history of such erosion. In recent years, sophisticated new techniques have allowed researchers to do this with greater accuracy.

Through a process called thermochronology, scientists analyze canyon rock samples by microscope in search of crystals known as apatite, which contain helium-producing uranium. When the apatite is hotter than around 122 degrees Fahrenheit (50 degrees Celsius), the helium escapes, but when the rocks cool–which happens when they are exposed to the surface by a river, for example–the helium remains trapped inside the crystals. By measuring the amount of helium in the apatite, then, scientists can estimate how long the rock has been exposed near the surface.

'Old Canyon' Research Claim: 70 Million Years

In November 2012, geologists Rebecca Flowers of the University of Colorado and Brian Wernicke of the California Institute of Technology used thermochronological data to back up their controversial challenge to the prevailing view that the Colorado River carved the Grand Canyon less than 6 million years ago. Writing in the journal Science, they claimed that an ancient river flowing west to east carved the western part of the Grand Canyon almost to modern depths some 70 million years ago, during the Late Cretaceous period.

'New Canyon' Research Claim: 60 Million Years

In 2014, a team led by Karl Karlstrom, a geologist at the University of New Mexico in Albuquerque and a leading proponent of the “Young Canyon” school of thought, released a study of their own in the journal Nature Geoscience. Karlstrom and his colleagues challenged the idea that the gorge we know as the Grand Canyon was around in the age of the dinosaurs. In research also backed by thermochronology, they argued that though some sections of it are older, the canyon as it exists today is no more than 6 million years old.

This study proposed that a section of the eastern canyon was excised between 15 to 25 million years ago, while an even older section further west—known as Hurricane Canyon—was carved by a northwest-flowing river between 50 and 70 million years ago. The easternmost and westernmost portions, however, were less than 6 million years old. 

As Karlstrom and his colleagues interpret the data, the Colorado River integrated the two older “paleocanyons” with the younger sections between 5 and 6 million years ago, after which it began flowing into the Gulf of California. The gorge then got much wider and deeper, and morphed into the Grand Canyon we see today. Since then, they say, the canyon has deepened at a fairly steady rate of roughly 100 to 200 meters (328 to 656 feet) every million years.

The new study did not resolve the debate over the formation's age, however, as Flowers and Wernicke abided by their “Old Canyon” argument. Flowers told that both her team and Karlstrom’s discovered 70 million-year-old cooling ages near the westernmost canyon, a section that the new study claims is less than 6 million years old. 

According to Flowers, “It will take a bit more time to understand fully why their interpretations are so different from ours.” Her co-author, Wernicke, is sticking to his guns as well, but points out that what’s important is that the “Young Canyon” view is not universally accepted without question, as “We’ve all learned that it’s a lot more complicated than that.”