It has been believed for some time that the Grand Canyon formed 6 million years ago. But now two geologists have evidence that it is closer to 70 million years old.
Rebecca Flowers at the University of Colorado, Boulder, and Ken Farley at the California Institute of Technology calculated the canyon's age by examining helium levels in the mineral apatite in the rocks under the western part of the canyon's floor. Apatite contains uranium and thorium, which decay into helium over time. At high temperatures, like those found deep underground, helium can dissipate. But if surface erosion brings these rocks closer to the surface, as happened at the Grand Canyon, then the cooler temperatures they are exposed to can cause the mineral to hold on to its helium.
Flowers and Farley found that the helium levels were higher than would have been expected if the rocks of the canyon had been exposed at the surface for only 6 million years, and concluded that the erosion that shaped the canyon began 70 million years ago, in the late Cretaceous (Science, doi.org/jvq). This raises the prospect that dinosaurs could once have stood on the rim of the canyon and gazed into its depths, just as humans do today.
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