Science  30 Nov 2012:
Vol. 338, Issue 6111, pp. 1134
  1. A Grand Old Canyon

    Parts of Arizona's Grand Canyon may be millions of years older than previously thought, suggests a study of helium atoms in mineral samples from the western reaches of the canyon.


    Helium, formed in minerals called apatites as a result of radioactive decay of uranium, diffuses through the mineral's crystal structure at deep underground temperatures above 70°C. But as the minerals rise toward Earth's surface—or as erosion carves a canyon toward the minerals—the rocks surrounding them cool, trapping more of the helium within the apatite grains. Helium concentrations in apatite can, therefore, serve as a geological time clock.

    The new study finds that much of the western portion of the Grand Canyon was carved to within a few hundred meters of its current depth by about 70 million years ago, scientists reported online this week in Science. That age is a far cry from the 5 to 6 million years previous researchers estimated by identifying when sediment from eastern portions of the canyon first appeared at the western end of the gorge.

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