Parts of the Grand Canyon are a mere five to six million years old, far younger than was previously thought. Formerly, the geophysical wonder of the world was dated along its entire length at 70 million years.
Professor Karl Karlstrom from the University of New Mexico, Albuquerque, has been working in the canyon on dating the various segments. He has now published results of his investigation in Nature Geoscience. His evidence contradicts what was called the “old canyon model” which assumed that the whole structure was carved out at one time and to its current depth.
He also refutes an alternative theory, the so-called “young canyon model” which argued that the whole canyon was cut out in the last six thousand millenia. Karlstrom’s results show a pattern which incorporates both of the theories.
He thinks that the Colorado River used some of the old segments in its path as it cut its way through from high in the Rockies in the last five to six million years. Therefore, the Grand Canyon is both “relatively” new as well as being extremely ancient in its formation
The enormity of the Grand Canyon has always posed problems for those who have come to study it, and scientists have long debated over its age. It runs for 450 km and goes to depths of 1,800m. Gathering samples of the mineral crystals in the rocks is the only way to assess the chronology. The structure of these crystals alters over time as they cool down. This occurs very slowly as they make their way closer to the surface by the effects of erosion.
The mineral apatite is the most useful in ascertaining the history of a rock’s temperature. Radioactive uranium decays and this in turn produces helium atoms within the apatite. The helium then diffuses its way out depending on how warm the rock is. It was the study of apatite that led previous researchers to date the canyon at 70 million years old.
There are five main segments to the Grand Canyon. Karlstom focused on four of these, using thermochronology techniques, including the helium dating, on the rocks. Karlstrom and his team have now concluded that the two end furthermost sections, the Marble Canyon and the Westernmost Grand Canyon are carved most recently. This, they say, is when the Colorado River has broken through and linked up the whole system.
Two of the middle portions, the Hurricane, and the Eastern Grand Canyon, are confirmed palaeocanyons, with Hurricane by far the oldest. They dated Hurricane at 50 to 70 million years, while Eastern Grand Canyon was found to be cut out around 15 to 25 million years ago.
Karlstrom summarized his results with the analogy of a human lifespan. He says that half of the Grand Canyon is young, and a quarter of it is middle-aged. The remaining parts of the chasm are incredibly old.
These results will never be “set in stone.” The Grand Canyon has not finished growing, it does so at the rate of the width of a piece of paper year on year. For now though, Karlstrom feels that he has accomplished an analysis that “honours all the hard-won data” and he has done so by working in one of the most beautiful places on earth. He is claiming to have solved the 140 year debate among geologists about the age of this most iconic of natural landscape features.
He may find that professional colleagues will beg to differ. Rebecca Flowers of the University of Colorado Boulder and Brian Wernicke, a Caltech geologist, are both puzzled and somewhat dubious about the findings. Wernicke has gone so far as to say, “They’re not thinking this through.” Flowers, who has collected similar data from the Grand Canyon, yet come to quite different conclusions, thinks that it will take more time to figure out why they have come to such divergent opinions on the matter.
The Grand Canyon may not be so ancient, but the old debate over her past looks set to continue well into the future.
By Kate Henderson