The Grand Canyon, which formed when four separate canyons joined up, is much younger than a study in 2012 indicated. This earlier study, by Rebecca Flowers of the University of Colorado in Boulder, suggested that it formed 70 million years ago, when dinosaurs roamed the earth. That is far older than geologists had previously believed. However it now seems that the youngest part of the canyon appeared just 5 or 6 million years ago.
The new analysis relies on a technique called thermochronology. This reveals when hot rocks from deep underground first approached the surface, and then cooled down as overlying rock eroded away. Karl Karlstrom of the University of New Mexico in Albuquerque and colleagues made thermochronology measurements at the base and rim in four sections of the canyon. Then they calculated when each set of rocks rose to the surface.
They found that one section of the Grand Canyon, the Music Mountain formation, became a canyon 50 to 70 million years ago – in line with Flowers' findings. But at that time, the other three segments were still deep underground. The rim of the youngest, eastern part of the Grand Canyon would have been covered by 2 kilometres of rock at that time.
The second section of canyon, the East Kaibab palaeocanyon, formed 15 to 25 million years ago. But the Grand Canyon as we know it today only came into existence 6 million years ago when the final two outer sections reached the surface: the Eastern Grand Canyon and the western Marble Canyon.
The previous canyons were all drained by different rivers, but after the last section had surfaced the Colorado river was able to gouge a path along all four, uniting them on its way to the Gulf of California.
Flowers says that Karlstrom's conclusions agree with hers except for the Marble Canyon, which she thinks is older. Her thermochronologic readings for the Marble Canyon are similar to Karlstrom's, yet seem to have been interpreted differently. "It will take a bit more time to understand fully why their interpretations are so different from ours and why they conclude that the erosion history varied so dramatically within this short reach of the canyon," Flowers says.
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo2065
This story is based on an article in New Scientist.