What is the Grand Canyon? HOW and WHEN did it form? A new scientific study proposes that the American Southwest’s iconic gorge is composed of several ancient canyons of different ages, which stitched together by erosion six million years ago, before being sculpted into its modern form.
One of the biggest mysteries of the Grand Canyon is how it formed. A new study published in the journal Nature Geoscience is helping researchers decipher a geological tale that began unfolding when dinosaurs roamed the landscape.
For nearly 150 years, scientists have been debating how and when the Grand Canyon formed. In recent decades they’ve mostly split into two camps: those proposing a “young canyon” model in which the Colorado River alone carved much of the gorge in the past five million years or so, and those suggesting an “old canyon” model in which a series of ancient rivers carved ancestral canyons along more or less the same route. The new research results say what actually happened probably lies between these two extremes.
Analyzing the Rocks
In the new study, the researchers used a variety of techniques (apatite fission-track dating, radiogenic helium measurements, length of fission tracks measurements) to analyze samples of phosphate-bearing rocks taken from four of the five major sections of the canyon, both from river level and from the canyon rim, which typically lies almost a mile (1.5 kilometers) above the river.
How Old Is the Grand Canyon?
- Different parts of the canyon formed at different times. One of the oldest segments, named the Hurricane segment after a famed geological fault, lies in the western portion of the canyon had been carved to about half its current depth between 70 million and 55 million years ago.
- Erosion hadn’t started etching a section that geologists have dubbed “Eastern Grand Canyon,” immediately downstream of where the Little Colorado River joins the Colorado, until some 25 million years ago.
- The westernmost and easternmost segments of the canyon were largely carved in the past five million or six million years.
- While the Hurricane and Eastern Grand Canyon segments were originally sculpted by different rivers, the Colorado took over the job in the past six million years, joining the disparate canyons and carving them wider and deeper.
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