The Grand Canyon in the southwestern USA is a young geologic canyon that formed in the past 5 to 6 million years, more recently than previously thought, reports a study published online this week in Nature Geoscience. The timing of its formation has been strongly debated, with some arguing it formed as long as 70 million years ago.

Karl Karlstrom and colleagues used thermochronology, a dating method that can measure the time when rocks were brought to Earth’s surface due to erosion removing the layers of rock above, to constrain the timing of the formation of four segments of the Grand Canyon. They find that two of the segments are ancient palaeocanyons - one formed between 50 and 70 million years ago and the other formed 15 to 25 million years ago. However, the remaining two palaeocanyon segments are young, carved out only in the past 5 to 6 million years. So even though parts of the canyon are old, the researchers conclude that the Grand Canyon can only have become one giant throughgoing canyon, similar to what we see today, when all of the smaller canyon segments, including those only formed in the last 5 to 6 million years, became linked due to erosion from the Colorado River.