Which of the following features of the Grand Canyon did Researcher 2 use to date the Grand Canyon?
Age of the Grand Canyon
A lot of research has been done on the age of the Grand Canyon. There are two competing ideas about the age of the
canyon. Two researchers explain their hypotheses on the age of the Grand Canyon below.
The deeper a rock is underground, the warmer it generally is. As uranium trapped in rock crystals decays, it forms
helium. When the rock is warmer than around 120°C, the helium can escape from the rock. By measuring the helium
isotopes in the rock crystals, it is possible to determine the time when the rock sample cooled below 120° C. This
method is know as thermochronometry, and it can be used to measure how long ago the rock sample cooled. This
cooling is generally caused by erosion bringing the rock layers closer to the surface.
Based on this method, several rock samples from the western half of the Grand Canyon must have been near
the surface approximately 70 million years ago. This clearly indicates that much of the current Grand Canyon must
have been in existence by 65 million years ago. Then about 6 million years ago, what is now the Colorado River
shifted and began to feed into this more ancient canyon system.
While there is definitely evidence for a few very early canyons in the western part, it is clear that the majority of the
current Grand Canyon was carved 6 million years ago. Dating the sediments deposited at the mouth of the Grand
Canyon reveals that no samples have been found that are more than 6 million years old. Several samples from the
western canyon system also indicate a date of exposure that is consistent with an age of 6 million years.
Finally, if the majority of the Grand Canyon was formed more than 65 million years ago, the canyon would have been
exposed to much greater erosion and the shape of the canyon should be much wider. The current canyon is very
narrow considering its depth, which is consistent with an age of 6 million years when the Colorado river shifted