The canyon's age is surprisingly controversial. Thermochronology suggests parts may be 70 million years old, not 5-6 million as traditionally thought, upending textbook geology.
Lennart Sikkema, CC BY 3.0, via Wikimedia Commons
The rocks vs. the river
The Grand Canyon presents two different geological stories. One story is about the rocks themselves, and the other is about the carving of the canyon. The rocks are ancient. At the bottom of the gorge, the Vishnu Schist is a metamorphic rock that is approximately 1.7 billion years old. The layered sedimentary rocks stacked above it get progressively younger, ending with the Kaibab Limestone at the rim, which formed about 270 million years ago. For most of geologic time, this entire region was a flat, unremarkable plain.
The second story, the carving of the massive chasm we see today, is scientific debate. For over a century, the accepted age of the Grand Canyon was 5 to 6 million years. Geologists based this on several pieces of evidence, including gravel deposits washed downstream by an ancestral Colorado River. This "young canyon" model proposes that the modern Colorado River became established and began aggressively cutting through the Colorado Plateau just a few million years ago, a very recent event in geological terms.
A clock in the crystals
The "old canyon" hypothesis gained significant traction with the application of a technique called apatite thermochronology. Apatite is a phosphate mineral found within granite and other rocks in the canyon's depths. These tiny apatite crystals contain trace amounts of uranium, which decays at a known rate. One decay process is spontaneous fission, which leaves microscopic damage trails, or "tracks," inside the crystal. Another process produces helium atoms that get trapped within the crystal lattice.
These damage tracks and helium atoms are a geologic clock, although one that is sensitive to heat. When the apatite is deep within the Earth's crust, temperatures are high (above 110°C), and any fission tracks that form are naturally repaired, or "annealed." Similarly, helium diffuses out of the hot crystal. As erosion removes kilometers of rock from above, the apatite cools as it gets closer to the surface. Below a certain temperature, the fission tracks and helium begin to accumulate. By measuring the density of these tracks and the amount of helium, scientists can determine when the rock cooled, and by extension, when deep erosion carved the canyon.
Data from apatite in the western Grand Canyon suggests the rocks cooled dramatically around 70 million years ago, during the time of the dinosaurs. This implies a deep canyon already existed then, carved by a different, ancient river system.
Stitching together an ancient landscape
The conflicting data has led to a hybrid model that is gaining acceptance. This model suggests the Grand Canyon as we know it is a composite feature. Ancient, disconnected canyon segments were carved at different times by different rivers. One segment, the "Hurricane," may have been etched out 70 to 55 million years ago. Another section, the "Eastern Grand Canyon," may have formed between 25 and 15 million years ago.
According to this hypothesis, the modern Colorado River integrated these older paleocanyons only within the last 5 to 6 million years. The river linked the separate segments, re-excavated them, and carved new sections like Marble Canyon to create the single, continuous Grand Canyon we see today. This explanation accounts for both the ancient cooling ages found in some parts of the canyon and the evidence for a young, fully integrated river system.
💡Fun Facts
The oldest rocks at the bottom of the canyon, the Vishnu Schist, are about 1.7 billion years old, nearly half the age of the Earth itself.
The age debate is not about the age of the rocks, but the age of the carving of the chasm.
The mineral apatite, used to date the canyon's carving, is the same mineral group that makes up the enamel on your teeth.
Even proponents of the "old canyon" theory agree that the final integration of the Colorado River through the entire length of the canyon happened only 5-6 million years ago.
The South Rim is open 24 hours a day, 365 days a year. The North Rim is open from mid-May to mid-October. Visitor center hours vary seasonally.
Admission
$35 per private vehicle, valid for 7 days. The park is cashless.
Accessibility
The South Rim has many paved, wheelchair-accessible trails and viewpoints along the rim. Inner canyon trails are steep, rugged, and not accessible. Accessible shuttle buses operate in the park.
