The Mystery of the Grand Canyon's Age: How is it 70 Million Years Old While the Colorado River is Only 6 Million Years Old?
It's a common misconception that the Grand Canyon and the Colorado River are the same age. In reality, the rocks in the Grand Canyon are much older than the river that carves through them, with some rock formations dating back hundreds of millions of years. But how does this align with the fact that the Colorado River, which shapes the canyon, is only about 6 million years old?
The Rocks Beneath the Grand Canyon
The rocks that you see in the Grand Canyon are far older than the river that cuts through it. The oldest rock formations at the bottom of the canyon, which are also the basement rock of the surrounding region, are approximately a billion or more years old. These layers of rock gradually become younger as you move upward in the canyon.
Imagine a cake baked three days ago. The cake itself is 3 days old, but if you were to cut a piece out of it today, the cut exposing the layers would be much newer. Similarly, the rocks in the Grand Canyon are millions to billions of years old, while the canyon and the river cutting through it are relatively new phenomena.
Formation Theories of the Grand Canyon and Colorado River
The age of the Grand Canyon and the Colorado River remains a subject of debate. Here are four primary theories that have been proposed to explain the canyon's formation:
The Little Colorado River Hypothesis
In the 1960s, Edwin McKee suggested that the Colorado River in the Grand Canyon formed when two different rivers met. He proposed that the ancestral Colorado River followed its present course until reaching the Kaibab Plateau, where it flowed down the channel of the Little Colorado River to the Gulf of Mexico. The young Colorado River would have slowly captured the ancestral portion by headward erosion. However, there is no evidence in the Little Colorado River drainage to support the idea of an eastward-flowing river.
The Northwest Flowing River Hypothesis
Geologists like Ivo Lucchitta believe that the ancestral Colorado River crossed the Kaibab Plateau as it does today and then veered off to the northwest alongside a ridge. For this to work, the Kaibab Plateau couldn't look like it does today—it may have been covered by Mesozoic rocks or didn't exist yet. While Lucchitta proposes that the young Colorado River captured the ancestral portion by headward erosion, no sedimentary deposits support this theory.
The Spillover Theory: The Catastrophic Draining of Ancient Lakes
Geologists have hypothesized that the ancestral Colorado River was temporarily dammed behind the Kaibab Plateau and other high points, forming a chain of ancient lakes. Water from these lakes would have overflowed, cutting a path through the plateau and creating the Grand Canyon. This theory proposes that a catastrophic event, such as an overflow or the capture of a young river, carved the canyon.
The Collapse of a Groundwater Karst System
Carol Hill, a geologist from the University of New Mexico, proposed a more recent theory that the "ancestral" Colorado River integrated with the young Colorado River through the collapse of a groundwater-karst system. Precipitation would have drained into the Redwall aquifer, flowing through the Kaibab Uplift and connecting with the young Colorado River. This theory suggests that the collapse of the karst system led to the formation of the Grand Canyon.
Despite the various theories, the common denominator is that the Kaibab Plateau was uplifted about 65 to 70 million years ago, creating a powerful pool of potential energy for water to drop to sea level. This uplift was a significant event, leading to the formation of the Grand Canyon as the youngest and most dynamic part of the landscape.
While the exact recipe for the formation of the Grand Canyon remains uncertain, each theory provides a different perspective on how the Colorado River could have cut through the older rock formations to create the spectacular Grand Canyon that we see today.