Since I live in Northern Arizona, the creation of the Grand Canyon is a common topic of conversation. The truth is no one knows for sure. Even the local geologists are divided on this topic.
There are currently four different theories: 2 "official" and 2 "unofficial".
The two official theories are:
1) "Around 20 million years ago the Colorado River begins to carve into the Grand Canyon at its eastern end, Marble Canyon, and probably exiting via Kanab Canyon.
At 17 million years ago the Colorado Plateau begins to uplift and causes the river to cut deeper.
Around 5 million years ago the uplift ceases and another river working its way northward along the San Andreas fault and eastward along the western Colorado Plateau captures the Colorado River."
2) "Around 35 million years ago the Kaibab Plateau begins to uplift and diverts the ancestral Colorado, which was already established on a course very similar to that of today, to the southeast. The cut-off western portion, now named the Hualapai Drainage System, continues to drain the western region.
About 12 million years ago the Colorado's path to the sea is blocked and a huge lake, Lake Bidahochi, is formed.
Eventually the Hualapai cuts back through the southern portion of the plateau and recaptures the Colorado. Lake Bidahochi is drained and becomes the Little Colorado River"
The unofficial theories are:
1) The main channel of the Grand Canyon was formed by the last Ice Age.
As the glaciers moved south they carved the main channel and than as the glaciers melted they left behind the Colorado River.
After that erosion by wind and water took over and made the canyon deeper.
2) A massive earthquake opened up the canyon at the same time that the Rocky Mountains started to form.
As the Rocky Mountains get higher, the canyon gets deeper. Wind and water erosion have added to the formation of the canyon.
At some point in the future, the earth will either crack open or close up. Either way the Grand Canyon will no longer exist and the entire region will suffer massive earthquake damage.
Personally, I think this last one was made up by insurance agents who want to sell earthquake insurance in Utah, Nevada and Northern Arizona.
Whichever, theory is true, the result is truly magnificent and a trip to the Grand Canyon will leave you with memories you will never forget.