There are several theories for how the Grand Canyon was formed

One theory is the standard uniformterrianism theory. The theory starts with a the hypothesis of an inland ocean in a geosyncline on a continental plate. Then there was a major uplift, pushing the horizontal layers vertically upward over 4.000 feet. The Colorado river then slowly cut through the plateau forming the Grand Canyon.

An Alternative theory was that the Grand Canyon was cut very rapidly as an large inland lake formed at the end of the last ice age was suddenly released cutting through the relatively soft layers of sediments formed during the ice ages.

The canyons formed by the release of the Lake formed by the eruption of Mr. St. Helens seems to support the alternative theory but is not widely accepted.