What Process Formed the Grand Canyon?
Erosion is defined as the removal of dirt and rock by ice, gravity, water and wind. Gravity can cause erosion by pulling soil and stone down a slope. Ice causes erosion through the pressure exerted by cycles of freezing and thawing. Because water has greater volume when frozen, ice tends to crack things apart. Wind and water both apply friction to the surfaces they come into contact with, and over time these can cause erosion as well. Wind and ice erosion did have some effect on the Grand Canyon, but this is merely cosmetic. The Grand Canyon was formed principally by water erosion and the Colorado River.
The other force that helped to form the Grand Canyon was geologic uplift, one of the processes that create plateaus and mountain chains. There were two major periods of uplift that resulted in the creation of the Colorado Plateaus. One was 75 million years ago, and the other 17 million years ago. Between them, they raised the area from roughly 1,000 feet above sea level to roughly 9,000 feet above sea level. The more recent of the two uplifts resulted in changes that gradually steepened and increased the water flowing into the Colorado River, greatly strengthening its power to erode its way through rock.
The Colorado River
That the Colorado River is directly responsible for much of the Grand Canyon is visible to the naked eye, since it sits at the bottom of it. When geological uplift created a bigger, more powerful Colorado River, it cut the canyon by means of water erosion, starting between 5 million and 6 million years ago.
Magnifying this erosive effect were the multiple eruptions from the Uinkarat volcanic field, starting roughly 750,000 years ago. Lava from these eruptions would flow into the western reaches of the Grand Canyon and literally dam the Colorado River with basalt. These volcanic dams would form reservoirs, shaping the land behind them, and then eventually fail, resulting in massive flooding downriver and further eroding that area as well. This activity only came to a halt roughly 100,000 years ago.