There are many ideas of how the Grand Canyon was formed. The most common idea involves the Colorado River.
The Grand Canyon is said to have existed over 1.84 billion years ago. The knowledge we have is not based on empirical science, but historical science. We do not have actual proof that any of the ideas commonly accepted for the creation of the Grand Canyon are actually true. There is no experience that can show that idea was the one that formed the Grand Canyon, but the commonly accepted theory is from erosion due to the Colorado River.
The Grand Canyon has 12 distinctive rock layers. The layers each come from their own time period. For example, the top layer of rock; the Kaibab formation, is said to have been formed about 270 million years ago. For reference of how long ago this was, the newest layer occurred before dinosaurs roamed the Earth. The list continues on. Next is the Toroweap Formation (273 million years ago), the Coconino Sandstone (275 million years ago), the Hermit Formation (275 million years ago), the Supai Group (315-285 million years ago), the Redwall Limestone (340 million years ago), the Temple Butte Formation (340 million years ago), the Muav Limestone (505 million years ago), the Bright Angel Shale (515 million years ago), the Tapeats Sandstone (525 million years ago), the Grand Canyon Supergroup (740 – 1,250 million years ago), and at the very bottom, the oldest layer of rock, is the Vishnu Basement Rocks which are 1,680 – 1,840 million years old.
The bottom layer consists of ancient igneous and metamorphic rock said to have formed deep in the Earth when island arcs collided with continental masses.
Some of the rocks that are here include schist, gneiss, and granite. The height from the rim (the top) to the very bottom in certain parts can be over 5,000 feet tall. These rock layers do not all exist in the same location. Some rock layers began to erode away in some areas before new layers were deposited on top, whereas in other areas the rock layers continued to build up with less erosion.
The Colorado River is said to have eroded the land into an arid plateau. It carved the depth through the Kaibab Plateau which is 7,000+ feet above sea level. The smaller side canyons are said to have formed from glaciers melting or summer thunderstorms. The thought is that the actual formation of the Grand Canyon itself (disregarding the age of the rocks) happened about 6 million years ago. The reason this idea is still a theory is because the Colorado River no longer follows this course and it is unclear how or when the altered course of the Colorado River occurred.