Grand Canyon National Park

Grand Canyon National Park

Grand Canyon Geology

Grand Canyon attracts the attention of the world for many reasons, but perhaps its greatest significance lies in the geologic record preserved and exposed here. The rocks at Grand Canyon are not inherently unique. Similar rocks are found throughout the world. What is unique about the geologic record at Grand Canyon is the variety of rocks present, the clarity with which they are exposed, and the complex geologic story they tell.

Two separate geologic stories exist at Grand Canyon. The older story is the one revealed in the thick sequence of rocks exposed in the walls of the canyon. These rocks provide a remarkable record of the Paleozoic Era (550—250 million years ago). At the bottom of the canyon there are remnants of Precambrian rocks that are two billion years old! The story these rocks tell is far older than the canyon itself. Mesozoic and Cenozoic rocks (250 million years old to the present) are largely missing at Grand Canyon. They have either been worn away or were never deposited.

The second geologic story at Grand Canyon concerns the origin of the canyon itself: when and how did it come to be? On one level the answer is simple: Grand Canyon owes its existence to the erosional forces of the Colorado River. Of equal importance are the forces of erosion that have shaped and continue to shape the canyon today. These include running water from rain, snowmelt, and tributary streams which enter the canyon throughout its length. The climate at Grand Canyon is classified as semi-arid and the South Rim receives only 15 inches (38 cm) of precipitation each year. The bottom of the canyon receives even less—8 inches (20 cm). The rain comes suddenly in violent storms, particularly in the late summer of each year. The power of erosion is therefore more evident here than in other places which receive more rain.

Grand Canyon owes its distinctive shape to the different rock layers in the canyon walls. Each responds to erosion in a different way: some form slopes, some form cliffs, some erode more quickly than others. The vivid colors of many of these layers are due mainly to trace amounts of various minerals. Most contain iron, which imparts subtle hues of red, yellow, and green to the canyon walls. Climate plays an important role in the appearance of the canyon. If the Grand Canyon received more precipitation, the plants and trees that grow here would be very different. The cacti and shrubs that grow here today would be replaced with lush vegetation.

How old is the canyon itself? The early history and evolution of the Colorado River (of which Grand Canyon is only a part) is the most complex aspect of Grand Canyon geology. The erosion that shaped the canyon occurred only in the past five to six million years, a blink of the eye in geological terms, and insignificant relative to the two billion-year-old rocks at the canyon bottom. 

Grand Canyon continues to grow and change. As long as rain and snow continue to fall in northern Arizona, the forces of erosion will continue to shape Grand Canyon.

Carving the Canyon

At one time, most of the western portion of the North American continent was at or below sea level, and the ancestral Colorado River meandered over a large plain. About 70 million years ago, a 130,000-square-mile area of the southwestern United States called the Colorado Plateau was gradually squeezed up thousands of feet high as the Pacific continental plate crashed against and went under the North American plate (close to the modern-day California coastline), sending powerful geologic reverberations eastward that created the Rockies. The Grand Canyon began to appear 5 to 6 million years ago as the sediment-laden Colorado and its tributaries worked to deepen and, with the aid of rain, ice and gravity, widen the canyon to its present 10-mile average width.

The Canyon Today

Since 1964, Glen Canyon Dam has harnessed the mighty Colorado River for water storage and hydroelectric generation, and controlled the seasonal floodwaters that did much to form the Grand Canyon. The dam purges the powerful river of many of its erosive sediments by leaving them behind in Lake Powell. 

For more information, visit Yavapai Observation Station, take a ranger-led geology walk, and stop at the bookstores where you can find books about the park's geologic story.