All of Arizona lies in the Intermontane Plateaus, which form one of the major physiographic divisions of the United States. The state can be divided into three physiographic regions: the Colorado Plateau, the Basin and Range region, and the Transition Zone, or Central Highlands.

The Colorado Plateau is a flat, dry, semidesert region that covers the northern two-fifths of the state. Many rivers have carved deep canyons into this region, with the most famous example being the Grand Canyon of the Colorado River. The river in the Grand Canyon is as much as 1,500 m (5,000 ft) below the level of the surrounding plateau. Canyon de Chelly and Oak Creek Canyon are also beautiful, but lesser known, canyons carved into the plateau by tributaries of the Colorado River.

About 2 billion years ago this area, now mostly 1,500 to 2,400 m (5,000 to 8,000 ft) above sea level, lay under a vast sea. Through the ages the land emerged and resubmerged repeatedly, and many different rock types, including igneous, sedimentary, and metamorphic, formed. Rivers cut through layers of soil and rock to reach the ancient granites, quartzites, and rocks now revealed. Water and wind have eroded the edges of canyons and the surface of the plateaus, carving isolated, steep-sided, flat-topped hills called mesas. Underlying rocks such as sandstones, shales, and limestones have also been exposed by erosion, creating a kaleidoscope of brilliant colors on canyon and mesa walls.

North of the Grand Canyon is the Kaibab Plateau, which is an area extending from Utah into Arizona. The Kaibab Plateau resembles a peninsula.

The section of the Colorado Plateau located in the northern and northeastern part of the state is a maze of valleys and mesas. Carved knobs, rounded domes, and tall rock spires that pierce broad valleys earned the area its name of Monument Valley. The Painted Desert, where red, yellow, purple, blue, brown, and gray rocks alternate in a vivid display of colors, extends south from the Grand Canyon to the Mogollon Rim. Within the Painted Desert is the Petrified Forest National Park, an area of giant, ancient fallen trees that slowly petrified over thousands of years.

South of the Grand Canyon lies the San Francisco Plateau, which is covered by ancient lava flows and dotted with extinct volcanic cones such as Humphreys Peak, 3,851 m (12,633 ft) tall, the highest point in Arizona. The southeastern part of the Colorado Plateau in Arizona is part of the Datil section, noted for its solidified lava flows and other volcanic features. The southern border of the Colorado Plateau is distinguished by an extensive volcanic escarpment known as the Mogollon Rim. The Rim, which extends from central Arizona toward the southeast and terminates in the Mogollon Mountains, was originally created by tectonic pressure, uplift of the plateau, and, most important, erosion of the Transition Zone. The steep rock wall reaches about 600 m (about 2,000 ft) high in some places. To the south of the Mogollon Rim is a narrow strip of land known as the Transition Zone.

The Transition Zone is characterized by mountain ranges so close together that the area appears as a cluster of rugged peaks separated by steep, narrow valleys. The Mazatzal, Santa Maria, Sierra Ancha, and White mountain ranges are found in this zone, which occupies part of the area once known as the Central Highlands. So uninviting was the landscape that prospectors did not explore the region until the late 19th century. Since then, more than 90 percent of Arizona’s mining activity has taken place in this area.

The Basin and Range region, known to Arizona residents as the Sonoran Desert, occupies most of the southern part of Arizona. It is composed of a series of smooth-floored desert basins separated by mountain ranges that extend from northwest to southeast. Mountains in this region include the Chiricahua, Gila, Pinaleno, Huachuca, Hualapai, Santa Catalina, Santa Rita and Superstition ranges. The portion of the Basin and Range region that lies to the south and west is a low, dry landscape. Elevations in this area range from as low as 43 m (141 ft) at Yuma to 3,267 m (10,717 ft) atop Mount Graham. While the land has little rain, along the western border of Arizona farms irrigated with waters from the Colorado River produce abundant crops. Most of the loose material on the mountains in this region has been carried down by infrequent but violent cloudbursts to form thick fans of sand and gravel where the steep slopes meet the basin floor. When irrigated, this area produces excellent crops. Also, the state’s largest cities are located in this region. Most elevations in the Basin and Range region are from 150 to 1,500 m (500 to 5,000 ft), but some mountains rise to more than 3,400 m (11,000 ft). The region is generally higher in the east than in the west. Its desert plains are drained by the lower courses of the Colorado and Gila rivers.

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