Arizona Regional Geology

Colorado Plateau Geology

In many areas the great sedimentary beds of the Colorado Plateau have remained intact as flat-lying rock layers, undisturbed by faulting. However, in other places the horizontally-layered rocks have been warped and folded, often forming what are known as monoclines. These monoclines form great "steps" on the earth' surface. An excellent example is the East Kaibab Monocline, which bounds the high Kaibab Plateau on its east side. The lack of vegetative cover in many areas makes these great folds quite apparent.

Not all of the rocks exposed on the Colorado Plateau are sedimentary in origin. Numerous volcanoes dot the region, adding variety to the landscape. The Uinkaret Mountains (Mt. Trumbull area), the Hopi Buttes, Vulcan's Throne in the Grand Canyon, and Sunset Crater and the striking San Francisco Peaks near Flagstaff are all volcanoes found in the area. Sunset Crater erupted less than 1000 years ago, very recently considering geologic time extends back billions of years! The highest point in the state is atop 12,633 foot Mt.Humphreys in the San Francisco Peaks. The "peaks", as locals call them, supported glaciers about 15,000 years ago during the last major glacial advance, but today only scattered snowfields manage to survive into the warm summer months.

Despite abundant precipitation in certain areas, surface water is rare on the Colorado Plateau. Much of the bedrock is limestone, which tends to "swallow" water into solution cavities. So where does the water go? It sinks through the ground and eventually reaches the surface again as springs at lower elevations. Many springs in the Grand Canyon and at the base of the Mogollon Rim are fed by water falling high atop the Colorado Plateau.

Basin and range topography forms as the earth is slowly pulled apart. To compensate for this stretching, the rocks break up along faults. Some blocks "rise" while adjacent areas "drop". Those which rise form fault block ranges while those that drop form intervening basins or valleys. Thus the term "basin and range."

The geology of the Mojave landscape is quite easily seen because of the sparse vegetative cover so common in this desert. Sedimentary bedding and faults or folds are particularly noticeable. In the mountains of the area one can often see a discrete break between the bedrock of the range and the eroded sands and gravels which form the relatively smooth skirt at their base. These areas of sediment are termed alluvial fans. They form as rain washes weathered rock down into the valley from the slopes of the mountains above. Often a whole series of these fans spreads outward from the mountain front, coalescing and forming what geologists term a bajada.

Unlike the Colorado Plateau to the north, igneous and metamorphic rocks are well exposed in many areas in this region. In several areas granitic plutons have intruded into overlying sedimentary rocks. The heat and water associated with this magma caused intense mineralization of nearby rocks, particularly limestone, and copper minerals formed. These valuable copper deposits have been mined historically and are being mined today in the Clifton-Morenci area and the Globe-Miami area. The mines contain low-grade ore (not very concentrated) so huge open-pit mines have been dug to extract enough rock to gather the copper.

Interestingly, some quartzite pebbles associated with ranges south and below the Mogollon Rim have been found in stream deposits atop the rim and to the north. How did they get there? It appears that at one time at least one of the ranges of the Central Highlands, the Mazatzal, was once above and connected to the Colorado Plateau. The Mogollon Rim did not yet exist, and sediment from this range was shed northward onto the plateau.

In the Sonoran Desert many craggy low to mid elevation mountain ranges rise above vast basins. These ranges generally trend northwest-southeast and parallel one another. With the limited vegetative cover, one can often see a discrete break between the bedrock of the range and the eroded sands and gravels which form the relatively smooth skirt at their base. These areas of sediment are termed alluvial fans. They form as rain washes weathered rock down into the valley from the slopes of the mountains above.

The dry desert environment can produce a whitish deposit called caliche which can cover the surface for miles. Caliche forms as water, rich in dissolved minerals such as calcium carbonate, evaporates at the surface leaving the solid mineral behind. Caliche produces a hardpan preventing root growth for many plants and greatly decreasing a soil's permeability. It can form naturally from rainwater but also can form in irrigated farmlands, adversely affecting the productivity of the soil. Similarly, playas form as runoff collects at the bottom of a basin and evaporates, leaving behind salts.

Large copper-bearing plutonic bodies intruded into the rocks of this area during the Laramide Orogeny (a massive mountain-building event which affected much of the Rocky Mountain region from about 80 to 35 million years ago). Copper minerals dissolved in hot solutions associated with rising magma penetrated into cracks in the surrounding rocks and then crystallized. Both high-grade and low-grade copper ore deposits formed. These have been mined historically and in many cases are still being mined today. As a result, Arizona is the leading producer of copper ore in the United States today.

A major period of volcanic activity occurred in southern Arizona about 25 million years ago. Glowing avalanches of hot gas and fragmented rock erupted from large volcanoes called calderas and flowed across the landscape incinerating everything in their path. Many of the volcanic deposits in the Sonoran Desert, including some exposed in Organ Pipe Cactus National Monument, are from this period of intense volcanism.

The Basin and Range topography of southern Arizona reaches its climax in the Sky Islands region in the southeastern corner of the state. Here several mountain ranges rise more than 6000 feet above the surrounding basins, with Mt.Graham in the Pinaleno Mountains rising nearly 8000 feet. Though still considered part of the Basin and Range, the geology of the Sky Island region is quite complex and not easily categorized as such. A great variety of rock types and geologic features are found in the Sky Island region. Sedimentary, metamorphic, and igneous (volcanic and plutonic) rocks are exposed in several different areas. The Rincon, Santa Catalina, and Pinaleno Mountains are considered by geologists to be metamorphic core complexes. These mountains are "cored" with a mixture of igneous and metamorphic rock, some of which has been highly deformed. The "inside" of these mountains has been revealed as faulting uplifted the ranges and erosion stripped away the overlying rocks, exposing their core.

A period of intense volcanism, beginning about 25 million years ago and continuing for several million years, occurred in the region depositing large amounts of volcanic ash and lava across the landscape. A volcanic rock called tuff, which is essentially a solidified hot ash flow, formed during some of these eruptions. The fantastic rock spires and hoodoos of Chiricahua National Monument in the Chiracahua Mountains formed from the erosion of this welded tuff.