Physical Geography: Tucson, Az, USA
Southwest, region, southwestern United States, historically denoting several geographic areas in turn and changing over the years as the nation expanded. After the War of 1812, the Southwest generally meant Missouri, Arkansas, and Louisiana; after Texas was annexed, it, too, was included. In the wake of the war with Mexico, the Southwest embraced most, but not all, of the territory that was acquired under the Treaty of Guadalupe Hidalgo (1848), including land often considered part of the “West”—i.e., New Mexico, Arizona, and all or parts of Oklahoma, Colorado, Utah, and Nevada, as suited the convenience of the user of the term. It ordinarily excludes California.
Arizona is a land of contradictions. Although widely reputed for its hot low-elevation desert covered with cacti and creosote bushes, more than half of the state lies at an elevation of at least 4,000 feet (1,200 metres) above sea level, and it possesses the largest stand of evergreen ponderosa pine trees in the world. Arizona is well known for its waterless tracts of desert, but, thanks to many large man-made lakes, it has many more miles of shoreline than its reputation might suggest. Such spectacular landforms as the Grand Canyon and the Painted Desert have become international symbols of the region’s ruggedness, yet Arizona’s environment is so delicate that in many ways it is more threatened by pollution than are New York City and Los Angeles. Its romantic reputation as a wild desert and a place of old-fashioned close-to-the-earth simplicity is at variance with the fact that after the 1860s the state’s economy became industrial and technological long before it was pastoral or agrarian. Arizona is located in the southwestern quadrant of the conterminous states, bordered by California to the west, Nevada to the northwest, Utah to the north, New Mexico to the east, and the Mexican state of Sonora to the south. The Colorado River forms the boundary with California and Nevada. Phoenix, situated in the south-central part of the state, is the capital and largest city. Area 113,990 square miles (295,233 square km). Population (2010) 6,392,017; (2019 est.) 7,278,717. Plate tectonics—the shifting of large, relatively thin segments of Earth’s crust—and stream erosion have done the most to create Arizona’s spectacular topography. Specifically, the Pacific Plate and the North American Plate came into contact and created the major tectonic forces that uplifted, wrinkled, and stretched Arizona’s geologic crust, forming its mountain ranges, basins, and high plateaus. Over the course of millennia, rivers and their tributaries have carved distinctive landforms on these surfaces. To Arizona’s two major physiographic divisions, the Colorado Plateau and the Basin and Range Province, geologists add the Transition Zone (or Central Highlands). The northeastern two-fifths of Arizona is part of the scenic Colorado Plateau. Far less rugged than adjacent portions of the plateau in Utah, these tablelands in Arizona consist mainly of plains interrupted by steplike escarpments. Although they are labeled mesas and plateaus, their ruggedness and inaccessibility have been exaggerated. The incomparable Grand Canyon of the Colorado River provides the major exception to what has proved to be an area easily traversed. Forest-clad volcanic mountains atop the plateaus provide the state’s highest points: Humphreys Peak, 12,633 feet (3,851 metres), in the San Francisco Mountains, and Baldy Mountain, 11,403 feet (3,476 metres), in the White Mountains.
Summing up, the Southwest Region includes California, Nevada, and Arizona. The Regional Office, headquartered in Sacramento, provides Center oversight and support, facilitates internal and external collaborations, and works to further USGS strategic science directions.