Text Box: Colorado Plateaus Introduction | Geological History | Sections | The Grand Canyon | National Parks | More National Parks | National Monuments | Other Physical Features | Resources                                                       The Colorado Plateaus province is a somewhat circular region in the four states of Arizona, Utah, Colorado and New Mexico.  The center of the province is slightly west of the point where these states touch–the Four Corners, which is the only spot in the United States where four states touch one another.  This province is approximately half the size of Texas.  The name of the province is not derived from the portion of the province in the state of Colorado, which is relatively small, but from the Colorado River, which flows through the province and has carved massive canyons here. The name of the province is sometimes stated in the singular–Colorado Plateau–in reference to the entire region having been lifted as a cohesive entity.  However, there are many individual plateaus in the province characterized by rather definite boundaries, at various altitudes, many with local names.  The province as a whole has distinct boundaries all around except on the southeastern portion.  The western border is formed by significant faults, especially the Hurricane and Grand Wash faults, the latter having produced the 2,000-foot-high Grand Wash Cliffs;  there are also extrusive igneous rocks along a portion of the western border.  Most of the southern edge is formed by the very prominent Mogollon (pronounced muggy-own) Rim and extrusive igneous rocks; this boundary also generally coincides with the drainage divide between the Colorado River and that of the Gila and Rio Grande rivers.  The eastern and northern edges are formed by the Rocky Mountains, including the extrusive San Juan Mountains on the eastern border; part of the eastern border is also bounded by the Rio Grande rift (see Basin and Range province).  The southeastern border is not as distinct as other portions because lava covers the underlying structures, although the Rio Grande rift forms a part of this portion of the border.    General Characteristics The Colorado Plateaus province is one of the most distinctive regions in the country.  The early studies of this province, especially those conducted from 1847-1880, yielded some of the first truly comprehensive descriptions of a geomorphic region, and  “...laid the first paving stones on the road to the development of the science of American geomorphology.”   This province has more significant canyons and plateaus than any other, and the study of how they have formed led to the establishment of many scientific ideas of landscape development.  The many plateaus in the province generally range in elevation from 5,000 to 11,300 feet.  Very few areas in the province, except the bottoms of some of the deepest canyons, are less than 5,000 feet above sea level.  Greatest local relief in the province is 5,000-6,000 feet.  This occurs in the deepest canyons, including Grand Canyon in Arizona, which has a maximum depth of approximately 6,000 feet, and where volcanic cones, which dot the province, rise to about the same height above the plateaus as the Grand Canyon has cut down into the land.  Highest elevations exceed 12,000 feet above sea level at Humphreys Peak and Agassiz Peak in Arizona.  Horizontal or gently dipping sedimentary rocks dominate throughout most of the province.  Sandstone, shale and limestone are most common.  Analysis of zircon mineral grains in the sand indicates that the most logical source of at least some of the sand–perhaps half of it–was from as far away as the Appalachian Mountains.  Some researchers have conjectured that the sand was transported from the eastern to the western part of the country by a vast westward-flowing transcontinental river system, perhaps rivaling the Amazon system of today, between about 275-150 million years ago.  Sedimentary rocks representing all of the geological periods, and most of the epochs, are present, mostly revealed in the numerous canyons.  Notable exceptions include rocks of Oligocene and Miocene age:  there are essentially no Oligocene deposits (except very minor amounts in the northern part) and almost no Miocene deposits in the province.  These apparently were eroded away during the various uplifts that the region has undergone.  Igneous and metamorphic rocks also occur, but are far less abundant than sedimentary rocks.  Volcanic material covers about 10 percent of the province and occurs mostly at the edges.  The volcanic field near central Arizona and the southwestern edge of the province is referred to as the San Francisco field.  Basalt covers about 3,000 square miles and is relatively young, perhaps less than 1,000 years old, some showing virtually no erosion.  There are also almost 400 cinder cones; these are among the youngest volcanic features of the region.  San Francisco Peaks (also sometimes called the San Francisco Mountains or simply San Francisco Mountain) is a collection of peaks generally believed to have been produced by an explosion perhaps about 400,000 years ago and collapse of one massive stratovolcano.  Eruptions at this volcano ceased about 200,000 years ago.  Erosion has been significant here, some of it produced by past glaciation.  Humphreys Peak (12,633 feet) is the tallest peak in the volcanic field, and is also the highest point in Arizona and the province.  Sunset Crater, at the eastern margin of the San Francisco field, will be discussed in the National Monuments portion of this chapter.  Two other regions of significant volcanic material include southwestern Utah, which has many lava-capped plateaus, which are among the highest plateaus in the province, and west-central New Mexico.  The volcanic region in northwestern New Mexico is the Mt. Taylor volcanic field, much of which extends southward into the Basin and Range province.  Here, near the southeastern edge of the province, several outpourings of basalt and other lava types from many vents last erupted about 1.5 million years ago.  Other structures of volcanic origin, intrusive and extrusive, occur in scattered places within the province and will be discussed later in this chapter.  Metamorphic rocks occur only in limited areas within the province, most notably in the Inner Gorge at the bottom of the Grand Canyon, where schist nearly 2 billion years old has been cut into by the Colorado River.  There is also a small region of metamorphic rocks in the southeastern part of the province.    Colorado Plateaus Introduction | Geological History | Sections | The Grand Canyon | National Parks | More National Parks | National Monuments | Other Physical Features | Resources