Topography of the Southwestern US
Does your region have rolling hills? Mountainous areas? Flat land where you never have to bike up a hill? The answers to these questions can help others understand the basic topography of your region. The term topography is used to describe the shape of the land surface as measured by how elevation—height above sea level—varies over large and small areas. Over geologic time, topography changes as a result of weathering and erosion, as well as the type and structure of the underlying bedrock. It is also a story of plate tectonics, volcanoes, folding, faulting, uplift, and mountain building.
The Southwest’s topographic zones are under the influence of the destructive surface processes of weathering and erosion. Weathering includes both the mechanical and chemical processes that break down a rock. There are two types of weathering: physical and chemical. Physical weathering describes the physical or mechanical breakdown of a rock, during which the rock is broken into smaller pieces but no chemical changes take place. Water, ice, and wind all contribute to physical weathering, sculpting the landscape into characteristic forms determined by the climate. In most areas, water is the primary agent of erosion. Streams are constantly eroding their way down through bedrock to sea level, creating valleys; Arizona’s Grand Canyon provides a dramatic example of this process. Given sufficient time, streams can cut deeply and develop wide flat floodplains on valley floors. The pounding action of ocean waves on a coastline contributes to the erosion of coastal rocks and sediments, while the emptying of a river can lead to the formation of a delta. Rock material is carried by rivers and streams to the oceans or to an inland lake or basin where it is eventually deposited. In the case of the Basin and Range and parts of the desert in southern New Mexico and Arizona, streams end in basins within the region. The rest of the Southwest is drained by rivers that reach either the Gulf of Mexico (the Platte, Arkansas, Canadian, Pecos, and Rio Grande rivers) or the Gulf of California (the Colorado River and its major tributaries, the Green and San Juan rivers).
Pressure release can also cause rocks to crack. Growing plant roots can exert many pounds per square inch of pressure on rocks—think of tree roots uplifting and cracking a sidewalk. Additionally, since rocks buried kilometers (miles) beneath the surface are under considerable pressure, if those rocks become exposed at the Earth’s surface (where the rock is under less pressure), the rock may expand and crack in a process called exfoliation (Figure 4.1). At higher elevations, ice can also change the landscape due to frequent episodes of freezing and thawing, causing both temperature and pressure differentials within a rock. As water trapped in fractures within the rock freezes and thaws, the fractures continue to widen (Figure 4.2). This alone can induce significant breakdown of large rock bodies. During the Quaternary, ice has been an important agent of erosion throughout the highest parts of the Southwest.
Working in conjunction with physical (mechanical) weathering, chemical weathering also helps to break down rocks through changes in the chemical composition of their constituent minerals. Some minerals contained in igneous and metamorphic rocks that are formed at high temperatures and pressures (far below the surface of the Earth) become unstable when they are exposed at the surface or placed in contact with water, where the temperature and pressure are considerably lower. Unstable minerals transition into more stable minerals, resulting in the breakup of rock. Weak acids, such as carbonic acid found in rainwater, promote the disintegration of certain types of rocks. Limestone and marble may be chemically broken down as carbonic acid reacts with the carbonate mineral composition of these rocks, forming cavities and caverns in the rock. Other sedimentary rocks held together by carbonate cement are also particularly susceptible to chemical weathering, which expedites the process of soil formation.
Volcanic activity has shaped the land throughout the Southwest. Although there are no active volcanoes there today, evidence of past activity—such as volcanic cones and craters, lava flows, dikes, and plugs—can be seen in a variety of locations, including volcanic fields throughout the Basin and Range and along the southern edge of the Colorado Plateau, tuff beds in and along the Rio Grande Rift, and the Carrizozo Malpais lava field in New Mexico.
See Chapter 2: Rocks for more information about the igneous, metamorphic, and sedimentary rocks of the Southwest.
The specific rock type at the surface has an important influence on the topography of a region. Certain rocks are able to resist weathering and erosion more easily than are others; resistant rocks that overlie weaker layers act as caps and form ridges. The great continental seas that advanced across the face of the continent during the Paleozoic and Mesozoic collected and preserved sediments that became sedimentary rocks, such as the deposits exposed along the walls of the Grand Canyon. Sedimentary rocks weather and erode differently than do crystalline (and generally harder) igneous and metamorphic rocks, such as those found at the base of the Grand Canyon and in the Rocky Mountains. Silica-rich igneous rocks have a crystalline nature and mineral composition that resists weathering far better than do the cemented grains of a sedimentary rock. The metamorphic equivalents of sedimentary and igneous rocks are often even more resistant than the original unmetamorphosed rocks, due to recrystallization. There are exceptions, however, such as schist, which is much weaker than its premetamorphic limestone or sandstone state.
The underlying structure of the rock layers also plays an important role in the topography at the surface. In the Southwest, with its semi-arid climates and lack of dense vegetative cover, the rock structures that influence the area’s topography are clearly revealed. Sedimentary rocks are originally deposited in flat-lying layers that rest on top of one another. The movement of tectonic plates creates stress and tension within the crust, especially at plate boundaries. Intrusions beneath the surface may also cause deformation of the crust. All these different sources of geological stress can deform flat sediment layers through folding, faulting, or overturning. These terms are collectively used to describe rock structure, and they can also be used to determine which forces have affected rocks in the past. The folding of horizontal rock beds followed by erosion and uplift brings layers of rock to the surface. Tilted rocks expose underlying layers; resistant layers stick out and remain as ridges, while surrounding layers of less resistant rock erode away. Faulting likewise exposes surface layers to erosion, as blocks of crust move and tilt along the fault plane. For example, the Basin and Range formed as a result of normal faulting (Figure 4.3A), which occurs due to extensional stresses that create uplifted ranges and downdropped basins. The Rocky Mountains provide another regional example of folding and faulting: this range formed as a result of uplift associated with subduction along the western edge of the North American plate. The shallow angle of the subducting plate generated thrust (reverse) faults (Figure 4.3B) and the onset of the Laramide Orogeny.
Just as we are able to make sense of the type of rocks in an area by knowing the geologic history of the Southwestern US, we are able to make sense of its topography (Figure 4.4) based on rocks and structures resulting from past geologic events. Topography is a central element of the broader concepts of geomorphology or physiography, which also include consideration of the shape (not just the height) of land forms, as well as the bedrock, soil, water, vegetation, and climate of an area, and how they interacted in the past to form the landscape we see today. A physiographic province is an area in which these features are similar, one in which these features are significantly different from those found in adjacent regions, and/or one that is separated from adjacent regions by major geological features. The "regions" of the Southwest that we use in this book are examples of major physiographic provinces. The topography unique to each region thus provides a set of clues to its extensive geologic history.
See Chapter 1: Geologic History for more information about the tectonics and mountain building that shaped the Southwest.
The rocks of the Southwest reach back into the Precambrian, but the area’s remarkable topography is primarily the result of dramatic Mesozoic and Cenozoic plate tectonics. Subduction of the Farallon plate during the late Cretaceous and Cenozoic, and the associated Sevier and Laramide orogenies, shaped the mountains of the Basin and Range as well as uplifiting the Rocky Mountains, Colorado Plateau, and the edge of the Great Plains. Late Cenozoic crustal extension led to faulting and volcanic activity that is most dramatically exhibited in the Basin and Range of Utah and Arizona and the Rio Grande Rift of New Mexico and Colorado. Southwestern topography particularly emphasizes broad epeirogenic uplift, one of the more contentious and least understood features of the Earth’s topography. In contrast to orogenic (mountain building) uplift, epeirogenic uplift raises the land surface without compressing and thus thickening the crust. During the Cenozoic, most of the Southwest was lifted by this process. Today, the elevation of nearly the entire Southwest lies above 1200 meters (4000 feet), except for the southwestern desert in Arizona (Figure 4.5). The north-south “backbone” of highest elevation, shown by areas above 1800 meters (6000 feet), stretches from New Mexico through Colorado and into Wyoming. The Rocky Mountains of Colorado define a particularly high area, where elevations exceed 2700 meters (9000 feet) over a large part of the state.