Explain how different desert landscapes develop.

To many, deserts are among Earth’s most beautiful and dramatic landscapes. In the desert, there is a contrast between the harsh, barren, and inhospitable nature of the place and its endless vistas, intriguing physical landforms, and sense of quiet timelessness. Prominent desert landscapes include regs, ergs, mesa-and-butte terrain, inselbergs, basin-and-range topography, and badlands. Each of these landscapes bears the signature of water, wind, and time.

A flat, stony plain, or reg, may be formed by either of two different processes. The first is aeolian sorting, which results in accumulations of similarly sized particles, ranging in size from fields of boulders to sand dunes composed of quartz grains to loess deposits composed of fine silt. Small particles blow farthest, while heavier particles travel shorter distances.

Most rocks are not moved at all by the wind. As lighter particles, such as clay and silt, are transported away in the process of deflation, an accumulation of rocks, called a lag deposit (Figure 18.12), may develop.

The second process forms regs consisting of stones that are tightly interlocked, as if they had been set in place by people. Desert pavement is a flat, rocky surface that resembles a cobblestone street. Beneath desert pavement are aridisol soils composed of clay and silt. These soils often contain few or no stones. If deflation formed desert pavement, then one would expect to find stones within the soil that would later be brought to the surface by deflation. Recent research indicates, however, that desert pavement may form as desert soils are built up on bare bedrock (Figure 18.13).

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Regs, whether formed as lag deposits or desert pavement, occur in all deserts. They are also called hamada in the Sahara and gibber plains in Australia.

The top sides of the rocks in regs are often coated with rock varnish. Overturning or removing the rocks reveals a lighter surface. In some areas of southern Peru, the desert’s surface was scraped and overturned by people of the ancient Nazca civilization to draw huge geoglyphs on the desert floor (Figure 18.14).

An erg (Arabic for “dune field”) is a desert landscape that is dominated by actively moving sand dunes. Ergs, also called sand seas, are among Earth’s most dramatic landscapes. They are devoid of vegetation and, by definition, are at least 125 km² (50 mi²) in extent. In all ergs, sand covers at least 20% of the desert floor. Any type of sand dune may form in an erg, and some ergs are flat sheets of sand with few dunes.

Ergs are most common where the subtropical high creates arid conditions. Ergs cannot develop in humid climates because vegetation will become established and stabilize the sand. The source of sand for ergs varies, but it is often derived from glacial outwash plains, dry lake beds, or river deltas. In much of the Middle East, such as Saudi Arabia, geologic uplift exposed marine sediments that were eroded to form ergs.

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It takes hundreds of thousands to millions of years for large ergs to develop. Large ergs are found in the Namib Desert of Namibia, the Taklamakan Desert in China, the Gobi Desert in Mongolia, interior Australia, and portions of southwestern North America. Earth’s largest erg is the Rub’ al Khali (or the Empty Quarter) on the Arabian Peninsula. It is estimated to have been active for over a million years. It covers 583,000 km² (225,000 mi²) and stretches from Saudi Arabia into Yemen, Oman, and the United Arab Emirates. After the Rub’ al Khali, Earth’s largest ergs are found in the Sahara, particularly in Algeria and Libya (Figure 18.15).

Like sand seas, mesas and buttes come to mind for many when they picture a typical desert landscape. Mesa-and-butte terrain, however, is determined more by the composition and structure of the bedrock than by climate. Mesa-and-butte terrain also occurs in humid climates, but it is easiest to see in arid desert climates because it is not obscured by vegetation. The tepuis in Venezuela and western Guyana, for example, form mesa-and-butte terrain that is covered by tropical savanna and tropical rainforest.

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Most mesas, buttes, and chimneys occur where a plateau has been uplifted, weathered, and eroded by flowing water. A mesa is flat-topped elevated area with one or more cliff-face sides and is wider than it is tall. A butte is a medium-sized flat-topped hill with cliff-face sides and is taller than it is wide. A chimney is a slender spire of rock with a flat top. Chimneys are far less common than mesas and buttes.

Although mesa-and-butte terrain can form on lava flows without geologic uplift, it is most common in sedimentary rocks that have been uplifted. Examples of mesa-and-butte terrain are found throughout the southwestern United States, particularly in Monument Valley on the Colorado Plateau. Figure 18.16 illustrates how mesa-and-butte terrain develops.

The surface of a flat-topped mesa has little or no dip (or slope). Dipping occurs where sedimentary rock has been folded or tilted through faulting (see Section 12.4). Where sedimentary rock at the ground surface dips gently, a cuesta is formed (Figure 18.17A). Hogbacks form where sedimentary rock at the ground surface dips steeply (Figure 18.17B).

An inselberg (meaning “island mountain”) is a large, weathered outcrop of bedrock surrounded by a flat plain. Inselbergs are exposed at the land surface through the process of exhumation (the removal of overlying sediments and rock). Inselbergs form because they are more resistant to erosion than the sediments and rocks surrounding them. Like mesas and buttes, inselbergs are not restricted to desert climates, but they are easiest to observe there. Kata Tjuta (or the Olgas) and Uluru (or Ayers Rock) in central Australia are two dramatic examples of inselbergs formed from sedimentary rocks (Figure 18.18).

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Bornhardts are inselbergs that take on a more rounded form and are usually derived from erosion-resistant crystalline rocks such as granite or gneiss. As the land surface is lowered through erosion, a crystalline rock body, such as a pluton, batholith, or laccolith (see Section 13.2), is exposed and rounded through weathering (Figure 18.19).

The Basin-and-Range Province is located in the Great Basin of the western United States, where the crust is being stretched through rifting (see Figure 14.21), forming a large internal drainage basin containing a series of tilted fault blocks called horsts and a series of valleys called grabens. Figure 18.20 illustrates the process that has created the Basin-and-Range Province.

Basin-and-range topography is not restricted to deserts. It is included in this section, however, because it is a prominent desert landscape in western North America. The only comparable region of widespread extension of continental crust is in the East African rift zone.

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Badlands are landscapes riddled by rills and gullies (Figure 18.21). They occur mostly in arid to semiarid environments where slopes are largely unvegetated and weak rocks, such as shale, are eroded by infrequent but intense rainfall. They often have loose, dry, clay-rich soils. Badlands are “bad” because they cannot be farmed and because they are difficult to cross. They range in size from a few grouped hillsides to thousands of square kilometers. Badlands often have many fossils preserved in their sedimentary rocks.