By the end of this chapter, students should be able to:
- Explain the defining characteristic of a desert and distinguish between the three broad categories of deserts.
- Explain how geographic features, , atmospheric circulation, and Coriolis Effect influence where deserts are located.
- List the primary desert and processes.
- Identify desert landforms.
- Explain how desert landforms are formed by and .
- Describe the main types of sand and the conditions that form them.
- Identify the main features of the desert (United States).
Approximately 30% of the Earth’s surface consists of deserts, which are defined as locations of low . While extremes are often associated with deserts, they do not define them. Deserts exhibit extreme temperatures because of the lack of moisture in the , including low humidity and scarce cloud cover. Without cloud cover, the Earth’s surface absorbs more of the Sun’s energy during the day and emits more heat at night.
Deserts are not randomly located on the Earth’s surface. Many deserts are located at latitudes between 15° and 30° in both hemispheres and at both the North and South Poles, created by prevailing wind circulation in the . Sinking, dry air currents occurring at 30° north and south of the equator produce that create deserts like the African Sahara and Australian Outback.
Another type of desert is found in the rain shadow created from prevailing winds blowing over mountain ranges. As the wind drives air up and over mountains, atmospheric moisture is released as snow or rain. Atmospheric pressure is lower at higher elevations, causing the moisture-laden air to cool. Cool air holds less moisture than hot air, and occurs as the wind rises up the mountain. After releasing its moisture on the windward side of the mountains, the dry air descends on the leeward or downwind side of the mountains to create an arid region with little called a rain shadow. Examples of rain-shadow deserts include the Western Interior Desert of North America and of Chile, which is the Earth’s driest, warm desert.
Finally, , such as vast areas of the Antarctic and Arctic, are created from sinking cold air that is too cold to hold much moisture. Although they are covered with ice and snow, these deserts have very low average annual . As a result, Antarctica is Earth’s driest .
Video 13.1: Rain shadow.
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13.1 The Origin of Deserts
13.1.1 Atmospheric Circulation
Geographic location, atmospheric circulation, and the Earth’s rotation are the primary causal factors of deserts. Solar energy converted to heat is the engine that drives the circulation of air in the and water in the oceans. The strength of the circulation is determined by how much energy is absorbed by the Earth’s surface, which in turn is dependent on the average position of the Sun relative to the Earth. In other words, the Earth is heated unevenly depending on and . is a line circling the Earth parallel to the equator and is measured in degrees. The equator is 0° and the North and South Poles are 90° N and 90° S respectively (see the diagram of generalized atmospheric circulation on Earth). is the angle made by a ray of sunlight shining on the Earth’s surface. Tropical zones are located near the equator, where the and are close to 0°, and receive high amounts of solar energy. The poles, which have latitudes and angles of incidence approaching 90°, receive little or almost no energy.
The figure shows the generalized air circulation within the . Three cells of circulating air span the space between the equator and poles in both hemispheres, the , the Ferrel or Midlatitude Cell, and the . In the located over the tropics and closest to the equatorial belt, the sun heats the air and causes it to rise. The rising air cools and releases its contained moisture as tropical rain. The rising dried air spreads away from the equator and toward the north and south poles, where it collides with dry air in the Ferrel Cell. The combined dry air sinks back to the Earth at 30° . This sinking drier air creates belts of predominantly high pressure at approximately 30° north and south of the equator, called the horse latitudes. Arid zones between 15o and 30o north and south of the equator thus exist within which desert conditions predominate. The descending air flowing north and south in the Hadley and Ferrel cells also creates prevailing winds called near the equator, and in the temperate zone. Note the arrows indicating general directions of winds in these zones.
Other deserts, like the that covers parts of Utah and Nevada, owe at least part of their origin to other atmospheric phenomena. The , while somewhat affected by sinking air effects from global circulation, is a rain-shadow desert. As westerly moist air from the Pacific rises over the Sierra Nevada and other mountains, it cools and loses moisture as condensation and on the upwind or rainy side of the mountains.
One of the driest places on Earth is the of northern Chile. The occupies a strip of land along Chile’s just north of 30°S, at the southern edge of the trade-wind belt. The desert lies west of the Andes Mountains, in the rain shadow created by prevailing blowing west. As this warm moist air crossing the Amazon meets the eastern edge of the mountains, it rises, cools, and precipitates much of its water out as rain. Once over the mountains, the cool, dry air descends onto the . Onshore winds from the Pacific are cooled by the Peru (Humboldt) ocean current. This super-cooled air holds almost no moisture and, with these three factors, some locations in the have received no measured for several years. This desert is the driest, non-polar location on Earth.
Notice in the figure that the polar regions are also areas of predominantly high pressure created by descending cold dry air, the . As with the other cells, cold air, which holds much less moisture than warm air, descends to create . This is why historically, land near the north and south poles has always been so dry.
13.1.2 Coriolis Effect
The Earth rotates toward the east where the sun rises. Think of spinning a weight on a string around your head. The speed of the weight depends on the length of the string. The speed of an object on the rotating Earth depends on its horizontal distance from the Earth’s of rotation. Higher latitudes are a smaller distance from the Earth’s rotational , and therefore do not travel as fast eastward as lower that are closer to the equator. When a fluid like air or water moves from a lower to a higher , the fluid maintains its momentum from moving at a higher speed, so it will travel relatively faster eastward than the Earth beneath at the higher latitudes. This factor causes deflection of movements that occur in north-south directions.
Another factor in the Coriolis effect also causes deflection of east-west movement due to the angle between the centripetal effect of Earth’s spin and gravity pulling toward the earth’s center (see figure 13.9). This produces a net deflection toward the equator. The total Coriolis deflection on a mass moving in any direction on the rotating Earth results from a combination of these two factors.
Since each hemisphere has three atmospheric cells moving respectively north and south relative to the Earth beneath them, the Coriolis effect deflects these moving air masses to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect also deflects moving masses of water in the ocean currents.
For example, in the northern hemisphere , the lower altitude air currents are flowing south towards the equator. These are deflected to the right (or west) by the Coriolis effect. This deflected air generates the prevailing that European sailors used to cross the Atlantic Ocean and reach South America and the Caribbean Islands in their tall ships. This air movement is mirrored in the in the southern hemisphere; the lower altitude air current flowing equatorward is deflected to the left, creating that blow to the northwest.
In the northern Mid-Latitude or Ferrel Cell, surface air currents flow from the horse latitudes ( 30°) toward the North Pole, and the Coriolis effect deflects them toward the east, or to the right, producing the zone of westerly winds. In the southern hemisphere Mid-Latitude or Ferrel Cell, the poleward flowing surface air is deflected to the left and flows southeast creating the Southern Hemisphere .
Another Coriolis-generated deflection produces the . At 60o north and south , relatively warmer rising air flows poleward cooling and converging at the poles where it sinks in the polar high. This sinking dry air creates the , the driest deserts on Earth. Persistence of ice and snow is a result of cold temperatures at these dry locations.
The Coriolis effect operates on all motions on the Earth. Artillerymen must take the Coriolis effect into account on ballistic trajectories when making long-distance targeting calculations. Geologists note how its effect on air and currents creates deserts in designated zones around the Earth as well as the surface currents in the ocean. The Coriolis effect causes the ocean to turn clockwise in the northern hemisphere and counterclockwise in the southern. It also affects by creating high-altitude, polar jet that sometimes push lobes of cold arctic air into the temperate zone, down to as far as 30° from the usual 60°. It also causes low pressure systems and intense tropical storms to rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Video 13.2: The Coriolis effect.
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13.2 Desert Weathering and Erosion
takes place in desert climates by the same means as other climates, only at a slower rate. While higher temperatures typically spur faster , water is the main agent of , and lack of water slows both mechanical and . Low levels also mean less as well as . When does occur in the desert, it is often heavy and may result in in which a lot of material may be dislodged and moved quickly.
One unique product in deserts is . Also known as desert patina or rock rust, this is thin dark brown layers of clay and iron and manganese that form on very stable surfaces within arid environments. The exact way this material forms is still unknown, though cosmogenic and biologic mechanisms have been proposed.
While water is still the dominant agent of in most desert environments, wind is a notable agent of and in many deserts. This includes suspended traveling in , or large dust storms, that frequent deserts. Deposits of windblown dust are called . deposits cover wide areas of the midwestern United States, much of it from rock flour that melted out of the during the last . Loess was also blown from desert regions in the West. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust. As noted in chapter 11, the load carried by a fluid (air is a fluid like water) is distributed among and . As with water, in wind these components depend on wind velocity.
Sand size material moves by a process called in which sand grains are lifted into the moving air and carried a short distance where they drop and splash into the surface dislodging other sand grains which are then carried a short distance and splash dislodging still others.
Since saltating sand grains are constantly impacting other sand grains, wind blown sand grains are commonly quite well rounded with frosted surfaces. is a cascading effect of sand movement creating a zone of wind blown sand up to a meter or so above the ground. This zone of saltating sand is a powerful agent in which features are effectively sandblasted. The fine-grained is effectively sorted from the sand near the surface carrying the silt and dust into . Wind is thus an effective agent separating sand and dust sized (≤70 µm) particles (see chapter 5). When wind velocity is high enough to slide or roll materials along the surface, the process is called .
One extreme version of movement was shrouded in mystery for years: . Also called and , these are large moving boulders along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with little definitive results. In recent years, several experimental and observational studies have confirmed that the stones, imbedded in thin layers of ice, are propelled by friction from high winds. These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab.
The zone of saltating sand is an effective agent of through sand abrasion. A outcrop which has such a sandblasted shape is called a . Rocks and boulders lying on the surface may be blasted and polished by saltating sand. When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such wind abraded desert rocks are called .
In places with sand and silt accumulations, clumps of vegetation often anchor on the desert surface. Yet, winds may be sufficient to remove materials not anchored by vegetation. The bowl-shaped depression remaining on the surface is called a .
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13.3 Desert Landforms
In the American Southwest, as emerge into the valleys from the adjacent mountains, they create desert landforms called fans. When the emerges from the narrow canyon, the flow is no longer constrained by the canyon walls and spreads out. At the lower slope angle, the water slows down and drops its coarser load. As the channel fills with this conglomeratic material, the is deflected around it. This deposited material deflects the into a of radial distributary channels in a process similar to how a is made by a entering a body of water. This process develops a of radial distributaries and constructs a fan shaped feature called an fan.
fans continue to grow and may eventually coalesce with neighboring fans to form an apron of along the mountain front called a .
As the mountains erode away and their accumulates first in fans, then , the mountains eventually are buried in their own erosional debris. Such buried mountain remnants are called , “island mountains,” as first described by the German geologist Wilhelm Bornhardt (1864–1946).
Where the desert valley is an enclosed , i.e. entering it do not drain out, the water is removed by evaporation and a dry lake is formed called a .
are among the flattest of all landforms. Such a dry lake may cover a large area and be filled after a heavy thunderstorm to only a few inches deep. lakes and desert that contain water only after rainstorms are called intermittent or . Because of intense thunderstorms, the volume of water transported by drainage in arid environments can be substantial during a short of time. Desert soil structures lack organic matter that promotes infiltration by absorbing water. Instead of percolating into the , the compacts the ground surface, making the hydrophobic or water-repellant. Because of this hardpan surface, may gather water across large areas, suddenly filling with water from storms many miles away.
High-volume flows, called , may move as sheet flows or , as well as be channeled through normally dry or canyons. Flash floods are a major factor in desert . Dry channels can fill quickly with , creating a mass of water and debris that charges down the , and even overflowing the banks. pose a serious hazard for desert travelers because the storm activity feeding the may be miles away. People hiking or camping in that have been bone dry for months, or years, have been swept away by sudden .
The popular concept of a typical desert is a broad expanse of sand. Geologically, deserts are defined by a lack of water and arid regions resembling a sea of sand belong to the category of desert called an . An consists of fine-grained, loose sand grains, often blown by wind, or forces, into . Probably the best known is the Rub’ al Khali, which means Empty Quarter, of the Arabian Peninsula. Ergs are also found in the Great Sand Dunes National Park (Colorado), Little Sahara Recreation Area (Utah), White Sands National Monument (New Mexico), and parts of Death Valley National Park (California). are not restricted to deserts, but may form anywhere there is a substantial supply of sand, including as far north as 60° N in Saskatchewan, Canada, in the Athabasca Sand Dunes Provincial Park. Coastal exist along lakes and oceans as well, and examples are found in Oregon, Michigan, and Indiana.
An internal cross section of a sand shows a feature called . As wind blows up the windward side of the , it carries sand to the crest depositing layers of sand parallel to the windward (or “stoss”) side. The sand builds up the crest of the and pours over the top until the leeward (downwind or slip) face of the reaches the , the maximum angle which will support the slip face. are unstable features and move as the sand erodes from the stoss side and continues to drop down the leeward side covering previous stoss and slip-face layers and creating the cross . Mostly, these are reworked over and over again, but occasionally, the features are preserved in a depression, then lithified. Shifting wind directions and abundant sand sources create chaotic patterns of cross like those seen in Zion National Park of Utah.
In the , Utah was covered by a series of , with the thickest being in Southern Utah, which lithified into (see chapter 5). Perhaps the best known of these is the Navajo Sandstone of age. This formation consists of dramatic cliffs and spires in Zion National Park and covers a large part of the Colorado Plateau. In Arches National Park, a later series of sand covered the Navajo Sandstone and lithified to become the Entrada Formation also during the . of overlying layers exposed fins of the underlying Entrada Sandstone and carved out weaker parts of the fins forming the arches.
As the cements that hold the grains together in these modern sand cliffs disintegrate and the freed grains gather at the base of the cliffs and move down the washes, sand grains may be recycled and redeposited. These sand may represent ancient sands recycled many times from origins in the early , just passing now through another cycle of and . An example of this is Coral Pink Sand Dunes State Park in Southwestern Utah, which contains sand that is being eroded from the Navajo Sandstone to form new .
are complex features formed by a combination of wind direction and sand supply, in some cases interacting with vegetation. There are several types of representing variables of wind direction, sand supply and vegetative anchoring. Barchan or crescent form where sand supply is limited and there is a fairly constant wind direction. Barchans move downwind and develop a crescent shape with wings on either side of a crest. Barchans are known to actually move over homes, even towns.
or form where sand supply is greater and the wind blows around a dominant direction, in a back-and-forth manner. They may form ridges tens of meters high lined up with the predominant wind directions.
form where vegetation anchors parts of the sand and unanchored parts . shape may be similar to barchan but usually reversed, and it is determined more by the anchoring vegetation than a strict parabolic form.
form where the wind direction is variable in all directions. Sand supply can range from limited to quite abundant. It is the variation in wind direction that forms the star.
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13.4 The Great Basin and the Basin and Range
The Great Basin is the largest area of interior in North America, meaning there is no outlet to the ocean and all remains in the or is evaporated. It covers western Utah, most of Nevada, and extends into southeastern California, southern Oregon, and southern Idaho. Because there is not outlet to the ocean, in the Great Basin deliver to lakes and within the . A subregion within the Great Basin is the which extends from the Wasatch Front in Utah west across Nevada to the Sierra Nevada Mountains of California. The basins and ranges referred to in the name are and , formed by blocks from crustal , as discussed in chapter 2 and chapter 9. The of the entire area has stretched by a factor of about 2, meaning from end to end, the distance has doubled over the past 30 million years or so. Valleys without outlets form individual basins, each of which is filled with leading into . During the recent , The climate was more humid and while glaciers were forming in some of the mountains, formed covering large areas (see section 14.4.3). During the , valleys in much of western Utah and eastern Nevada were covered by Lake Bonneville. As the became arid after the , Lake Bonneville dried leaving as a remnant the Great Salt Lake in Utah.
The desert of the extends from about 35° to near 40° and results from a rain shadow effect created by westerly winds from the Pacific rising and cooling over the Sierras becoming depleted of moisture by on the western side. The result is relatively dry air descending across Nevada and western Utah.
A journey from the Wasatch Front southwest to the Pacific Ocean will show stages of desert landscape evolution from the block mountains of Utah with sharp peaks and fans at the mouths of canyons, through landscapes in Southern Nevada with along the mountain fronts, to the landscapes in the Mojave Desert of California with subdued sticking up through a sea of coalesced . These landscapes illustrate the evolutionary stages of desert landscape development.
When previously arable land suitable for agriculture transforms into desert, this process is called . Plants and -rich (see chapter 5) promote and water retention. When an area becomes more arid due to changing environmental conditions, the plants and become less effective in retaining water, creating a loop of . This self-reinforcing loop spirals into increasingly arid conditions and further enlarges the desert regions.
may be caused by human activities, such as unsustainable crop cultivation practices, overgrazing by livestock, overuse of groundwater, and global change. Human-caused desertification is a serious worldwide problem. The world map figure above shows what areas are most vulnerable to . Note the red and orange areas in the western and midwestern regions of the United States, which also cover large areas of arable land used for raising food crops and animals. The creation of the Dust Bowl in the 1930s (see chapter 5) is a classic example of a high-vulnerability region impacted by human–caused . As demonstrated in the Dust Bowl, conflicts may arise between agricultural practices and conservation measures. Mitigating while allowing farmers to make a survivable living requires public and individual education to create community support and understanding of sustainable agriculture alternatives.
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Approximately 30% of Earth’s surface is arid lands, the location of which is determined by , atmospheric circulation, and terrain. The arid belts between 15o and 30o north and south latitudes are produced by descending air masses associated with major cells in the and include the major deserts like the Sahara in Africa and the Middle East. lie behind mountain ranges or long land expanses in zones of prevailing winds like the deserts of western North America, the Atacama of South America, and the Gobi of Asia. Dry descending air also creates the at the poles.
Major atmospheric circulation involves the , midlatitude or Ferrel cells, and the in each hemisphere. Warmed and rising air in the rains back on the tropics and moves toward the poles as dryer air. That air meets the dryer equatorward moving air of the Ferrel cells. This dry air descends in the arid zones, called the horse latitudes, to produce the arid belts in each hemisphere. Rotation of the Earth creates the Coriolis Effect that deflects these moving air masses to produce zones of prevailing winds, the in the subtropics and the in the midlatitudes. A combination of , rain shadow, and cold adjacent ocean currents causes the of northern Chile, the driest desert on Earth.
in deserts takes place just as in other climes only slower because of less water. is a product unique to desert environments. As in more humid climes, water is the main agent of although wind is a notable agent. Large dust storms called transport large amounts of that may accumulate in sand seas called or finer grained deposits. Sand transport occurs mainly by in which grain to grain impact causes frosting of grain surfaces. Sandblasting by persistent winds produces stones with polished surfaces called and sculpted features called .
Landforms produced in desert environments include fans, , , and . Windblown sand can accumulate as . The forms of , like barchans, parabolic, longitudinal, and , relate to the abundance of sand supply and wind direction as well as presence of vegetation. The internal structure of shows . in an ancient desert leave in places like Zion and Arches national parks in Utah showing shifting wind directions in these ancient environments. in desert regions may carry water only after storms and pose risk of .
The Great Basin of North America is an enclosed with no to the ocean. The only exit for there is by evaporation. Travels in the Great Basin show stages of development of desert landscapes from and fans, to , to which are eroded mountains buried in their own erosional debris.
Poor land management can result in dying vegetation and loss of moisture producing an accelerating process of in which once productive land is degraded into unproductive desert. This is a serious worldwide problem.
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Figure 13.1: World hot deserts (Koppen BWh). Peel, M. C., Finlayson, B. L., and McMahon, T. A.; adapted by Me ne frego. 2011. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Koppen_World_Map_BWh.png
Figure 13.2: Mountainous areas in front of the prevailing winds create a rain shadow. domdomegg. 2015. CC BY 4.0. https://commons.wikimedia.org/wiki/File:Rainshadow_copy.svg
Figure 13.3: In this image from the ISS, the Sierra Nevada Mountains are perpendicular to prevailing westerly winds, creating a rain shadow to the east (down in the image). NASA. 2003. Public domain. https://earthobservatory.nasa.gov/images/3273/southern-sierra-nevada-and-owens-lake
Figure 13.4: Generalized atmospheric circulation. NASA. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Earth_Global_Circulation.jpg
Figure 13.5: USGS Map of the Great Basin Desert. USGS. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Central_Basin_and_Range_ecoregion.gif
Figure 13.6: Map of the Atacama desert (yellow) and surrounding related climate areas (orange). cobaltcigs. 2010. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Atacama_map.svg
Figure 13.7: The polar vortex of mid-November, 2013. NOAA. 2014. Public domain. https://commons.wikimedia.org/wiki/File:November2013_polar_vortex_geopotentialheight_mean_Large.jpg
Figure 13.8: In the inertial frame of reference of the top picture, the ball moves in a straight line. Hubi. 2003. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Corioliskraftanimation.gif
Figure 13.9: Forces acting on a mass moving East–West on the rotating Earth that produce the Coriolis Effect. Kindred Grey. 2022. CC BY 4.0.
Figure 13.10: Inertia of air masses caused by the Coriolis Effect in the absence of other forces. Kes47. 2015. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Coriolis_effect.svg
Figure 13.11: Gyres of the Earth’s oceans. Canuckguy; adapted by Shadowxfox and Popadius. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Corrientes-oceanicas-en.svg
Figure 13.12: Weathering and erosion of Canyonlands National Park has created a unique landscape, including arches, cliffs, and spires. Rick. 2005. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Mesa_Arch_Canyonlands_National_Park.jpg
Figure 13.13: Newspaper rock, near Canyonlands National Park, has many petroglyphs carved into desert varnish. Cacophony. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Newspaper_Rock_Full.jpg
Figure 13.14: A dust storm (haboob) hits Texas in 2019. Jakeorin. 2019. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Haboob_in_Big_Spring,_TX.jpg
Figure 13.15: Diagram showing the mechanics of saltation. NASA. 2002. Public domain. https://commons.wikimedia.org/wiki/File:Saltation-mechanics.gif
Figure 13.16: Enlarged image of frosted and rounded windblown sand grains. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:CoralPinkSandDunesSand.JPG
Figure 13.17: A “sailing stone” at Racetrack Playa in Death Valley National Park, California. Lgcharlot. 2006. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Racetrack_Playa_in_Death_Valley_National_Park.jpg
Figure 13.18: Wind-carved ventifact in White Desert National Park, Egypt. Christine Schultz. 2003. Public domain. https://commons.wikimedia.org/wiki/File:Weisse_W%C3%BCste.jpg
Figure 13.19: A yardang near Meadow, Texas. United States Department of Agriculture. 2000. Public domain. https://commons.wikimedia.org/wiki/File:Yardang_Lea-Yoakum_Dunes.jpg
Figure 13.20: Blowout in Texas. Leaflet. 1996. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Blowout_Earth_TX.jpg
Figure 13.21: Aerial image of alluvial fan in Death Valley. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Alluvial_Fan.jpg
Figure 13.22: Bajada along Frisco Peak in Utah. GerthMichael. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:FriscoMountainUT.jpg
Figure 13.23: Inselbergs in the Western Sahara. Nick Brooks. 2007. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Breast-Shaped_Hill.jpg
Figure 13.24: Satellite image of desert playa surrounded by mountains. Robert Simmon, using Landsat data from the U.S. Geological Survey and NASA. 2013. Public domain. https://earthobservatory.nasa.gov/images/80913/eye-exam-for-a-satellite
Figure 13.25: Dry wash (or ephemeral stream). Finetooth. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Dry_Wash_in_PEFO_NP.jpg
Figure 13.26: Flash flood in a dry wash. USGS. 2016. Public domain. https://www.usgs.gov/media/images/a-flooded-river-australia
Figure 13.27: Sahara Desert, a sand sea or erg. Wsx. 2004. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Sahara_Desert_in_Jalu,_Libya.jpeg
Figure 13.28: Formation of cross bedding in sand dunes. David Tarailo, GSA, GeoCorps Program. 2015. Public domain. https://commons.wikimedia.org/wiki/File:Formation_of_cross-bedding.jpg
Figure 13.29: Cross beds in the Navajo Sandstone at Zion National Park. Roy Luck. 2010. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Sandstone_showing_Cross-bedding_Zion_National_Park_Utah_USA.jpg
Figure 13.30: Enlarged image of frosted and rounded windblown sand grains. Wilson44691. 2008. Public domain. https://commons.wikimedia.org/wiki/File:CoralPinkSandDunesSand.JPG
Figure 13.31: NASA image of barchan dune field in coastal Brazil. NASA. 2003. Public domain. https://earthobservatory.nasa.gov/images/3889/coastal-dunes-brazil
Figure 13.32: Satellite image of longitudinal dunes in Egypt. NASA. 2012. Public domain. https://earthobservatory.nasa.gov/images/78151/linear-dunes-great-sand-sea-egypt
Figure 13.33: Parabolic dunes. Po ke jung. 2011. CC BY 3.0. https://commons.wikimedia.org/wiki/File:Parabolic_dune.jpg
Figure 13.35: The Great Basin. Kmusser; adapted by Hike395. 2022. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Great_Basin_map.gif
Figure 13.36: Typical Basin and Range scene. Matt Affolter (QFL247). 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:RidgecrestCA.JPG
Figure 13.37: World map showing desertification vulnerability. USDA. 1998. Public domain. https://commons.wikimedia.org/wiki/File:Desertification_map.png
The measure of degrees north or south from the equator, which has a latitude of 0 degrees. The Earth's north and south poles have latitudes of 90 degrees north and south, respectively.
Breaking down rocks into small pieces by chemical or mechanical means.
The transport and movement of weathered sediments.
Sediment gathering together and collecting, typically in a topographic low point.
A large pile of sediment, deposited perpendicular to flow. Internal bedding in dunes dips toward flow direction (i.e. cross bedding). Formed in the upper part of the lower flow regime.
Term for the extensional tectonic province that extends from California’s Sierra Nevada Mountains in the west, to Utah’s Wasatch Mountains to the east, to southern Oregon and Idaho to the north, to northern Mexico to the south. Known as a wide rift, each graben (basin) is bounded by horsts (ranges).
Depositional environments that are on land.
The act of a solid coming out of solution, typically resulting from a drop in temperature or a decrease of the dissolving material.
The measure of the vibrational (kinetic) energy of a substance.
The gases that are part of the Earth, which are mainly nitrogen and oxygen.
Wind patterns that move from east to west near the equator, due to global circulation patterns.
Driest nonpolar desert on Earth, located in west-central South America.
Deserts formed by descending air at the poles.
The layers of igneous, sedimentary, and metamorphic rocks that form the continents. Continental crust is much thicker than oceanic crust. Continental crust is defined as having higher concentrations of very light elements like K, Na, and Ca, and is the lowest density rocky layer of Earth. Its average composition is similar to granite.
Angle from perpendicular to the ground surface at which light rays hit the ground. If the sun is directly above a point and light rays are hitting the ground directly, then the angle of incidence is 0.
A part of the global circulation system that rises at the equator and sinks at 30°.
Part of the global circulation pattern where air sinks at the poles (90° latitude) and rises at 60° latitude.
Winds that move from west to east between 30° and 60° latitude due to global circulation patterns.
Desert area stretching from California to the west, Utah to the east, and Idaho/Oregon to the north. Partially caused by latitude, partially caused by rain shadow.
The entire area which is related to land-sea interactions.
A down-warped feature in the crust.
Dividing two-dimensional line between the two sides of a fold.
The thin, outer layer of the Earth which makes up the rocky bottom of the ocean basins. Oceanic crust is much thinner (but denser) than continental crust. Oceanic crust is made of rocks similar to basalt and as it cools, becomes more dense.
Large circular ocean currents formed by global atmospheric circulation patters.
Current conditions within the atmosphere.
A channelled body of water.
Breaking down of mineral material via chemical methods, like dissolution and oxidation.
Water that flows over the surface.
A process where water freezes inside cracks in rocks, causing expansion and mechanical weathering.
Dangerous flooding that occurs in arid regions.
Dark mineralization that forms on rocks in desert environments.
A natural substance that is typically solid, has a crystalline structure, and is typically formed by inorganic processes. Minerals are the building blocks of most rocks.
Minerals in which ions are bonded to oxygen, such as hematite (Fe2O3).
Pieces of rock that have been weathered and possibly eroded.
Dust storms that occur in desert areas.
Wind-blown silt, mainly formed from glacial processes.
Thick glaciers that cover continents during ice ages.
A period of cooler temperatures on Earth in which ice sheets can grow on continents.
Sediment that large and dense, typically sits on the bottom of stream channels, and is only moved with higher-speed flows.
Bedload sediments that can be carried by higher-velocity flows.
Silt and sand that is lifted from the bed and transported for short distances.
Term for the underlying lithified rocks that make up the geologic record in an area. This term can sometimes refer to only the deeper, crystalline (non-layered) rocks.
The range of sediment sizes within a sediment or sediment within sedimentary rocks. Well sorted means the sediment has the same sizes, poorly sorted means many different sizes are present.
A slow and steady movement. Can occur as part of faults, mass wasting, and grain movement.
Pieces of rock that have been weathered and possibly eroded.
Rocks that move along thin ice sheets with high winds.
A dry lake bed in a desert valley.
Erosional rock face caused by sand abrasion.
Rock with abraded surfaces formed in deserts.
A depression in dune sediment formed because of a lack of anchoring vegetation.
Depositional environments that are associated with running water.
An interconnected set of parts that combine and make up a whole.
Place where rivers enter a large body of water, forming a triangular shape as the river deposits sediment and switches course.
Loose sediment deposited from running water.
A group of several alluvial fans that have come together and formed a single surface.
Isolated piece of bedrock which sticks above an alluvial surface.
A specific layer of rock with identifiable properties.
A stream or river that can be wet or dry depending on the season.
A unit of the geologic time scale; smaller than an era, larger than an epoch. We are currently in the Quaternary period.
A type of non-eroded sediment mixed with organic matter, used by plants. Many essential elements for life, like nitrogen, are delivered to organisms via the soil.
Planar flow of water over land surfaces.
Dry riverbed in an arid region.
The area within a topographic basin or drainage divide in which water collects.
A vast stretch of sand dunes.
Deposition with wind-blown sediment.
A sedimentary structure that has inclined layers within an overall layer. Forms commonly in dunes, larger in eolian dunes.
Slope angle where shear forces and normal forces are equal.
Meaning "middle life," it is the middle era of the Phanerozoic, starting at 252 million years ago and ending 66 million years ago. Known as the Age of Reptiles.
The second largest span of time recognized by geologists; smaller than an eon, larger than a period. We are currently in the Cenozoic era.
A rock primarily made of sand.
An extensive, distinct, and mapped set of geologic layers.
The middle period of the Mesozoic era, 201-145 million years ago.
SiO2. Transparent, but can be any color imaginable with impurities. No cleavage, hard, and commonly forms equant masses. Perfect crystals are hexagonal prisms topped with pyramidal shapes. One of the most common minerals, and is found in many different geologic settings, including the dominant component of sand on the surface of Earth. Structure is a three-dimensional network of silica tetrahedra, connected as much as possible to each other.
Rocks that are formed from liquid rock, i.e. from volcanic processes.
A term for the collective time before the Phanerozoic (pre-541 million years ago), including the Hadean, Archean, and Proterozoic. Known for a lack of easy-to-find fossils.
Dunes that are much longer than wide, forming from wind that varies in two opposite directions.
Dunes that form semicircular shapes due to anchoring vegetation.
Dunes that form from many different wind directions.
Uplifted mountain block caused by normal faulting.
A valley formed by normal faulting.
A dip-slip fault in which the hanging wall drops relative to the footwall, caused by extensional forces.
Stresses that pull objects apart into a larger surface area or volume; stretching forces.
The outermost physical layer of the Earth, made of the entire crust and upper mantle. It is brittle and broken into a series of plates, and these plates move in various ways (relative to one another), causing the features of the theory of plate tectonics.
An interpretation of the rock record which describes the cause of sedimentation (i.e. ancient beach, river, swamp, etc.).
Lakes that form via increased precipitation with glacial climate shifts.
Long term averages and variations within the conditions of the atmosphere.
Planar feature where two blocks of bedrock move past each other via earthquakes.
The process that turns non-desert land into desert.
Organic rich material found in soil.
Water that is below the surface.
Water that works its way down into the subsurface.
A process that exacerbates the effects of an input, amplifying the output. That is, a one-directional loop that self-reinforces change.
Deserts that form as air loses moisture traveling over mountains.
Any evidence of ancient life.