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Introduction to Geology

Chapter 13 - Deserts

A desert is an arid, often sandy region of little rainfall, extreme temperatures, and with sparse vegetation or no vegetation at all. Dry regions comprise approximate about 30 percent of Earth's land area (Figure 13-1).

Factors influencing the location of deserts and arid lands include:
* Global wind patterns control regional weather patterns.
* Rising winds (low pressure) cool, causing precipitation; sinking winds (high pressure) warm and become unsaturated and dry.
* Arid environments occur in both warm and cool regions.
* Seasonal weather patterns also influence arid weather patterns (summer drought and winter monsoons).
* Mountain ranges block atmospheric flow, forcing precipitation on the upwind side, and rainshadow conditions may prevail downwind.
* Drought may create temporary desert conditions. Regional and global climate changes can expand or shrink desert regions.

Click on thumbnail images to get a larger view.
World Deserts
Fig. 13-1. Desert regions of the world. The largest desert region includes the African Sahara and Arabian Peninsula where large sand seas dominate the landscape

Global atmospheric circulation patterns and desert

Desert regions are not limited to hot climates, but it is related to the amounts of precipitation a region receives (Figures 13-2 and 13-3). The location of desert regions has most to do with the global atmospheric circulation system. The global "wind engine" is fueled by incoming solar radiation and moderated by water temperatures in the global oceans. Near the equator, where solar heating is direct and most intense, the air is warm and picks up moisture from the oceans. Along the Intertropical Convergence Zone, warm moist air rises, expands, cools, and produces precipitation (Figure 13-4). The air depleted of its heat and moisture moves northward and southward from the equatorial region to a region roughly 30º north and south of the equator to a region where the winds sink back to the surface. As they sink they warm up and become even dryer. This generally explains the location of most of the desert regions around the world.

A rainshadow is a dry region that occurs on the downwind side of a mountain range (or high volcano) that partially blocks the the flow of moist air, forcing precipitation on the prevailing windward side, and creates more arid conditions on the downwind side. Figure 13-5 illustrates the rainshadow effects caused by California's mountain ranges. High regions block or modify atmospheric circulation patterns. For instance the Himalayas and Tibetan Plateau produce the rainshadow effects experienced by Mongolia and western China (Gobi Desert).
World Temperature World Rainfall
Fig. 13-2. World average temperatures Fig. 13-3. World average rainfall
World wind zones California rainshadow
Fig. 13-5. California's desert regions: Prevailing winds from the ocean produce seasonal precipitation in the mountainous areas, but dry desert conditions prevail in the rainshadow regions east of the mountain ranges.
Fig. 13-4. Global wind circulation patterns impact regional climates. The world's large deserts occur where "sinking winds" produce arid conditions.
Other desert-like areas include steppe regions and arctic tundra regions. A steppe is a large semiarid area of flat unforested grassland, such as the Great Plains region of the United States and Canada. Large steppe regions occur in southern Russian and Siberia, and parts of Argentina and Australia.

Tundra is a region of permanent cold that is largely or entirely devoid of life. Much of northern Alaska, Canada, and Siberia are regions that receive very little precipitation. Parts of Antarctica also are very dry despite being blanketed with ice.

Drought

Drought can cause temporary desert-like conditions. Many parts of the American southwest from Texas to California have experienced major drought conditions. Long-term changes in the prevailing weather patterns in a region can set up drought conditions that my last for months to many years. During droughts, soil dries out and plant cover dies back. Drought causes desert regions to expand, especially when poor agricultural practices impact how the land responds to drought. The "Dust Bowl" of the 1930s in Oklahoma and Texas is an example when desert conditions prevailed in a region that under normal conditions received seasonal precipitation (Figure 13-6). Poor arid-land agricultural practices contributed to the Dust Bowl disasters.

Microclimate regions: The orientation of a mountain slope can influence local climate conditions. For instance south-facing slopes in the northern hemisphere are typically warmer and dryer than north facing slopes. This is because the south-facing slopes receive more direct sunlight and experience more daytime heating and drying. It is common for mountain slopes in the American West to have grassy or barren slopes on the south side of mountains and forested slopes (or at least more vegetation) on their cooler, moister northern slopes (Figure 13-7).
Dust Bowl, Texas, 1936
Fig. 13-6. Drought can create temporary or long-term desert-like conditions
Fremont Peak
Fig. 13-7. Dry microclimates:
south-facing slopes are drier
than north-facing slopes in
the northern hemisphere.
How does precipitation and weathering affect surface topography of arid regions?

Despite arid conditions in desert regions, precipitation in the form of infrequent thunderstorms is a primary cause of erosion in desert regions. Factors influencing weathering and erosion in desert regions include:

* Bedrock composition and topography influence development of desert landforms.
* Less weathering occurs in deserts because there is a lack of moisture and because there is reduced organic acids from plants.
* Infrequent storms cause rapid erosion and prevent the establishment of plants and soil formation.
* Intense rainfall causes flash floods and reduces infiltration.
* Desert environmental conditions allow salts, particularly calcium carbonate, to accumulate and reduce porosity and permeability in surficial sediments. Caliche is a hardened zone in soils and surficial deposits found in semiarid regions where of calcium carbonate and possibly other carbonates, clay minerals, or crystalline salts such as sodium chloride or sodium nitrate impregnated the pore spaces in the sediment or soil.

Landscape Characteristics in Desert Regions

The lack of plant cover and prevailing arid conditions are apparent in desert regions, but it is the influence of infrequent but heavy rainstorms that shape the landscape in arid environments. Erosion produces a number of landforms unique to desert environments.

Badlands
are the name applied to uncultivatable land with typically very rugged relief, a heavily eroded appearance, and bares little or no vegetation (Figures 13-8 to 13-10). Badlands typically occur in soft or poorly consolidated sediments that erode quickly when infrequent rains occur.

An arroyo
is a watercourse (water-carved gully, channel) in an arid region (Figures 13-11 to 13-13). Arroyos are typically dry (ephemeral) but are prone to flash floods after rare seasonal thunderstorms. When flash floods occur in desert regions, the flow is often more a mix of flowing mud than water, and large amounts of material can be transported long distances, including large boulders (such as in Figure 13-10).
Badlands in the Painted Desert, Arizona Gower Gulch, Death Valley, California Golden Canyon, Death Valley, California
Fig. 13-8. Badlands in
Petrified Forest National Park in the Painted Desert region of Arizona
Fig. 13-9. Badlands
(with Manly Beacon point)
Death Valley National Park,
California
Fig. 13-10. Badlands of
Golden Canyon in
Death Valley National Park,
California
Afton Canyon, Mojave Desert, California Gower Gulch arroyo, Death Valley, California Boulder in Marble Canyon, Death Valley, California
Fig. 13-11. Arroyo in Afton Canyon located in the Mojave Desert, California Fig. 13-12. Gower Wash (an arroyo) in Death Valley National Park, California Fig. 13-13. Granite boulder in bed of Marble Canyon in
Death Valley National Park,
California

Pediments and Inselbergs

A pediment is a gently inclined erosional surface carved into bedrock, typically covered with stream gravel that has developed at the foot of mountains (Figures 13-14 and 13-15). A pediment develops when running water erodes most of the mass of the mountain down to a base-level consistent with surrounding alluvial fans in an arid or semiarid region (alluvial fans are discussed below).

Ilsenberg, Joshua Tree National Park, California Sima Dome
Fig. 13-14. Inselbergs of on a pediment surface in Joshua Tree National Park, California. These inselbergs are mostly remnants of granitic plutons. Fig. 13-15. Pediment surface with inselbergs (small bumps) on Sima Dome, Mojave National Preserve, CA. Sima Dome is a spectacular example of a mountainous landscape that has eroded down to a base-level pediment surface. Only several inselbergs remain on a surface that was probably very rugged early in its geologic history. The sculpting of this unique landscape has been ongoing for many millions of years.
An inselberg is an isolated rocky hill or mountain rising above a plain-like landscape in a typically hot, dry region. Inselbergs are typically patches of resistant bedrock that withstand the effects of weathering and erosion that other rocks associated with a pediment surface.

Spheroidal granite boulder piles

In many desert regions where there is granitic bedrock (having a uniform texture and composition), weathering at or below the surface creates naturally rounded boulders. Weathering takes of the sharp edges created by intersecting fractures in the bedrock. Water and air seeps in along fractures and gradually degrades the granite into sandy sediment that easily washes away when infrequent rainstorms occur, leaving being stacks of granitic boulders (Figures 13-16 to 13-18). Grus is the name for grainy sand and fine gravel (sediments) derived from the weathering of granitic rocks, typically in arid or semiarid regions.
Chemical weathering Quail Springs, Joshua Tree National Park, California Ryans Peak, Joshua Tree National Park, California
Fig. 13-16. Formation of spheroidal boulders by weathering and erosion of granite. Fig. 13-17. Spheroidally weathered boulders, Quail Springs, Joshua Tree National Park, California Fig. 13-18. Spheroidally weathered boulders along I-8 in eastern San Diego County, California

Desert Pavement

In many desert regions, the ground surface is covered by gravel-sized rock fragments. A desert pavement is a gravel layer that forms on the surface of many desert landscapes where the wind has removed finer materials (sand, silt, dust). Desert pavements are common on old pediment surfaces and are well developed on old alluvial fan surfaces that have not been disturbed by flashflood erosion for a long period of time (Figures 13-19 and 13-20). The gravel on the surface not only protects the finer material (below) from being eroded, but can actually trap dust which get washed by rain into the layer immediately below a layer of gravel on the surface. The rock fragments on the surface of a desert pavement typically have dull-dark appearance from being exposed to long periods of solar radiation. The rocks may also appear abraded by the wind into interesting shaped rocks called ventifacts.
Desert pavement on a pediment surface Desert pavement
Fig. 13-19. A desert pavement partly covers a pediment surface (Mojave Preserve, CA). Fig. 13-20. Close-up view of a desert pavement in the Mojave Preserve, CA.

Desert Depositional Environments

Despite arid conditions, flowing water is the main mechanism eroding, transporting and depositing sediments. Mountain in desert regions frequently show a pattern called hour-glass canyons. Precipitation falls most intensely in upland regions. Coalescing stream carve bowl-shape headwater regions. As more stream merge to form a principle stream, the rate and intensity of erosion increases downslope carving a deep narrow confining canyon. Once the stream reaches the valley, the floodwaters slow down and drop their sediments, building up gently-sloped alluvial fans (Figures 13-21 to 13-24).
Desert Environments hour glass canyon
Fig. 13-23. Formation of an "hour-glass" canyon on a mountainside in a desert region.
alluvial fans
Fig. 13-24. Example of an hour-glass canyon and alluvial fan on the eastern flank of Telescope Peak in the Panamint Range (located on the west side of Death Valley, California).
Fig. 13-21. Desert depositional environments
Cross section of Death Valley
Fig. 13-22. Cross section of Death Valley

Formation of alluvial fans and bajadas

An alluvial fan is an outspread, gently sloping mass of sediment deposited by a stream where it issues out of the mouth of a narrow canyon draining from an upland area (Figures 13-25 to 13-27). Viewed from above, an alluvial fan typically has the shape of an open fan with the apex being at the mouth of the canyon. Alluvial fans are common in arid to semi-arid regions, but can be covered with forests in the California Coast Ranges. Alluvial fans may merge together to form an apron-like slope along the base of a mountain front, forming a bajada. A bajada is an alluvial plain along the base of a mountain front formed from the accumulation and coalescing of alluvial fans.
Furnace Creek, Death Valley, California Panamint Valley, California Copper Canyon fan, Death Valley, California
Fig. 13-25. Furnace Creek alluvial fan in Death Valley National Park, California Fig. 13-26. Alluvial fan with a recent debris flow deposit, Panamint Valley in Death Valley National Park, CA Fig. 13-27. Copper Canyon Fan, a large alluvial fan in Death Valley National Park, California.
Where a stream exits a canyon and dumps sediments on an alluvial fan, the upland portion of the alluvial fan tends to consist mostly of a mix boulders, cobbles, and gravel. Farther from the mouth of the canyon, the size of the sediment fraction decreases in size as the sediment is distributed by migrating braided channels spreading over an increasingly large area with a increasingly gentle slope profile. The toe region of alluvial fans are typically dominated by finer-grained sediments (sand and mud). The toe of an alluvial fan may merge with a valley trunk stream (a principle stream draining through a valley) of may merge directly with an ephemeral lake bed (called a playa) filling the lowest part of a valley.

Salt Lakes, Salt Pans, and Playas

A playa is an ephemeral (intermittently flooded) lake bed that occupies the lowest part of a internally drained, isolated valley in an arid or semiarid region. Many playas in desert regions are also "salt pans" (Figures 13-28 to 13-30).

A salt pan is a shallow basin, usually in a desert region, containing salt, gypsum, sodium carbonate, or other evaporite minerals that was deposited from an evaporated salt lake (playa) setting. Salt pans form in valleys where there is no external drainage. The Bonnieville Salt Flats on the west side of the Great Salt Lake is an example of an extensive salt pan.
Racetrack Playa, Death Valley, California Dantes View Death Valley salt pan
Fig. 13-28. Racetrack Playa in Death Valley National Park
California is surrounded by alluvial fans (a bajada).
Fig. 13-29. Death Valley's playa is a salt pan in the lowest part of Death Valley National Park, California Fig. 13-30. Death Valley playa (salt pan) showing salt polygons forming along the principle stream in the valley.
Glacial lakes in the Mojave Desert region

Lakes in Desert Regions

During the last glaciation period, lakes filled many of the valleys in the western United States. Figure 12-31 shows locations of many of the ancestral lakes that existed in the greater Mojave Desert region. Through the Holocene Epoch, the climate got warmer and drier, and all the lakes eventually dried up. These low areas still become flooded during wet periods, but they typically completely dry up during drought periods, leaving behind crusty layers of salty, fine-grained sediments on their surfaces.
Mono Lake, California Saratoga Spring oasis, Death Valley, California
Fig. 13-32. "Tufa Towers" in Mono Lake, a large salt lake on the east side of the Sierra Nevada Range in California Fig. 13-33. Saratoga Springs oasis in southern Death Valley National Park, California
Fig. 13-31. Ancient lakes of the Mojave Desert region An oasis is a fertile or green spot in a desert or wasteland, made so by the presence of water. Oases exist in locations where groundwater rises to the surface in arid environments. They may be the only source of freshwater in a large region, and are essential to supporting desert wildlife.
How does wind erode and transport materials?

Wind in arid regions lifts and carries away dust-sized particles. Sand grains are carried in the "bed load" moving by rolling and bouncing along the surface (a process called saltation).

Abrasion is the process of wearing down or rubbing away by means of friction, typically by wind-blown dust or sand.

Deflation is the removal of particles of rock, sand, soil or dust, by the wind, often leaving a rocky crust (a desert pavement) on a desert landscape.

Sediments deposited by the wind

A dune is an accumulation of wind-blown sand and/or silt found in association with deserts or sandy beach settings where there is a constant supply of silt to sand grain-sized sediments. A collection of dunes is called a dune field. Dune fields typically occur in the down-wind end of a valley where winds slow down or are unable to lift fine-grained materials over a barrier, such as an adjacent mountain range. A star dune is typically the highest dune in a dune field. Star dunes typically have three or four arms radiating from its usually higher center so that it resembles a star in shape. Star dunes form when winds blow from three or four directions, or when the wind direction shifts frequently or changes with the seasons. Extensive dune fields cover large portions of the Sahara Desert and Arabian Peninsula and other desert regions of the world.
Death Valley dust storm Ripple bedding
Fig. 13-34. Dust devils, Death Valley National Park, California Fig. 13-35. Ripples in wind-blown sand, Death Valley National Park
dunes with ripples Kelso Dunes
Fig. 13-36. Sand dunes with ripple bedding, near Stovepipe Wells, Death Valley National Park Fig. 13-37. Star dune in Kelso Dunes in the Mojave National Reserve,
California
In some desert regions, it may be years between significant rainfall events. However, in many of these locations the wind can dominate processes moving fine-grained sediments. Sand particles carried in the wind will wear away at outcrops creating unusual landscapes. Over time, exposed rock in desert environments can take on an wind-abraded appearance (Figures 13-38 and 13-39).

Pebbles, cobbles, and boulders too heavy to be moved by the wind accumulate on the land surface forming desert pavements (discussed above). A ventifact is a stone on the surface in a desert environment that is unusually shaped, abraded by wind-blown sediment. Markings on ventifacts typically reflect seasonal prevailing wind patterns.
Wind-abraided outcrops of Navajo Sandstone on the Navajo Reservation Wind-abraided sandstone outcrops near Tuba City, Arizona Ventifacts
Fig. 13-38. Wind-abraded outcrops of Navajo Sandstone near Tuba City, Arizona Fig. 13-39. Wind-abraded outcrops of Navajo Sandstone near Tuba City, Arizona Fig. 13-40. Ventifacts (wind abraded cobbles) from Panamint Valley, California
Loess is a tan, buff to gray windblown deposit of fine-grained, loamy, calcareous silt or clay; fine-grained deposits typically derived from glacial outwash plains or dust derived from arid regions. Loess deposits blanket large regions of the northern Great Plains and the Midwest (Figure 13-41).

The Sliding Rocks of Racetrack Playa: Perhaps one of the most unusual phenomena related to desert processes involving wind are the Sliding Rocks of the Racetrack Playa (in Death Valley National Park, CA, Figure 13-42). Hundred of cobble to boulder-sized rocks have been observed leaving trails of where they have slid across the surface of this remote playa. The rocks slide on a rain-slickened surface, sometime embedded in a thin ice sheet that is moved by the wind. Other times, hurricane-force winds may be capable of the moving rocks. Most of the trails follow the prevailing wind direction.
Map of the Sand Hills region of Nebraska Racetrack Playa sliding rocks, Death Valley, California California
Fig. 13-41. Map showing the Sand Hills region of Nebraska. The region was an extensive dune field currently stabilized by grasses. Fig. 13-42. Sliding rocks with trails on Racetrack Playa located in a remote area of Death Valley National Park, California
Desert varnish is a dark, hard film (or "patina") of metal oxides formed on exposed rock surfaces in arid regions (Figure 13-43). In many places the the metal oxides layers are most obviously deposited by a thin films of rainwater streaming down cliff surfaces, but in other locations they may be related to films associated with microbial activity and possibly metals deposited directly from the air. Desert varnish layers are typically a few millimeters thick, or less. Evaluation of prehistoric indian petroglyphs carved into desert varnish show that the layers form very slowly over time. Figure 13-44 shows a collection of petroglyphs on "Newspaper Rock" (Utah). More recent petroglyphs (including an indian riding on a horse) are light colored, but more ancient images, probably carved thousands of years ago, are darker from a new layer of desert varnish (patina) becoming re-established on the rock surface. Desert varnish in Canyon de Chelley National Monument Newspaper Rock, Utah
Fig. 13-43. Desert varnish on a sandstone cliff. Canyon de Chelley National Monument, Arizona. Fig. 13-44. Prehistoric indian petroglyphs carved in desert varnish, Newspaper Rock, Utah.

Deserts and semiarid regions of North America (and Hawaii)

Chihuanhuan Desert—North America's largest desert, considered a "rainshadow" desert on the east side of the Sierra Madre (the mountain range of central Mexico. Extends from near Mexico City northward into west Texas and New Mexico.

Sonoran Desert—
a desert with a unique ecosystem that covers large parts of the Southwestern United States in Arizona and California, and Northwest Mexico in Sonora and Baja California, best know for its Saguaro and Organ Pipe Cactus, generally a frost free region.

Colorado Desert—western part of the Sonoran Desert, located in low elevations along the lower Colorado River and southern California encompassing the Salton Sea region.

Mojave Desert—a high desert region located in southern California and parts of southern Nevada, southwestern Utah and northwestern Arizona. It is a region with "Basin and Range" topography.

Great Plains—high, semi-arid grasslands (steppe) region in the rainshadow east of the Rocky Mountains extending from central Canada to central Texas.

Colorado Plateau—high, semi-arid region between the Great Basin and the Rocky Mountains in the 4Corners region (Colorado, Utah, Arizona, and New Mexico. Much of the region is covered with pinyon-pine and juniper shrub forests.

Hawaii Rainshadow Arid Regions—
on the leeward (downwind) side of the larger Hawaiian Islands arid to semiarid conditions prevail. The largest arid region is the Kao Desert in the rainshadow region of Mauna Loa, Hawaii's largest volcano.
Desert of North America Chijuajuan Desert, Big Bend National Park Sonoran Desert, Saguaro National Park
Fig. 13-45. Deserts of North America Fig. 13-46. Chihuanhuan Desert (North America's largest desert) Fig. 13-47. Sonoran Desert
(with Saguaro cactus)
Mojave Desert, Joshua Tree National Park Colorado River Desert, Lake Mead National Recreation Area Colorado Plateau
Fig. 13-48. Mojave Desert
(with Joshua Trees)
Fig. 13-49. Colorado Desert
(with Teddybear Cholla cactus)
Fig. 13-50. Colorado Plateau
(with Pinyon Pine-Juniper shrub forest)
Great Basin, Great Basin National Park Steppe, Badlands National Park Kao Desert on Hawaii
Fig. 13-51. Great Basin
(grasslands with Basin and Range topography)
Fig. 13-52. Great Plains (grasslands in the rainshadow of the Rocky Mountains) Fig. 13-53. The Kao Desert is in the rainshadow region of Mauna Loa volcano on Hawaii.
Selected Resources

Desert Landforms and Surface Processes in the Mojave National Preserve and Vicinity

National Parks in Desert Environments

Death Valley National Park
Mojave National Preserve
Joshua Tree National Park
Saguaro National Park
Lake Mead National Recreation Area
Big Bend National Park
Badlands National Park

Parks of the Colorado Plateau
Arches National Park
Canyonlands National Park
Natural Bridges National Monument
Capital Reef National Park
Bryce Canyon National Park
Cedar Breaks National Monument
Grand Canyon National Park
Glen Canyon National Recreation Area


Chapter 13 Quiz Questions


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