A view overlooking Death Valley, California.A view overlooking Death Valley, California.

Fig. 14-CO, p.333

Why do Why do deserts deserts exist?exist?

Falling air Falling air creates creates deserts at 30 deserts at 30 degrees north degrees north and south and south latitudes. The latitudes. The red arrows red arrows inside the inside the globe show globe show surface surface winds; the winds; the blue arrows blue arrows (right) show (right) show air flow on air flow on the surface the surface and at higher and at higher elevations. elevations.

Fig. 14-1, p.335

The major deserts of the world. Where are they concentrated?The major deserts of the world. Where are they concentrated?

Deserts cover 25% of the Earth’s land surface where 13% of the world’s population Deserts cover 25% of the Earth’s land surface where 13% of the world’s population lives. A desert is any region that receives less than _______ inches of rain per year.lives. A desert is any region that receives less than _______ inches of rain per year.

Fig. 14-2, p.335

Rain-shadow deserts form where warm, moist air from the Rain-shadow deserts form where warm, moist air from the ocean rises as it flows over mountains. As it rises, it cools and ocean rises as it flows over mountains. As it rises, it cools and water vapor condenses to form rain. The dry descending air water vapor condenses to form rain. The dry descending air on the lee side absorbs moisture, forming a desert. on the lee side absorbs moisture, forming a desert.

Fig. 14-3, p.335

Rainfall patterns in the Rainfall patterns in the State of California, State of California, where prevailing winds where prevailing winds carry moist Pacific air carry moist Pacific air eastward over the eastward over the mountains. Note that mountains. Note that rain-shadow deserts lie rain-shadow deserts lie east of the mountain east of the mountain ranges.ranges.

What about the Atacama What about the Atacama and Gobi deserts? and Gobi deserts?

Rainfall given in Rainfall given in cm/year.cm/year.

Fig. 14-4, p.336

Water and Deserts.Water and Deserts. Water reaches the desert from three sources: Water reaches the desert from three sources: streams (from wetter regions), groundwater and rain/snowfall. The streams (from wetter regions), groundwater and rain/snowfall. The Colorado River (above) flows from the Rockies through the arid SW Colorado River (above) flows from the Rockies through the arid SW United States and empties (where?). United States and empties (where?). Fig. 14-5, p.337

Desert StreamsDesert Streams: Courthouse : Courthouse Wash, Utah. In the spring, when Wash, Utah. In the spring, when rain and melting snow fill the rain and melting snow fill the channel with water…channel with water…

Fig. 14-6a, p.337

……same wash, in the summer, same wash, in the summer, when the creek bed is dry…when the creek bed is dry…

A stream bed that is dry for most A stream bed that is dry for most of the year is called a wash (or of the year is called a wash (or arroyo)…arroyo)…

where does the water go? what is where does the water go? what is a “water table” and where is it in a “water table” and where is it in this photo?this photo?

Fig. 14-6b, p.337

Desert Lakes:Desert Lakes:

Mud cracks pattern Mud cracks pattern the floor of a playa the floor of a playa in Utah.in Utah.

An intermittent An intermittent desert lake is called desert lake is called a playa…how does it a playa…how does it form?form?

Is the Salton Sea a Is the Salton Sea a playa lake?playa lake?

Fig. 14-7, p.338

When lakes evaporate, ions precipitate to deposit salts on the playa. When lakes evaporate, ions precipitate to deposit salts on the playa. Economically valuable mineral deposits can accumulate over the Economically valuable mineral deposits can accumulate over the years. Here, a mule team is hauling valuable mineral deposits from years. Here, a mule team is hauling valuable mineral deposits from Death Valley during the 1800s.Death Valley during the 1800s.

Fig. 14-8, p.338

Flash Floods:Flash Floods:

In August 1997, 11 hikers In August 1997, 11 hikers perished when a flash flood perished when a flash flood filled a slot canyon similar to filled a slot canyon similar to this one in the Utah desert. The this one in the Utah desert. The steep walls made escape steep walls made escape impossible.impossible.

Fig. 14-9, p.338

Pediments and Bajadas: Pediments and Bajadas: An alluvial fan in Death Valley forms An alluvial fan in Death Valley forms where a steep mountain stream deposits sediment where is enters a where a steep mountain stream deposits sediment where is enters a valley. A Bajada is a depositional surface. valley. A Bajada is a depositional surface.

Fig. 14-10, p.339

The Bajada in the foreground merges with a gently sloping pediment The Bajada in the foreground merges with a gently sloping pediment to form a continuous surface in front of mountains in Mongolia. The to form a continuous surface in front of mountains in Mongolia. The basin has no external drainage. A pediment is an erosional surface basin has no external drainage. A pediment is an erosional surface where sediment is transported from the mountains to the bajada.where sediment is transported from the mountains to the bajada.

Fig. 14-11, p.339

Two American Deserts: Two American Deserts: The Colorado Plateau; The Colorado Plateau; Death Valley and the Death Valley and the Great BasinGreat Basin

Features of the Colorado Features of the Colorado Plateau include spires and Plateau include spires and buttes formed when buttes formed when streams reach a temporary streams reach a temporary base level and erode base level and erode laterally. The streams laterally. The streams transport the eroded transport the eroded sediment away from the sediment away from the region. Bottom photo is region. Bottom photo is spires and buttes in spires and buttes in Monument Valley, Az. A Monument Valley, Az. A plateau is a large elevated plateau is a large elevated area of fairly flat land. It area of fairly flat land. It is a larger area than mesas is a larger area than mesas and buttes. and buttes.

Fig. 14-12, p.340

Location of Great Basin shown in red…Colorado Plateau is to the right of Location of Great Basin shown in red…Colorado Plateau is to the right of the great basin and takes in parts of Utah, Colorado, Arizona and New the great basin and takes in parts of Utah, Colorado, Arizona and New Mexico.Mexico. Fig. 14-13b, p.341

Fig. 14-12a, p.340

Fig. 14-12b, p.340

Death Valley and the Great Basin: sediment eroded from Death Valley and the Great Basin: sediment eroded from surrounding mountains is slowly filling Death Valley, in the surrounding mountains is slowly filling Death Valley, in the rain shadow of the Sierra Nevadas. DV has no external rain shadow of the Sierra Nevadas. DV has no external drainage, unlike the Colorado Plateau which drains to the drainage, unlike the Colorado Plateau which drains to the Gulf of California. Gulf of California.

Fig. 14-13, p.341

Fig. 14-13a, p.341

Bajadas and pediments are common features of the Great Basin. The form from a Bajadas and pediments are common features of the Great Basin. The form from a combination of tectonic, erosional and depositional processes. Mountains a valleys combination of tectonic, erosional and depositional processes. Mountains a valleys commonly form by block faulting (horst and graben or basin and range commonly form by block faulting (horst and graben or basin and range topography). Mountains slowly drown in their own sediment…topography). Mountains slowly drown in their own sediment…

Fig. 14-14, p.341

Fig. 14-14a, p.341

Fig. 14-14b, p.341

Fig. 14-14c, p.341

Wind: it blows across and Wind: it blows across and erodes bare, unprotected desert erodes bare, unprotected desert soil and forms features such as soil and forms features such as dunes, desert pavement and dunes, desert pavement and loess.loess.

To right, wind erodes silt and To right, wind erodes silt and sand (called deflation) but sand (called deflation) but leaves larger rocks behind to leaves larger rocks behind to form desert pavement. This is form desert pavement. This is a continuous cover of stones a continuous cover of stones that protects the desert surface that protects the desert surface from further erosion. from further erosion.

Fig. 14-15, p.342

Fig. 14-15a, p.342

Fig. 14-15b, p.342

Wind also moves sand Wind also moves sand grains by saltation grains by saltation (usually not lifted more (usually not lifted more than 1 meter off the than 1 meter off the ground) which can carve ground) which can carve features by abrasion such features by abrasion such as the pinnacle to right in as the pinnacle to right in the Grand Canyon. the Grand Canyon.

Fig. 14-16, p.343

Dunes: a Dunes: a mound or mound or ridge of wind-ridge of wind-deposited deposited sand. They sand. They form when form when wind erodes wind erodes sand from one sand from one location and location and deposits it in deposits it in another. another.

To right, To right, dunes near dunes near Lago Poopo, Lago Poopo, Bolivia.Bolivia.

Fig. 14-17, p.343

Blowouts can form (saucer or trough-shaped depression). They can Blowouts can form (saucer or trough-shaped depression). They can very large, such as the Qattara Depression in Egypt (100 meters deep, very large, such as the Qattara Depression in Egypt (100 meters deep, 10 km in diameter).10 km in diameter).

Fig. 14-18, p.343

Most dunes are assymetrical; wind erodes sand from the windward Most dunes are assymetrical; wind erodes sand from the windward side of a dune, carries it up to the dune crest and the sand slides down side of a dune, carries it up to the dune crest and the sand slides down on the sheltered leeward side (slip face) at the angle of repose for sand on the sheltered leeward side (slip face) at the angle of repose for sand (approx. 35 degrees). (approx. 35 degrees).

Fig. 14-19, p.343

What’s this? When dunes become buried by younger sediment and What’s this? When dunes become buried by younger sediment and lithified over geologic time, the sandstone retains the original lithified over geologic time, the sandstone retains the original sedimentary structures of the dunes. Steeply dipping layers of the sedimentary structures of the dunes. Steeply dipping layers of the dune face are preserved here as cross-bedding (Zion National Park).dune face are preserved here as cross-bedding (Zion National Park).

Fig. 14-20, p.344

Types of Dunes: When sand Types of Dunes: When sand supply is limited (rocky supply is limited (rocky deserts), the tips of Barchan deserts), the tips of Barchan dunes travel faster than the dunes travel faster than the center and point downwind. center and point downwind. Barchans migrate Barchans migrate independently. independently.

Fig. 14-21, p.344

Fig. 14-21ab, p.344

Fig. 14-21c, p.344

If sand is plentiful and evenly If sand is plentiful and evenly dispersed, it accumulates in long dispersed, it accumulates in long ridges called transverse dunes ridges called transverse dunes aligned perpendicular to the aligned perpendicular to the prevailing wind. prevailing wind.

Fig. 14-22, p.346

Fig. 14-22a, p.345

Fig. 14-22b, p.345

If sparse desert vegetation is If sparse desert vegetation is present, like along a seacoast present, like along a seacoast or semiarid desert, a blowout or semiarid desert, a blowout might form at bare areas might form at bare areas between vegetation as sand between vegetation as sand is eroded while the tips are is eroded while the tips are anchored by plants anchored by plants surrounding the blowout. A surrounding the blowout. A parabolic dune is similar to a parabolic dune is similar to a barchan, except that the tips barchan, except that the tips of a parabolic dune point of a parabolic dune point into the wind. into the wind.

Fig. 14-23, p.346

Fig. 14-23a, p.346

Fig. 14-23b, p.345

If the wind direction is If the wind direction is erratic but prevails from erratic but prevails from the same general the same general compass direction and compass direction and sand supply is limited, sand supply is limited, then long, straight then long, straight longitudinal dunes form longitudinal dunes form parallel to the prevailing parallel to the prevailing wind direction. In the wind direction. In the Sahara Desert, they can Sahara Desert, they can reach 100 km long. reach 100 km long.

Fig. 14-24, p.346

Fig. 14-24a, p.346

Fig. 14-24b, p.346

Loess: wind can carry silt for hundreds or thousands of miles and deposit it as silt. Loess: wind can carry silt for hundreds or thousands of miles and deposit it as silt. Silt is porous, uniform and typically lacks layering, and particles can interlock. It’s Silt is porous, uniform and typically lacks layering, and particles can interlock. It’s not cemented, but can form vertical cliffs and bluffs. The largest deposits in China, not cemented, but can form vertical cliffs and bluffs. The largest deposits in China, more than 300 meters thick, were used as dwellings. In 1920, a great earthquake more than 300 meters thick, were used as dwellings. In 1920, a great earthquake collapsed the cave system and killed an estimated 100,000 people. Above are loess collapsed the cave system and killed an estimated 100,000 people. Above are loess caves in Pakistan.caves in Pakistan.

Fig. 14-25, p.346

Loess deposits in the United States. Soils formed on Loess are Loess deposits in the United States. Soils formed on Loess are generally fertile and make good farmland.generally fertile and make good farmland.

Fig. 14-26, p.347

Desertification: Desertification: What is it?What is it?

Fig. 14-27, p.348

p.349