Deserts

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*Deserts cover about

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30% of the Earth’s land surface (42 million square kilometers)

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*Semiarid lands

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also called steppes, typically receives 250 to 500 millimeters (10 to 20 inches) of rain per year

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Arid lands

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receive less than 250 millimeters (10 inches) of rain per year

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Deserts

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are formally defined as regions that rarely receive precipitation, typically less than 100 millimeters (4 inches) per year

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Polar deserts on Earth

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cover nearly 5 million square kilometers and range from bedrock to gravel plains to ice sheets

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The Sahara Desert in North Africa

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considered to be the largest desert on Earth

In fact it is just part of an even greater desert environment, the low-latitude deserts

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The low-latitude deserts

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a virtually unbroken desert environment stretching for more than 9300 kilometers (5800 miles) from the Atlantic coast of North Africa across the Middle East and into India

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* Most desert regions are created by the

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pattern of the Earth’s air circulation and the topography of the land

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The Sahara Desert occurs

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where the very dry air from the subtropical high sinks downward
With no clouds and no rain, the Sahara is dry

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The world record high temperature

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of 137o F (57.8o C) was record in El Azizia, Libya, in North Africa’s Sahara Desert on September 13, 1922

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Many deserts in the middle latitudes, such as in the Western U.S., are

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rainshadow deserts
Moisture laden clouds blowing in from the Pacific Ocean encounter the Coast Range and the Sierra Nevada

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Badwater Basin, located within Death Valley,

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is the lowest point in North America with a depth of -282 feet (87 meters) below sea level

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The highest temperature ever recorded in the Western Hemisphere

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57oC (134oF), occurred on July 10, 1913 in Death Valley

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*The Atacama Desert in South America

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is in the rainshadow of the Andes Mountains and is the driest desert on Earth (some areas have not had rain for 400 years)

The Atacama Desert is so dry and lifeless, that it is used by researchers at UTK and Carnegie Melon to test Mars rovers for NASA

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In dry lands, rock weathering of any type is greatly reduced due to the lack of water

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Most of the weathering that does occur is due to mechanical weathering, which tends to produce sharp, angular rock particles

Chemical weather can occur over long periods of time, forming thin layers of clays and soils
Iron-bearing silicate minerals can slowly oxidize, producing the rust-colored stain that tints some desert landscapes

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*Deserts have ephemeral streams,

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which means that they only carry water in response to specific episodes of rainfall

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Over the years, as the fans grow larger, they can coalesce forming and apron of sediments along the front of the mountains called

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a bajada

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On the rare occurrence of heavy rains, water and sediments may flow out into the center of the valleys, converting the basin floors into a shallow

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playa lake

Playa lakes are temporary features that only last a few days or at best a few weeks before evaporation and infiltration remove the water
The dry, flat lake bed that remains is called a playa

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A playa from which there is no outflow of water (either on the surface as rivers, or underground) is referred to as an

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endorheic basin

In particular, if the playa has no natural drainage to the sea, it is referred to as interior drainage

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Eventually the mountain are all but eroded away leaving only a few isolated knobs projecting above the surrounding sediment-filled basins

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These isolated knobs are called inselbergs

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*Mount Uluru (formally called Ayers Rock)

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is a major tourist attraction in the dry, central desert of Australia
It is a classic example of an inselberg

Mount Uluru is 350 meters (1200 feet) high and has a circumference of 9.5 kilometers (6 miles)
It is formed of weathering-resistant quartz sandstone and is the last remnant of a massive mountain range

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*Windblown sand moves by skipping and bouncing along on the surface, a process called

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saltation

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*The “Dust Bowl” era

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refers to a series of dust storms in the central U.S. and Canada from 1931 to 1939, caused by decades of inappropriate farming techniques

A month of especially vicious dust storms destroyed 5 million acres of wheat in March of 1935

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The “Black Sunday”

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dust storm of April 14, 1935 was so bad that day was turned into pitch black night and most people thought they would die as the storm raged on and on

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*Wind erosion consists of

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either deflation or abrasion (test question).

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deflation

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is the lifting and removal of loose sediment, especially silt and sand

The most noticeable results of deflation are shallow depressions called blowouts
Blowouts can range in size from small dimples less than a meter wide to depression that approach 50 meters in depth and several kilometers across

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In portions of many deserts, the surface is a closely packed layer of coarse pebbles and cobbles too large to be moved by wind

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This stony veneer is called desert pavement

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abrasion

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is the wearing-away of rocks and boulders by the repeated impact of particles carried by the wind
It is very similar to sandblasting

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*Ventifacts

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are rocks that have been abraded, grooved, or polished by wind driven sand

Ventifact are most typically found in arid environments where there is little vegetation and the winds are strong

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*Wind abrasion can create a large feature called a

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yardang, which is a streamlined, wind sculpted ridge that is oriented parallel to the prevailing wind

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A deposit of windblown dust is called

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loess

The fine dust in loess is typically only 0.01 to 0.06 millimeter (0.0004 to 0.0024 inch) in diameter
There are two primary sources for loess deposits
Deserts
Glacial outwash deposits

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*The Yellow River (Huang Ho)

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cuts through the Loess Plateau and get its name from the yellow brown sediment of the eroded loess

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*Barchan sand dunes

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are extremely common on Mars

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*Radially symmetrical, star dunes

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are pyramidal sand mounds with slipfaces on three or four or more arms that radiate from the high center of the mound (test question)

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The highest dunes in the U.S. are

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in the Great Sand Dunes National Park in southern Colorado and rise 210 meters (805 feet) above the surrounding plains

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*The highest dunes in the world

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located along the southwest coast of Africa in the Namib Desert in the country of Namibia

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many dunes

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have an asymmetrical profile, with the windward slide having a gentle slope and the leenward side being steep