Page 557 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 16 Oceans, Coastal Systems, and Wind Processes 521
Longitudinal dunes
Barchan dune
Star dune
Issaouane Erg
Illizi
Sabha
LIBYA
SAHARA
Ghat Djanet
ALGERIA
Murzuq
Sand Sea
0
200 KM
NIGER
Bilma
Tibesti
Zouar
CHAD
(a) The Issaouane Erg of eastern Algeria consists of star dunes, barchan dunes, and longitudinal dunes, disclosing the prevailing wind history of the region.
(b) The Sand Hills in central Nebraska are sand and silt deposits derived from glaciated regions to the north and west. These densely packed barchan dunes, inactive for at least 600 years, are now stabilized by vegetation (green). Water is in blue.
ridge dunes. Linear dunes include longitudinal dunes and seif dunes (not pictured). Seif dunes are named after the Arabic word for “sword” and are shorter with a more sinuous crest than longitudinal dunes. Star dunes are
◀Figure 16.31 Examples of sand seas, or ergs. [(a) ISS astro- naut photo, NASA/ GSFC. (b) NASA.]
the largest in size. Dome dunes are rare, and often occur at the margins of sand seas. Reversing dunes form where winds frequently reverse direction.
Active sand dunes cover about 10% of Earth’s deserts and dune migration can threaten popu- lated areas (see The Human Denominator, HD 16d, ahead). Dune fields are also present in many humid climates, typically midlatitude coastal regions, in- cluding the coasts of large lakes. In North America, coastal dunes are found on coastlines adjacent to the Great Lakes, Gulf of St. Lawrence, Oregon, Gulf of Mexico, and Atlantic Ocean. Climatic conditions tend to limit the occurrence of coastal dunes on tropical and high-latitude coasts where higher hu- midity restricts windblown sand transport.
These same dune-forming principles and dune terminology apply to snow-covered land- scapes. Snow dunes are formed as wind deposits snow in drifts. In semiarid farming areas, drift- ing snow captured by fences and by tall stubble left in fields contributes significantly to soil moisture when the snow melts.
In some ergs, winds from varying direc- tions produce star dunes with multiple slip- faces (Figure 16.31a). Star dunes are the moun- tainous giants of the sandy desert. They are pinwheel-shaped, with several radiating arms rising and joining to form a common central peak that can approach 200 m in height. In the image, you can also see some crescentic dunes, suggesting that the region has experi- enced changing wind patterns over time, as, in
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Twin Lakes
Diamond Lake
Barchan dunes
NEBRASKA
Black Lake
0 2 4 KILOMETRES
Georeport 16.5 Human Activities Disturb Eolian Landforms
In the United States, recreational off-road vehicles (ORVs) and all-terrain vehicles (ATVs) currently number more than
15 million. Such vehicles erode desert dunes, disrupt desert pavement, and create ruts that easily concentrate surface runoff
into gullies that deepen with continuing erosion.
Military activities such as explosions and the movement of heavy vehicles also destroy desert pavement, as in Iraq and Afghanistan,
where the breakdown of thousands of square kilometres of stable desert pavement has sent dust and sand into nearby cities and onto farmland. In the Registan Desert of southern Afghanistan, south of the city of Kandahar, drought conditions were exacerbated by military activities. The disruption of desert pavement surfaces resulted in sand movement that has overtaken areas of sparse agricultural activity and has covered more than 100 villages.