Geology Reference
In-Depth Information
Chapter Summary
Desertifi cation is the expansion of deserts into formerly
productive lands. It destroys croplands and rangelands,
causing massive starvation and forcing hundreds of thou-
sands of people from their homelands.
Wind transports sediment in suspension or as bed load.
Suspended load is the material that is carried in sus-
pension by water or wind. Silt- and clay-sized particles
constitute most of a wind's suspended load. Bed load is
the material that is too large or heavy to be carried in sus-
pension by water or wind. Particles are moved along the
surface by saltation, rolling, or sliding.
Wind erodes material by either abrasion or defl ation.
Abrasion is the impact of saltating sand grains on an
object. Ventifacts are common products of wind abrasion.
Defl ation is the removal of loose surface material by
wind. Defl ation hollows resulting from differential ero-
sion of surface material are common features of many
deserts, as is desert pavement, which effectively protects
the underlying surface from additional defl ation.
The two major wind deposits are dunes and loess. Dunes
are mounds or ridges of wind-deposited sand that form
when wind fl ows over and around an obstruction, result-
ing in the deposition of sand grains, which accumulate
and build up a deposit of sand. Loess is wind-blown silt
and clay deposits composed of angular quartz grains,
feldspar, micas, and calcite.
The four major dune types are barchan, longitudinal,
transverse, and parabolic. The amount of sand available,
prevailing wind direction, wind velocity, and amount of
vegetation determine which type of dune will form.
Loess is derived from deserts, Pleistocene glacial outwash
deposits, and river fl oodplains in semiarid regions. Loess
covers approximately 10% of Earth's land surface and
weathers to a rich, productive soil.
The winds of the major air-pressure belts, oriented east-
west, result from the rising and cooling of air. The winds
are defl ected clockwise in the Northern Hemisphere and
counterclockwise in the Southern Hemisphere by the Cori-
olis effect to produce Earth's global wind patterns.
Dry climates, located in the low and middle latitudes
where the potential loss of water by evaporation exceeds
the yearly precipitation, cover 30% of Earth's land sur-
face and are subdivided into semiarid and arid regions.
Semiarid regions receive more precipitation than arid
regions, yet are moderately dry. Arid regions, generally
described as deserts, are dry and receive less than 25 cm
of rain per year.
The majority of the world's deserts are in the low-latitude
dry-climate zone between 20 and 30 degrees north and
south latitudes. Their dry climate results from a high-
pressure belt of descending dry air. The remaining deserts
are in the middle latitudes, where their distribution is
related to the rain-shadow effect, and in the dry polar
regions.
Deserts are characterized by high temperatures, little pre-
cipitation, and sparse plant cover. Rainfall is unpredict-
able and, when it does occur, tends to be intense and of
short duration.
Mechanical weathering is the dominant form of weather-
ing in deserts, and coupled with slow rates of chemical
weathering, results in poorly developed soils.
Running water is the major agent of erosion in deserts
and was even more important during the Pleistocene,
when wetter climates resulted in humid conditions.
Wind is an erosional agent in deserts and is very effec-
tive in transporting and depositing unconsolidated fi ne-
grained sediments.
Important desert landforms include playas, which are dry
lakebeds; when temporarily fi lled with water, they form
playa lakes. Alluvial fans are fan-shaped sedimentary
deposits that may coalesce to form bajadas.
Pediments are erosional bedrock surfaces of low relief
that slope gently away from mountain bases.
Inselbergs are isolated steep-sided erosional remnants
that rise above the surrounding desert plains. Buttes and
mesas are, respectively, pinnacle-like and fl at-topped ero-
sional remnants with steep sides.
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
Important Terms
abrasion (p. 385)
alluvial fan (p. 400)
barchan dune (p. 388)
butte (p. 402)
Coriolis effect (p. 391)
defl ation (p. 386)
desert (p. 392)
desert pavement (p. 386)
desertifi cation (p. 384)
dune (p. 387)
loess (p. 389)
longitudinal dune (p. 389)
mesa (p. 402)
parabolic dune (p. 389)
pediment (p. 401)
playa (p. 400)
rain-shadow desert (p. 396)
transverse dune (p. 389)
ventifact (p. 385)
Search WWH ::
Custom Search