CHAPTER 12 Deserts and Wind
304
C H A P T E R
T W E L V E
Deserts and Wind
in Review
The concept of dryness is relative; it refers to any situation in
which a water deficiency exists. Dry regions encompass about 30
percent of Earth’ s land surface. Two climatic types are commonly
recognized: desert, which is arid, and steppe (a marginal and
more humid variant of desert), which is semiarid. Low-latitude
deserts coincide with the zones of subtropical highs in lower latitudes. In contrast, middle-latitude deserts exist principally because
of their positions in the deep interiors of large landmasses far
removed from the ocean.
The same geologic processes that operate in humid regions
also operate in deserts, but under contrasting climatic conditions.
In dry lands rock weathering of any type is greatly reduced because
of the lack of moisture and the scarcity of organic acids from
decaying plants. Much of the weathered debris in deserts is the
result of mechanical weathering. Practically all desert streams are
dry most of the time and are said to be ephemeral. Stream
courses in deserts are seldom well integrated and lack an extensive system of tributaries. Nevertheless, running water is responsible
for most of the erosional work in a desert. Although wind erosion
is more significant in dry areas than elsewhere, the main role
of wind in a desert is in the transportation and deposition of
sediment.
Because arid regions typically lack permanent streams,
they are characterized as having interior drainage. Many of the
landscapes of the Basin and Range region of the western and
southwestern United States are the result of streams eroding
uplifted mountain blocks and depositing the sediment in interior
basins. Alluvial fans, playas, and playa lakes are features often
associated with these landscapes. In the late stages of erosion, the
mountain areas are reduced to a few large bedrock knobs, called
inselbergs, projecting above the sediment-filled basin.
The transport of sediment by wind differs from that by running water in two ways. First, wind has a low density compared
to water; thus, it is not capable of picking up and transporting
coarse materials. Second, because wind is not confined to
channels, it can spread sediment over large areas. The bed load
of wind consists of sand grains skipping and bouncing along
the surface in a process termed saltation. Fine dust particles are
capable of being carried by the wind great distances as
suspended load.
Compared to running water and glaciers, wind is a less significant erosional agent. Deflation, the lifting and removal of loose
material, often produces shallow depressions called blowouts.
Wind also erodes by abrasion, sometimes creating interestingly
shaped stones termed ventifacts. Yardangs are narrow, streamlined,
wind-sculpted landforms.
Desert pavement is a thin layer of coarse pebbles and cobbles
that covers some desert surfaces. Once established, it protects the
surface from deflation. Depending on circumstances, it may
develop as a result of deflation or deposition of fine particles.
Wind deposits are of two distinct types: (1) mounds and ridges
of sand, called dunes, that are formed from sediment carried as
part of the wind’ s bed load; and (2) extensive blankets of silt,
called loess, that once were carried by wind in suspension. The
profiles of many dunes show an asymmetrical shape, with the
leeward (sheltered) slope being steep and the windward slope
being more gently inclined. The types of sand dunes include
(1) barchan dunes, (2) transverse dunes, (3) barchanoid dunes,
(4) longitudinal dunes, (5) parabolic dunes, and (6) star dunes.
The thickest and most extensive deposits of loess occur in
western and northern China. Unlike the deposits in China,
which originated in deserts, the loess in the United States and
Europe is an indirect product of glaciation.
Key Terms
abrasion (p. 298)
alluvial fan (p. 294)
bajada (p. 294)
barchan dunes (p. 300)
barchanoid dunes (p. 302)
bed load (p. 295)
blowouts (p. 297)
cross beds (p. 300)
deflation (p. 297)
desert (p. 288)
desert pavement (p. 297)
dry climate (p. 288)
dunes (p. 299)
ephemeral streams (p. 291)
interior drainage (p. 293)
inselbergs (p. 294)
loess (p. 303)
longitudinal dunes (p. 302)
parabolic dunes (p. 302)
playa (p. 294)
playa lake (p. 294)
rainshadow desert (p. 290)
saltation (p. 295)
slip face (p. 299)
star dunes (p. 302)
steppe (p. 288)
suspended load (p. 296)
transverse dunes (p. 300)
ventifacts (p. 299)
yardang (p. 299)
304
C H A P T E R
T W E L V E
Deserts and Wind
in Review
The concept of dryness is relative; it refers to any situation in
which a water deficiency exists. Dry regions encompass about 30
percent of Earth’ s land surface. Two climatic types are commonly
recognized: desert, which is arid, and steppe (a marginal and
more humid variant of desert), which is semiarid. Low-latitude
deserts coincide with the zones of subtropical highs in lower latitudes. In contrast, middle-latitude deserts exist principally because
of their positions in the deep interiors of large landmasses far
removed from the ocean.
The same geologic processes that operate in humid regions
also operate in deserts, but under contrasting climatic conditions.
In dry lands rock weathering of any type is greatly reduced because
of the lack of moisture and the scarcity of organic acids from
decaying plants. Much of the weathered debris in deserts is the
result of mechanical weathering. Practically all desert streams are
dry most of the time and are said to be ephemeral. Stream
courses in deserts are seldom well integrated and lack an extensive system of tributaries. Nevertheless, running water is responsible
for most of the erosional work in a desert. Although wind erosion
is more significant in dry areas than elsewhere, the main role
of wind in a desert is in the transportation and deposition of
sediment.
Because arid regions typically lack permanent streams,
they are characterized as having interior drainage. Many of the
landscapes of the Basin and Range region of the western and
southwestern United States are the result of streams eroding
uplifted mountain blocks and depositing the sediment in interior
basins. Alluvial fans, playas, and playa lakes are features often
associated with these landscapes. In the late stages of erosion, the
mountain areas are reduced to a few large bedrock knobs, called
inselbergs, projecting above the sediment-filled basin.
The transport of sediment by wind differs from that by running water in two ways. First, wind has a low density compared
to water; thus, it is not capable of picking up and transporting
coarse materials. Second, because wind is not confined to
channels, it can spread sediment over large areas. The bed load
of wind consists of sand grains skipping and bouncing along
the surface in a process termed saltation. Fine dust particles are
capable of being carried by the wind great distances as
suspended load.
Compared to running water and glaciers, wind is a less significant erosional agent. Deflation, the lifting and removal of loose
material, often produces shallow depressions called blowouts.
Wind also erodes by abrasion, sometimes creating interestingly
shaped stones termed ventifacts. Yardangs are narrow, streamlined,
wind-sculpted landforms.
Desert pavement is a thin layer of coarse pebbles and cobbles
that covers some desert surfaces. Once established, it protects the
surface from deflation. Depending on circumstances, it may
develop as a result of deflation or deposition of fine particles.
Wind deposits are of two distinct types: (1) mounds and ridges
of sand, called dunes, that are formed from sediment carried as
part of the wind’ s bed load; and (2) extensive blankets of silt,
called loess, that once were carried by wind in suspension. The
profiles of many dunes show an asymmetrical shape, with the
leeward (sheltered) slope being steep and the windward slope
being more gently inclined. The types of sand dunes include
(1) barchan dunes, (2) transverse dunes, (3) barchanoid dunes,
(4) longitudinal dunes, (5) parabolic dunes, and (6) star dunes.
The thickest and most extensive deposits of loess occur in
western and northern China. Unlike the deposits in China,
which originated in deserts, the loess in the United States and
Europe is an indirect product of glaciation.
Key Terms
abrasion (p. 298)
alluvial fan (p. 294)
bajada (p. 294)
barchan dunes (p. 300)
barchanoid dunes (p. 302)
bed load (p. 295)
blowouts (p. 297)
cross beds (p. 300)
deflation (p. 297)
desert (p. 288)
desert pavement (p. 297)
dry climate (p. 288)
dunes (p. 299)
ephemeral streams (p. 291)
interior drainage (p. 293)
inselbergs (p. 294)
loess (p. 303)
longitudinal dunes (p. 302)
parabolic dunes (p. 302)
playa (p. 294)
playa lake (p. 294)
rainshadow desert (p. 290)
saltation (p. 295)
slip face (p. 299)
star dunes (p. 302)
steppe (p. 288)
suspended load (p. 296)
transverse dunes (p. 300)
ventifacts (p. 299)
yardang (p. 299)