Geoscience Reference
In-Depth Information
of the southwestern United States, the scale of erosion is
much less, but small yardang fields and widespread ven-
tifaction speak of localised wind erosion, much of it relict
from an earlier time period.
velocity are packed into cracks, and then are hit by succes-
sive impacts to cause a wedging effect. Although it may
potentially occur in deserts, it has not been recorded.
21.2.1.1
Abrasion
Ventifacts, abraded surfaces and the bases of yardangs de-
velop within a curtain of saltating sand grains. The devel-
opment and form of these geomorphic features is thereby
dependent on particle fluxes within a zone that ranges
from the surface up to a height of about 1-2 m (Hobbs,
1917). Within this general zone of abrasion, there will be a
height at which the kinetic energy flux is maximised (An-
derson, 1986). In general, particles travelling at greater
heights have higher velocities, as wind velocity increases
with height and the longer saltation paths allow for greater
acceleration by the wind (Greeley
et al.
, 1984). However,
the number of particles also diminishes with height and
therefore the maximum kinetic energy flux must take into
account both particle flux and speed.
At the elevation of the greatest kinetic energy flux,
erosion profiles develop with distinct maxima of mass
removal (Sharp, 1964, 1980; Wilshire, Nakata and
Hallet, 1981; Anderson, 1986; Laity and Bridges, 2009),
the height of which is influenced by such factors as wind
speed and the degree of grain bounce. For any given sur-
face, the height of maximum abrasion shifts upward as
wind velocity increases (Jianjun
et al.
, 2001; Liu
et al.
,
2003). Winds are accelerated in constrictions, such as val-
leys and passes, and where they ascend over hills or es-
carpments, promoting erosion in these areas (Laity, 1987).
Sand streams erode at greater heights when passing over
hard surfaces, which promotes grain bounce (Wilshire,
Nakata and Hallet, 1981). For example, the height dis-
tribution of windblown sand over Chinese gobi (desert
pavement) surfaces reaches up to 2.3 m, whereas above
mobile sand beds 95 % of the sand is concentrated in
the lower 20 cm (Jianjun
et al.
, 2001). However, even on
hard surfaces, the percentage of sand at high elevations is
quite low (1-3.4 % in the Chinese example) and therefore
other processes are required to explain observed abrasion
at elevations greater than
21.2
The physical setting: conditions for
wind erosion
Wind erosion requires extreme aridity, persistent and
sometimes strong winds and a supply of abrasive sedi-
ment. Aridity limits vegetation cover, thereby allowing
the free sweep of the wind and the movement of sedi-
ment. The abrasive sand is usually supplied from areas
where water actively provides a sediment source, either
from coastal zones (Corbett, 1993), river systems that dis-
gorge from mountainous regions, alluvial bajadas or sandy
beaches (Wilson, 1971). Its transport may be regionally
enhanced by topographic acceleration of the wind.
21.2.1
Processes of aeolian erosion
There are two principle processes involved in wind ero-
sion: abrasion and deflation.
Abrasion
refers to the me-
chanical wear of rock or sediments by the impact of par-
ticles in saltation (Greeley
et al.
, 1984). The removal of
loose, fine material from a surface, often pre-weathered
by salt weathering or other processes, is referred to as
de-
flation
. The deflated material is transported as fine grains
in atmospheric suspension (Greeley and Iversen, 1985).
Although the terms abrasion and deflation are clearly
defined and signify two quite separate processes, they are
not always well differentiated in the literature. The terms
deflation hollow or deflation basin are commonly used,
e.g. even when both abrasion and deflation are involved,
with abrasion sometimes dominant. Bristow, Drake and
Armitage (2009) refer to the deflation of sediments from
the Bodele Depression, while photographs and accompa-
nying text provide evidence of abrasion on the lakebed.
In this sense, deflation is often used to refer to a general
lowering of the land surface by wind erosion or to the
removal of fine materials produced by abrasion.
The relative significance of abrasion and deflation in
landform development is rarely studied and not well un-
derstood. In the formation of ventifacts, abrasion is the
key player. However, for depressions and yardangs, both
processes may be important. A third process of wind ero-
sion,
rock wedging
, has been recorded in Antarctica (Hall,
1989). This process occurs when grains moving at high
1 m (Laity and Bridges, 2009).
It is likely that abrasion elevations increase as the entire
sand surface rises as, for example, when dune sand moves
across a hill or through an interyardang passage.
The magnitude of erosion depends on the
suscepti-
bility to abrasion
(Greeley
et al.
, 1984),
S
a
(a function
of rock density, hardness, fracture-mechanical properties,
primary texture and shape) and on the properties of the
impacting particle (diameter
D
, density
∼
ρ
p
, speed
V
and
