Wind erosion in drylands - Arid Zone Geomorphology - page 546

little difference between the effectiveness of quartz and
basalt particles. Ash appears to be less effective, but the
data for both basalt and ash particles are limited (Greeley
et al. , 1984). Aggregated grains tend to plaster against the
target, creating mass gain rather than loss.
Field studies of target materials and sand-abraded rocks
indicate a mass loss of between 30 and 1630 µm/yr (Sharp,
1964, 1980; Greeley et al. , 1984; Kuenen, 1960; Knight
and Burningham, 2003). High-speed video (HSV) exper-
iments help to elucidate the process, demonstrating that:
(a) the windward side of rocks are subject to abrasion,
(b) sand hits the targets directly and is not deflected by
vortices that affect dust, (c) the outgoing particle velocity
is less than the incoming, showing that kinetic energy is
transferred to the target surface (Banks, Bridges and Ben-
zit, 2005; Laity and Bridges, 2009), and (d) some grains
rebound into the airstream and therefore provide a second
impact to the surface, amplifying the abrasion (Bridges
et al. , 2005).
The mechanisms of aeolian abrasion are based largely
on the interpretation of scanning electron micrograph
(SEM) images of disrupted surfaces. Greeley et al. (1984)
examined brittle materials abraded under laboratory con-
ditions and Laity (1995) and Laity and Bridges (2009)
examined active ventifacts abraded in the field. Accord-
ingtoGreeley et al. (1984), not all impacts will remove
material; some damage the surface and prepare it for future
removal. When well-rounded particles impact, a circular
crack, or Hertzian fracture, develops around the indenta-
tion site. The crack diameter increases with both particle
diameter and velocity. Angular particles produce a more
diverse style of erosion.
Scanning electron micrographs of field specimens in
an advanced stage of abrasion suggest that chipping is the
most important mechanism of mass removal. A secondary
mechanism, less frequently observed and perhaps only
significant in relatively weak rock, involves gouging by
sand grains that move essentially parallel to the rock sur-
face (Laity, 1995). Thus, while ventifacts appear macro-
scopically smooth and polished, they are microscopically
rough. The nature of the topographic roughness depends
on material type. Basaltic ventifacts sampled from Pis-
gah Crater have a microcrystalline or glassy groundmass,
which at high magnifications is seen to be damaged by mi-
crocracking. Exposed plagioclase phenocrysts have cleav-
age fractures. By contrast, marble ventifacts from the Lit-
tle Cowhole Mountains, California, show a different abra-
sional texture (Figure 21.20(a)). At high magnifications
(

2.4

suspended

saltated and
suspended

1.6

0.8

0

0 100 200

Kinetic Energy Flux (J m -2 s -1 )

Figure 21.19 Kinetic energy flux profiles of sand and dust.
The profiles of saltating grains have distinctive maxima at
a height of several tens of centimetres above the surface.
This profile is reflected in both ventifact and yardang forms.
The height of maximum kinetic energy in sand increases with
mean liftoff velocity. The kinetic energy flux of dust increases
steadily with height and does not show a distinct maxima
(from Laity and Bridges, 2009, after Anderson, 1986; courtesy
of Elsevier Limited).

of yardangs or fence posts, a marked abrasion zone, or
re-entrant, develops near the surface. This form can be
explained with reference to the kinetic energy profiles
of sand (Figure 21.19). Wind tunnel studies, fieldwork
and analytical models indicate that the decreasing flux of
sand with height combined with increasing velocity re-
sults in a kinetic energy profile peak within the lower tens
of centimetres above a level surface (Sharp, 1964, 1980;
Wilshire, Nakata and Hallet, 1981; Anderson, 1986; Liu
et al. , 2003; Bridges et al. , 2005). The cover photograph
in Greeley and Iversen (1985) shows this profile exem-
plified in a notched ventifact. By contrast, in the case of
dust, the erosional profile should increase steadily with
height, as the velocity of the dust matches closely that of
the wind and the flux is fairly invariant (Anderson, 1986)
(Figure 21.19). Such an erosional profile is not observed
in nature.

21.2.5.5 Mass loss by sand abrasion

2000-5000), the surface topography is shown to be
formed by impact-generated cleavage fracture of the mar-
ble crystallites, with angular marble debris lying on the

×

There has been little study of the role of particle composi-
tion on the susceptibility of the rock to abrasion. On Earth,