Geoscience Reference
In-Depth Information
21.2.5.4
The nature of the abrading agent:
sand versus dust
protrudes into the higher velocity windstream. There
are three possible explanations for the greater feature
scale near the top of the rock: (1) particle impact veloc-
ity is highest here, (2) the duration of erosion is greater
or (3) the features have migrated up the rock, becoming
bigger with time. Quantitative studies will be necessary
to unravel the answers to these questions.
Both sand and dust are abundant in deserts. Their relative
abrasional efficacy depends upon particle velocity, mass,
flux and interaction with the target (does the particle strike
the target or is it deflected around it?). While dust has a
high flux and velocity, particle mass is low and dust is
easily deflected around an obstacle, rather than impacting
it directly. By contrast, sand has a lower flux and velocity
but a much higher mass and directly impacts the rock
surface, often more than once owing to rebound effects.
A theoretical consideration of these factors, discussed in
more detail below, suggests that dust is unlikely to play a
significant role in abrasion.
In contrast to dust, sand in saltation achieves sufficient
momentum to be decoupled from the airstream around
obstacles and impacts the target directly. Its velocity at
the point of impact is
21.2.5.3
Processes of erosion
Ventifacts form by the process of abrasion. The efficacy
of this process is a function of the abradant. Although
Whitney (1979) suggested that wind alone could erode
rock mass, her experiments used a pressurised airblaster,
which applies more force than is the case in nature. To
date, no ventifacts have been recorded in an area subject
to pure airflow alone. Pure air is most likely to deflate
particles that have been separated from the rock mass by
weathering.
Several authors have suggested that dust abrasion may
have a role to play in the formation of ventifacts (Max-
son, 1940; Sharp, 1949; Whitney, 1978; Lancaster, 1984;
Breed, McCauley and Whitney, 1989; Schlyter, 1994).
Dust particles are composed of silts and clays, less than
0.0625 mm (62.5 µm) in diameter, and transported in sus-
pension. In dust storms in Kuwait and Saudi Arabia, for
example, most particles were less than 10 µmindiame-
ter, although 40 % fell in the 10-30 µm range (Draxler
et al.
, 2001). Dust has been suggested as an abradant be-
cause it was thought more likely to explain polish (Sharp,
1949; Lancaster, 1984) and might be able to follow vortex
currents more easily and thereby cut flutes or lineations
(Maxson, 1940; Whitney, 1978). However, ventifacts ap-
pear not to be found in areas subject to dust influx alone.
In the Mojave Desert, a comparison may be made between
the young (
∼
50 % or less of the wind velocity
(Bridges
et al.
, 2005) and, considering the velocity con-
tribution to kinetic energy (
v
2
), the kinetic energy (KE) is
much less than that of dust. However, the mass is much
greater and this increases as a cube of the particle size.
Therefore, a 100 µm sand grain has 1000 times the mass of
a10µm dust particle. Upon impact, the mass loss of a rock
by abrasion varies with particle diameter,
D
, by approxi-
mately
D
3
. Taking velocity and mass into consideration,
the KE of sand is therefore 50-100 times that of dust
(Laity and Bridges, 2009). Furthermore, dust is coupled
to the airflow and deflected around the obstacle, whereas
sand impacts directly. Anderson (1986) demonstrated that
deflection of dust around obstacles decreased the number
of impacts, such that the number of impacts from 10 µm
dust is about 10 % of that of 100 µm sand, given the same
number of initial particles. Laity and Bridges (2009) con-
clude that on a per particle basis, about 1000 times more
energy is transferred to rock surfaces by sand than dust.
Given the potentially greater density of dust particles,
can the additive KE contribution of dust impacts equal that
of sand grains? Griffith's Theory (Griffith 1921) suggests
that rocks and other solids fail from particle abrasion after
a certain energy limit is exceeded (Lawn, 1995), with
the criterion for failure being the size of the stress field
induced by impact relative to the characteristic spacing of
critical microflaws in the rock. Dust induces a stress field
smaller than these microflaw spacings and is therefore
much less likely to be able to abrade rock (Laity and
Bridges, 2009).
Another factor that needs to be considered is the form
of the ventifact relative to the kinetic energy abrasion pro-
file. The facets on ventifacts typically slope away from
the wind. On other abraded materials, such as the prows
18 kyr) surface of the Pisgah volcanic flow,
which is traversed by sand and has abundant ventifacts,
and the much older Cima volcanic field (with cones and
flows formed from the Miocene to Holocene), which has
accumulated up to 3 metres of aeolian dust but lacks ven-
tifacts.
The majority of ventifact field studies have invoked
sand as the abradant. Sand is a loose, granular material,
0.0635-2 mm (622.5-2000 µm) in diameter, which moves
by saltation, travelling on a characteristic path (the salta-
tion trajectory), reaching 1-2 m above the surface and
extending several metres downwind. Maximum velocity
is achieved at the top of the trajectory, which is approx-
imately 0.5 to 0.66 of the wind speed. Typical abrasion
heights on both ventifacts and yardangs rise to 1 m (Hobbs,
1917), although greater heights may be achieved as dunes
∼
