Geoscience Reference
In-Depth Information
fine-grained xenolith inclusions that are resistant to ero-
sion may stand out several centimetres in relief from the
matrix.
21.2.5.2
Feature formation and feature scale
There are presently few data on the development of ven-
tifact features. Factors that influence the type and nature
of the features include rock size, homogeneity and hard-
ness, wind velocity and face angle. It can be observed
that certain features, such as flutes and helical forms, are
universal in basic geometric form and occur on a range
of rock types including granites, basalts, tuffs, limestones
and other lithologies. In general, very small rocks (a few
centimetres in diameter) are faceted and polished, but lack
additional features.
The homogeneity of the rock plays an important role in
feature formation. The development of features may be re-
lated to the relationship of any irregularities in the rock to
the size-scale of the sand grains. This has been observed by
placing manmade materials in the field. In 1993, six blocks
of very fine-grained, highly homogeneous modelling foam
were placed at an angle of
45 to the horizontal adjacent
to ventifacts in an active aeolian environment. Over the
past 17 years, all of the blocks have lost considerable mass,
with the softest almost completely abraded away (Laity
and Bridges, 2009), but erosion has been essentially uni-
form and no flutes or lineations have formed. By contrast,
formed plaster targets that were homogeneous in compo-
sition, but contained inherent air vesicles, developed flutes
within a few months. Moreover, Styrofoam (a heteroge-
neous foam product) placed on the actively saltating bed
of the Mojave River rapidly developed lineations (Laity,
1995). These observations suggest that initial irregulari-
ties large enough to be 'seen' by the sand grains will grow
to macroscopic forms even in fairly homogeneous mate-
rials (P. L. Varkonyi, personal communication, 2010).
In the field, dense and texturally homogeneous rocks
such as cherts, orthoquartzites or weakly vesicular basalts
are faceted, with very small features that have little im-
pact on the overall form of the ventifact. Rocks that are
not homogeneous in texture develop larger and more var-
ious surface textures. In macrocrystalline or layered ma-
terials, the weaker minerals are more easily eroded and
differential erosion is well developed. For example, in
coarse-grained granitic rocks, near-vertical faces will be
intensely pitted, with the feldspars forming the low areas,
whereas at lower angles or where the wind sweeps by the
sides of the rock, the more resistant quartz mineral may
shield the feldspars, creating a positive relief in grooved
surfaces (Fisher, 1996). Soft rocks, such as tuffs, develop
Figure 21.17 Soft rocks, such as this tuff, develop large and
deep features. The surfaces of these high angle rocks are cov-
ered in deep, short flutes. A case-hardened surface on the
ventifact to the right of the figure has protected much of the
face of the rock, but where it has breached, significant mass
has been lost. Location: Mono Basin, California.
Owing to the timescales involved in ventifact forma-
tion, it is difficult to observe feature development in the
field. Therefore, laboratory studies or mathematical mod-
elling are necessary to understand feature growth. To date,
however, little work has been done in this area. The ex-
perimental work of Schoewe (1932) demonstrated that the
rate of abrasion and angle of the abraded surface decrease
over time. Work by Bridges et al. (2010) on rock simulant
targets abraded by sand in a boundary layer wind tunnel
indicates that the surface of a rock is initially roughened,
with the surface area-to-volume ratio increasing at a rate
greater than log (2/3). Initially, many small pits are pro-
duced, but over time these merge to produce larger ones.
Over time, the larger pits tend to become more elongate
Search WWH ::




Custom Search