Weathering systems - Arid Zone Geomorphology - page 94

6.7 Weathering rates in arid
environments

6.8 Arid weathering and landscape
evolution

In 1950 Louis Peltier produced a conceptual graph illus-
trating the nature and rate of weathering in different envi-
ronments around the world (Peltier, 1950). This identified
arid environments as not only environments dominated
by physical weathering but also as being places where
weathering was extremely slow. Such a view of desert
landscapes as being fossilised because of slow weath-
ering rates was indeed another geomorphological ortho-
doxy during the first half of the twentieth century. The
supposedly slow rates of weathering were deduced from
the lack of moisture and thus the limited array of poten-
tial weathering processes. Furthermore, slow weathering
rates were inferred from the thin soils present (as higher
rates of weathering usually lead to the accumulation of
thicker soil layers built up from the debris and residue
produced by weathering). The more recent focus on salt
weathering as the key agent of weathering in arid environ-
ments has been coupled with a growing belief that, in fact,
many desert areas are characterised by rather rapid rates of
weathering.
Despite the wide array of techniques used to study
weathering in the field and laboratory, it is still very hard
to quantify weathering rates in any general way. The mea-
surements that do exist tend to be highly place- and time-
specific. For example, Viles and Goudie (2004) monitored
weathering rates at a range of different locations in and
around a coastal salt pan in Namibia using test blocks
(some of which were cut from local rocks) and found
high rates in some rock types in the most saline parts.
Because some sensitive rock tablets disintegrated during
the two year exposure period while others showed only
minimal weight loss (and in some cases weight gain), it
proved very difficult to find a common metric for rock
breakdown rates. Thus, it was impossible to go beyond
saying that weathering was fast for some rock types in
some locations and slow in others. However, one clear
finding was that there were indeed hot spots of weathering
activity within salty, near-coastal environments, lending
support to the view that in some parts of arid environments
weathering rates can be rapid. Rather conflicting views of
the rates of weathering in deserts are obtained from cos-
mogenic dating, which estimates denudation rates over
millions of years. Over these timespans weathering in the
Namib and other deserts has been found to be slow relative
to other environments. These findings confirm the impor-
tance of scale in answering questions about arid zone
weathering.

How weathering contributes to overall landscape evo-
lution within desert environments remains a subject of
debate. While it is clear that progress is being made
in linking weathering to the development of individual
small-scale landforms, the bigger picture is much more
complex and blurred. Several studies have illustrated,
often through laboratory experimentation, that weather-
ing is capable of producing fine-grained sediments such
as desert loess (Smith, Wright and Whalley, 2002). Salt
weathering 'hotspots' thus might be important sources of
silt-sized sediment or dust, as evidenced along the coastal
fringes of the Namib Desert by Viles and Goudie (2007).
Studies have also indicated that weathering contributes to
rock fall and the evolution of rock slope profiles within
arid areas, as, for example, found by Goudie et al. (2002)
and Migon et al. (2005) in the Wadi Rum area of Jordan.
In cliffed slopes in jointed rocks, salt weathering and other
processes become concentrated along joints and fractures,
contributing to the destabilisation of blocks and so encour-
aging rock fall. Once the fallen boulders are concentrated
along the foot of the cliff, in the absence of effective flu-
vial erosion, the prevailing weathering rate will determine
whether or not they remain in situ protecting the foot of
the cliff from undercutting. Very different slope profiles
will result depending on the rates of weathering in joints
as compared with those on boulders.
Large closed depressions in arid environments may also
owe much of their development to weathering, as recorded
by Aref, El-Khoriby and Hamdan (2002) for the Qattara
depression in Egypt. Here, differences in groundwater
salinity between east and west portions of the basin control
the activity of salt weathering. Within the western part of
the basin, highly saline groundwater leads to very active
salt weathering, causing denudation and the production of
fine-grained sediment, which are then blown away to form
nearby lunette dunes. This process has been going on since
the start of the Quaternary, although the initial excavation
of the depression started much earlier and was caused by
denudation under wetter environmental conditions.
A big debate has emerged in the geomorphological lit-
erature in recent years over the role of groundwater sap-
ping in the formation of amphitheatre-headed canyons
in many arid areas, which exemplifies the complexity of
linking weathering processes to the development of larger-
scale features. Amphitheatre-headed valleys have steep
headwalls and show little evidence that precipitation-fed
overland flow has been involved in their creation. Several