Weathering systems - Arid Zone Geomorphology - page 89

(such as putting out test blocks whose starting proper-
ties are well characterised) allow the creation of a field
experiment, but natural conditions free us from having
to simulate the environment during the experiment. Such
field experiments, which have a long history in arid weath-
ering studies (e.g. Goudie and Watson, 1984), now benefit
hugely from the improvements in microclimatic monitor-
ing and so on, mentioned above, which now allow a really
good assessment of conditions during the experiment.
Computer-based modelling allows virtual experimen-
tation and is a very powerful tool for areas such as
weathering systems research where processes of inter-
est are slow and hard to monitor. However, weathering
geomorphologists have been relatively slow to take up
such techniques beyond conceptual modelling (usually
in the form of diagrams illustrating potential weathering
systems evolutionary pathways). Some exceptions are the
work of Huinink, Pel and Kopinga (2004) and Turking-
ton and Phillips ((2004)), both of whom have addressed
cavernous weathering, and Moores, Pelletier and Smith
(2008), who have focused on solar radation, moisture and
cracking. Modelling has the huge advantage that it can
address questions at any temporal and spatial scale. The
growth of microclimatic data collection in recent years
can help the development and testing of models of arid
weathering systems.
The laboratory and field-based methods discussed
above provide a largely small-scale and short-term un-
derstanding of arid weathering. In distinction, dating tech-
niques allow us to determine the age of weathering-related
phenomena and thus can enlarge our view of the rates of
weathering. Cosmogenic dating techniques in particular
have proved to be very helpful in answering questions
about long-term rates of weathering and have been widely
used in arid environments. Cosmogenic dating works by
measuring the amount of cosmogenic isotopes produced
by cosmic ray bombardment of minerals. The longer a
mineral has been exposed at the Earth's surface the more
cosmic rays it has received and thus the more cosmogenic
isotopes are found. Commonly used cosmogenic isotopes
for weathering and denudation studies are 10 Be and 26 Al,
which are produced by cosmic ray bombardment of sili-
cate minerals. Studies have found extremely low rates of
denudation over million year timespans in arid environ-
ments, such as in Namibia, where Van der Wateren and
Dunai (2001) found rates of 0.1-1 m/Ma over the past
5 Myr and Bierman and Caffee (2001) found rates of
1-16 m/Ma over the Pleistocene. It is also worth mention-
ing here that, as well as using dating techniques to quan-
tify rates of weathering, the development of weathering-
related features such as weathering rinds has been used as

has also been used to investigate weathering and palaeoen-
vironmental histories (Dorn, 1998; Zerboni, 2008).

6.5 Linking processes to form in arid
weathering systems

At the dawn of the twentieth century it was largely en-
visaged that physical weathering processes dominated in
arid environments. Indeed, the picture was even simpler
than this, with an orthodox view emerging that heating
and cooling as a result of changing receipt of solar radi-
ation diurnally and over shorter timespans was the major
(sometimes the only) process operating. This process is
often called insolation weathering. The perceived domi-
nance of insolation weathering (while easy to understand
given the lack of techniques to probe more deeply) was
based on flimsy evidence and a degree of circular reason-
ing. Extremes of temperature are frequently observed in
arid environments, as is cracking and flaking and other
weathering features that could be caused by temperature
fluctuations. A plausible link between the two could thus
be made. Early experimental work by Griggs queried the
efficacy of insolation weathering and indicated that it was
much more effective in the presence of moisture (Griggs,
1936). More recently there has been a resurgence of in-
terest in insolation weathering and its role in cracking
boulders on desert pavements (McFadden et al. , 2005).
Similar thermal cracking can occur in arid environments
as a result of fires (Dorn, 2003). As well as cracking,
insolation weathering has been linked to exfoliation of
the outer layers of boulders and the granular disintegra-
tion of rocks comprised of minerals with different thermal
properties.
A century of research has now confirmed that a much
larger array of weathering processes operates in arid en-
vironments, as depicted in Table 6.2. The prevalence of
extreme fluctuations of temperature and relative humid-
ity in many desert areas means that physical weathering
processes are preeminent. However, the presence of salts
in groundwater and their concentration by evaporation of
rainfall and other moisture sources means that salt weath-
ering has now come to be seen as a dominant process in
many deserts. Indeed, salt weathering has now become
the new orthodoxy. Salt weathering operates as salts ex-
ert pressure on pore walls within the near-surface zone
of a rock in three main ways - through repeated crys-
tallisation and dissolution, through hydration and through
expansion on heating (see Goudie and Viles, 1997, for a
general overview of salt weathering). Cooke and Sperling