Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
20 microns
Figure 6.5
Evidence for arid weathering processes: (a) salt crystallization on the underside of a tafone roof, Messak Settafet,
Libya; (b) lichens growing on the leeward side of pebbles in the Namib Desert; (c) SEM micrograph of a cross-section through the
lower parts of a lichen thallus growing on marble in the Namib Desert showing the production of boreholes by fungal hyphae; (d)
microsolutional features on basalt pebble, northern Namib Desert (image courtesy of Dr Mary Bourke).
in arid environments has been associated with small-scale
karren on soluble rocks, case hardening and the produc-
tion of calcrete, gypcrete and other indurated layers in
soils, as well as the growth of tufa and speleothem de-
posits. However, in several places there is plausible evi-
dence that such indicators of chemical weathering date
back to periods when wetter conditions prevailed, as
they are now starting to become deteriorated through
the action of subsequent weathering processes, as found
on limestone outcrops in southern Tunisia by Smith,
Warke and Moses (2000). Thus, large-scale data (from
solute loads) and small-scale observations of weather-
ing processes and features can give different pictures of
the importance of chemical weathering in today's arid
environments.
Much of our knowledge of weathering processes in
arid environments comes from observations in the field,
in the field. This approach might be called 'abductive
weathering science', where one deduces the processes
that operated from a detailed analysis of the material left
behind. Laboratory experiments have also been used to
corroborate such field evidence and test whether or not
particular processes are effective under certain conditions
and what kinds of deterioration they cause. Monitoring of
microenvironmental conditions in the field has also helped
to test whether specific processes could be operating, but
as yet there has been no real monitoring of individual
weathering processes under field conditions. The small-
scale nature of the weathering processes discussed above
has, with currently available technologies, precluded this.
For example, while we can use time-lapse photography to
watch flakes become detached from rock surfaces in the
field we cannot, as yet, monitor salt crystallisation and
dissolution events repeatedly within cracks and pores to
