Geoscience Reference
In-Depth Information
Table 6.2
Weathering processes known to operate in arid environments.
Physical
Biological
Chemical
Insolation weathering
Biophysical
Hydrolysis
Fire and thermal shock
Root wedging
Dissolution (of limestone)
Salt weathering
Lichen thalli growth pressures
Iron oxidation
Crystallisation
Lichen thalli wetting and drying
Hydration
Hydration
Biochemical
Thermal expansion
Biodissolution
Chelation
demonstrated the relative importance of salt crystallisa-
tion versus hydration under controlled conditions. Since
their experiments, a range of other workers have tried to
isolate and investigate different facets of the salt weath-
ering role of a variety of salts on different rocks (e.g. Yu
and Oguchi, 2009). While salt weathering in arid environ-
ments is basically a physical weathering process it does,
of course, rely on chemical transformations (the dissolu-
tion of salts in water and their recrystallisation) and re-
quires the presence of water. Some salts are also capable of
hydration (a chemical process whereby water molecules
are taken up into their crystalline structure), which has
also been shown to be an effective agent of salt weather-
ing. Salt weathering has been linked to flaking, blistering
and granular disintegration of salt-affected rock surfaces
(Figure 6.5(a)).
Other important weathering processes thought to oper-
ate in arid environments are biochemical and biophysical
weathering by rock surface biofilm communities. A rich
microflora inhabits many desert areas, even where higher
vegetation growth is prohibited by the harsh conditions.
Lichens, algae, fungi and bacteria can exist on very lit-
tle water, and can in some cases extract moisture from
humid air without the need for any sort of liquid water.
They can also often tolerate quite saline environments,
and many microorganisms have evolved effective chem-
ical sunscreens to enable them to survive in conditions
where they receive very high solar radiation. A range of
microorganisms and lichens can also extract nutrients di-
rectly from the rock surface and are thus highly able to
survive in nutrient-limited arid environments. Many of
these physiological adaptations mean that these biofilm
communities are highly effective agents of weathering. In
some cases they can bore their way into the rock surface
(up to a few millimetres in depth) through a combination
of chemical and physical means, in order to seek shade,
nutrients and water. This process of boring causes weath-
can be exploited by other agents of weathering. Figures
6.5(b) and (c) depict examples of biological weathering
by rock surface biofilms in arid environments. Biofilms
and lichens can cause weathering in a number of ways.
Paradise (1997) illustrates, through a detailed microscope
study of lichens from the genus
Xanthoparmelia
on Red
Mountain, Arizona, that lichens can produce both bio-
physical weathering (towards the centre of the thallus) and
biochemical weathering (towards the edges). Biological
weathering has been linked to exfoliation of thin flakes
and granular disintegration of rocks where biofilms oc-
cur, as well as the case hardening of sandstones in deserts
(Viles and Goudie, 2004).
Chemical weathering processes, despite the large-scale
lack of moisture, can play an important role in arid en-
vironments, but one that often leads to the accumula-
tion and consolidation of minerals and rocks rather than
their breakdown and removal. A suite of chemical weath-
ering processes has been noted from a range of envi-
ronments, including the dissolution of soluble minerals,
iron oxidation, hydrolysis of silicate minerals and a num-
ber of other chemical transformations. Where even small
amounts of moisture are present any susceptible miner-
als will be affected by such processes, often producing
very small weathering features diagnostic of chemical
weathering (e.g. microscale karren features as shown in
Figure 6.5(d)). In desert environments, the high rates of
evaporation mean that water is often only transient and
thus the products of chemical weathering tend to be de-
posited within or on the affected soils and rocks, rather
than being washed away as dissolved or suspended load.
However, Pope, Dorn and Dixon (1995) reanalysed data
on chemical denudation across climatic regions (drawn
from solute loads of rivers) and found no clear relation-
ship with climate - arid and semi-arid areas do not ap-
pear to have distinctly lower rates of chemical denuda-
tion and so chemical weathering may be more important
