Geoscience Reference
In-Depth Information
from other sources may be significant (section
5.1.2.1). Such sources of moisture tend to be
alkali as they are sourced from salt-rich waters
such as the sea or salt lakes. In the latter cases
pH may be as high as 9.9 (Van Gulu, Turkey;
Goudie 1997). This high pH greatly increases
silica mobility during rock weathering. Where
moisture is trapped within rocks the thermal
expansivity of the rock (Goudie 1997) is in-
creased. Heating of 10 -50°C can develop 250
atmospheres of tensile strength (Winkler 1977),
thus exacerbating breakdown of the rock.
during anhydrite (dehydrated calcium sulphate)
to gypsum (hydrated calcium sulphate) trans-
formation (Winkler & Wilhelm 1970).
3
Thermal expansion.
When halite (sodium
chloride) is heated from 0°C to 60°C it expands
by 0.5%, whereas granite minerals only expand
by up to 0.2%. This differential expansion can
contribute to rock disintegration (Goudie 1997).
5.2.2 Zones of net erosion
Within arid environments sediment production
will be dominated by source areas susceptible to
erosion. These most commonly include areas that
generate conditions of high erosivity (slopes) or
areas of high erodibility reflecting lithologies less
resistant to erosion, such as lake-bed sediments.
5.2.1.3 Salt weathering
In arid environments salt can be sourced from a
number of potential areas. These may include:
1
sea water or relict sea water which may be
found in bodies of water (e.g. lakes) formerly
connected to the sea;
2
'cyclic salts' derived from atmospheric inputs
such as dust and rainfall or volcanic emissions;
3
the release of salts through rock weathering,
especially where the bedrock is evaporitic in
origin (e.g. halite).
These abundant salts in arid environments mean
that salt weathering is not only key in weathering,
but also creates problems in the built environ-
ment (section 5.6.3).
Weakening of sediment by salt weathering
predisposes many types of sediment in arid areas
to deflation. In some cases the hollows gener-
ated by deflation attract grazing animals, which
in turn increase erosion (Goudie 1989). Erosion
of the pans may be limited at depth by ground-
water (Goudie & Wells 1995), which will also
contribute to the salt weathering. Salt weather-
ing can occur through a number of processes.
1
Salt crystal growth.
This may result from
changes in solubility with temperature, evapora-
tional concentration of solutions, and mixing
of salt solutions with the same ion (see Goudie
1997).
2
Hydration.
Some salts will hydrate and
rehydrate in response to changes in temperature
and humidity. As salt changes to its hydrated
form it takes up water, increasing its volume.
Some of the largest hydration pressures occur
5.2.2.1 Slopes
Many arid areas are associated with landforms
such as pediments, cuestas and mesas that are
protected by a caprock. This caprock may be
part of the geological sequence, or it could be
a crust developed
in situ
(section 5.3.5). What-
ever the cause of the crust, caprock failure is
not uncommon, producing slope talus and a
potential sediment supply. Rates of scarp retreat,
and thus sediment production, vary accord-
ing to the local balance between the geological
characteristics and the climate. Lower rates
tend to be reported from hyper-arid regions,
for example 100 mm kyr
−1
from limestone has
been reported from the Sinai, Israel (Yair &
Gerson 1974). However, the overall control on
rates of retreat is local lithology. This varies
greatly between different lithologies, with the
highest retreat rates reported from conglo-
merates (6700 mm kyr
−1
, Lucchitta 1975) and
sandstone (500 - 6700 mm kyr
−1
, Schmidt 1980,
1989). Similarly retreat rates within the same
overall lithology may vary dramatically as a func-
tion of microlithological variations in the caprock,
and overall stratigraphy. For example, limestone
scarp-retreat rates of 100 - 2000 mm kyr
−1
have
been reported in hyper-arid areas (Sinai, Israel;
Yair & Gerson 1974) and 160 - 400 mm in
semi-arid areas (Arizona, USA; Cole & Mayer
