Weathering processes and resulting forms
249
thenardite (Na2SO4) to mirabilite (Na2SO4.10H20) or that of anhydrite (CaSO4) to
gypsum (SO4.2H20). This last produces a volume increase of about 39% (Jauzein, 1974).
The kind of salts in a region depends upon the lithology of the source area and,
therefore, varies from one region to other. In Australia NaC1 predominates, in the salt flats
of the Atacama Desert the dominant salts are NaC1 and sodium and calcium sulphates, in
the endorheic area of the Ebro Depression of Spain the most common salts are sodium
chlorides and sulphates (Pueyo, 1978 to 1979), and in some salt lakes of mid-eastern
Africa the preponderant salt is Na2CO3.
Salts come fundamentally from the freed ions in the chemical weathering processes of
rocks, which are mobilized in solution by run-off waters and also by subterranean fluxes.
In littoral zones, aquifer overpumping can cause saltwater intrusion and salinity increase
of the extracted waters. Another salt mobilization source is the water and aeolian erosion
of salt deposits. Salts can also come from volcanic dust or gases. In the littoral deserts,
marine aerosols coming from the sea commonly constitute an important source of salts
(Goudie and Viles, 1997). In these areas there is a clear gradual decrease of salt content
towards inland areas (Goudie, 1989a). In Aftout, Mauritania, and in the Senegal delta,
where waves strike against the littoral bars they produce water drops that evaporate and
generate small salt crystals, which float in suspension giving origin to a whitish fog that the
wind pushes toward the continent. Afterwards they can be washed out by the rain or, what
is more frequent, by an increase in relative humidity during the night that favours the water
fixation in these hydroscopic crystals that fall as a consequence of these enlargements
(Tricart, 1969).
Salts coming from these different feeding sources accumulate in a great variety of
situations (Cooke et al., 1993). In littoral deserts (Peru-Chile, Namibia and western
Australia) salt coming from oceans produced a salt contribution of 150 kg/ha/yr. In tidal
flats of seas of high salinity as at the Red Sea, Persian Gulf and Mediterranean Sea an
important saline sedimentation takes place in littoral sabkhas. In continental interiors,
evaporation of lacustrine waters supplied for run-off, as well as groundwater and aeolian
contributions, generate saltlakes. The different solubility of the salts controls their
precipitation and results in a zonation that shows the presence of chlorides in the inner
part, sulphates in the middle zones, and less soluble carbonates along the external borders
(Figure 11.8).
2.3.4. Physical-chemical processes
The mechanisms that involve salt weathering are of chemical and physical character. They
include crystallization, hydration, and thermal expansion of high-soluble salts in confined
and semiconfined spaces, such as in pores and cracks of the rocks.
Of these three mechanisms, crystal growth is the most important (Goudie, 1974) and it
happens that as saline solutions become saturated, precipitates and salt crystals are
produced in the pore spaces (Winkler and Singer, 1972). These crystals can continue
growing between the walls that confine them, if there is a film of saline solution at
the crystal/rock interface. Crystallization generates important pressures that are
transmitted to the rock (Evans, 1969 to 1970). Precipitation is caused by the evaporation,
cooling, or mixing of solutions with a common ion. Some salt solubility (NaaSO4,
NaaCO3, NaNO3 and MgSO4) diminishes with the descent of temperature, which during
249
thenardite (Na2SO4) to mirabilite (Na2SO4.10H20) or that of anhydrite (CaSO4) to
gypsum (SO4.2H20). This last produces a volume increase of about 39% (Jauzein, 1974).
The kind of salts in a region depends upon the lithology of the source area and,
therefore, varies from one region to other. In Australia NaC1 predominates, in the salt flats
of the Atacama Desert the dominant salts are NaC1 and sodium and calcium sulphates, in
the endorheic area of the Ebro Depression of Spain the most common salts are sodium
chlorides and sulphates (Pueyo, 1978 to 1979), and in some salt lakes of mid-eastern
Africa the preponderant salt is Na2CO3.
Salts come fundamentally from the freed ions in the chemical weathering processes of
rocks, which are mobilized in solution by run-off waters and also by subterranean fluxes.
In littoral zones, aquifer overpumping can cause saltwater intrusion and salinity increase
of the extracted waters. Another salt mobilization source is the water and aeolian erosion
of salt deposits. Salts can also come from volcanic dust or gases. In the littoral deserts,
marine aerosols coming from the sea commonly constitute an important source of salts
(Goudie and Viles, 1997). In these areas there is a clear gradual decrease of salt content
towards inland areas (Goudie, 1989a). In Aftout, Mauritania, and in the Senegal delta,
where waves strike against the littoral bars they produce water drops that evaporate and
generate small salt crystals, which float in suspension giving origin to a whitish fog that the
wind pushes toward the continent. Afterwards they can be washed out by the rain or, what
is more frequent, by an increase in relative humidity during the night that favours the water
fixation in these hydroscopic crystals that fall as a consequence of these enlargements
(Tricart, 1969).
Salts coming from these different feeding sources accumulate in a great variety of
situations (Cooke et al., 1993). In littoral deserts (Peru-Chile, Namibia and western
Australia) salt coming from oceans produced a salt contribution of 150 kg/ha/yr. In tidal
flats of seas of high salinity as at the Red Sea, Persian Gulf and Mediterranean Sea an
important saline sedimentation takes place in littoral sabkhas. In continental interiors,
evaporation of lacustrine waters supplied for run-off, as well as groundwater and aeolian
contributions, generate saltlakes. The different solubility of the salts controls their
precipitation and results in a zonation that shows the presence of chlorides in the inner
part, sulphates in the middle zones, and less soluble carbonates along the external borders
(Figure 11.8).
2.3.4. Physical-chemical processes
The mechanisms that involve salt weathering are of chemical and physical character. They
include crystallization, hydration, and thermal expansion of high-soluble salts in confined
and semiconfined spaces, such as in pores and cracks of the rocks.
Of these three mechanisms, crystal growth is the most important (Goudie, 1974) and it
happens that as saline solutions become saturated, precipitates and salt crystals are
produced in the pore spaces (Winkler and Singer, 1972). These crystals can continue
growing between the walls that confine them, if there is a film of saline solution at
the crystal/rock interface. Crystallization generates important pressures that are
transmitted to the rock (Evans, 1969 to 1970). Precipitation is caused by the evaporation,
cooling, or mixing of solutions with a common ion. Some salt solubility (NaaSO4,
NaaCO3, NaNO3 and MgSO4) diminishes with the descent of temperature, which during