462
Climatic Geomorphology
he (or she) must be able to communicate ideas comprehensibly to others who may have no
geomorphological knowledge.
2. Weathering
The weathering of desert areas is mainly driven by strong variations in temperature and
humidity. Fluctuations in temperature give rise to insolation weathering, and at high
altitude to freeze-thaw phenomena. Variations in humidity encourage wetting and drying
processes as well as haloclasty. The lack of water impedes chemical modification, and
biotic weathering is mainly limited to the physical and chemical degradation caused by
algae and lichens. Weathering in these areas is therefore rather peculiar and gives rise to a
set of micro-landforms such as honeycomb, tafoni, caverns and gnammas, which can be
very abundant in medium-grained rocks (Mabbutt, 1977).
2.1. Weathering by wetting and drying cycles and salt weathering
Weathering by wetting and drying cycles leads to the disintegration of rocks by flaking and
cracking (Oilier, 1984). It is an important agent of erosion in desert areas, causing
considerable breakage of rocks (Oilier, 1977). The water circulating in the pores of rocks
carries very soluble salts, especially chlorides and sulphates, and when these precipitate
because of evaporation they can have an even greater disruptive influence than freezethaw. This process, known as haloclasty, acts alongside wetting and drying. Of all the
weathering processes at work in desert areas, haloclasty, which involves the growth of
crystals, volumetric expansion, and salt hydration (Cooke, 1986; Cooke et al., 1993), is the
most important (Doornkamp and Ibrahim, 1990). It can disintegrate rocks into particles the
size of silt. Evidence of this is mainly seen along the edges of salt lakes, in ephemeral
channels, and where the area of capillarity intersects the soil surface. The builders of
ancient civilizations knew the disruptive effects of salts on rock very well (Oilier, 1977).
The Great Pyramid of Egypt (Fig. 19.1), for example, was originally covered in a
protective layer of resistant limestone slabs. Unfortunately, about 1000 years ago part of
this covering was destroyed and the weathering of the pyramid's core began (Emery,
1960). Although natural materials are still used in construction today, many structures are
made of concrete, but the weathering behaviour of this material can be studied in the same
way.
The effectiveness of haloclasty (Cooke et al., 1982) depends on environmental
conditions such as those produced by daily fluctuations in temperature and humidity
(which give rise to the crystallization or hydration of salts), and on fluctuations of the
capillary front. Equally important are the properties of the materials affected, especially
those related to porosity and pore distribution. Finally, different salts have different effects:
the most aggressive are sodium and magnesium sulphate (Fig. 11.7) (Goudie et al., 1970;
Goudie, 1974 and Cooke, 1979).
The most common and most serious problems related to engineering structures (Cooke,
1986; Goudie, 1994a; Goudie and Viles, 1997; Goudie, 1998) concern fluctuations in the
capillary front. These can affect the foundations of buildings, roads, canals, oil pipelines
Climatic Geomorphology
he (or she) must be able to communicate ideas comprehensibly to others who may have no
geomorphological knowledge.
2. Weathering
The weathering of desert areas is mainly driven by strong variations in temperature and
humidity. Fluctuations in temperature give rise to insolation weathering, and at high
altitude to freeze-thaw phenomena. Variations in humidity encourage wetting and drying
processes as well as haloclasty. The lack of water impedes chemical modification, and
biotic weathering is mainly limited to the physical and chemical degradation caused by
algae and lichens. Weathering in these areas is therefore rather peculiar and gives rise to a
set of micro-landforms such as honeycomb, tafoni, caverns and gnammas, which can be
very abundant in medium-grained rocks (Mabbutt, 1977).
2.1. Weathering by wetting and drying cycles and salt weathering
Weathering by wetting and drying cycles leads to the disintegration of rocks by flaking and
cracking (Oilier, 1984). It is an important agent of erosion in desert areas, causing
considerable breakage of rocks (Oilier, 1977). The water circulating in the pores of rocks
carries very soluble salts, especially chlorides and sulphates, and when these precipitate
because of evaporation they can have an even greater disruptive influence than freezethaw. This process, known as haloclasty, acts alongside wetting and drying. Of all the
weathering processes at work in desert areas, haloclasty, which involves the growth of
crystals, volumetric expansion, and salt hydration (Cooke, 1986; Cooke et al., 1993), is the
most important (Doornkamp and Ibrahim, 1990). It can disintegrate rocks into particles the
size of silt. Evidence of this is mainly seen along the edges of salt lakes, in ephemeral
channels, and where the area of capillarity intersects the soil surface. The builders of
ancient civilizations knew the disruptive effects of salts on rock very well (Oilier, 1977).
The Great Pyramid of Egypt (Fig. 19.1), for example, was originally covered in a
protective layer of resistant limestone slabs. Unfortunately, about 1000 years ago part of
this covering was destroyed and the weathering of the pyramid's core began (Emery,
1960). Although natural materials are still used in construction today, many structures are
made of concrete, but the weathering behaviour of this material can be studied in the same
way.
The effectiveness of haloclasty (Cooke et al., 1982) depends on environmental
conditions such as those produced by daily fluctuations in temperature and humidity
(which give rise to the crystallization or hydration of salts), and on fluctuations of the
capillary front. Equally important are the properties of the materials affected, especially
those related to porosity and pore distribution. Finally, different salts have different effects:
the most aggressive are sodium and magnesium sulphate (Fig. 11.7) (Goudie et al., 1970;
Goudie, 1974 and Cooke, 1979).
The most common and most serious problems related to engineering structures (Cooke,
1986; Goudie, 1994a; Goudie and Viles, 1997; Goudie, 1998) concern fluctuations in the
capillary front. These can affect the foundations of buildings, roads, canals, oil pipelines