Chapter 11
Weathering processes and resulting forms
1. Introduction
The alteration of the rocks in arid zones is above all the result of the wide daily and
seasonal variations of temperature and relative humidity. Moreover, in deserts the rock
outcrops are more common than in other climatic zones. The lithologic and structural
characteristics of these outcrops influence the action of the processes and the resultant
morphologies (Figure 11.1). In these desert environments physical weathering has a more
specific importance than the biological and chemical weathering processes. Already it has
been pointed out that in cold deserts during the cold months, rock fracture by frost action is
an important process and the fundamental characteristics of it have been analysed in the
section on periglacial processes. The scarcity of water and high temperatures give rise to
weak development in animal life and vegetation and, therefore, biological weathering is
much reduced. Equally, the moisture deficit affects the activity and intensity of chemical
weathering, because water is necessary in attacking the crystalline structures and for the
later mobilization of the ions in dissolution. Some processes like salt weathering, however,
are of a physical-chemical type and this implies dissolution and precipitation of highly
soluble salts that cause the rock to break later (Cooke et al., 1993). Also in the arid zones
can be recognized the formation of iron oxides by capillarity and the mobilization of silica
and alumina in alkaline media (Smith, 1994). All this together appears to indicate that the
main part of the chemical weathering processes takes place in deserts, but its intensity
diminishes considerably in comparison to more humid environments. On the other hand, in
arid regions one can find some forms and deposits inherited from different past climatic
conditions, such as alteration profiles, laterites, sinkholes, karstic cavities, and so forth.
They have all been formed in past climates that were much more humid and in which
chemical weathering was more aggressive.
The results of the activity of weathering processes results in granular disintegration,
flaking (Figure 11.2), and splitting. The fluvial and aeolian erosive processes mobilize the
freed particles, leaving an imprint on the diverse rock morphologies, as a consequence of
the unequal activity of the alteration processes. These forms are customarily of small size
and are characterized by the presence of shallow hollows in the rock surface (Goudie,
1989a).
In the past few decades there has been considerable progress in the knowledge of the
action of weathering processes in desert areas and in the development of the resultant
morphologies. The investigations have been carried out in the field or in the laboratory by
investigations on durability. The first ones studied detailed temperature and relative
humidity variation registers for several years and the influence of this variability on
different types of rock weathering. Laboratory studies were designed to investigate
Weathering processes and resulting forms
1. Introduction
The alteration of the rocks in arid zones is above all the result of the wide daily and
seasonal variations of temperature and relative humidity. Moreover, in deserts the rock
outcrops are more common than in other climatic zones. The lithologic and structural
characteristics of these outcrops influence the action of the processes and the resultant
morphologies (Figure 11.1). In these desert environments physical weathering has a more
specific importance than the biological and chemical weathering processes. Already it has
been pointed out that in cold deserts during the cold months, rock fracture by frost action is
an important process and the fundamental characteristics of it have been analysed in the
section on periglacial processes. The scarcity of water and high temperatures give rise to
weak development in animal life and vegetation and, therefore, biological weathering is
much reduced. Equally, the moisture deficit affects the activity and intensity of chemical
weathering, because water is necessary in attacking the crystalline structures and for the
later mobilization of the ions in dissolution. Some processes like salt weathering, however,
are of a physical-chemical type and this implies dissolution and precipitation of highly
soluble salts that cause the rock to break later (Cooke et al., 1993). Also in the arid zones
can be recognized the formation of iron oxides by capillarity and the mobilization of silica
and alumina in alkaline media (Smith, 1994). All this together appears to indicate that the
main part of the chemical weathering processes takes place in deserts, but its intensity
diminishes considerably in comparison to more humid environments. On the other hand, in
arid regions one can find some forms and deposits inherited from different past climatic
conditions, such as alteration profiles, laterites, sinkholes, karstic cavities, and so forth.
They have all been formed in past climates that were much more humid and in which
chemical weathering was more aggressive.
The results of the activity of weathering processes results in granular disintegration,
flaking (Figure 11.2), and splitting. The fluvial and aeolian erosive processes mobilize the
freed particles, leaving an imprint on the diverse rock morphologies, as a consequence of
the unequal activity of the alteration processes. These forms are customarily of small size
and are characterized by the presence of shallow hollows in the rock surface (Goudie,
1989a).
In the past few decades there has been considerable progress in the knowledge of the
action of weathering processes in desert areas and in the development of the resultant
morphologies. The investigations have been carried out in the field or in the laboratory by
investigations on durability. The first ones studied detailed temperature and relative
humidity variation registers for several years and the influence of this variability on
different types of rock weathering. Laboratory studies were designed to investigate