236
Climatic Geomorphology
names (alisio, irifi, sirocco, harmattan, and other names) (Goudie and Wilkinson, 1977).
In particular cases they can evolve into tornado-like, dust-devils, which are hot air masses,
whirling and particle loaded. They can reach tens or hundreds of metres in altitude with
durations of several minutes. Much more important are the dust storms (haboob), that are
air walls full of particles. They can produce catastrophic effects on animal life and human
activities (Pew6, 1981).
4. Geomorphological differentiation of desert areas
The geomorphic criteria that can be used to establish classifications of arid zones can vary
a great deal. The application of rules about dominant processes and general
morphostructural types in desert environments allows the establishment of simple
differentiations that in general show the main geomorphological features of these
morphoclimatic zones.
Two important dominions can be recognised in deserts as a function of the main acting
processes (Cooke et al., 1993). The first is the aeolian dominion in which predominates the
erosive action and sedimentation of the wind. Erosion can create extended groups of
yardangs, as those of the Libyan and Arabian Deserts, in which the rock outcrops are
clearly lined up by the abrasive action of the wind. Erosive action can also been shown in
the blow-out fields or deflation areas present in some desert areas such as the Kalahari
Desert. The accumulation of particles transported by the wind gives rise to extensive sand
seas that cover approximately a quarter of the deserts. A significant portion of these sand
seas are nowadays stabilised by vegetation, as in the southern Sahara, and indicate a major
aridity in past periods. The fluvial dominion results from the erosive and accumulative
activity of run-off waters, which is more evident in the semi-arid zones. Fluvial action
remains limited to the valleys, in which erosion and sedimentation are a part of their load,
or to the alluvial fan systems, which distribute the water and solid load to the foot of
mountainous zones, with a general diminution of the size of the particles towards the distal
areas. Both dominions can be superimposed, because the activity of the aeolian and fluvial
processes coexists many times in the same zone. Therefore, in the sedimentary
record aeolian and fluvial sequences can be interbedded (Figure 10.6). On the other
hand, the climatic changes that have taken place during the Quaternary have produced
important modifications in the aridity of numerous areas. As a consequence, the activity
and intensity of aeolian and fluvial processes seems to be modified considerably. In some
regions, such as in the Sahel, the diminution of the aridity emphasised the fluvial activity
and it paralyzed the aeolian action, stabilising and partially wasting the systems of dunes
by water erosion.
The application of the morphostructural criterion, based on the type of dominant
morphologies and in the geologic structure, along with the degree of tectonic stability,
permits distinguising between shield and platform deserts and mountain and basin deserts
(Mabbutt, 1977). The first comprises the African, Arabian, Indian and Australian deserts.
They are formed fundamentally from cratonic areas basically preformed by Precambrian
rocks and platforms of later age. Their main characteristic is the flat relief, interrupted by
recent volcanic massifs, like the mountains of the Hoggar and of the Tibesti in the central
Sahara, and by important fractures such as that relative to rift-valleys system in Eastern
Climatic Geomorphology
names (alisio, irifi, sirocco, harmattan, and other names) (Goudie and Wilkinson, 1977).
In particular cases they can evolve into tornado-like, dust-devils, which are hot air masses,
whirling and particle loaded. They can reach tens or hundreds of metres in altitude with
durations of several minutes. Much more important are the dust storms (haboob), that are
air walls full of particles. They can produce catastrophic effects on animal life and human
activities (Pew6, 1981).
4. Geomorphological differentiation of desert areas
The geomorphic criteria that can be used to establish classifications of arid zones can vary
a great deal. The application of rules about dominant processes and general
morphostructural types in desert environments allows the establishment of simple
differentiations that in general show the main geomorphological features of these
morphoclimatic zones.
Two important dominions can be recognised in deserts as a function of the main acting
processes (Cooke et al., 1993). The first is the aeolian dominion in which predominates the
erosive action and sedimentation of the wind. Erosion can create extended groups of
yardangs, as those of the Libyan and Arabian Deserts, in which the rock outcrops are
clearly lined up by the abrasive action of the wind. Erosive action can also been shown in
the blow-out fields or deflation areas present in some desert areas such as the Kalahari
Desert. The accumulation of particles transported by the wind gives rise to extensive sand
seas that cover approximately a quarter of the deserts. A significant portion of these sand
seas are nowadays stabilised by vegetation, as in the southern Sahara, and indicate a major
aridity in past periods. The fluvial dominion results from the erosive and accumulative
activity of run-off waters, which is more evident in the semi-arid zones. Fluvial action
remains limited to the valleys, in which erosion and sedimentation are a part of their load,
or to the alluvial fan systems, which distribute the water and solid load to the foot of
mountainous zones, with a general diminution of the size of the particles towards the distal
areas. Both dominions can be superimposed, because the activity of the aeolian and fluvial
processes coexists many times in the same zone. Therefore, in the sedimentary
record aeolian and fluvial sequences can be interbedded (Figure 10.6). On the other
hand, the climatic changes that have taken place during the Quaternary have produced
important modifications in the aridity of numerous areas. As a consequence, the activity
and intensity of aeolian and fluvial processes seems to be modified considerably. In some
regions, such as in the Sahel, the diminution of the aridity emphasised the fluvial activity
and it paralyzed the aeolian action, stabilising and partially wasting the systems of dunes
by water erosion.
The application of the morphostructural criterion, based on the type of dominant
morphologies and in the geologic structure, along with the degree of tectonic stability,
permits distinguising between shield and platform deserts and mountain and basin deserts
(Mabbutt, 1977). The first comprises the African, Arabian, Indian and Australian deserts.
They are formed fundamentally from cratonic areas basically preformed by Precambrian
rocks and platforms of later age. Their main characteristic is the flat relief, interrupted by
recent volcanic massifs, like the mountains of the Hoggar and of the Tibesti in the central
Sahara, and by important fractures such as that relative to rift-valleys system in Eastern