Chapter 14
Slopes in arid zones
In arid regions the production of particles by weathering processes is generally smaller
than those mobilized by superficial run-off or by the wind. Evidently, vegetation cover,
tightly related to the climate, exerts a manifest influence in this balance. Therefore, it can
be said that the degree of aridity controls the balance between supply and evacuation of
particles. In the cold mountainous deserts, however, where frost action is the dominant
process, the generation of frost shattering is higher than exportation (Figure 8.13), and
moreover if aridity is strong and, as a consequence, water erosion is not as important, it is
unable to mobilize all the clasts produced.
Although the influence of climate and vegetation is evident in the evolution of the
desert slopes, the lithologic composition, along with the frequency of the existing planes of
discontinuity, are the most fundamental factors in the development of the modelling of
the slopes under these arid environments. These geologic characteristics directly affect the
activity and intensity of the weathering processes. These factors, therefore, determine
the character and make possible the differentiation of three types of slopes developed on
crystalline rocks (Figure 14.1), stratified formations, and highly erodable materials
(Mabbutt, 1977). Some authors refer to this first slope type as "massive rocks" and include
in it sandstones and conglomerates (Howard and Selby, 1994).
1. Slopes on crystalline rocks
The chemical composition of the magma exerts a direct influence on the morphology of
the expelled volcanic materials. The basic products produce extensive basaltic flows,
which alternate with pyroclastic deposits. The differential erosion of these formations
produces stepped slopes (trap), characteristic of the big plateau basalts outcroppings, of
different ages, existent in the world. Similarly, morphologies of this type occur in the rift
valleys of East Africa. The emission of magmas of acid composition, as a consequence of
their greater viscosity, generates explosive eruptions that are accompanied by domes and
needles, as in the trachytes and phonolites of the Hoggar massif, in the central region of the
Sahara.
In plutonic rocks two types of forms are recognized. Some are controlled by sheeting
and other by cross-jointing systems. Sheeting develops in massive rocks that are
fundamentally crystalline, is formed by curved joints parallel to the surface, and is denoted
also as topographical jointing. Their origin is attributed to decompression or unloading of
rocks that have being subjected to major force inside of the Earth's crust. Later where the
rocks ascend to the surface and lose load, they relax and fracture of the rock massif takes
Slopes in arid zones
In arid regions the production of particles by weathering processes is generally smaller
than those mobilized by superficial run-off or by the wind. Evidently, vegetation cover,
tightly related to the climate, exerts a manifest influence in this balance. Therefore, it can
be said that the degree of aridity controls the balance between supply and evacuation of
particles. In the cold mountainous deserts, however, where frost action is the dominant
process, the generation of frost shattering is higher than exportation (Figure 8.13), and
moreover if aridity is strong and, as a consequence, water erosion is not as important, it is
unable to mobilize all the clasts produced.
Although the influence of climate and vegetation is evident in the evolution of the
desert slopes, the lithologic composition, along with the frequency of the existing planes of
discontinuity, are the most fundamental factors in the development of the modelling of
the slopes under these arid environments. These geologic characteristics directly affect the
activity and intensity of the weathering processes. These factors, therefore, determine
the character and make possible the differentiation of three types of slopes developed on
crystalline rocks (Figure 14.1), stratified formations, and highly erodable materials
(Mabbutt, 1977). Some authors refer to this first slope type as "massive rocks" and include
in it sandstones and conglomerates (Howard and Selby, 1994).
1. Slopes on crystalline rocks
The chemical composition of the magma exerts a direct influence on the morphology of
the expelled volcanic materials. The basic products produce extensive basaltic flows,
which alternate with pyroclastic deposits. The differential erosion of these formations
produces stepped slopes (trap), characteristic of the big plateau basalts outcroppings, of
different ages, existent in the world. Similarly, morphologies of this type occur in the rift
valleys of East Africa. The emission of magmas of acid composition, as a consequence of
their greater viscosity, generates explosive eruptions that are accompanied by domes and
needles, as in the trachytes and phonolites of the Hoggar massif, in the central region of the
Sahara.
In plutonic rocks two types of forms are recognized. Some are controlled by sheeting
and other by cross-jointing systems. Sheeting develops in massive rocks that are
fundamentally crystalline, is formed by curved joints parallel to the surface, and is denoted
also as topographical jointing. Their origin is attributed to decompression or unloading of
rocks that have being subjected to major force inside of the Earth's crust. Later where the
rocks ascend to the surface and lose load, they relax and fracture of the rock massif takes