518
Climatic Geomorphology
In the arid and semi-arid zones of Africa (Thomas, 1974) and Australia (Mabbutt,
1965b), in temperate regions of the USA (Kaye, 1967), and Europe (Demek, 1965;
Guti6rrez and Rodrfguez, 1978; Molina and Blanco, 1980) deep weathering profiles are
relatively common. It is obvious that the origin of such thickness is not possible in the
climatic conditions under which they remain. In the opinion of numerous authors, they
must be interpreted as generated under humid tropical climates, with a greater chemical
activity. Therefore, the existence of thick weathering profiles located in zones far from the
humid tropics is a good indicator of climatic change.
Thomas (1974) has analysed the spatial distribution of regoliths. The influence of rock
structure is apparent because it affects weathering depth. In this sense, the density of
splitting planes existing on the rock massif controls weathering thickness. The maximum
depth is related to the maximum density (Thomas, 1966) (Figure 20.13), which sometimes corresponds to intersection zones between fracturing systems. For this reason the
weathering front is very irregular and bears no relation to the topographical profile.
On the contrary, the deepest weathering develops on the fluvial divides. This can be due to
the fluvial erosion of a previously thick weathering profile (Figure 20.14) or to a faster
weathering on the divides (Thomas, 1994).
3.6. Weathering zones
The alteration profile is not an unstructured regolith. Instead, several differentiations can
be made within the profile, especially apparent on granitic rocks. The works by Ruxton
and Berry (1957, 1961) and Berry and Ruxton (1959) on granite rocks in Hong Kong were
the first key studies in establishing a zonation of weathering profiles. Wilhelmy (1958)
studied similar topics in eastern Asia, Africa, Central and South America, the Iberian
Peninsula, and Corsica. Mabbutt (1961b) analysed them in western Australia. Inside the
profile the number of flesh rock blocks decreases towards the higher parts whereas their
angularity increases with depth. In some cases joint blocks present a concentric structure
formed by shells of residual materials, with a corestone that can remain totally flesh
and coherent. This disposition is known as spheroidal weathering (Figure 20.15 and
Figure 20.16). Its origin is thought to be due to more rapid chemical attack on comers and
edges of joint blocks, compared to joint-block faces, as well as the migrations of elements
within the rock (Ollier, 1984). The upper parts of the profile are easily eroded down to the
zone where blocks predominate. The differentiation into zones is based on the alteration
degree of the granite rock and on the presence of unaltered blocks inside the profile
(Figure 20.17). All the previously cited authors apply these criteria to establish divisions
within the weathering profile. Gerrard (1988) made a wide compilation of classifications
of weathering profiles presented by different authors. Ollier (1984) proposed the most
widely used zonation (Gerrard, 1988):
1. Soil.
2. Structureless regolith.
3. Regolith retaining rock structure (Figure 20.18).
4. Regolith with rounded corestones.
5. Regolith with angular, locked corestones (Figure 20.19).
6. Fresh, unweathered rock.
Climatic Geomorphology
In the arid and semi-arid zones of Africa (Thomas, 1974) and Australia (Mabbutt,
1965b), in temperate regions of the USA (Kaye, 1967), and Europe (Demek, 1965;
Guti6rrez and Rodrfguez, 1978; Molina and Blanco, 1980) deep weathering profiles are
relatively common. It is obvious that the origin of such thickness is not possible in the
climatic conditions under which they remain. In the opinion of numerous authors, they
must be interpreted as generated under humid tropical climates, with a greater chemical
activity. Therefore, the existence of thick weathering profiles located in zones far from the
humid tropics is a good indicator of climatic change.
Thomas (1974) has analysed the spatial distribution of regoliths. The influence of rock
structure is apparent because it affects weathering depth. In this sense, the density of
splitting planes existing on the rock massif controls weathering thickness. The maximum
depth is related to the maximum density (Thomas, 1966) (Figure 20.13), which sometimes corresponds to intersection zones between fracturing systems. For this reason the
weathering front is very irregular and bears no relation to the topographical profile.
On the contrary, the deepest weathering develops on the fluvial divides. This can be due to
the fluvial erosion of a previously thick weathering profile (Figure 20.14) or to a faster
weathering on the divides (Thomas, 1994).
3.6. Weathering zones
The alteration profile is not an unstructured regolith. Instead, several differentiations can
be made within the profile, especially apparent on granitic rocks. The works by Ruxton
and Berry (1957, 1961) and Berry and Ruxton (1959) on granite rocks in Hong Kong were
the first key studies in establishing a zonation of weathering profiles. Wilhelmy (1958)
studied similar topics in eastern Asia, Africa, Central and South America, the Iberian
Peninsula, and Corsica. Mabbutt (1961b) analysed them in western Australia. Inside the
profile the number of flesh rock blocks decreases towards the higher parts whereas their
angularity increases with depth. In some cases joint blocks present a concentric structure
formed by shells of residual materials, with a corestone that can remain totally flesh
and coherent. This disposition is known as spheroidal weathering (Figure 20.15 and
Figure 20.16). Its origin is thought to be due to more rapid chemical attack on comers and
edges of joint blocks, compared to joint-block faces, as well as the migrations of elements
within the rock (Ollier, 1984). The upper parts of the profile are easily eroded down to the
zone where blocks predominate. The differentiation into zones is based on the alteration
degree of the granite rock and on the presence of unaltered blocks inside the profile
(Figure 20.17). All the previously cited authors apply these criteria to establish divisions
within the weathering profile. Gerrard (1988) made a wide compilation of classifications
of weathering profiles presented by different authors. Ollier (1984) proposed the most
widely used zonation (Gerrard, 1988):
1. Soil.
2. Structureless regolith.
3. Regolith retaining rock structure (Figure 20.18).
4. Regolith with rounded corestones.
5. Regolith with angular, locked corestones (Figure 20.19).
6. Fresh, unweathered rock.