504
Climatic Geomorphology
Because temperatures are very homogeneous, climatic subdivisions are made as a
function of precipitation. For the humid tropics and transitional areas a limiting value is
established with annual precipitation higher than 750 mm (Thomas, 1994). It includes rain
forest (Af and Am) and savanna (Aw and As) climates (K6ppen, 1936).
Morphoclimatic differentiation is quite similar, because precipitation and the resulting
vegetation cover play a determinant role in humid tropic regions. The numerous geomorphic inheritances from other climates existing both in equatorial and in humid and dry
climates, however, must be taken into account, although more obvious in the latter, near
zones of dry climate. Dominant geomorphological processes in the humid tropics are
chemical weathering, mass movements, and fluvial erosion, the latter being much more
important in regions of humid and dry climate (Wilson, 1968, 1969).
Without any doubt, the intense deep weathering and its profile evolution constitute one
of the most characteristic features of this morphoclimatic zone. Some authors, however,
doubt the existence of a tropical climatic geomorphology (Stoddart, 1969c), and others the
differentiation of the tropical zone within the global morphoclimatic system (Douglas,
1978; Gupta, 1993). This last author indicated that processes and forms in the tropics
depend only partially on climate, and that there exist other important factors such as
neotectonics, lithology, episodic and catastrophic events (cyclones), and anthropic activities. Although this statement is true, it must be kept in mind that all these non-climatic
processes and factors act throughout the world, and they operate at the same time as
climatic causes. This vision would correspond to very populated regions undergoing
important neotectonics. This is a quite different situation from the one existing in African
and Brazilian regions, where old planation surfaces have favoured the development of
thick regoliths, while environmental changes have produced lateritic landforms, etch
surfaces, and other features typical of humid tropical zones.
Following Btidel (1977, 1982), humid tropical zones are completely different from
other morphoclimatic zones. They do not develop fluvial terraces, gravel plains, or flood
plains such as in the ectropics (temperate regions), nor loess mantles. The soil is much
deeper than in the ectropics, and contains plenty of duricrusts and hardpans. Inselbergs of
variable size are common in the tropics, standing out on extensive erosion surfaces, which
are weathered at the same time as they form. Mountain zones are characterized by deep
incisions. This author assumes that morphodynamics in tropical lowlands are completely
different, where landforms are controlled by the generation of etch surfaces.
Tricart and Cailleux (1965) differentiated an intertropical zone with warm temperatures
and constant fluvial flows. Depending the rainfall distribution, its annual amount, and
the vegetation cover density, they divide this zone into a savanna domain, with less
precipitation and less dense vegetation cover. Sheet wash is considerable and chemical
weathering is strong and discontinuous in time. The rainforest domain is characterized by
dense vegetation cover and higher precipitation. In this domain chemical and biochemical
processes reach a maximum intensity.
3. Weathering in the humid tropics
Weathering is the response of materials that were initially in equilibrium inside the
lithosphere to conditions at or near their contact with the atmosphere, hydrosphere and,
Climatic Geomorphology
Because temperatures are very homogeneous, climatic subdivisions are made as a
function of precipitation. For the humid tropics and transitional areas a limiting value is
established with annual precipitation higher than 750 mm (Thomas, 1994). It includes rain
forest (Af and Am) and savanna (Aw and As) climates (K6ppen, 1936).
Morphoclimatic differentiation is quite similar, because precipitation and the resulting
vegetation cover play a determinant role in humid tropic regions. The numerous geomorphic inheritances from other climates existing both in equatorial and in humid and dry
climates, however, must be taken into account, although more obvious in the latter, near
zones of dry climate. Dominant geomorphological processes in the humid tropics are
chemical weathering, mass movements, and fluvial erosion, the latter being much more
important in regions of humid and dry climate (Wilson, 1968, 1969).
Without any doubt, the intense deep weathering and its profile evolution constitute one
of the most characteristic features of this morphoclimatic zone. Some authors, however,
doubt the existence of a tropical climatic geomorphology (Stoddart, 1969c), and others the
differentiation of the tropical zone within the global morphoclimatic system (Douglas,
1978; Gupta, 1993). This last author indicated that processes and forms in the tropics
depend only partially on climate, and that there exist other important factors such as
neotectonics, lithology, episodic and catastrophic events (cyclones), and anthropic activities. Although this statement is true, it must be kept in mind that all these non-climatic
processes and factors act throughout the world, and they operate at the same time as
climatic causes. This vision would correspond to very populated regions undergoing
important neotectonics. This is a quite different situation from the one existing in African
and Brazilian regions, where old planation surfaces have favoured the development of
thick regoliths, while environmental changes have produced lateritic landforms, etch
surfaces, and other features typical of humid tropical zones.
Following Btidel (1977, 1982), humid tropical zones are completely different from
other morphoclimatic zones. They do not develop fluvial terraces, gravel plains, or flood
plains such as in the ectropics (temperate regions), nor loess mantles. The soil is much
deeper than in the ectropics, and contains plenty of duricrusts and hardpans. Inselbergs of
variable size are common in the tropics, standing out on extensive erosion surfaces, which
are weathered at the same time as they form. Mountain zones are characterized by deep
incisions. This author assumes that morphodynamics in tropical lowlands are completely
different, where landforms are controlled by the generation of etch surfaces.
Tricart and Cailleux (1965) differentiated an intertropical zone with warm temperatures
and constant fluvial flows. Depending the rainfall distribution, its annual amount, and
the vegetation cover density, they divide this zone into a savanna domain, with less
precipitation and less dense vegetation cover. Sheet wash is considerable and chemical
weathering is strong and discontinuous in time. The rainforest domain is characterized by
dense vegetation cover and higher precipitation. In this domain chemical and biochemical
processes reach a maximum intensity.
3. Weathering in the humid tropics
Weathering is the response of materials that were initially in equilibrium inside the
lithosphere to conditions at or near their contact with the atmosphere, hydrosphere and,