Chapter 20
The humid tropics: weathering and laterites
1. Introduction
The tropics are the regions located between the Tropic of Cancer (23.5~ and the Tropic
of Capricorn (23.5~ These limits involve areas where the sun can reach the zenith. As a
result, these zones of low latitudes receive a great amount of solar radiation and continuous
heat, with important biological consequences such as the development of a dense and
widespread vegetal cover, which has a strong influence upon the morphogenesis of these
regions. In the intertropical zones there exist both large areas warm deserts and important
zones with high rainfall. A separation between different environments is needed, which
can be based upon several criteria such as climate, biology or geomorphology, among
others. The subdivision is always difficult, especially in continental areas (Reading
et al., 1995).
Some authors discriminate the humid tropics by using vegetation criteria, concluding
that these regions are characterized by the scarcity or absence of xerophytes and the
development of rainforests, deciduous forests and savannas (Fosberg et al., 1961). Other
authors use climatic criteria and establish a limit on an average temperature of 18~ for
the coldest month (Krppen, 1936). The mean annual precipitation has also been proposed,
but great discrepancies exist among researchers. Krppen (1936) used the range of
450-600 mm and Tricart (1974b) between 750 and 800 mm.
About 2400 million inhabitants live in the tropical zones, which means nearly 45 per
cent of the world' s population. Nearly all these people live in the humid tropics, and about
60% in western and southern Asia. The most important economic sector is agriculture,
which obviously depends a great deal on the climatic fluctuations and also on catastrophic
events (McGregor and Nieuwolt, 1998).
Most parts of the surface occupied by the non-arid tropics are located on stable cratonic
zones, which are basically constituted by Precambrian plutonic and metamorphic rocks,
mainly in central Africa and the Brazilian shield. These materials underwent long-lasting
denudational processes, which favoured the development of extensive planation surfaces.
Due to the low topographic gradient, the erosion velocity is slow and weathering
progresses with no difficulty. For this reason, landform evolution over long periods of time
can be studied, under conditions of crustal stability (Thomas, 1974; Wirthmann, 2000).
In the opposite situation, regions affected by alpine tectonics and with high relief,
mainly in Indonesia, SE Asia, Central America and the Antilles, suffer high denudation
rates and weathering cannot deepen. In addition, prevailing rocks are different from the
stable regions, like the great calcareous outcrops where singular karst landforms develop.
As previously indicated in preceding chapters, in the arid regions, and especially in
the semi-arid zones, typical humid tropical weathering profiles are quite common.
The humid tropics: weathering and laterites
1. Introduction
The tropics are the regions located between the Tropic of Cancer (23.5~ and the Tropic
of Capricorn (23.5~ These limits involve areas where the sun can reach the zenith. As a
result, these zones of low latitudes receive a great amount of solar radiation and continuous
heat, with important biological consequences such as the development of a dense and
widespread vegetal cover, which has a strong influence upon the morphogenesis of these
regions. In the intertropical zones there exist both large areas warm deserts and important
zones with high rainfall. A separation between different environments is needed, which
can be based upon several criteria such as climate, biology or geomorphology, among
others. The subdivision is always difficult, especially in continental areas (Reading
et al., 1995).
Some authors discriminate the humid tropics by using vegetation criteria, concluding
that these regions are characterized by the scarcity or absence of xerophytes and the
development of rainforests, deciduous forests and savannas (Fosberg et al., 1961). Other
authors use climatic criteria and establish a limit on an average temperature of 18~ for
the coldest month (Krppen, 1936). The mean annual precipitation has also been proposed,
but great discrepancies exist among researchers. Krppen (1936) used the range of
450-600 mm and Tricart (1974b) between 750 and 800 mm.
About 2400 million inhabitants live in the tropical zones, which means nearly 45 per
cent of the world' s population. Nearly all these people live in the humid tropics, and about
60% in western and southern Asia. The most important economic sector is agriculture,
which obviously depends a great deal on the climatic fluctuations and also on catastrophic
events (McGregor and Nieuwolt, 1998).
Most parts of the surface occupied by the non-arid tropics are located on stable cratonic
zones, which are basically constituted by Precambrian plutonic and metamorphic rocks,
mainly in central Africa and the Brazilian shield. These materials underwent long-lasting
denudational processes, which favoured the development of extensive planation surfaces.
Due to the low topographic gradient, the erosion velocity is slow and weathering
progresses with no difficulty. For this reason, landform evolution over long periods of time
can be studied, under conditions of crustal stability (Thomas, 1974; Wirthmann, 2000).
In the opposite situation, regions affected by alpine tectonics and with high relief,
mainly in Indonesia, SE Asia, Central America and the Antilles, suffer high denudation
rates and weathering cannot deepen. In addition, prevailing rocks are different from the
stable regions, like the great calcareous outcrops where singular karst landforms develop.
As previously indicated in preceding chapters, in the arid regions, and especially in
the semi-arid zones, typical humid tropical weathering profiles are quite common.