The humid tropics: weathering and laterites
505
perhaps more important, the biosphere (Reiche, 1950). The importance of the biosphere is
perhaps overemphasized. A great part of the rocks have been submitted to high pressures
and temperatures, in the absence of air and water. After outcropping from the surface, they
change towards an equilibrium with pressures of about 1 atm, much lower temperatures
and the presence of air and water, which are the conditions existing in the contact
lithosphere-atmosphere (Carroll, 1970).
Climate constitutes one of the most significant factors in weathering development.
Water from precipitation is the most important reactive agent in the weathering process
and its quantity is a prime factor in the intensity and type of alteration. In humid tropical
regions water attacks crystalline structures and evacuates bases (Na +, K +, Ca 2+, and
Mg 2+) as well as silica to some extent. It also affects the velocity of biochemical reactions
in plants. Temperature has direct and indirect effects on weathering processes. High
temperatures accelerate chemical reactions. Following the Van't Hoff Law, every 10~
increase in temperature duplicates or triplicates the rates of chemical reactions (Tricart
and Cailleux, 1965). Moreover, heat increases biological activity. As a result of these
temperatures, bacteria consume great quantities of organic matter in decomposition, which
brings about the scarcity of humus in these humid tropical environments.
The result of the actuation of weathering processes, especially those related to
chemical alteration, gives rise to an intensively weathered rock (Taylor and Eggleton,
2001). This rock decomposition is a consequence of intense precipitation and temperature
and also of the vegetation response to these conditions. In these areas natural waters
reach pH values ranging between 4 and 6, and very possibly these acid conditions may be
due to silicate hydrolysis (Curtis, 1976). The important release of biogenic carbon dioxide
supplies great quantities of bicarbonates to the waters. Humification processes produce
humic and fulvic acids, which export cations as organometallic compounds (Thomas,
1994). The activity of all these processes produces thick alteration profiles that will be
analysed later in this chapter.
3.1. Physical and biological weathering
Weathering processes can be divided in three types: physical or mechanical, biological,
and chemical, and all of them can act simultaneously. Physical weathering breaks the rock
by different mechanical mechanisms. Most of the forces applied to the geological
materials is external, although they can also accumulate inside the rock.
Many rocks were originated or remained for a long time at considerable depths in
the lithosphere and hence, were submitted to high confining pressures. When uplifted to
the surface, these rocks suffered a pressure-release by unloading that generates joint-like
fractures parallel to the topography; a process known as sheeting (Figure 20.3) (Gilbert,
1904). Joint spacing increases with depth and is better recognizable in massive rocks such
as granites, conglomerates, sandstones, and so forth. The difficulty of recognizing this type
ofjointing in non-massive rocks is probably due to the existence of old bedding planes that
would distribute expansion stress (Ollier, 1965). Other authors suggest that sheeting is
a consequence of lateral compression (Twidale, 1973, 1982a; Vidal Romanf et al., 1995;
Vidal Romanf and Twidale, 1998). As will be discussed later, this jointing has great
importance in the landform development of tropical cratonic areas (Birot, 1968a,b).
505
perhaps more important, the biosphere (Reiche, 1950). The importance of the biosphere is
perhaps overemphasized. A great part of the rocks have been submitted to high pressures
and temperatures, in the absence of air and water. After outcropping from the surface, they
change towards an equilibrium with pressures of about 1 atm, much lower temperatures
and the presence of air and water, which are the conditions existing in the contact
lithosphere-atmosphere (Carroll, 1970).
Climate constitutes one of the most significant factors in weathering development.
Water from precipitation is the most important reactive agent in the weathering process
and its quantity is a prime factor in the intensity and type of alteration. In humid tropical
regions water attacks crystalline structures and evacuates bases (Na +, K +, Ca 2+, and
Mg 2+) as well as silica to some extent. It also affects the velocity of biochemical reactions
in plants. Temperature has direct and indirect effects on weathering processes. High
temperatures accelerate chemical reactions. Following the Van't Hoff Law, every 10~
increase in temperature duplicates or triplicates the rates of chemical reactions (Tricart
and Cailleux, 1965). Moreover, heat increases biological activity. As a result of these
temperatures, bacteria consume great quantities of organic matter in decomposition, which
brings about the scarcity of humus in these humid tropical environments.
The result of the actuation of weathering processes, especially those related to
chemical alteration, gives rise to an intensively weathered rock (Taylor and Eggleton,
2001). This rock decomposition is a consequence of intense precipitation and temperature
and also of the vegetation response to these conditions. In these areas natural waters
reach pH values ranging between 4 and 6, and very possibly these acid conditions may be
due to silicate hydrolysis (Curtis, 1976). The important release of biogenic carbon dioxide
supplies great quantities of bicarbonates to the waters. Humification processes produce
humic and fulvic acids, which export cations as organometallic compounds (Thomas,
1994). The activity of all these processes produces thick alteration profiles that will be
analysed later in this chapter.
3.1. Physical and biological weathering
Weathering processes can be divided in three types: physical or mechanical, biological,
and chemical, and all of them can act simultaneously. Physical weathering breaks the rock
by different mechanical mechanisms. Most of the forces applied to the geological
materials is external, although they can also accumulate inside the rock.
Many rocks were originated or remained for a long time at considerable depths in
the lithosphere and hence, were submitted to high confining pressures. When uplifted to
the surface, these rocks suffered a pressure-release by unloading that generates joint-like
fractures parallel to the topography; a process known as sheeting (Figure 20.3) (Gilbert,
1904). Joint spacing increases with depth and is better recognizable in massive rocks such
as granites, conglomerates, sandstones, and so forth. The difficulty of recognizing this type
ofjointing in non-massive rocks is probably due to the existence of old bedding planes that
would distribute expansion stress (Ollier, 1965). Other authors suggest that sheeting is
a consequence of lateral compression (Twidale, 1973, 1982a; Vidal Romanf et al., 1995;
Vidal Romanf and Twidale, 1998). As will be discussed later, this jointing has great
importance in the landform development of tropical cratonic areas (Birot, 1968a,b).