Chapter 21
Tropical landforms
1. Introduction
Tropical zones affected by the Alpine Orogeny are constituted of high relief with great
volcanic cones like Nevado del Ruiz in Colombia, Cotopaxi in Ecuador, La Soufri~re
in Guadalupe (Figure 21.1), Pinatubo in the Philippines, Merapi in Java, and many others.
These tropical regions correspond to the mountains of Central America, the Antilles,
southeast Asia and Indonesia. In the three last areas dissolution processes affect great
extensions of carbonate rocks, giving rise to characteristic tropical karst landforms with
towers and conical hills. These tropical zones present a remarkable tectonic control
because they still undergo Quaternary crustal instability. Weathering products generated
in these areas are easily exported by fluvial action. Hence, these regions are characterized
by undergoing vertical tectonic movements that generate relief, important volcanic
activity of the explosive type, and an important erosive fluvial activity (Wirthmann, 2000).
Instead, regions on shields or cratons of Africa, Brazil-Guyana, India and Australia
are characterized by tectonic stability, at least during recent geologic times. These
circumstances have favoured the development of extensive planation surfaces mainly
upon Precambrian rocks, typical of these tropical shield regions. As a consequence, relief
is commonly smooth and weathering products undergo weak water erosion. The result
of these conditions is the development of thick weathering profiles that sometimes can
exceed 100 m. Base-level lowering produces deep incisions in the regolith, originating a
set of landforms related to differential weathering and erosion processes (Faniran and
Jeje, 1983). Finally, the rift valley system is located in eastern Africa and is associated with
important volcanism. The most outstanding examples of this igneous activity are Mount
Kenya (5194 m), and Kilimanjaro (5895 m) (Figure 21.2) in Tanzania.
2. Erosion and sedimentation in fluvial systems
Water courses in the humid tropics transport little coarse bed load. This is due to intense
alteration and that the regions were not glaciated in the recent past as were so many
of the temperate areas of the Northern Hemisphere. The regolith is constituted of sand,
silt, and clay particles and, therefore, rivers mainly evacuate materials of these sizes
and dissolved products. These sediments have little abrasive power and small erosive
capability. Little research exists on the bed load of tropical rivers and hence very few
data are available about this topic. More research work was produced for mountain
zones, where sediments are rapidly removed by major flows which are able to transport
all the solid discharge to lower parts of the basin by incising into the existing rocks.
Tropical landforms
1. Introduction
Tropical zones affected by the Alpine Orogeny are constituted of high relief with great
volcanic cones like Nevado del Ruiz in Colombia, Cotopaxi in Ecuador, La Soufri~re
in Guadalupe (Figure 21.1), Pinatubo in the Philippines, Merapi in Java, and many others.
These tropical regions correspond to the mountains of Central America, the Antilles,
southeast Asia and Indonesia. In the three last areas dissolution processes affect great
extensions of carbonate rocks, giving rise to characteristic tropical karst landforms with
towers and conical hills. These tropical zones present a remarkable tectonic control
because they still undergo Quaternary crustal instability. Weathering products generated
in these areas are easily exported by fluvial action. Hence, these regions are characterized
by undergoing vertical tectonic movements that generate relief, important volcanic
activity of the explosive type, and an important erosive fluvial activity (Wirthmann, 2000).
Instead, regions on shields or cratons of Africa, Brazil-Guyana, India and Australia
are characterized by tectonic stability, at least during recent geologic times. These
circumstances have favoured the development of extensive planation surfaces mainly
upon Precambrian rocks, typical of these tropical shield regions. As a consequence, relief
is commonly smooth and weathering products undergo weak water erosion. The result
of these conditions is the development of thick weathering profiles that sometimes can
exceed 100 m. Base-level lowering produces deep incisions in the regolith, originating a
set of landforms related to differential weathering and erosion processes (Faniran and
Jeje, 1983). Finally, the rift valley system is located in eastern Africa and is associated with
important volcanism. The most outstanding examples of this igneous activity are Mount
Kenya (5194 m), and Kilimanjaro (5895 m) (Figure 21.2) in Tanzania.
2. Erosion and sedimentation in fluvial systems
Water courses in the humid tropics transport little coarse bed load. This is due to intense
alteration and that the regions were not glaciated in the recent past as were so many
of the temperate areas of the Northern Hemisphere. The regolith is constituted of sand,
silt, and clay particles and, therefore, rivers mainly evacuate materials of these sizes
and dissolved products. These sediments have little abrasive power and small erosive
capability. Little research exists on the bed load of tropical rivers and hence very few
data are available about this topic. More research work was produced for mountain
zones, where sediments are rapidly removed by major flows which are able to transport
all the solid discharge to lower parts of the basin by incising into the existing rocks.