Environmental Engineering Reference
In-Depth Information
weathering caused by the higher rates of chemical processes at warmer temperatures
where moisture is
Figure 28.3
Climatic data for Manaus,
Brazil.
available; the wetter an area is, the deeper the weathering should be. The processes
operating on Earth's surface have been identified in Chapters 10-15. In the tropics the
intensity of these processes will be different from that in other environments, though it is
difficult to prove that these generate landforms or landscapes specifically tropical.
Many parts of the tropics are based on relatively stable continental plates. Such areas
have not been exposed to glaciation and therefore weathered material has been able to
accumulate rather than being dispersed, as happened in many parts of the temperate and
subpolar lands. In extreme cases the weathering horizon may be as deep as 30 m in the
more humid tropics, though its depth decreases where moisture is less available and
normal depths are about 3 m. How long such weathering horizons have been developing
is difficult to decipher but many are believed to be long established. In the more
tectonically active areas, such as Indonesia, volcanism can bury existing weathering
horizons. The new lava is then rapidly weathered in turn. Erosion of the weathered
material may take place through fluvial processes or by mass movement. Where slopes
are steep, this may be significant, but on gentler slopes the density of biomass produces a
protective zone on which slope movement is slow. One of the most important processes
of the tropics is run-off. Most of the material eroded from the humid forest is carried by
run-off in the form of solution. Little coarse sediment moves as bed load, but the density
of vegetation prevents all except the finest particles being carried by overland flow. Only
bank erosion will produce an sudden increase in the amount of sediment transport.
In steeper areas associated with the recent volcanic chains, such as eastern Asia,
Indonesia, New Guinea and the Andes, mass movement may become more significant.
Slides and flows can strip away vegetation and weathered material to expose regolith and
even bedrock. The effects of these processes can be seen from satellite images of the
