Water table
Peats
cm 0 20 40 60 80 100 120
Groundwater laterites River
Swamp Tropical black soils (groundwater gleys) cm 0 Ahg Black-brown clay Ah Bg1 Black massive peaty clay 30 BL Bg2 Black massive blocky clay 60 Bg3 C
Brown-black peaty clay Calcareous clay; various colours
90 120
Vertisol (tropical black soil)
Throughflow Duricrust
Freely draining Ferruginous soils
Thin immature soils
Red-brown loamy sand Red-brown gravelly loam with ironstone concretions
Bg
Mottled red-grey clay; massive structure
C
Shale
Plinthite
7 Tropical environments
A typical soil catena for these areas can be identified, with ferruginous soils on the upper slopes, vertisols (tropical black clays) on the lower slopes, gleyed soils where drainage is impeded and alluvial soils close to rivers (Figure 7.4).
cm 0 20 40 60 80 100 120
Ah
Bw
B/C
Groundwater laterite
Dark red-brown sandy clay loam with some quartz gravel; weak crumbling structure, pH 5.6 Red sandy clay loam with hard ferruginous soil with some gravel and quartz gravel; weak sub-angular blocky structure at top; massive and compact at base; pH 5.4 Red clay loam; massive and compact; pH 5.9; merging to loose friable rotting bedrock Ferruginous soil
Figure 7.4 Soil catena in the savanna
7.3 Tropical landforms Tropical landforms are diverse and complex. They are the result of many interrelated factors including climate, rock type, tectonics, time, vegetation, drainage, topography and, increasingly, human impact.
Weathering Mechanical and chemical weathering occur widely in the tropics. l In the humid tropics, the availability of water and the consistently high temperatures maximise the efficiency of chemical reactions, and in the oldest part of the tropics these have been operating for a very long period. In contrast, in many savanna areas, where there is less moisture, exfoliation or disintegration occurs. l In many regions, weathering is complete and the weathering profile is very deep. As the depth of the weathered profile increases, slopes can become less stable. Rapid mass movements are likely to take place in a cyclical pattern, once a certain amount of weathering has occurred. l Weathering profiles vary widely. The idealised weathering profile has three zones – residual soil, weathered rock and relatively unweathered bedrock. Weathered rock is also known as saprolite. l In the weathered zone, at least 10% of the rock is unweathered corestones. This zone is typically highly permeable, especially in the upper sections, and contains minerals in a wide range of weathering stages. l The ‘weathering front’ or ‘basal surface of weathering’ between solid rock and saprolite (weathered rock) can be very irregular. Typically, deep weathering occurs to depths of 30–60 m, but because of variations in jointing density and rock composition, the depth varies widely over short distances.
110
Revised
Now test yourself 14 Describe the main types of weathering that occur in tropical environments.
Answer on p.217
Cambridge International AS and A Level Geography Revision Guide
Tested
