7.3 Tropical landforms

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).

Water table Throughflow

Black-brown clay Black massive peaty clay Black massive blocky clay Brown-black peaty clay Calcareous clay; various colours Vertisol (tropical black soil) Groundwater laterite Red-brown loamy sand Red-brown gravelly loam with ironstone concretions Mottled red-grey clay; massive structure Shale Plinthite Duricrust Freely draining Ferruginous soils Groundwater laterites Tropical black soils (groundwater gleys) River Peats Thin immature soils Ah Bw B/C 120 80 60 100 40 20 0 cm

Ahg Bg3 Bg2 Bg1 C

Swamp 90 120 60 30 0

0 20 40 60 80 100 120 Ah Bg BL C

Figure 7.4 Soil catena in the savanna

Dark red-brown sandy clay loam with some quartz gravel; weak crumbling structure, pH 5.6 Red clay loam; massive and compact; pH 5.9; merging to loose friable rotting bedrock 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

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.

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 30–60m, but because of variations in jointing density and rock composition, the depth varies widely over short distances.

Revised

14 Describe the main types of weathering that occur in tropical environments.

Tested

Most tors are found in strongly jointed rock. Tors vary in height from 20m to 35m and have core stones of up to 8m diameter. They are formed by chemical weathering of the rock along joints and bedding planes beneath the surface. If the joints are widely spaced the core stones are large whereas if the joints are close together the amount of weathering increases and the corestones are much smaller. Good examples of tors are found on the Jos Plateau of Nigeria and in the Matopas region and around Harare in Zimbabwe.

Inselbergs are best developed on volcanic materials, especially granite and gneiss, with widely spaced joints and a high potassium content. These residual hills are the result of stripping weathered regolith from a differentially weathered surface.

The major debate is whether deep weathering is needed for hill formation. The two-stage model requires the development of a mass of weathered material beneath the ground and its subsequent removal. Alternatively, weathering and erosion could occur simultaneously. The diversity of residual hills suggests that both mechanisms operate simultaneously.

Monolithic domed inselbergs called bornhardts are characteristic landforms of granite plateaus of the African savanna, but can also be found in tropical humid regions. They are characterised by steep slopes and a convex upper slope. Bornhardts are eventually broken down into residual hills called castle kopjes. Bornhardts occur in igneous and metamorphic rocks. Granite, an igneous rock, develops joints, up to 35 m below the surface, during the process of pressure release. Vertical jointing in granite is responsible for the formation of castle kopjes.

The two main theories for the formation of bornhardts include: l the stripping or exhumation theory – increased removal of regolith occurs so that unweathered rocks beneath the surface are revealed l parallel retreat, which states that the retreat of valley sides occurs until only remnant inselbergs are left

Classic examples of bornhardts include Mt Hora, in the Mzimba District of Malawi, and Mt Abuja in northern Nigeria.

Pediplains are low-angled plains (pediments) separated by rocky hills known as kopjes. They are formed as a result of the parallel retreat of slopes (Figure 7.5). Pediplanation begins with tectonic uplift, resulting in accelerating river erosion forming knick points, falls, rapids and gorges along river valleys. When base level is reached, lateral erosion begins to occur.

Tors are ridges or piles of spheroidically weathered boulders that have their bases in the bedrock and are surrounded by weathered debris. Inselbergs are isolated residual hills that stand prominently over a level surface.

Many features, such as tors and inselbergs, can be formed in different ways. Make sure you can give alternative explanations for their formation.

scarps retreating

Figure 7.5 Pediplanation formed by parallel retreat

inselberg pediplain

An etchplain is an area of stripped and exposed unweathered bedrock (Figure 7.6). Etchplains occurs in ancient shield areas and are associated with deep weathering. Periods when weathering was more efficient resulted in the accumulation of great depths of weathered material. In contrast, there were periods when erosion was more rapid than weathering, leading to erosion of the weathered material, and exposure of the unweathered rock.

a The regolith surface is being lowered at a faster rate than the subterranean surface.

Smooth savanna plain Shifting courses of shallow rivers Marginal inselberg

Upper subaerial surface being lowered by ‘wash’ Lower subterranean surface being lowered by chemical weathering Deep, intensely weathered regolith

b The irregular subterranean surface is being exposed as inselbergs.

Savanna plain studded with inselbergs Marginal inselberg

Figure 7.6 Etchplain formation

There are two major landform features associated with tropical karst (Figure 7.7): l Polygonal or cockpit karst is a landscape pitted with smooth-sided, soilcovered depressions and conical hills. l Tower karst is a landscape characterised by upstanding rounded blocks set in a region of low relief.

Revised

Some students consider that the types of limestone features found in temperate areas will be found in tropical areas – tropical limestone scenery is dominated by cockpit and tower karst.

The surface is broken up by many small solution holes but the overall surface remains generally level.

Cockpit karst is usually a hilly area in which many deep solution holes have developed to give it an ’eggbox’ appearance. Cockpit

Water table

The widening and deepening of the cockpits has destroyed much of the limestone above the water table. Only a few limestone towers remain, sticking up from a flat plain of sediments that have filled in the cockpits at a level just above the water table. Eventually the towers will be entirely eroded, and disappear. Water table Towers

Sediments

Figure 7.7 Cockpit karst and tower karst

Polygonal or cockpit karst is characterised by groups of hills, fairly uniform in height. These can be up to 160m high in Jamaica, with a base of up to 300m. They develop mainly as a result of solution. Polygonal karst tends to occur in areas that have been subjected to:

l high rates of tectonic uplift l intense vertical erosion by rivers

The spacing of the hills may be related to the original stream network. Concentrated solution along preferred routes, such as wider joints, leads to accelerated weathering of certain sections of the limestone, especially during times of high flow. Water will continue to weather the limestone as far down as the water table. This creates closed depressions and dolines. Once the water table is reached, water will flow laterally rather than vertically, developing a flat plain. By contrast, tower karst is much more variable in size than the conical hills of cockpit karst, with towers ranging from just a few metres to over 150m in height in Sarawak. Other areas of tower karst include southern China, Malaysia, Indonesia and the Caribbean. The towers are characterised by steep sides, with cliffs and overhangs, and with caves and solution notches at their base. The steepest towers are found on massive, gently tilted limestone.

Where the water table is close to the surface, rivers will be able to maintain their flow over limestone, erode the surface and leave residual blocks set in a river plain. Other important processes include:

l differential solution along lines of weakness l the retreat of cockpit karst slopes to produce isolated tower karst l lateral erosion

15 Explain why tors can be described as ‘joint-controlled’. 16 Outline the two theories for the formation of bornhardts. 17 What is the difference between cockpit karst and tower karst?

Tested