MAJOR LANDFORMS IN THE (SUB-)HUMID TROPICS Large parts of the humid and sub-humid tropics belong to one of three morphostructural units: 1 Precambrian shields constitute major parts of eastern South America, equatorial Africa, and central and southern India; 2 Young alpine fold belts e.g. the equatorial Andes and Central America, and greater parts of Southeast Asia; 3 Tropical alluvial plains comprising fluvial sedimentary basins such as the Amazon basin, the Congo basin and the Indus-Ganges basin, and coastal plains, e.g. the coastal plains of the Guyana s, the Niger delta and the Mekong delta. Landforms in high mountain areas were discussed in an earlier chapter as were the alluvial lowlands. The present chapter discusses common landforms on Precambrian shields and in the lower ranges of Alpine fold belts (below 3000 meters) in the humid and seasonally dry tropics. Landforms on Precambrian shields Precambrian shields, or cratons, constitute the oldest cores of continents; they are remnants of mountains that formed more than 600 million years ago and that have since eroded to undulating plains that rise up to only a few hundred meters above the present sea level. The lithospheric plates on which the shields rest move over the Earth's surface at a rate of several centimetres per year. In places, this movement produces weak stretches that become subject to rifting and subsidence. Such locations are preferential sites for formation of sedimentary river basins (e.g. the Amazon basin) or for deposition of rocks (e.g. during the Mesozoic in South Sweden). The Precambrian era spans 80 percent of the geological history of the Earth and includes many periods of mountain building, erosion and sedimentation. Igneous, sedimentary and metamorphic rocks of Precambrian age exist in great variety but crystalline (plutonic and metamorphic) rocks predominate.
Precambrian formations consist of one or more of the following: High-grade metamorphic belts. These are normally narrow belts (only tens of kilometres across) that consist for the greater part of strongly metamorphosed rocks, which originate from sedimentary rocks. The lithology of these belts is diverse with metamorphosed limestone (marbles) and/or metamorphosed sandstone (quartzites) alongside rocks that are not of sedimentary origin such as metamorphosed basalt flows or dykes (amphibolites) and strongly metamorphosed rocks, e.g. gneiss, granulites and granitoid gneiss. The considerable variation in mineralogical and chemical/ physical properties of these rocks explains the wide variety of landforms and soils. Greenstone belts. These are narrow belts (a few tens or hundreds of kilometres across) that can stretch over thousands of kilometres. Greenstone belts consist mainly of metamorphosed volcanic rocks, notably basalt and andesite, with varying proportions of intercalated sedimentary rocks that have normally been converted to schist and phyllites by low-grade metamorphism. A characteristic feature of greenstone belts is the occurrence of tonalite intrusions, normally with oval outlines on the geological map. (Tonalite is a granite-like rock of plutonic origin and usually has plagioclase as the sole feldspar. As plagioclase weathers easily, tonalite areas are more deeply weathered than nearby granite areas). Examples are the Koidu Basin in Sierra Leone and the Brokopondo lake area in Surinam. Granite areas, often associated with either migmatites (i.e. banded rocks formed through partial melting of sediments deep in the crust), or granitoid gneiss. Platform areas with horizontal sedimentary rocks, commonly sandstones, on top of the Precambrian shield; adjacent uncovered shield areas are referred to as 'basement areas. Tropical shield areas were mainly modified by chemical weathering and by fluvial and marine processes (glacial, periglacial and aeolian processes were insignificant in the recent past). How water could shape the surface in tropical shield areas is largely explained by the amount and intensity of precipitation, and the presence or absence of a protective vegetation cover.
In areas under rain forest, most precipitation is intercepted by the canopy from where it trickles down to the forest floor and infiltrates into the soil. There, it promotes rapid chemical weathering of rocks because its low ionic strength and comparatively high temperature promote hydrolysation processes. The 'saprolite' (i.e. 'rotten rock') under a rain forest may extend down to a depth of tens or even hundreds of metres. The saprolite is usually less thick on granite (say, 10-20 metres) than on metamorphic rock (40-70 metres; data from Surinam). Long periods of strong chemical weathering and little, if any, surface runoff have gradually deepened the weathering front, a process known as 'etching'. The saprolite is normally clayey because feldspars and ferromagnesian minerals have weathered to clay minerals and (sesqui-)oxides. The sand content of the saprolite reflects the content of coarse quartz in the original parent rock. Thoroughly weathered saprolites are chemically very poor, despite their lush (rain forest) vegetation cover. Note that the vegetation is less densely spaced in arid areas because each individual plant needs a larger volume of soil for its water supply. A single downpour on such open land can cause torrential 'sheet floods'. In the intermediate situation, i.e. in semi-arid savannah and prairie areas, surface runoff and denudation are particularly severe. On sites with sparse vegetation and distinct surface relief, the rate of topsoil erosion may well exceed the rate of weathering. This results in 'stripping' of the land; etching is more common under protective vegetation types, e.g. under rain forest. Figure 1 explains how summit levels were formed after prolonged, differential etching and stripping. Note that the balance between etching and stripping was almost certainly different from the present situation during long periods in the past.
1 Figure 1. Development of summit levels by differential etching and stripping (Kroonenberg and Melitz, 1983). The photo at the right shows an inselberg in southern Africa. 2 3 4 Many shield areas in tropical regions include vast, dissected etch-plains with solitary elevated remnants that are either bare, dome-shaped granite hills ('inselbergs') such as the sugar loaf of Rio de Janeiro, or heaps of huge granite boulders known as 'tors'. The etch-plains consist of deep, flat or undulating, residual weathering crust dissected by a network of V-shaped valleys that are only a few metres deep. Where the natural drainage pattern is widely spaced, remnants of the original flat surface may still be in place but only rounded, convex hills remain in areas with more densely spaced gullies. Note that natural drainage patterns are normally conditioned by underlying bedrock: low ridges and depressions form upon differential etching and stripping of weathering-resistant rocks.
The occurrence of isolated inselbergs and tors amidst vast expanses of undulating lowland is more common in savannah regions than in areas with rain forest. Valleys in savannah regions (called 'dambos' or 'vleis' in large parts of Africa) tend to be broad and shallow as a result of colluviation and slope wash. The widespread occurrence of laterite plateaus is an indication of climate fluctuations in the past. Strongly weathered saprolite with quartz-rich clays ('plinthite') formed during humid eras. In the (now) semi-arid tropics, much plinthite has subsequently hardened to 'ironstone'. Many plateaux are weathering residues protected by an ironstone cap. In places they formed through relief inversion of iron-cemented valley fills. Landforms in alpine fold belts (lower than 3000 metres) High mountain areas in tropical regions became glaciated in the Pleistocene but tracts lower than 3,000 meters above the present mean sea level were never reached by descending valley glaciers. In lower mountain areas, the relation between rainfall and land surface transformation is similar to that in shield areas. Rain forest is the preponderant vegetation type and infiltration water reaches great depths. Weathering is rapid and fresh rock is difficult to find, even in deeply dissected terrain. The lower foot slopes of the Andes and Himalayas and uplands in Africa present numerous examples. The dominant geomorphic controls in humid tropical mountain areas are: strong tectonic uplift; rapid incision of rivers, and undercutting of slopes and subsequent mass movement. Landslides and mudflows have shaped many slope sites in the humid and seasonally dry tropics. These phenomena were triggered by torrential rainfall that saturated the weathering crust with water. Often, seismic events such as earthquakes gave the final stimulus for sliding. Shallow landslides are common in forested mountain areas, e.g. in New Guinea, Sulawesi, Hawaii or the Andes; a provisional chronology can often be established by simply considering degrees of forest regeneration.
Note that, contrary to common belief, forest vegetation cannot prevent landslides from happening because the sliding landmass detaches itself at the 'weathering front', i.e. the contact plane between saprolite and fresh rock that is beyond the reach of tree roots. It is generally true that regions with crystalline rocks have symmetrical hills with sharp crests and rectilinear slopes, separated by steep V-shaped valleys. Joints and faults in the underlying rocks determine the drainage pattern. Weathering mantles tend to be less deep over siliceous sedimentary rocks than over crystalline rocks. The alternation of resistant and less resistant strata is the main controlling factor in folded sedimentary rocks. Nice examples can be seen in areas with alternating limestone and sandstone ridges as extend from India through Burma, Thailand and Laos all the way to Vietnam. Humid tropical areas with calcareous rocks may show abundant 'karst' phenomena such as sink holes and caves formed upon dissolution of limestone. 'Tower karst' with residual limestone rocks standing in the landscape as towers (e.g. in Guilin, China) formed upon advanced dissolution of limestone. Similarly convincing are the 'cockpit' or 'mogote' hills at Bohol (Philippines) and the razor-sharp limestone ridges of the 'brokenbottle country' of New Guinea. Note that such extreme karstic landscapes can only develop in uplifting areas. The advanced weathering of rocks in the (sub-) humid tropics produced typical tropical soils: red or yellow in colour and strongly leached. Additional features: they are deep, finely textured, contain no more than traces of weatherable minerals, have low-activity clays, less than 5 percent recognisable rock structure and gradual soil boundaries. Differences between soils in the (sub-)humid tropics can often be attributed to differences in lithology and/or (past) moisture regime. Figure 2. Towerkarst in China