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7.2 - Landforms of tropical environments - content
7.2 - Landforms of tropical environments - content
The formation of characteristic landforms:
Granite: (deep weathering profiles) tors, inselbergs, and bornhardts
Limestone: tropical karst (cone karst, tower karst, and cockpit karst).
The Tropical Rainforest made up 14% of the Earth's surface, now there are only about 6% covering the land. This area contains mountains, valleys, flood plains, streams, rivers, and wetlands.
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.
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.
Limestone in tropical environments
Limestone in tropical environments
Karst (limestone) landscapes are scattered with sink holes and bumpy hills. Karst landscapes are shaped by the dissolving action of water. These landforms are created by the action of carbonated water derived from rain and soil. Precipitation collects CO2 when it falls through the atmosphere and from the soil. This mild carbonic acid dissolves the bed rock over long periods of time. Openings in the bedrock allow water to pass through the developing drainage system, forming and transforming the landscape.
The landforms result mostly from chemical weathering, in particular carbonation and solution. These landforms can occur in a variety of climates, but in accordance with Van't Hoff's law and the Peltier Model they are particularly present in Tropical Environments.
Karst Landforms
Karst Landforms
Karst landforms are created by water sinking and circulating underground, and the resulting chemical erosion of bedrock. For this reason, the development of karst landforms is limited to areas where comparatively soluble rocks — principally limestone — exist. Approximately 8 per cent of the earth's land surface is karst terrain. Karst landforms occur at both the surface and subsurface.
Inselberg or Bornhardt?
Inselberg or Bornhardt?
While bornhardt was originally used to sometimes denote a type of inselberg, the term bornhardt is used in modern literature to refer to domed hills and mountains regardless of isolation in the landscape; thus, not all bornhardts are inselbergs and not all inselbergs are bornhardts. For example Sugar Loaf Mountain (Rio De Janiero, Brazil) is a non-inselberg bornhardt.
Granite in tropical environments
Granite in tropical environments
Granite is one of the most common constituents of the earth's crust. On the continents it forms extensive outcrops, covering large areas of shield-lands. One of the most characteristic granite landforms is the tor. Tors are isolated masses of rock consisting of either a single or of numerous joint blocks displaying varying degrees of angularity or roundness.
In Tropical Environments granite landforms are largey shaped by hydrolysis, which operates along joints in the granite. This process is accelerated as a result of the hot wet climate and decaying vegetation. The processes involved typically lead to deep regolith and undulating Basal Surface Weathering This ‘weathering front’, between solid rock and saprolite, 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.
Inselberg: ‘Inselberg’ is a German term, which literally means an ‘island hill’. It is an isolated hill that stands above an extensive plain of erosion. They are steep-sided and dome-shaped. Inselbergs arise from rocks that erode at a slower rate than that of the surrounding rocks. Volcanic processes are responsible for the rising of the resistant rock above the surrounding area. The resistant rock is able to resist erosion due to its tight joints. Once formed, inselbergs appear steep-sided.
Bornhardt: 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 . Bornhardts occur in igneous and metamorphic rocks.
Granite, an igneous rock, develops off-loading joints, up to 35 m below the surface, during the process of pressure release. Vertical joints (contractual joints) in granite are responsible for the formation of tors (also known as castle kopjes). The two main theories for the formation of bornhardts include:
• the stripping or exhumation theory – increased removal of regolith occurs so that unweathered rocks beneath the surface are revealed
• 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.
Tor: A blocky hill rising abruptly from its surroundings. It is likely to have an irregular summit, 20-35m in height and undergoes block disintegration. Most tors are found in strongly jointed rock with corestones of up to 8 m 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 in Tropical Environments are found on the Jos Plateau of Nigeria and in the Matopas region and around Harare in Zimbabwe.
Diagrams from Cambridge AS and A Level Geography (Collins, 2016) pp 164/5
Questions and Answers - Granite
Questions and Answers - Granite
Explain the nature of weathering processes in humid tropical areas and describe their influence on the development of granite landforms. [15]
Chemical weathering is dominant in the humid tropics due to the high temperatures and ready supply of water. The high temperatures increase the rate and the water allows the introduction of the weathering agents into rocks. It is inhibited to some degree by the luxuriance of the vegetation such that much chemical weathering is most effective at depth giving rise to a basal surface of weathering. Rotting vegetation contributes humic acids. Mechanical weathering is less dominant due to vegetation and climate and is limited to biological weathering, dilatation (where surface is removed) and possibly some limited thermal fracture. Granite landforms are generally the result of deep weathering (hydrolysis) along a basal surface of weathering. This exploits joints and cracks within the granite to bring about block disintegration. Periods of climatic change may bring about the stripping of regolith allowing the production of features such as inselbergs, castle kopjes, spheroidal blocks, etc.
With the aid of diagrams, explain how the processes of weathering and erosion lead to the development of granite landforms in tropical environments. [10]
In granite geology, jointing and bedding planes allow for rapid chemical weathering, especially hydrolysis, accelerated by high temperatures and precipitation. Landforms such as ruwares, bornhardts, inselbergs, tors and castle kopjes form. Weathering occurs underground and will attack granite along joints and cracks, producing a weathering profile with core stones and a weathering front (basal surface of weathering). Erosional stripping, due to sub aerial processes, possibly the result of climatic change, will allow landforms to emerge.
Much of the explanation can be contained in well annotated diagrams. If no diagram, max. 5 marks.
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