A2GEOGRAPHYREVISIONTROPICAL ENVIRONMENTS7.3 TROPICALLANDFORMS

KEYTERMSANDDEFINITIONSTors are ridges or piles of spheroidically weathered boulders that have their bases in the bedrock and are surrounded by weathered debris.Inselbergs are any isolated hill or hills that stand prominently over a level surface.Pediplanation is the extension of plains at the expense of interfluves through the process of parallel retreat.Peneplanation is the formation of an area of low relief (peneplain) separated by low erosional residuals.Etchplain is an area of stripped and exposed unweathered bedrock.Tower karst refers to upstanding rounded blocks of limestone set in a region of low relief.

KEYTERMSANDDEFINITIONSCockpit karst is a characteristic of tropical areas. It is a landscape pitted with smooth-sided soil-covered depressions and cone-like hills.Basal surface of weathering, also known as the weathering front, represents the depth to which rocks have been weathered; i.e. the boundary between the weathered rock (saprolite) and the solid unweathered rock.Saprolite is weathered rock.Relict features are landforms formed under conditions not currently experienced.Duricrusts are concentrations of minerals such as iron, aluminium, silica and calcium carbonate within the zone of weathering.

TOPICSUMMARYTropical landforms are diverse and complex.

● In the humid tropics, the availability of water and the consistently high temperatures maximise the efficiency of chemical weathering.● The ‘weathering front’ or ‘basal surface of weathering’ between sound rock and saprolite (weathered rock) may be very irregular.● Typically, deep weathering occurs down to 30–60 m.● Duricrusts are concentrations of iron (ferricrete), aluminium (alcrete), silica (silcrete) and calcium carbonate (calcrete) found within the zone of weathering.● A laterite horizon is formed under the surface when iron and aluminium oxides are concentrated there.

TOPICSUMMARYA peneplain is an area of low relief, separated by low erosional residuals (monadnocks).

● Pediplanation proposes the extension of plains at the expense of interfluves through the retreat of slopes though parallel retreat.● An etchplain is an area of stripped and exposed unweathered bedrock.● An inselberg is any isolated hill or hills that stand prominently over a level surface.

TOPICSUMMARYTors are ridges or piles of spheroidically weathered boulders that have their bases in the bedrock and are surrounded by weathered debris.

● An inselberg or bornhardt is characterised by steep slopes and a convex upper slope.● There are two major landform features associated with tropical karst: cockpit karst and tower karst.● Underground karst features include caves and tunnels formed by carbonation-solution and erosion by rivers.

ADDITIONALWORK1. Describe and account for the main types of weathering in tropical environments. How effective are they and why?2. Compare and contrast the processes of pediplanation, peneplanation and etchplain formation. Comment on the landforms they produce.3. Describe and explain the formation of tors and inselbergs.4. Account for the features associated with tropical karst.5. Explain the term ‘basal surface of weathering’. How is the development of tropical landforms influenced by the basal surface of weathering?6. To what extent are tropical landforms relict features?

SUGGESTEDWEBSITESwww.rswa.org.au/content/work/journals/PDF/80(3)/80(3)campbell.pdf for an article on granite landforms in tropical environments (including photographs).www.geomorph.org/gt/regional/nam.html for granite inselbergs of the central Namib desert.www.tourmalaysia.com/category/sarawak/sarawaktoursit-spot/mulu-national-park-sarawak-toursit-spotsarawak-2 for Gunung Mulu National Park, with video. This is one of the world’s most intensively studied areas of tropical karst.www.agc.army.mil/research/products/desert_guide/lsmsheet/lsduri.htm for information on duricrusts.