Wednesday, February 7, 2007
Tampines JC Prelim Paper 2005
(a) Explain what is meant by 'basal surface of weathering' and pedimentation as illustrated in Figure 5. (4m)
(b) Explain how different sets of proceeses might have led to the formation of tors besides that shown in Figure 5. (6m)
Our Ans:
(a) Basal surface of weathering refers to the boundary between weathered and fresh rocks. It shows the max depth at which weathering can take place. It tends to be irregular in outline, as illustrated in the figure.
As seen from the figure, pedimentation is illustrated by the scarp retreat. This allows the pediments to coalesce, and form an extensive plain known as the pediplain.
(b)Alternate formation of tors, (besides the method shown in fig 5)
Firstly, the geological conditions required are the presence of extensive and differential joints. According to the Etchplanation theory of tor formation, it involves the deep and differential weathering where those areas with close network of joints undergo greater weathering and to greater depths relative to those with widely-spaced joints. In this case, such intense chemical weathering will include hydrolysis and hydration. When the rock is eventually weakened chemically, physical exertion would help in its disintegration. Weathering extends all the way to the basal surface of weathering, hense the basal platform is irregular with rises and troughs due to the uneven distribution of joints. The rises are the areas which have undergone a lesser degree of weathering. In the formation of tors, the lower parts usually perserve rounded, detached boulder or corse of unweathered rock. The exhumation of this regolith will take the form of river erosion, incision, wind erosion and surface erosion. This process causes piles of corestones to be exposed, which are also known as tors.
you, 10:23 PM
(a) Explain what is meant by 'basal surface of weathering' and pedimentation as illustrated in Figure 5. (4m)
(b) Explain how different sets of proceeses might have led to the formation of tors besides that shown in Figure 5. (6m)
Our Ans:
(a) Basal surface of weathering refers to the boundary between weathered and fresh rocks. It shows the max depth at which weathering can take place. It tends to be irregular in outline, as illustrated in the figure.
As seen from the figure, pedimentation is illustrated by the scarp retreat. This allows the pediments to coalesce, and form an extensive plain known as the pediplain.
(b)Alternate formation of tors, (besides the method shown in fig 5)
Firstly, the geological conditions required are the presence of extensive and differential joints. According to the Etchplanation theory of tor formation, it involves the deep and differential weathering where those areas with close network of joints undergo greater weathering and to greater depths relative to those with widely-spaced joints. In this case, such intense chemical weathering will include hydrolysis and hydration. When the rock is eventually weakened chemically, physical exertion would help in its disintegration. Weathering extends all the way to the basal surface of weathering, hense the basal platform is irregular with rises and troughs due to the uneven distribution of joints. The rises are the areas which have undergone a lesser degree of weathering. In the formation of tors, the lower parts usually perserve rounded, detached boulder or corse of unweathered rock. The exhumation of this regolith will take the form of river erosion, incision, wind erosion and surface erosion. This process causes piles of corestones to be exposed, which are also known as tors.
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