Wednesday, February 7, 2007
MI Prelim Paper 2005
(a) Briefly describe the characteristics of granite and explain its impact on the way it is weathered. (12m)
(b) Discuss the necessity of a seasonal humid tropical climate for the formation of inselbergs. (13m)
Our Answer:
(a) Characteristics of Granite & Impact in Weathering
Chemical Composition: Chiefly made of quartz, orthoclase feldspars and biotite micas. Although relatively stable, hydrolysis can wear down feldspars and micas to kaolin, if water is provided entry into the granite.
Hardness: Very hard, (esp due to quartz), providing mechanical strength. This characteristic allows some resistance to weathering. However, presence of joints could decrease resistance.
Texture: Coarse grained, large crystal size (3-5mm), however, minerals interlocked tightly, therefore decreasing porosity, and increasing resistance to weathering.
Colour: Varies, from light to dark. Dark colours, however, with lower albedo encourages the absorption of light. Alternative heating and cooling results in expansion and contraction of the granitic rock, inducing stress and resulting in insolation weathering. *The effect also occurs when the rock is made of different coloured minerals, different grains would expand and contract alternatively, which would lead to granular disintegration.
Permeability: Poor primary permeability as minerals interlocked tightly. However, extensive presence of joints (secondary permeability). Joints develop during colling when it contracts, developing in 3 main directions, vertical, horizontal and curvilinear (due to pressure release). Jointing makes granite vulnerable to physical and chemical weathering, as it allows for entry of weathering agents such as water, and increases the surface area for weathering.
(b) A seasonal humid tropical climate is necessary to a large extent in the formation of inselbergs. According to the Etchplanation theory of formation, inselbergs require a period of humid tropical climate followed by a period of aridity. It is during the humid climate when intense deep and differential chemical weathering takes place and intense erosion and exhumation will take place during the period of aridity, thus allowing for the formation inselbergs. Therefore, it being 'seasonal' plays a very important task in the formation of the inselbergs.
Firstly, the geological conditions required are the presence of differential joints in order to form tors, ruwares, bornhardts or kopjes. To form tors, jointing should be more cuboidal and extensive, while the rock is usually more massive for the formation of the other types of inselbergs.
The first stage, according to the Etchplanation theory, 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. These weathering processes require the water from rainfall derived from the humid climate. 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. However, in the formation of the other inselbergs, the extent of weathering is less and still maintains the massiveness of the original granite rock.
However, regolith now overlies the troughs and does not uncover the inselbergs yet. Thus, the humid climate is required only to an extent of its formation, and has to be seasonal. To complete the process, a period of aridity is required for the erosion and exhumation of regolith to reveal the inselberg finally.
The exhumation will take the form of river erosion, incision, wind erosion and surface erosion. The first condition required for exhumation is during rapid uplft and incision of streams. Vertical downcutting of streams would resilt in the removal of regolith. Incision of the regolith occurs as a result of the climatic change to an arid one, with open vegetation vcover and a more rapid hillslope erosion. The loss of vegetation cover as a result of a drier climate implies the loss or soicl nutrients which binds regolith. The loss of vegetative cover also implies higher rates of rainsplash and gully erosion. The rate of surface wash erosion is also accelerated due to lack of veg. Wind erosion is significantly high due to high wind speed in arid climates, being loose and unconsolidated, regolith will be easily removed.
This process causes piles of corestones to be exposed, in the formation of tors, and the greater portion of the basal surface of weathering to be exposed, in the formation of the other inselbergs. When only a limited amount of regolith is removed, then only a small section of the summit of the rises would be exposed, this is known as the ruware. A bornhardt is exposed when more layers of regolith are exposed by weathering agents like insolation, acquired in an arid region. The removal of regolith also results in pressure release which leads to the formation sheet and curvilinear joints, further disintegrating the granite into kopjes.
Therefore, a "seasonal" humid tropical climate is highly necessary for the formation of inselbergs as there is also a need for more arid conditions to complete the its formation.
you, 9:04 PM
(a) Briefly describe the characteristics of granite and explain its impact on the way it is weathered. (12m)
(b) Discuss the necessity of a seasonal humid tropical climate for the formation of inselbergs. (13m)
Our Answer:
(a) Characteristics of Granite & Impact in Weathering
Chemical Composition: Chiefly made of quartz, orthoclase feldspars and biotite micas. Although relatively stable, hydrolysis can wear down feldspars and micas to kaolin, if water is provided entry into the granite.
Hardness: Very hard, (esp due to quartz), providing mechanical strength. This characteristic allows some resistance to weathering. However, presence of joints could decrease resistance.
Texture: Coarse grained, large crystal size (3-5mm), however, minerals interlocked tightly, therefore decreasing porosity, and increasing resistance to weathering.
Colour: Varies, from light to dark. Dark colours, however, with lower albedo encourages the absorption of light. Alternative heating and cooling results in expansion and contraction of the granitic rock, inducing stress and resulting in insolation weathering. *The effect also occurs when the rock is made of different coloured minerals, different grains would expand and contract alternatively, which would lead to granular disintegration.
Permeability: Poor primary permeability as minerals interlocked tightly. However, extensive presence of joints (secondary permeability). Joints develop during colling when it contracts, developing in 3 main directions, vertical, horizontal and curvilinear (due to pressure release). Jointing makes granite vulnerable to physical and chemical weathering, as it allows for entry of weathering agents such as water, and increases the surface area for weathering.
(b) A seasonal humid tropical climate is necessary to a large extent in the formation of inselbergs. According to the Etchplanation theory of formation, inselbergs require a period of humid tropical climate followed by a period of aridity. It is during the humid climate when intense deep and differential chemical weathering takes place and intense erosion and exhumation will take place during the period of aridity, thus allowing for the formation inselbergs. Therefore, it being 'seasonal' plays a very important task in the formation of the inselbergs.
Firstly, the geological conditions required are the presence of differential joints in order to form tors, ruwares, bornhardts or kopjes. To form tors, jointing should be more cuboidal and extensive, while the rock is usually more massive for the formation of the other types of inselbergs.
The first stage, according to the Etchplanation theory, 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. These weathering processes require the water from rainfall derived from the humid climate. 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. However, in the formation of the other inselbergs, the extent of weathering is less and still maintains the massiveness of the original granite rock.
However, regolith now overlies the troughs and does not uncover the inselbergs yet. Thus, the humid climate is required only to an extent of its formation, and has to be seasonal. To complete the process, a period of aridity is required for the erosion and exhumation of regolith to reveal the inselberg finally.
The exhumation will take the form of river erosion, incision, wind erosion and surface erosion. The first condition required for exhumation is during rapid uplft and incision of streams. Vertical downcutting of streams would resilt in the removal of regolith. Incision of the regolith occurs as a result of the climatic change to an arid one, with open vegetation vcover and a more rapid hillslope erosion. The loss of vegetation cover as a result of a drier climate implies the loss or soicl nutrients which binds regolith. The loss of vegetative cover also implies higher rates of rainsplash and gully erosion. The rate of surface wash erosion is also accelerated due to lack of veg. Wind erosion is significantly high due to high wind speed in arid climates, being loose and unconsolidated, regolith will be easily removed.
This process causes piles of corestones to be exposed, in the formation of tors, and the greater portion of the basal surface of weathering to be exposed, in the formation of the other inselbergs. When only a limited amount of regolith is removed, then only a small section of the summit of the rises would be exposed, this is known as the ruware. A bornhardt is exposed when more layers of regolith are exposed by weathering agents like insolation, acquired in an arid region. The removal of regolith also results in pressure release which leads to the formation sheet and curvilinear joints, further disintegrating the granite into kopjes.
Therefore, a "seasonal" humid tropical climate is highly necessary for the formation of inselbergs as there is also a need for more arid conditions to complete the its formation.
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