(a) "The weathering processes that granitic rock undergoes are largely influenced by its rock characteristics." How far do you agree with this statement? Discuss. (10)
(b) Using diagrams, describe the characteristic features of castle kopje and explain its formation. (8)Our Ans:
(a) Agree to large extent.
Chemical composition: though quartz is very resistant, feldspar and mica can be attacked, as feldspar is susceptible to hydrolysis and wiill break down into aluminosilic acid and potassium hydroxide and mica will undergo hydration
Hardness: of constituent minerals, provides mechanical strenght and prevents physical weathering.
Non-porosity: deters chem weathering
Colours of minerals: Various colour result in different coefficients of expansion and contraction
Texture: Coarse texture, with large minerals but tightly interlocked.
Jointing: its cuboidal, curvilinear jointing makes granite susceptible to
physical attack (freeze thaw, sheeting, exfoliation),
chemical attack (hydrolysis, hydration) and
biological weathering (roots penetrate, dislodging rocks and organic acids aid chem attack)
(b) Characteristics of castle kopje
- pile of rock fragments rooted on the bed rock
- castellated appearance
Formation on castle kopje
- explained with both pediplanation and etchplanation
- pediplanation :
>river rejuvenation
>uplift of the land
>vertical erosion
>lateral erosion
>interfluve undergoes slope retreat, forming a bornhardt
>physical weathering (sheeting, block disintegration) of bornhardt results in formation of castle kopje
-etchplanation:
>uneven distribution of joints in bed rock
>deep and differential chem weathering (hydrolysis, hydration)
>formation of regolith
>erosion and stripping away of regolith by m eans of river incisiion and wind erosion
>exposure of the summit of the basal surface of weathering, a ruware
>more of the bed rock is eventually exposed
>pressure release operation on bornhardt reults in formation of curvilinear joints, sheeting takes place
>physical weathering such as block disintegration further breaks down the bornhardt into castle kopje