Friday, February 23, 2007












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Thursday, February 22, 2007
Granite Island

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At the southeastern side of Fleurieu Peninsula, facing Encounter Bay, is the small island, Granite Island. It is made up of wonderful granite boulders altered into many different shapes by forces of marine erosion and chemical weathering (and influenced by jointing). Granite Island is visited by walking along a causeway from the resort centre of Victor Harbour, riding in a tram, or taking a boat cruise.

The granites of Granite Island were intruded into deep metamorphosed sediments about 500my ago. Over many millions of years that followed, the materials above were removed by erosion, exposing the granites at the surface. Permian glacial deposits can be found on the granites in places, indicating that the granites have been exposed for a long time.

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Granite tends to be quite resistant to breakdown; it is often found in areas that are higher than surrounding areas (of other rock types). However, if it is closely jointed, it will be broken down more rapidly, as weathering agents are offered more surfaces to destroy. Granite boulders (called tors) are typically rounded through the particular weathering processes that destroy the rock's 'corners' (see photos right).

Joints - usually vertical cracks in rock - can be seen clearly in Photo 1 (Joints often form when granite cools from magma or when materials above are removed or 'offloaded'.) When the joint planes are perpendicular to the direction of marine attack the rock will be broken down more rapidly than it would be otherwise.

Some rocks look as though portions have fallen off, forming cave-like hollows (Photo 2). This cavernous weathering results from chemical disruption of constituent minerals by salt spray. As granite is made of minerals with a range of hardness, it is likely that softer minerals are decomposed and removed first. Eventually only the resistant mineral quartz (which is generally a major component of sand) remains, explaining why granite areas mostly seem to produce sandy soils.

Granite can show variation in age and appearance, but there are always similarities between types of granite. At Granite Island, two forms of granite outcrop exists.

The Victor Harbour granite is porphyritic, with large phenocrysts of potash feldspar and xenoliths of schist and fine-granied granite. The rock of this area appears to be blue, due to the unusual colour of the quartz crystals.

The other granite that outcrops on Granite Island is the grey, fine-grained granite that appears as xenoliths within the Victor Harbour granite. A complex series of steps is believed to have formed both granites from the original intrusion.
you, 10:43 PM

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You write very well.

By Anonymous Laina, at November 12, 2008 at 10:18 AM  

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you, 10:30 PM

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Monday, February 12, 2007
Sorry for the delay guys, been busy.

Let me know what you think of this template, I can modify it or change it altogether.

Cheers.

James
you, 10:00 PM

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Testing the comments function.

By Blogger you, at February 22, 2007 at 10:25 PM  

it works! awesome!

- kerr

By Anonymous Anonymous, at February 22, 2007 at 10:34 PM  

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Saturday, February 10, 2007
Did you know that the Ancient Egyptians used granite to construct their pyramids? How the Egyptians worked with solid granite is still puzzling. Here are some photos which show how granite was used for antiquities.

Obelisk


Made with a single piece of stone


Egyptian writing

Obelisk-rejects


These are huge blocks of granite...



...that have been rejected for making obelisks...


... due to their cracks.

Can this be due to workmanship errors? Hmm...

Statue at a Museum

entirely made of granite

Pyramids


Pyramid made of granite and coated with limestone, which eventually got weathered.


The darker shade is actually limestone, because the granite here is of a lighter hue.

More pyramids & their closeups


Here lies some nicely-cut-out...


...granite blocks...


...piled onto one another...


...to form the amazing pyramids!


Pyramids = the beauty of grrrrrrrrrrrrranite!

That's all :)
you, 7:25 PM

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(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
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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.
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Types of landforms; Tors





General Description:
Small hills or heaps of spheroidally weathered rock boulders, with distinctive jointing, rising abruptly from the surrounding gentle ground surface(a.k.a "castellated/boulder/blocky inselbergs")

Shape: Rounded corestones, can be tabular

Height: 1m to 8m (usually 1m to 3m only) in diameter, and 30m to 50m in height

Summit:
Irregular, but rounded tors tend to have convex summit and tabular tors tend to have flat ones

Slope: Convex for rounded tors, rectilinear for tabular ones

Jointing: Horizontal, and vertical, cuboidal.

Vegetation: Absence

Surroundings: Etchplains, or on periglacial slopes



Granite Tors, Alaska (1998)
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Types of landforms; Kopje


Augrabies Falls National Park, South Africa

General Description:
Found on large domes with bedrock of similar origin, appear as small hills or heaps of weathere rock boulders, rooted in bedrock, rising abruptly from the surrounding gentle ground surface (a.k.a "koppies")

Shape: Irregular, largely angular, reflecting either massive structures or a well-developed vertical or near-vertical joints. Yet, sometimes rounded joint blocks as well.

Height: 1m to 8m in diameter (but usually til 3m only), 30m to 50m in height

Summit:
Irregular

Slope: Largely rectilinear, as a result og angular and irregularly shaped blocks

Jointing: Extensive. Vertical, horizontal, cuboidal joints, which serve as avenues of weaknesses where the weathering agents differentially attack the rock structure.

Vegetation: Absence

Surroundings: Summits, valley sides, valley floors and pediplains


Kopje, Spitskop Nature Reserve, Upington, South Africa



CASTLE KOPJES

Height: (main difference) Taller, 50m to 100m
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Types of landforms; Bornhardt
Africa, Mozambique, Niassa Reserve

General Description: Dome shaped (a.k.a. "domed inselbergs"), massive structures, steep-sided, isolated hills standing above surrounding plains

Shape: Dome shaped, circular in plan but in most cases exhibiting rectilinear plans which are controlled by fracture patterns. Asymmetrical due to internal structure patterns, but some are nearly symmetrical

Height: 30m to 300m

Summit: Pronounced convexity, with curvilinear and sheet jointing. Several cupola like culminations (depressions & holes) present, substantial regolith cover, shallow and broad pans of irregular shape observed

Slope: Near vertical or overhanging sides, slopes are rectilinear and gentle or convex, small tafonis (cave like forms) or overhanging caverns common at base of slope, lower slopes may be convex, constant, or concave


Jointing:
Massive & Extensive. Vertical and horizontal jointing, massive curvilinear joints (due to unloading).

Vegetation: Absence, but sparse veg present in tropical regions

Surroundings: An extensive plain, underlain by regolith and alluvium (etchplain/pediplain;depending on formation)
you, 9:17 PM

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Types of landforms; Ruware

General Description: Near level rock outcrops (a.k.a. "whale's back" and "dwala"), develops where the basal surface is undulating and reffered to as the summit of the rises of the b.s. of weathering

Shape: Gently sloping and rounded or elongated hills, low-lying

Height: 2m to 5m

Summit: Gentle convex summit, which has a very smooth surface initially but weathering takes advantage of vertical and curvilinear sheet joints to cause exfoliation and block disintegration over time

Slope: Convex or rectilinear slopes, very gentle

Jointing: Might not be very jointed. However, vertical and horizontal jointing might exist and older ruwares would have sheet and curvilinear jointing on the surface

Vegetation: Absence or little

Surroundings: Deeply weathered regolith materials
you, 9:12 PM

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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.
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By Anonymous Anonymous, at April 11, 2013 at 2:33 PM  

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Thursday, February 1, 2007
Project:
Granite Landscape


Scope of Project:
Rock characteristic
Types of landforms
Theories of formation
Case studies


Task Disintegration (break down):
David: blogskin, types of landform
Jenny: rock characteristic, pictures
Kerryn: PArts (performing arts), theories of formation
James: blogskin, pictures, case studies
Candy: Case studies, rock characteristics
Hilary: Updates, Q&A, types of landforms
Daphne: PArts (performing arts), theories of formation
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Wednesday, January 24, 2007
Helllooooo... it's DONE :D lalala..
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