Thursday, February 22, 2007
Granite Island
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.
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
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.
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.
1 Comments:
You write very well.
By 
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November 12, 2008 at 10:18 AM
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