GRANITE ISLAND: Matthew Flinders & the Coastal Landforms of SE Australia

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.	Photo 1: Granite boulders, Granite Island, SA (Victor Harbour and Causeway in distance) cavernous weathering Photo 2: Cavernous weathering on granite, Granite Island
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. The Victor Harbour granite (see right) 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.	Photo 3: Victor Harbour Granite, with phenocrysts and xenoliths visible (Granite Island) (see below)
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.
Landforms are utilised as habitats by living things. In the case of Granite Island, there is suitable 'living space' for the Little Penguin, Eudyptula minor (also known as the Fairy Penguin). This is the smallest of 18 penguin species, and the only one that breeds in Australia. Early in the morning penguins, in groups, head out to sea to catch fish. They return in the evening, waddling ashore to their burrows in dunes or rocky crevices. When raising chicks, parents disgorge fish they have caught to feed their two hungry offspring. Controlled tourism allows people to observe these natural 'rituals'. While appearing cumbersome (but cute) while moving on land, the penguin is a master of sea travel, swimming like a jet-propelled torpedo. The penguin, like other important creatures of the region, is totally protected. A further 'ritual' (an 'eco-ritual'?) may be said to have arisen, namely the visits by numerous people coming to see the penguins undergoing their 'rituals'.	Little penguins Photo 4: Information on penguin activities from signboards, Granite Island
Rosetta Head Rosetta Head, a mainland site adjacent to Granite Island, is most prominent where it is composed of resistant granite (see photo). Nearby areas, composed mainly of the metamorphic rock, schist, are less resistant, and therefore less prominent (see area to the right of the Head, in photo). There is also a complex band where the granite was intruded into the schist. The granite has schist mixed in with it (see photo 3, above); the feldspar variety is albite.	Photo 5: Granite Is (left), Rosetta Head (The Bluff) (right)
Rosetta Head's rounded profile is suggestive of a roche moutonnée (a glacial feature), but no direct evidence proves that glacial action carved the headland. Rocks from area, though, are found across Fleurieu Peninsula (to the west), evidence that glacial transport operated in this vicinity in the Permian, and was generally from east to west. It is believed that the Permian ice sheet extended over the Peninsula, northwards to Yorke Peninsula, to a depth of 1,000 metres at maximum intensity. At this time the South Pole was quite close by, perhaps near the site of present-day Tasmania. Australia was still part of Gondwana, as also were the present-day continents of Antarctica, South America and Africa. Australia's final split from Antarctica was completed by 45my ago; since then Australia has moved steadily northwards.

For further information

LANDFORM LINKS

Granite (rock type of Granite Island) Granite - the rock

Minerals of Granite:
Quartz Feldspar Hornblende Mica

Granite Minerals

Geolab: biotite granite

Introduction to Igneous Rocks

Weathering (mainly USA) - by Allen Glazner

ENVIRONMENTAL LINKS

Granite Island National Park

Granite Island's Natural Attractions - Parks Web

Granite Island Features (including Penguins)

Kidcyber: Penguins

Little Penguin Little Penguin: Fact Sheet

SA Whale Centre

PEOPLE LINKS

General Victor Harbour Information & History

Rowl Twidale - Granite Landform Geomorphologist

Printed Materials

Ed. Hasenohr,P. & Corbett,D. - A Field Guide to the Coastal Geology of Fleurieu Peninsula
Field Geology Club of SA, Adelaide. 1986

Clark, I.F. & Cook, B.J. - Geological Science: Perspectives of the Earth
Australian Academy of Science, Canberra ACT. 1983

Reader's Digest Guide to the Australian Coast
Reader's Digest Services, Surrey Hills NSW. 1983

and Matthew Flinders Books