ROCK LANDFORMS of Australia & NZ - IGNEOUS LANDFORMS

This site visits landform features developed in association with granite in southern Qld.
The role of rock jointing is the key area of interest.

Theme Study (A): Role of JOINTING in granite (SITE: 1) Feature: Dome weathered on widely jointed granite mass Location: Girraween National Park, southern Qld
KEY POINT: Dome on granite. Granite is resistant to breakdown when the joints are widely spaced.
The South Pyramid, a granite dome, is shown. This is a massive feature that has resisted weathering and erosion more successfully than the surrounding area, which has been lowered. In the area of the Pyramid joints (cracks) in the rock mass are widely spaced, and hence, less scope for weathering and removal of rock materials. Originally adjacent areas were believed to have been more closely jointed, thus suffering more weathering and erosion of material and therefore leaving the Pyramid (and other nearby domes) standing out. A large boulder is visible on the Pyramid slope. It may be the remnant of a thick rock sheet formed over the surface by large scale cracking. Frequently such boulders (and sheets) creep slowly downslope on granite domes. It has been said that vandals attempted to dislodge it many years ago, but without success. (Note: The granite type in this area is adamellite.) top of page
Theme Study (A): Role of JOINTING in granite (SITE: 2) Feature: Rounded boulders (some balancing) Location: Girraween National Park, southern Qld
KEY POINT: Rounded boulders formed on moderately jointed granite.
Sphinx Rock is shown on right of photo. When jointing is regular and closer than the widely spaced joints of domes, it is possible for smaller, but very interesting features, such as 'balancing rocks', to result. Much of the important weathering along joint planes is believed to occur underground. Areas of greatest exposure to the effects of moisture are weakened - this occurs at the intersection of joint planes and has the effect of breaking down the block corners. When exposed at the surface, the corners are readily removed, thus producing the rounded blocks (or boulders) characteristic of the area. The size and shape of the rounded blocks is mainly determined by original joint pattern and spacing. In some countries rounded blocks (boulders) of granite are called tors. top of page
Theme Study (A): Role of JOINTING in granite (SITE: 3) Feature: Tree root exploiting rock joint, thus increasing speed of breakdown Location: Girraween National Park, southern Qld
KEY POINT: Granite jointing - use and enlargement of joint by tree roots
A tree seedling has germinated on the surface of a granite block. The tree's roots, while seeking moisture and nutrients, have exploited a joint in the block. As the tree grows, the roots expand and force apart the block sections, thus widening the joint. This is an example of biological weathering and illustrates the power of living things in changing landforms. Chemical weathering (from the influence of water and the decomposition of dead materials) will also continue to act in the joint area. Because a joint is a weakness in a rock that allows moisture to seep through (and at times, tree roots), it can be concluded that rock with closely spaced joints will weather more rapidly than widely-jointed rock of the same composition. top of page
Additional Site: Theme Study (A) - Role of JOINTING in granite Feature: Granite Arch, formed from weathering along joint planes Location: Girraween National Park, southern Qld
	Granite Arch is another spectacular example of granite weathering along joint planes. It is believed that the major blocks formed underground first, and were exposed later as surrounding material was removed by erosion. It is likely that features such as this are presently being formed underground - to be revealed at the surface some time in the future! Present weathering causes flakes to break off the surface, slowly reducing boulder size. As a balancing rock gradually become smaller it can be expected to shift position.

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granite dome rounded boulders biological factor

		Jointing in granite influences landform development Granite, an igneous intrusive rock, forms from magma deep under the surface. It cools slowly, forming large crystals. As it is silica-rich, it shows considerable resistance to weathering. However, as the magma cools, and later, as covering materials are removed (i.e. off loaded), the granite mass cracks. These cracks are called joints. How widely spaced these joints are influences subsequent weathering rates. The closer the joints, the more rapid the weathering.
IGNEOUS LANDFORMS		- Theme Study (A) © C.Grant Granite Jointing (Sites 1-3)