Ruqayyah Snyder
Australia's oldest regions, such as the Pilbara and Yilgarn cratons of Western Australia, are composed almost exclusively of high-temperature metamorphic rocks. However, the difficulty in finding metamorphic rocks in Australia may lie in the fact that their identification is based on texture and mineral composition. Metamorphic rocks are formed due to variations in pressure, temperature, and chemically active fluids resulting from geological events, and their mineral assemblages and textural relationships provide a record of the P-T conditions that can be used to decipher the underlying tectonic and geodynamic processes that drove the metamorphism.
| Characteristics | Values |
|---|---|
| Texture | Non-foliated (or massive), knotted schist, gneiss |
| Mineralogy | Andalusite, kyanite, sillimanite, cordierite, staurolite, muscovite, biotite, talc, pyroxene, amphibole, olivine, plagioclase feldspar, etc. |
| Composition | Pelitic, Carbonate, ultramafic plutonic, etc. |
| Location | Western Australia, New South Wales, South Australia, Victoria |
| Use | Grindstones, millstones, stone axes, pigment ochre |
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What You'll Learn
- The cratonic regions of Australia are composed almost exclusively of metamorphic rocks
- The Broken Hill region of NSW is composed of metamorphic rocks
- Aboriginal and Torres Strait Islander peoples have used metamorphic rocks for grindstones and millstones
- The Bama people of Queensland used metamorphic slate to process toxic cycad kernels
- The distribution and location of valuable rock deposits were well known to Aboriginal and Torres Strait Islander peoples
The cratonic regions of Australia are composed almost exclusively of metamorphic rocks
The cratonic regions of Australia, like those of other continents, are composed almost exclusively of metamorphic rocks. This is because cratonic regions are the oldest and most stable parts of continents. In fact, the cratonic regions of Australia are among the oldest continental crust rocks on Earth, formed in the Archaean era (4 to 2.5 billion years ago).
The Gawler Craton in South Australia, for example, is an Archaean craton. The Yilgarn Craton and Pilbara Craton in Western Australia are also cratonic regions composed of high-temperature metamorphic rocks and metamorphosed granites. The Glenburgh Orogeny, which is partly responsible for the assembly of the West Australian landmass, exposed the rocks of the Glenburgh terrane and the Yarlarweelor Gneiss Complex. The Gawler Craton and the Willyama Block are flanked by several Proterozoic orogenic belts and sedimentary basins, including the Musgrave Block and the Arunta Block.
The Broken Hill region of far western New South Wales is another area in Australia composed almost exclusively of metamorphic rocks. The Broken Hill deposit is an example of how metamorphism is strongly associated with ore deposits. This is because metallic elements like lead, zinc, and copper are highly mobile during metamorphism, especially when fluid is involved.
Metamorphic rocks are identified by their texture and mineral composition. They can be divided into non-foliated (or massive) rocks, like knotted schist, and gneiss, which has layers of different compositions. The minerals that make up metamorphic rocks are listed in decreasing order of abundance and then given a suffix relating to their texture. For example, a rock composed of 60% muscovite and 40% biotite with interlocking platy grains is called a muscovite-biotite schist.
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The Broken Hill region of NSW is composed of metamorphic rocks
The Broken Hill region of NSW is composed almost exclusively of metamorphic rocks. This is because the region has three zones of high-grade metamorphism, with boundaries running north-east to south-west, parallel to the structural grain of the region. As the metamorphic grade increases from zone A to the much higher grade of zone C, crystal sizes tend to increase, while the number of hydrous minerals decreases.
The Broken Hill region's metamorphic rocks are part of the Willyama Complex, which covers an area of some 5,600 square miles (14,500 square kilometres). The province of this complex extends from the Euriowie Inlier and unconformity (50 miles or 80 kilometres north of Broken Hill) to the unconformity just west of Radium Hill in South Australia, 60 miles (95 kilometres) to the southwest of Broken Hill.
The original rocks in the Broken Hill region were once layered arenaceous to argillaceous sediments deposited under marine conditions. These rocks were metamorphosed by deep-seated metamorphism into gneisses, schists, granulites, and quartzites containing minerals such as almandine, annite, feldspar, hornblende, quartz, sillimanite, and staurolite. This high-grade metamorphism is described as being mainly of the sillimanite-almandine sub-facies of the almandine-amphibolite facies type.
Metamorphic rocks are pre-existing rocks whose mineral composition and/or texture has been changed by processes within the Earth. They can be classified according to their texture and mineral composition. Non-foliated (or massive) metamorphic rocks include knotted schist (with large non-platy minerals surrounded by platy minerals) and gneiss (with alternating layers of different compositions). Soapstone, a metamorphic rock composed predominantly of talc, is an example of a rock formed by the metamorphism of magnesian minerals.
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Aboriginal and Torres Strait Islander peoples have used metamorphic rocks for grindstones and millstones
Aboriginal and Torres Strait Islander peoples have a deep understanding of the properties of various minerals and use different rock types for different applications. Through direct observation and trial and error, First Nations Australians recognised that igneous rocks such as basalt or volcanic greenstone are very hard minerals with high tensile strength, making them ideal for stone axes.
Sedimentary rocks such as sandstone, or metamorphic rocks such as quartzite, were more commonly used to manufacture grindstones and millstones for food sources. This is because they provided a more abrasive surface. Grindstones and millstones were used to crush, grind or pound different materials, such as food, seeds, and medicines.
The Bama people of northern Queensland, however, chose an unlikely stone type for their grindstone: metamorphic slate. Slate has a smooth and generally non-abrasive surface, so cross-cuts were incised into the stone to make it more effective for grinding toxic cycad kernels to produce an edible source of carbohydrates. The Bama people understood that slate's mildly hydrophobic properties prevented the accumulation of toxins in the grinding tool.
Aboriginal people also used small grinding stones to crush soft rocks and clays (such as ochre) to make pigments. The pigments were used to decorate bodies for ceremonies, to paint rock art, and to decorate objects such as possum skin cloaks and weapons.
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The Bama people of Queensland used metamorphic slate to process toxic cycad kernels
While there is no direct evidence of the Bama people of Queensland using metamorphic slate to process toxic cycad kernels, there is extensive documentation of the detoxification processes employed by the Aboriginal peoples of Queensland to convert toxic cycad seeds into edible food products. The cycad seeds, also known as cycad kernels, contain a toxic substance called cycasin, which is known to cause acute symptoms such as vomiting and nausea and has also been linked to various types of cancer.
The Aboriginal people of Queensland had developed various methods to remove these toxins, allowing the cycad seeds to become a valuable food source. One common method involved cutting open the kernels and leaching out the toxins in water. The kernels were then ground into a powder and baked to make cycad bread. Another method utilised fermentation, where the kernels were left in large containers or pits for several months until they frothed or became mouldy.
Metamorphic rocks, on the other hand, are prevalent in the oldest regions of Australia, such as the Pilbara and Yilgarn cratons of Western Australia. These regions are composed almost exclusively of high-temperature metamorphic rocks and metamorphosed granites. The Broken Hill region of far western New South Wales is another area composed predominantly of metamorphic rocks.
The Bama people, as indigenous custodians of the land and with their intimate knowledge of the natural environment, likely had access to and utilised metamorphic rocks for various purposes, including food preparation. While there is no specific mention of using metamorphic slate for cycad kernel processing, it is plausible that they employed these rocks in their detoxification processes due to their availability and suitability for grinding or crushing the kernels.
In summary, while there is no direct evidence of the Bama people of Queensland using metamorphic slate specifically for processing toxic cycad kernels, it is probable that they had access to and utilised metamorphic rocks in their detoxification processes. The Aboriginal peoples of Queensland have demonstrated their scientific knowledge and inquiry skills in developing various methods to render toxic cycad seeds edible, ensuring a sustainable and nutritious food source for their communities.
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The distribution and location of valuable rock deposits were well known to Aboriginal and Torres Strait Islander peoples
Aboriginal and Torres Strait Islander peoples have a profound understanding of the properties of various minerals and rocks, including sedimentary, igneous, and metamorphic types. They recognised that igneous rocks, such as basalt or volcanic greenstone, possess high tensile strength and hardness, making them ideal for crafting durable tools like stone axes. On the other hand, sedimentary rocks like sandstone and metamorphic rocks like quartzite were preferred for creating grindstones and millstones due to their abrasive surfaces, which were essential for food preparation. The Bama people of northern Queensland, however, innovatively used metamorphic slate for grinding toxic cycad kernels to produce a carbohydrate source, demonstrating their ability to adapt rock types to specific needs.
The extraction of valuable rock resources was a sophisticated process for Aboriginal and Torres Strait Islander peoples. Quarrying and mining were the two most common techniques employed, with several hundred extraction sites recorded in eastern Australia alone. One notable example is Wilgie Mia in Western Australia, recognised as the world's oldest continuous mining operation, showcasing the antiquity and complexity of their mining practices.
The knowledge of rock types and their applications extended beyond practical purposes and played a significant role in the cultural and spiritual aspects of Aboriginal and Torres Strait Islander communities. Ochre, a sedimentary rock composed of ferric oxide, clay, and sand, was highly prized for thousands of years as a pigment in cosmetics, body decoration, and rock painting. The distribution and quality of ochre deposits were well known, and this knowledge facilitated trade over large distances.
The understanding of rock types and their applications was integral to the self-determination and resource management practices of Aboriginal and Torres Strait Islander communities. Their complex kinship systems, social interactions, and spiritual beliefs were deeply intertwined with their natural environment, and their extensive knowledge of rocks and minerals was a fundamental aspect of their holistic worldview.
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Frequently asked questions
Metamorphic rocks are not necessarily hard to find in Australia. In fact, the oldest regions in Australia, such as the Pilbara and Yilgarn cratons of Western Australia, are composed almost exclusively of high-temperature metamorphic rocks. The Broken Hill region of far western New South Wales is another example of an area composed almost entirely of metamorphic rocks.
The Lachlan Fold Belt of New South Wales comprises many metamorphic sequences. Within New South Wales, most metamorphic rocks occur west of the Sydney Basin, from Oberon outwards to Broken Hill. The Bama people of northern Queensland also used metamorphic slate as a grindstone.
Metamorphic rocks are formed by changes in rocks due to variations in pressure, temperature, or chemically active fluids resulting from geological events. These changes directly express the evolving thermal regime, which reflects the geodynamics driving the metamorphism.
