Tasmanian Rocks

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1. SLATE –a metamorphic rock formed deep in the Earth’s crust when fine grained sediment is subjected to enormous pressure, producing the characteristic slatey cleavage. Slate mining has taken place at Bangor and Pipers River.

QVM1999:GRM:13 		2. QUARTZITE – seen here combined with slate, is a metamorphic rock formed when pressure deep within the Earth compacts quartz-rich sediments under great pressure. Just as layers of sand and mud are common in today’s oceans and streams, ancient sands and muds formed today’s quartzite and slate layers.

As can be seen in this specimen, the pressure often leads to buckling of the slate and quartzite layers. Rocks of this type in Tasmania are of Precambrian (over 700 million years old) and occur near Burnie, at Cradle Mountain and in much of S.W. Tasmania. This specimen comes from Strathgordon.

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3. SCHIST – a high grade metamorphic rock which forms deep in the Earth’s crust. Pressure there is so high that minerals may change into difference forms with the same composition, more stable at the higher pressure. The most common visible minerals in schists are mica flakes. This muscovite schist occurs near Strathgordon.
QVM:1999GRI:46 4. GRANITE – is an igneous rock, formed when magma solidifies deep within the crust. The rock’s colour varies depending on the main minerals present. Significant amounts of orange orthoclase feldspar produce a red granite, as found around Scottsdale and Coles Bay. A high percentage of darker minerals produces a variety of granite called granodiorite, as found north of St.Helens and in Mt.Heemskirk near Zeehan. A good deal of what is commonly described as granite actually has another name. If microscopic examination shows the presence of two feldspar minerals, the granitic rock is termed an adamellite. Extremely coarse grained material of granitic composition is called pegmatite.

QVM1992GRI39		 5. DOLERITE – is the most common igneous rock occurring in Tasmania, found making up the Central Plateau and the tops of many mountains including Barrow and Wellington. It may also be found close to sea level, making up the Bluff at Devonport and in the Cataract Gorge. This rock forms when mafic magma cools a few hundred metres beneath the Earth’s surface. In the Jurassic Period, about 160 million years ago, layers of magma up to hundreds of metres thick were intruded into existing Permian & Triassic age rocks. With its thick overblanket of strata, the magma took thousands of years to cool, leaving a very uniform medium grained rock with the characteristic columnar jointing pattern which creates the obvious columns on our mountain tops.

QVM1989GRI37		 6. BASALT – a volcanic igneous rock of very similar chemical content to dolerite, but cooling on the Earth’s surface. In Tasmania over a period between 40 million and 10 million years ago (Tertiary) there was widespread volcanic activity in. In many cases the flows were relatively thin and have long weathered away, so often only the characteristic “chocolate” soil remains – as around Scottsdale. The lava frequently contains gas holes (vesicles), which may become partly or totally filled with mineral material (amygdules). In this amygdaloidal basalt from near Scottsdale the white mineral is chabazite, a calcium silicate and one of the zeolite group of minerals.

QVM20000GRS102		 7. CONGLOMERATE – a clastic sedimentary rock formed from sand and gravel, commonly deposited in fresh or salt water. This specimen is from Mt. Roland and has been sectioned with a diamond-bladed saw to show the structure clearly. The overall pink colouration is due to iron minerals in the rock’s cement. Similar conglomerates are found in the Dial and West Coast ranges. In some other locations, such as Beaconsfield and Mt. Tim Shea on the Gordon Road, the rock is a bluish grey because it has chromic oxide in its cement, rather than iron oxide. All the rocks mentioned are of Ordovician age – about 520 million years old.

QVM1956GRS6 		8. SANDSTONE – a clastic sedimentary rock composed mainly of sand sized material (0.15 to 2.00mm grains). In Tasmania there are sandstones of many ages, but probably the best known is the Triassic age (230 million years) sandstone used to construct the buildings in Salamanca Place and to build the bridges at Ross and Richmond. This specimen, which has been partially dressed by a stone mason, comes from the now filled in quarry at Patersonia. This rock was used to build several structures in Launceston, including the two banks on the corner of Paterson & St. John streets.

QVM2000GRS35 		9. SHALE – a fine grained clastic sedimentary rock of sorted material and having, as a result, a relatively flat fracture. Unsorted fine grained material makes the rock a siltstone, which only breaks into totally angular pieces. The shale’s colour depends on the minerals present in the cement, in this case iron oxides resulting in the pinkish to chocolate colour. This specimen comes from near Lefroy and is part of the Mathinna Group of sedimentary rocks – of Silurian age – about 430 million years ago.

QVM1992GRS7 		10. COAL – is a sedimentary rock termed organic, because its main component is plant material. There are several types of coal and geologists recognize several 'coal minerals'. This piece of bituminous coal, from near Coalbrook, shows shiny layers of vitrain and dull layers of fusain. There are two major ages of coal in Tasmania. Permian coal was mined near Latrobe and at Preolenna SW of Wynyard. Although burning with a hotter fire than our other main coal, it has a high sulphur content which makes its use today uneconomic. Triassic age coal is mined today at Fingal and Cornwall, near St.Marys. There were once many mines producing this age coal including near Bicheno and at many places in S.E. Tasmania, including Sandfly and Ouse.

QVM1957GRS16 		11. LIMESTONE – is usually of organic origin, having been formed from accumulated shells, coral and other marine (and rarely freshwater) organisms. This fine grained, uniform limestone is from the Smelters Quarry at Queenstown and is of Ordovician (about 470 million years ago) age, the most common for a Tasmanian limestone. Similar Ordovician limestones are found near Gunns Planes, Mole Creek, Railton, Beaconsfield and Maydena. As with the majority of organic limestones, the specimen is devoid of fossils despite its origin. Over millions of years the calcium carbonate is repeatedly dissolved and re-deposited, destroying all signs of the original organism.

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 Photos: A.Vains 		12. BAUXITE / LATERITE – are residual sedimentary rocks, formed by the slow chemical breakdown of dolerite and basalt (Tasmania has lots of both rocks) on the Earth’s surface in hot, steamy conditions. The heavy metallic oxides remain behind while the lighter and soluble minerals are carried away by running water. Pure LATERITE is iron oxide and usually dark red. Pure BAUXITE is aluminium oxide and is light yellow. This specimen, from St.Leonards has both bauxite and laterite.

Many hill tops in north and northwest Tasmania have undergone laterite/bauxite formation several million years ago when the climate was sub-tropical.


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