Geology and Geologists

Early interest in geology: James' Ross sketch of 'A singular Rock...' (Tasmaniana Library, SLT)

Past geological events have provided a unique legacy of mineral wealth, fertile soils and natural beauty based on a variety of landforms which contributes much to the Tasmanian lifestyle. Most of those events occurred before the island became isolated from the rest of the Australian continent by Bass Strait.

The oldest rocks in Tasmania were deposited west of what is now 147E as sands, muds and calcareous sediments on a shallow sea floor possibly as much as 1000 million years ago (mya). In many places the original sandy rocks include grains of zircon derived from rocks formed as long as 3100 mya. The sands have been metamorphosed to quartzites and quartz schists which, because of their resistance to erosion, cap many of the peaks (for example Frenchmans Cap) and ranges (for example Frankland) in south-western Tasmania. The oldest fossils in the island, stromatolites made by blue-green algae, occur in calcareous sedimentary rocks near Mount Mueller.

A very important event for Tasmania occurred over an interval of only about 10 million years (510500 mya) during which submarine volcanoes poured out lava and threw out fragments of volcanic material. Hot volcanic fluids and sea water carried metallic elements in solution and leached more out of the lavas and fragmentary volcanic rocks they passed through. When the fluids reached the sea floor, the metals were deposited mainly as sulphides (of iron, copper, silver, zinc and lead) and some gold. These volcanoes lay in an arcuate belt about 15 km wide from Elliott Bay, south of Macquarie Harbour, to north-west of Cradle Mountain, and east to near Deloraine. These volcanic rocks, the Mount Read Volcanics, are host to valuable mineral deposits at Mount Lyell, Henty, Rosebery, Hellyer, Que River and elsewhere.

Granite, a resistant rock, also caps many Tasmanian mountains (for example The Hazards at Coles Bay). It was intruded as molten rock into strongly folded and fractured sedimentary rocks a few kilometres below the earth's surface. The intrusions (during mountain-building episodes) occurred at several widely separated times 760 mya (King Island), about 490 mya (western and northern Tasmania), and 395 to 360 mya on the main island of Tasmania and on many Bass Strait islands. Tin and tungsten deposits are closely associated with the granite, gold less closely.

A dark, coarse-grained rock with prominent, large columns (dolerite) underlies much of the eastern half of Tasmania and, being resistant to erosion, caps most of the higher mountains, for example Mount Wellington and Cradle Mountain. The dolerite was emplaced about 175 mya just below the earth's surface as sheets of molten rock in almost horizontally layered, very fossiliferous rocks 300 to 200 million years old. The blanket of sedimentary rocks over the dolerite has been removed by erosion in many places.

Tasmania is so mountainous partly because it contains many rock types resistant to erosion, but also because of a phase of fracturing and associated uplift which began in what is now south-eastern Bass Strait almost 100 mya. The uplift spread to Boobyalla where, about 85 mya, a boulder bed accumulated, presumably at the foot of a slope. Further fracturing and uplift, mainly on NNW or N trends, led to down-faulted blocks which became the sites of major river systems, for example Tamar, Coal, Derwent, and embayments, for example Oyster Bay, D'Entrecasteaux Channel, Macquarie Harbour. Fracturing is still taking place but neither frequently nor severely.

Vulcanism occurred sporadically in Tasmania from about 100 mya but the major vulcanism, associated with the fracturing, began in south-east Tasmania 64 mya and continued until 8.5 mya (at Stanley). This vulcanism produced magnesium- and iron-rich lavas which cooled to a fine-grained, dark rock (basalt), and showers of volcanic ash. At least 120 volcanic centres have been recognised. The basaltic rocks occur north-east of a line from Granville Harbour to Huonville, and are particularly abundant in north-western Tasmania. The basalts weather to a very fertile red soil, much valued for farming in north-western Tasmania.

The delineation of Tasmania as an island near the edge of the Australian continent began about 145 mya with the initiation of depressions south of a line from Casterton to Lakes Entrance in Victoria. As Gondwana broke up, Antarctica began to move south from Tasmania about 120 mya and the sea, spreading from the north, reached Strahan about 55 mya; final separation, allowing the ocean south of Australia to connect with the Tasman Sea south of Tasmania occurred about 33.5 mya. As New Zealand pulled away, the Tasman Sea began to open about 80 mya and the sea had reached 65 km SSE of Tasman Island 75 mya. Tasmania was not finally isolated until sea level rose about 23 mya and has since been separate, except for brief intervals of low sea level.

Of the many men and women prominent in the study of Tasmanian geology, some deserve particular mention because of their prominence among their contemporaries for the contribution they made or are making. Dr Joseph Milligan made a systematic study of the coal basins late in the 1840s; knowledge of the succession and structure of Tasmanian rocks was increased significantly by Charles Gould as first Geological Surveyor (18591869); much of his own contribution to knowledge of rocks younger than 300 million years was included by Robert Mackenzie Johnston in his summary of the geology as then known in his book, Systematic Account of the Geology of Tasmania (1888); William Twelvetrees, Government Geologist 18991919, led the Geological Survey in many advances in geological knowledge of mines and mineral fields; fresh insights were revealed by Dr S Warren Carey in a critical review of the geology published as part of his Annual Report of the Government Geologist for 1945. Carey was appointed Foundation Professor of Geology at the University of Tasmania in 1946 and in the next thirty years he, his staff and research students made major advances in almost every aspect of the topic.

In the late twentieth century, staff of the Geological Survey of Tasmania under the leadership of Ian Jennings and Dr Tony Brown, University staff and research students under Professors David Green and Ross Large, and company geologists and consultants all contributed to a flood of new knowledge. More has been learned of the geology of Tasmania in the last fifty years than in the first 150 years after European settlement.

Further reading: C Burrett & E Martin (eds), Geology and mineral resources of Tasmania, Brisbane, 1989; D Seymour & C Calver, 'Explanatory notes for the time-space diagram and stratotectonic elements map of Tasmania', Mineral Resources of Tasmania Geological Survey Records 1995/01; and 'Time-space diagram, version 2', Mineral Resources Tasmania, 1998.

Max Banks