It’s a cliché to say that the geology of a place defines so much of what occurs on it, but the case of dolerite in Tasmania truly bears this theory out. This igneous rock is prevalent throughout the island, defining many of its distinctive mountains and soaring sea cliffs, and was among the first physical attributes noted by seventeenth and eighteenth century seafarers – indeed, Abel Tasman saw little else other than dolerite coastal scenery in his 1642 voyage.
Tasmania’s dolerite correlates with similar formations in Antarctica and South Africa – evidence of Gondwana’s violent break-up during the Jurassic period, when vast quantities of molten rock pushed up from deep in the earth through weak points in the Earth’s crust. Tasmania’s dolerite-capped highlands were at the right latitude to experience erosion by the freeze-thaw phenomenon during the most recent Ice Age.
Dolerite has influenced the settlement and development of Tasmania, including the location of roads, rail, mines and agriculture. The rock has also directly affected the biology of the island, creating unique habitats for plants and animals; indeed, it was the botanist, Robert Brown, who made the first recorded geophysical observation related to dolerite – when he climbed Mt Wellington in 1804 and noted that the “perpendicular rocks atop Table Mountain affected the compass needle”.
Despite its majestic appearance, dolerite’s physical properties have not always been warmly received, being a very hard rock, difficult to drill, shape or quarry, and forming only thin, rocky soils. A government report in 1937, noted:
“... in the central plateau and the higher portions of the State all the overlying sedimentaries have been denuded away and the diabase [i.e. dolerite] forms the surface. It destroyed the coal measures and itself is barren of all mineral wealth and use, except as road metal. It is so dense and hard that the soil washes away as rapidly as the rock decays. It is Tasmania’s curse.”