Dolerite
by Peter Grant
If you knew none of that, but were simply curious about what makes the Tasmanian landscape so different from the rest of Australia, the words “mountains” and “dolerite” would be among your top answers. Above the city of Hobart, for instance, sits the mountain we call kunanyi/Mount Wellington. It’s a classic doleritic mountain. On Launceston’s eastern skyline are three large mountain masses: Mount Arthur, Mount Barrow, and Ben Lomond. Each one is capped by dolerite.
Dolerite boulders on kunanyi/Mount Wellington
Drive from the north towards the north-west of the State, and the high cliffs of the Great Western Tiers that sit above the northern Midlands are nearly all doleritic. Walk the famous Overland Track and you’ll be amongst dolerite-capped mountains – including Barn Bluff, the Pelions, Mount Ossa, Cathedral Mountain and Mount Olympus – virtually all the way.
Sail around the waters, or walk the tracks, of the Tasman Peninsula, and you’ll see some of the most spectacular sea cliffs in the world at places like Cape Hauy and Tasman Island. And yes, those cliffs are made of dolerite.
Dolerite cliffs by Em Wilson
One of the spectacular features of dolerite, including on kunanyi/Mount Wellington, is steep columnar cliffs. These are the result of the magma cooling quite quickly, relatively close to the surface, and forming vertical joints. At the edges of dolerite masses these joints have eroded to become spectacular cliffs. The Organ Pipes of kunanyi are a classic example.
Geological changes are usually so ponderous by human chronology, that they seem out of our realm of experience. You might as well ask a bush fly to compare a series of winter cold snaps. Its life is so fleeting that the first cold snap is its last. Yet geologists believe that the massive upwelling of sub-surface dolerite here may have been so sudden as to be noticed by humans, had any existed at that time. Our imaginary proto-humans, living at sea-level in this region, may have found the ground around them elevated by as much as 600m over a period of years, perhaps even months.
Geology is seldom that dramatic, but perhaps this story of dolerite goes some way towards explaining why Tasmania’s landscape is so remarkable.
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Peter Grant is a nature writer and blogger, who lives with his wife in the foothills
of kunanyi/Mt Wellington. After studying Earth Science, Theology and Education,
he spent 24 years working with Tasmania Parks & Wildlife Service, much of it as the Manager of Interpretation and Education. Now semi-retired, Peter keeps
active by mixing writing with bushwalking, photography, mountain biking and
playing with grandchildren. More at naturescribe.com
Feature photo: Peter Grant