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Dolerite

Dolerite

Oct 10, 2022

There are so many things that make Tasmania special. But would any of us single out a rock, and specifically the rock called dolerite, as one of those? And why should we?
Dolerite mountains. [Photo: Peter Grant]
Here are the basics: Dolerite is an igneous rock formed when a large volume of subsurface material melted during the dramatic breakup of the former supercontinent of Gondwana, beginning about 170 million years ago. The resulting magma bubbled and oozed its way towards the Earth’s surface, becoming one of the largest upwellings of magma in Earth’s history. It cooled near the surface, but it took millions of years of erosion before it became visible at the surface. Today, by far the largest surface exposure of dolerite is found in Tasmania. In fact, a little under 40% of Tasmania is made up of dolerite. That’s an eye-watering 30,000 square kilometres of the stuff!
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 formation

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

Dolerite cliffs. [Photo: 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.

Peter Grant is a nature writer and blogger. You can read more of his writing at naturescribe.com

Feature photo: Peter Grant

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Joshua Pringle