Tasmania's Journal of Discovery

The Rock which makes Tasmania

The Rock which makes Tasmania
By David Leaman
Published by Leaman Geophysics
ISBN 0 95811 990 2

David Leaman is a geologist, an experienced bushwalker and writer of books on the two foregoing subjects. He grew up in Glenorchy on the very rock he later agreed to research. For him this rock is no hard place but a lodestar for our island.

The rock is dolerite, which has both put Tasmania on the map and shaped its economy and future. Not so many years ago it was called “Tasmania’s curse”.

David Leaman tells why in this unusual book that is, as he writes:

A very personal and doubtless idiosyncratic book presenting a view of one rock and how it has come to dominate a people, their homeland, their economy, their psychology and their tourism marketing often without knowing.

What exactly is dolerite? To quote the book again:

A fine to medium-grained aphyric hypabsyssal intrusive rock consisting essentially of calcic-plagioclase and pyroxene with or without olivine; a basaltic rock characterised by an ophitic texture.

As Leaman continues, “this is everything the professionals will want to know”. He handily gives the following translation:

An igneous rock (meaning that it has cooled and crystallised from high temperature magma) which is wholly crystalline and whose mineral crystals are less than 1-2mm across or long. It is composed principally of members of two rock-forming mineral families (plagioclase feldspars and pyroxenes complex silicates of sodium, calcium, magnesium and iron).

Part of the book is an account of Leaman’s own experiences, starting with his studies at the University of Tasmania and subsequent contribution to the Geological Survey of Tasmania as mapper, engineering and groundwater geologist and specialist geophysicist.

Some of the pages really benefit from a professional’s knowledge, though it’s written with the layman in mind. Which makes it fascinating, if not exactly a cover-to-cover romp-through read.

Organ pipes and fluted cliffs

The more you do read, however, the more you realise just how important this rock has been, and remains, to Tasmania.

The striking “organ pipes” of Mt Wellington, looming over Hobart, are dolerite, as are the fluted cliffs of the Tasman Peninsula, Mount Ossa (Tasmania’s tallest peak), Mount Olympus and the jagged peaks of Cradle Mountain, and much else of eastern, southern and central Tasmania. In fact, we have more dolerite exposed than anywhere else in the world.

Dolerite influenced the course of Tasmania’s economic history as it was implicated in many mining problems. Because its presence prevented easy coal extraction, Tasmania was passed over as a source of manufacturing.

By the time hydro power came along it was too late to make up the shortfall for which, with our tourism flourishing partly because of the distinctive dolerite land formations and our green and peaceful non-industrialised landscape, we can be thankful.

Not seen in many places elswhere in the world, dolerite was therefore remarkably different to the eyes of early explorers and visitors, from Captain Abel Janszoon Tasman to Charles Darwin. Patsy Hollis