THE COLUMNAR COAST OF TASMANIAThe dolerites of Tasmania are remnants of Gondwana, a “super con

THE COLUMNAR COAST OF TASMANIAThe dolerites of Tasmania are remnants of Gondwana, a “super continent” of all the lands of the southern hemisphere that rifted apart in the Jurassic.A dolerite (known in parts of the geologic world as diabase, though the definitions differ with degree of alteration) is a shallow intrusion of magmatic rock that crystallizes at just the right rate of cooling to have a fine-grained but optically visible texture of mineral grain sizes – this means that if you squint hard enough or use a hand lens that you’ll be able to see the individual minerals within the rock. Were these finer-grained, it could be termed a basaltic texture; if larger grained, it could be a diorite or (if even larger-grained) a gabbro. The rate of cooling proceeds rapidly from the outside in, and in lucky circumstances such as occurred in the Tasmanian dolerites, as this happens the more solid rocks “shrink” in volume, causing the creation of cracks. They cool; they crack; the cracks allow more rocks in the interior of the intrusion to cool, and then they crack. And they crack with a symmetry that forms columnar patterns, creating a massif full of what is termed “columnar joints.” Columnar jointed dolerites commonly have five or six sides throughout their massif, but three-sided, seven-sided, and even twelve sided columns are known. Columns can be just a few centimeters to over several meters in diameter. Sets of straight, regular columns are called “Colonades” and more irregular and curved sets of columns are called “entablature.”These impressive dolerite columns on the coast of Tasmania were once part of a continuous formation over 4000 km (known as the Ferrar Magmatic Province) extending from Australia through Tasmania and into Antarctica. These are Jurassic in age, and their intrusion probably aided to instigate the rifting apart of the gigantic Gondwana supercontinent into today’s more manageable parts. The doleritic clifts surpass 100 m in topography above the sea along much of the southern and eastern coast of Tasmania, and some singular columns occur as giant “totem poles” standing in the sea.Massifs of columnar joints are not uncommon; in addition to dolerites, they can occur in lava flows, flows of volcanic ash, and shallow intrusions of other compositions. There are hundreds of known localities throughout the world, and at least a dozen really spectacular formations that, like these in Tasmania, make glorious tourist destinations. Some of the columnar jointed areas mentioned on The Earth Story or well-known from other sources include: The Giant’s Causeway, Ireland (http://tinyurl.com/o5d7e6b The Devil’s Postpile, California; Devil’s Tower, Wyoming (http://tinyurl.com/od35y6s The Hebrides Islands, UK; The Organ Pipes, Namibia; and Aldeyjarfoss, Iceland.Annie RPhoto used with kind permission of Albert Hill http://alberthillphotography.com/?page_id=889More information on the Tasmanian Dolerites and columnar jointing:http://maps.unomaha.edu/maher/geo330/julia1.htmlhttp://volcano.oregonstate.edu/education/facts/col_joint.htmlhttp://www.tandfonline.com/doi/abs/10.1046/j.1440-0952.2001.00875.x#.UdwmAfnDBvA -- source link

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