Columnar Jointing on Mount Wellington

January 10, 2007


Provided and copyright by: Phillip Lachman
Summary author: Phillip Lachman

The photo above shows a fantastic view of the ever-present columnar jointing in dolerite found on the geologically diverse island of Tasmania. This view is of the 4,170 ft (1,270 m) summit of Mount Wellington, situated less than an hour from the heart of Hobart, the capital of the Australian state of Tasmania. Geologically, it was formed when a thick sheet of dolerite intruded during the Jurassic Period, 170 million years ago. Shrinkage cracks developed while the molten magma cooled, forming large vertical columns with polygonal cross-sections. Mount Wellington was formed some 150 million years ago by block faulting. Erosion has since exposed the 1,150 ft (350 m) thick horizontal sheet of dolerite, featuring the Organ Pipes, on the eastern face of the Mountain. This area has been eroded back approximately 1.2 mi (2 km) since the escarpment was produced 10-15 million years ago. Additionally, dolerite formations were further weathered as water froze and expanded within the columnar cracks, resulting in the dolerite talus evident on the higher slopes. Large dolerite sheets are globally unusual, though reasonably common in Tasmania. For more about this, see the Geodiversity Overview link as well as the additional links below.

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