The Dolerite Columns of Mount Wellington
Tasks:
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Are the columns mostly vertical or inclined? What does this indicate about their formation?
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Why do columnar structures form in dolerite?
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Which geological processes contribute to the weathering of the dolerite columns?
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Mount Wellington (Kunanyi) dominates the landscape of Hobart and is primarily composed of dolerite, a dark igneous rock. The impressive rock formations with their columnar structures are evidence of how the rock was formed and weathered over time.
In this Earthcache, you will learn how dolerite was formed, why it often appears as columns, and which geological processes are involved.
1. Formation of Dolerite on Mount Wellington
About 185 million years ago, during the Jurassic period, magma intruded into the Earth's crust but solidified below the surface. Because it cooled relatively slowly, large crystals formed, giving dolerite its characteristic coarse-grained appearance. These massive dolerite intrusions cover large parts of Tasmania and are unique worldwide.
2. Why Do Dolerite Columns Form?
As the dolerite magma cooled, it contracted, leading to the formation of cracks. These cracks often developed perpendicular to the cooling direction, creating a columnar pattern—similar to the famous Giant’s Causeway in Ireland or Devils Tower in the USA. Most of these columns have six sides, as this is the most efficient shape for relieving stress.
Later, erosion exposed the dolerite layers, allowing us to see the characteristic rock towers and block fields we observe today.
3. Weathering of the Dolerite Columns
Although dolerite is a very resistant rock, it is still affected by various weathering processes:
- Freeze-thaw weathering: Water seeps into cracks, freezes, and expands, slowly breaking the rock apart.
- Chemical weathering: Carbonic acid in rainwater can gradually dissolve minerals.
- Physical erosion: Wind and temperature fluctuations contribute to the fragmentation of the columns.
As a result, rounded edges and rock fragments accumulate on the slopes.
https://www.mineralatlas.eu/lexikon/index.php/RockData?lang=de&language=german&rock=Dolerit
https://epod.usra.edu/blog/2007/01/columnar-jointing-on-mt-wellington.html
https://de.wikipedia.org/wiki/Mount_Wellington_(Tasmanien)