Sunday, 4 January 2015

Kalkajaka


Black Mountain National Park, just south of Cooktown, is always a remarkable sight. The outcrops of lichen-covered granite boulders are unlike anything I've seen elsewhere. Photographs don't seem to do the scene justice, though I continue to try whenever I pass. These are yesterday's efforts.


The Department of National Parks, Recreation, Sport and Racing explains the Mountain's appearance thusly:
Black Mountain's structure resulted from slow geological processes. Around 260 million years ago, a mass of molten rock (magma) slowly solidified deep below the earth's surface, forming a body of hard granite rock. As softer land surfaces above eroded away, the sparsely fractured top of this granite was gradually exposed. Weathering and chemical decomposition removed loose material along weak fractures extending downward through the rock. More resistant rock remained as large rectangular blocks, their corners becoming progressively rounded into boulders. The solid granite core of the mountain now lies beneath the jumbled cover of boulders.

The granite rock is actually a light grey colour and composed of mineral such as feldspar, mica and hornblende. Black Mountain's distinctive dark appearance is due to a film of microscopic blue-green algae growing on the exposed surfaces. Grey patches and boulder fractures indicate ongoing rock disintegration—a process accelerated dramatically when cold rain hits rock, sometimes with explosive results.

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