Tuesday, 28 August 2012

Rates of Erosion of Tweed Shield Volcanic Rock

The inscription on a plaque at the Mt Cougal section of the Springbrook National Park, Queensland, claims that the landscape depicted below is the result of 23 million years of erosion.


It is alleged that in the small space between where this picture was taken and the carpark just a couple of hundred metres down the track, 5000 million tonnes (1.8 cubic kilometres) has been eroded away from a half-kilometre thick layer of basalt which flowed from the old Tweed Shield Volcano, allegedly over 23 million years ago.

The picture below shows another landform nearby at Fingal Head, NSW - a landform which it is claimed comes from the same volcanic eruption. As the lava cooled, it cracked in places, forming these vertical pillars.


Notice that apart from some rounded rock forms at their base, much of the exposed rock has so far maintained its polygonal shape - despite 23 million years having allegedly transpired, and despite being subjected to the erosive effects of coastal winds, rain and the Pacific Ocean.

And yet, up at Mt Cougal, lesser erosive forces than the Pacific Ocean - a mere waterfall - is thought to have eroded away 5000 million tonnes of the same material over the same period of time.

I need someone to explain to me how the wind, rain and a small waterfall has been able to erode away so much of the basalt at Mt Cougal, while over the same period of time, the wind, rain and the Pacific Ocean hasn't yet been able to erode away the polygonal shape of many of the exposed rocks at Fingal Head.

On the surface it seems to me that either the rocks at Fingal Head aren't as old, or the erosion at Mt Cougal didn't take that long.

Just wondering what the standard explanation for this might be.

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