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Originally Posted by ozark1
Eroica,
Are you on slightly dodgy ground with this one? Thomsonś calculations failed for two reasons - 1- he didnt know about radioactivity and 2- he also didnt know about convection within the molten core. The radioactivity bit explains about 60% of the problem and is the principal fault and was the reason that Thomson conceded defeat in 1905.
However I think youŕe getting at the fact that Thomson based his calculations on conduction (Fourier theory) and radiative cooling, neglecting the possibility that the interior of the earth would be molten and therefore convective.
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Correct!
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The real reason for Kelvin's low estimate of the age of the Earth was that his other major assumption, that the Earth cools by conduction alone, was also wrong. We now know from plate tectonics that the Earth's mantle moves and this movement is driven by thermal convection - hot material rises and cold material sinks. This process "evens out" the temperature within the deeper Earth but maintains a higher temperature gradient near the surface than is generated by purely conductive cooling. In other words, the Earth cools by conduction near the surface but mainly by convection iin the deeper layers. By using the surface gradient in a model which assumed that conduction is the only way of transporting heat throughout the whole Earth, Kelvin arrived at his erroneously short age; and he would have done so even if he had included the correct estimates of radiogenic heat production.
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Source: Kate Bradshaw et al. (1998), S103,
Discovering Science, Block 10,
Earth and life through time, The Open University, Milton Keynes.