In 1862, British physicist Lord Kelvin (William Thomson) tried to estimate the Earth's age, working from a simple theory of the cooling process from primordial times. He assumed a constant, temperature-independent conductivity of the Earth's interior, and no internal sources of heat, arriving at an estimate of 98 million years. Thirty years later, another physicist, John Perry, pointed out that, should the Earth's thermal conductivity decrease with temperature, the estimated age might become much longer, even as long as billions of years. Perry was right.
The presence of radiogenic heating — internal heat created by the decay of long-lived radioactive isotopes — was unknown to Kelvin, and had little to do with his poor estimate. Yet Kelvin was poking around some deep mysteries. Radiogenic heating does play an important role in the Earth's interior geophysical dynamics, and might even be a major driver of all mantle convection — responsible for volcanoes and plate tectonics. The evidence is only slowly accruing, as it relies on detecting the most elusive particles currently known — neutrinos.
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