Overview

Lord Kelvin assumed that the Earth started as a molten ball and calculated that it cooled to its present condition by thermal conduction. The still molten part was kept uniform by convection and the frozen bits sank to the center. Kelvin knew that Earth's temperature increased downward into deep mines and guessed that the Earth began as molten rock at 7000 °F. By solving Fourier's equation, Kelvin found that it would take a hundred million years for the Earth's temperature gradient to level out to one degree every 50 feet. In numbers haughty for their implied plus or minus nothing, Kelvin's final estimate, in 1897, for the age of the Earth was 24 million years. This calculation established that the Earth had a finite age rather than the prevailing geologic wisdom that there was ‘no vestige of a beginning, no prospect of an end’ or that Earth's age was ‘incomprehensibly vast’.

On hindsight we know that uncertainties in the assumptions and parameters are such that Kelvin could have concluded that an Earth cooled by conduction was tens to hundreds of times older than he published, even without internal heat sources.