The Radioactive Heating of the Earth
Since at some stage in the past we would presume that the Earth was in a hot , molten state, it would seem that the cooling of the Earth to its present temperature would take an extremely long time. When Lord Kelvin undertook the task of calculating the cooling time of the Earth, the result was surprisingly short - about 30,000 years. Compared to the apparent age of the Earth, this is just a moment.
Another obvious factor is the Sun's input of energy to the Earth, which would warm it and at least slow the cooling from a molten state. But when that is factored in, the Earth of today would be expected to be much cooler - in fact below the freezing point of water. Soon after Kelvin's time it was realized that there must be another source of heat within the Earth that is helping to maintain its temperature. We can account for the needed heating by including the heat provided by the radioactive elements uranium, thorium and potassium in the Earth's crust.