Radiogenic heating
Lord Kelvin (1824-1907) assumed the main sources of energy were from primordial heat and tidal friction. He concluded that the planet could not be much older than about 20 - 40 Ma. The conclusion was contrary to the ideas of James Hutton (1726-1797), Charles Lyell (1797-1875) and Charles Darwin (1809-1882) who argued that the Earth must be much older. Klevin however didn’t account for radioactive decay, which, leading to radiogenic heating, is an important heat source within the Earth.