How did the discovery of radioactivity change scientific estimates of the age of Earth?

William Thomson, Lord Kelvin, originally estimated the Earth to be between 20 and 40 million years old based on thermodynamic principles. His calculations relied on the assumption that the Earth was an inert body cooling from an initial molten state and was incapable of generating new internal heat.1

The discovery of radioactivity by Henri Becquerel and the subsequent work of Marie and Pierre Curie revealed that radioactive elements, such as uranium, continuously produce heat. This proved that the Earth’s interior is a dynamic system rather than a cooling, inert one, effectively invalidating Kelvin’s thermodynamic model.1

Following this, scientists like Ernest Rutherford and Bertram B. Boltwood utilized the process of radioactive decay to develop dating methods. These techniques provided numerical ages for minerals ranging from hundreds of millions to billions of years, demonstrating that the Earth was significantly older than Kelvin’s estimates had suggested.12