Finally, the solution comes along in 1895. Henri Becquerel discovered radioactivity. It was soon realized that the energy from radioactive decay was enough to keep the planet hot. But more than that, it could be used for dating rocks. Radiogenic dating works because radiogenic isotopes decay at a constant rate.
Decay rates vary greatly for different radioactive isotopes.
The rate of decay is defined in terms of half life . For 14 C, the half life is 5570 years. For 87 Rb, the half life is 47 billion years. A parent decays at a constant rate as shown at the diagram at the left. Each decays to a different isotope of lead.