Figure 24.1 Radioactive Isotopes Allow Us to Date Ancient Rocks The decay of radioactive isotopes into stable isotopes happens at a steady rate. A half-life is the time it takes for half of the remaining atoms to decay in this way. (A) The graph demonstrates the principle of half-life using carbon-14 (¹⁴C) as an example. The half-life of ¹⁴C is 5,700 years. (B) Different radioisotopes have different characteristic half-lives that allow us to estimate the ages of many rocks.