A technique for measuring the ages of rocks and minerals from the decay of certain radioactive elements within them; also known as radiometric dating. The technique involves comparing the amount of a long-lived radioactive parent isotope in a sample with the amount of the daughter isotope into which it decays. Isotopes used in radioactive age dating include uranium-238, which decays to thorium-230 with a half-life of 4.5 × 109 years; uranium-235, which decays to protactinium-231 with a half-life of 7.07× 108 years; potassium-40, which decays to argon-40 with a half-life of 1.27 × 109 years; and rubidium-87, which decays to strontium-87 with a half-life of 4.88 × 1010 years.