How are radioactive isotopes used to determine the absolute age of igneous rock?
When the isotopes decay, scientists can find out how old the rock is depending on the radioactive isotope's half-life.
Radioactive isotopes are unstable and will decay. For example, when humans die carbon-14 decays. The isotopes will decay into a stable isotope over time. Scientists can tell how old the rock was from looking at the radioactive isotope's half-life, which tells them how long it would take for there to be half the radioactive isotope and half the stable isotope. At the next half-life there will be 25% of the radioactive isotope and 75% of the stable isotope. At the next half life there will be 12.5% radioactive and 87.5% stable.
Carbon-14 is a radioactive isotope with a half life of 5,730 years. How old would carbon-14 be when there is 75% carbon-14 in the rock?
75% is half of the time before the half-life, so it would be 2,365 years.
Hope this helps. Half life helps scientists find how much the isotope has decayed and the age of the rock.