# How are radioactive isotopes used to determine the absolute age of igneous rock?

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When the isotopes decay, scientists can find out how old the rock is depending on the radioactive isotope's half-life.

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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.

Example:

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.

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