Uranium-lead dating and Potassium-argon dating. Radioactive isotopes by their nature are unstable and break down over time. This can be measured in half-lives. The half-life of a radioactive isotope is how long it takes for half of the nuclei in any given sample to decay. This process goes on at a steady, constant rate for each isotope and scientists are able to use that rate of decay to determine how long the process has been going on
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