- Other Radioactive Isotopes

The isotopes in Table below are used to date igneous rocks. These isotopes have much longer half-lives than carbon-14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago?

Unstable Isotope Decays to At a Half-Life of (years) Dates Rocks Aged (years old)
Potassium-40 Argon-40 1.3 billion 100 thousand – 1 billion
Uranium-235 Lead-207 700 million 1 million – 4.5 billion
Uranium-238 Lead-206 4.5 billion 1 million – 4.5 billion