Evidence About Earth’s Past (Book)
ABSOLUTE AGES OF ROCKS
- 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 |