Dating rocks with radioactivity

  • Exhibition Text

    • Several radioactive elements are useful for dating, depending on how rapidly they decay. For old rocks, a radioactive element with a very long half-life is needed. One such element is samarium, which is present in minuscule amounts in most rocks and minerals. Radio-active samarium transforms to neodymium with a half-life of 106 billion years. These elements have been used to determine the age of the Stillwater Complex, a body that solidified from molten rock 2.7 billion years ago.

      Samarium (Sm), with an atomic weight of 147, decays to neodymium  (Nd), with a weight of 143. But Nd has another isotope, 144Nd, which  is not radioactive and does not change in concentration with time.

      Dating the Stillwater Gabbro:  For the Stillwater gabbro, the line has a slope indicating an age between 2.693 and 2.709 billion years.

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  • For Educators

    • Topic: Earth Science

      Subtopic: Geologic Time

      Keywords: Gabbro, Radioactive substances, Geochemistry, Radioactive dating

      Audience: General

In This Section

Dating rocks by trails of destruction

Dating rocks by trails of destruction

This 1.85-billion-year-old gneiss and the one-million-year-old granite vein cutting through it were dated by counting the number of fission tracks in the apatite (calcium phosphate) crystals contained in each rock.