Dating rocks with radioactivity

Part of Hall of Planet Earth.

Several rock specimens and explanation signs about dating rocks and radioactivity.

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.

In This Section

Exhibit An exact age for an old rock This gabbro formed by crystallization of magma. Exhibit 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...