Fossils can be dated relatively by comparing strata that they are found in. To put an absolute date on that layer, scientist use radiometric dating. Isotopes of common elements decay radioactively over time and give scientist built in rock clocks. Examples of some of the most useful elements include:
- Rubidium-87 (1/2 life 50 billion yrs)
- Thorium-232 (1/2 life 13.9 billion yrs)
- Uranium-238 (1/2 life 4.5 billion yrs)
- Potassium-40 (1/2 life 1.3 billion yrs)
- Uranium-235 (1/2 life 713 million yrs)
- Carbon-14 (1/2 life 5770 yrs).
The Earth's 4.5 billion year age has been obtained from dating the oldest surviving rocks of the Earth's crust (3.8 billion year old rocks from Greenland), the Moon (4.6 billion from Apollo 17 mission) and meteorites (4.7 billion from Norton County meteorites). To see more information on radiometric dating of the Earth click here.
For more information on the details of radiometric dating I suggest the following:
