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Chronology

The age of the Earth and its inhabitants has been determined through two complementary lines of evidence: relative dating and numerical (or radiometric) dating.

Relative vs. absolute dating
  • Relative dating places fossils in a temporal sequence by noting their positions in layers of rocks, known as strata. As shown in the diagram, fossils found in lower strata were typically deposited first and are deemed to be older (this principle is known as superposition). Sometimes this method doesn't work, either because the layers weren't deposited horizontally to begin with, or because they have been overturned.
    If that's the case, we can use one of three other methods to date fossil-bearing layers relative to one another: faunal succession, crosscutting relationships, and inclusions.
    By studying and comparing strata from all over the world we can learn which came first and which came next, but we need further evidence to ascertain the specific, or numerical, ages of fossils.
  • Numerical dating relies on the decay of radioactive elements, such as uranium, potassium, rubidium and carbon. Very old rocks must be dated using volcanic material. By dating volcanic ash layers both above and below a fossil-bearing layer, as shown in the diagram, you can determine "older than X, but younger than Y" dates for the fossils. Sedimentary rocks less than 50,000 years old can be dated as well, using their radioactive carbon content. Geologists have assembled a geological time scale on the basis of numerical dating of rocks from around the world.


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More details
To read more about determining relative dates, visit the Utah Geological Survey website. And for more information on using radioactive isotopes to determine numerical dates, visit the USGS website.