Methods of Dating Fossils

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Fossils are the remains of plants and animals that have been preserved in rocks. Geologists have found fossils that are only a few thousand years old and others that are more than 600 million years old. Fossils are rare; it takes very specific conditions for an organism to be preserved as a fossil. Scientists use two different methods for determining the age of a fossil: absolute dating and relative dating.

Absolute Dating

  • In 1896, Henry Becquerel discovered in the laboratory that uranium undergoes radioactive decay. Lord Rutherford, who developed a model of the atom, determined that the radioactive decay of uranium can be used to measure the geologic age of rocks. Radioactive decay is a process where an isotope of an element spontaneously loses particles from its nucleus to create a daughter isotope. This decay occurs at specific rate. The half-life of a radioactive isotope is the amount of time it takes for one-half of the radioactive isotopes to decay. By comparing the ratio of parent isotopes to daughter isotopes, scientists are able to determine the age of the fossil and the rock.

    Isotopes that have long half-lives are necessary to date fossils because many fossils are millions of years old. Uranium-235 is used quite often in the absolute dating of fossils. Uranium-235 has a half-life of 704 million years. Therefore, Uranium-235 can be used to accurately date rocks that are hundreds of million years old. Many people are under the misconception that carbon-14 is used for dating of fossils. Carbon-14 has a half life of 5,730 years, which is too short to be useful in dating fossils. Carbon-14 is quite useful in dating artifacts, however.

Relative Dating

  • Relative dating is based on the laws of stratigraphy. Relative dating does not give you an specific age of the fossil but can tell you a general time period based on the geologic beds above and below the place the fossil was found. The laws of stratigraphy state that the older rocks are buried beneath younger rocks. Each rock layer was originally horizontal and was continuous across the Earth's surface. Essentially, by looking at fossils in a rock column, geologists are able to determine which fossils are geologically older than others. Evolutionary processes and extinctions can be seen when looking at the faunal succession. More accurate ages of fossils can be determined if layers in the column have undergone absolute dating.

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