Absolute Time
Earth’s history is divided into four eras: Precambrian, Paleozoic, Mesozoic and Cenozoic. We now know that the Precambrian Era comprises most of Earth history, around 78%, lasting from 4.54 billion years ago to around 540 million years ago. The lengths of time the other eras represent can be seen on a time scale. Where do these numbers come from? These numbers refer to absolute time, the other way geologists refer to geologic time.
Absolute time refers to a specific number of years before the present. Thus, an event may have happened 65 million years ago. Absolute dating techniques inform geologists of the age of a feature or event (within a calculated margin of error) just as inspecting a person’s birth certificate or driver’s license allows us to determine the absolute age of a person. These numerical ages in geology are determined by dating the rocks, a method that involves the laboratory analyses of elemental isotopes within the minerals of a rock. Because the determination of absolute time involves modern analytic methods, early versions of the geologic time scale were developed without the knowledge of exactly how long ago a rock formed, or how long a specific time period actually lasted. Slowly, absolute ages were assigned to the boundaries between all the time periods and eras. It is an absolute age date from a volcanic ash layer that exists in most locations around the world, which allows geologists to claim that the end of the Cretaceous Period occurred 65 million years ago.
To determine an absolute age, scientists analyze the radioactive isotopes found in rocks. These isotopes have unstable nuclei and decay at a predicted rate. This can be used to determine a more precise age of a rock. Radioactive decay occurs when the unstable nucleus of an atom spontaneously opens and ejects protons, neutrons, and other particles. The decaying nucleus is called the parent isotope. Because the atom is losing these subatomic particles, the parent isotope changes to a stable daughter isotope. A half-life is the time it takes for half of the parent to decay. With each subsequent half-life, half of the remaining atoms decay.
The image below illustrates radioactive decay. You can see that with each half-life, the parent isotope decreases by half.
Determining Absolute Age
How do scientists find out a more precise age of a rock or fossil sample? They analyze the sample using specialized equipment to determine the % of parent isotope remaining in the sample. The remaining % of the parent isotope is used to calculate the age of the sample.
The example below shows the steps taken to calculate the absolute age of a rock or fossil sample.
Approximately how old is the sample in the example above?
Scientists look for specific radioactive isotopes when analyzing rock or fossil samples. There are several abundant isotopes they can find. Below is a chart containing these isotopes, their half-life, and their stable daughter isotope.
Common radioactive isotopes used to date rocks and fossils include:
| Unstable Parent Isotope | Half-Life | Stable Daughter Isotope |
| Potassium-40 (K40) | 1.3 billion years | Argon-40 (Ar40) |
| Rubidum-87 (Rb87) | 49 billion years | Strontium-87 (Sr87) |
| Uranium-235 (U235) | 704 million years | Lead-207 (Pb207) |
| Uranium-238 (U238) | 4.5 billion years | Lead-206 (Pb206) |
| Carbon-14 (C14) | 5,730 years | Nitrogen-14 (N14) |
For the oldest rocks and fossils, K40, Rb87, U235, and U238 are most useful. For more recent rocks and fossils (less than 75,000 years old), C14 is used.
On the next page, you will have the opportunity to practice absolute dating!
Image Credits
- Half-Life-Diagram: https://commons.wikimedia.org/wiki/File:Radiometric_Dating.svg
- Absolute-Dating-Example (Sab): Sabrina Ewald