The technique -called radiometric dating- is based on the radioactivity of certain minerals. In these minerals, one or more chemical elements decay radioactively. Their atoms give off very small particles and other radiation. During this process the radioactive elements change slightly and become other elements. A given element may also have different forms, or isotopes. Isotopes have different atomic weights. Some of these isotopes may be radioactive and undergo change.
Each radioactive-decay process occurs regardless of external circumstances. Each decay process takes a fixed length of time, known as its half-life. The length of time depends on the isotope and its atomic weight. As the element decays, its quantity in the rock or mineral becomes smaller. The amount of the element it is changing into becomes greater. Scientists can therefore calculate the age of the rock or mineral. They simply need to know the decay times of elements and the proportions of these elements and of their end products.
Several radioactive elements and isotopes are commonly used to date ancient objects. These elements are: carbon-14 (carbon isotope with atomic weight 14); rubidium-87; potassium-40; strontium-90; and uranium-235 and -238. The quantities of isotopes in a sample are measured by radiation detectors and other methods.
The age of a rock may be based on one or more isotopes. However, these methods are not perfect. There may be an uncertainty of as much as several hundred million years. There may be disagreement in ages measured with different isotopes.
Nevertheless, the radioactive-decay method is the most exact dating method known to science. It has been used to determine the age of Earth. The oldest known rock was formed nearly 4.2 billion years ago. Planet Earth itself is thought to be about 4.6 billion years old.
