The standard geologic time scale was devised according to relative time relationships observed in rocks across the world. Determining the actual ages of these time spans, and thus establishing the beginning and ending dates of geologic eons, eras, periods, and epochs, became possible with the discovery of radioactivity. The important boundary between the Paleozoic era and the Precambrian era is dated at about 570 million years ago; the Mesozoic era (the “Age of the Dinosaurs”) started about 245 million years ago and ended 66 million years ago.
Radioactive elements decay at known rates of speed. This radioactive decay begins after the elements are locked into crystalline mineral structures. Some elements have variations called isotopes, which are atoms that contain different numbers of neutrons in their nuclei. For example, uranium has the isotopes U‐235 and U‐238; U‐238 has three more neutrons than does U‐235.
Radioactive decay is the breakdown of isotopes that contain unstable nuclei. As an element decays it creates a series of daughter products. For example, uranium‐238 loses protons and neutrons during its decay, going through a series of intermediate daughter products to form its end product lead‐206, a stable isotope. The rate of radioactive decay is constant. By determining the relative amounts of a radioactive isotope and its decay products in a mineral, the age of the mineral can be determined. Other decay reactions that are used to calculate absolute age are carbon‐14 to nitrogen‐14, potassium‐40 to argon‐40, rubidium‐87 to strontium‐87, thorium‐232 to lead‐208, and uranium‐235 to lead‐207.
An isotope's half‐life is the time it takes for half of a known quantity of radioactive material to convert to its daughter product. For example, the half‐life of U‐238 is 4.5 billion years. Thus, if you began with one gram of U‐238, 4.5 billion years later only one‐half gram would remain. After another 4.5 billion years, only one‐quarter of the original amount would remain.
Depending on the kind of isotope being analyzed, isotopic ratios are measured in single mineral crystals or larger pieces of rock.
Radiometric age dating works best on igneous, volcanic, or metamorphic rocks. It is important to select “fresh” rock that has not been chemically or structurally altered by deformation, weathering, hydrothermal alteration, or metamorphism. The calculated age is the age from which the rock or mineral stabilized. The interpretation of the age date also depends somewhat on the kind of rock being analyzed. For example, age dates from volcanic rocks that cooled quickly can give an age that is very close to the age of the eruption of the volcanic flow. Since it takes plutonic rocks millions of years to cool, the age from a mineral that formed in the pluton could be close to the age of intrusion or close to the age of the final crystallization, depending on when it formed in the intrusion. Minerals that could generate original age dates from rocks that have been metamorphosed are usually destroyed by the metamorphism; ages derived from metamorphic rocks typically indicate the time the rocks cooled and stabilized after metamorphism. Dates can usually be determined to within a few million years; in the scope of geologic time, that is a relatively small margin of error.
Absolute age dates have confirmed the basic principles of relative time—for example, a uranium‐lead date from a dike that intrudes into an older rock always yields an absolute age date that is younger than the absolute age date of the enclosing rock. Thousands of comparisons between absolute ages and relative time relationships have proven that radiometric age dating works.
Geologists have calculated the beginning and ending dates for the eons, eras, periods, and epochs of the geologic time scale, and major geologic events. To date, the oldest rocks discovered on the earth are from northwestern Canada and are 3.964 billion years old. The development of complex life in the Cambrian period occurred about 570 million years ago, or only in the last 13 percent of the earth's vast history. Age dating has permitted the age specificity of the internationally accepted geologic time scale.