Geochronometry

(geology) The study of the absolute age of the rocks of the earth based on the radioactive decay of isotopes, such as ²³⁸U, ²³⁵U, ²³²Th, ⁸⁷Rb, ⁴⁰K, and ¹⁴C, present in minerals and rocks.

The measurement of the age of rocks, minerals, water, and biological materials. Measurements are based primarily on the radioactive decay or fission of such naturally occurring isotopes as ²³⁸U, ²³⁵U, ²³²Th, ¹⁸⁷Re, ¹⁷⁶Lu, ¹⁴⁷Sm, ⁸⁷Rb, ⁴⁰K, ¹²⁹I, ³⁶Cl, ²⁶Al, ¹⁴C, and ¹⁰Be. These radioactive isotopes can be divided into two groups: primordial isotopes that are residual from early nucleosynthesis, and cosmogenic isotopes that are continuously produced by cosmic-ray-induced spallation reactions primarily within the Earth's atmosphere or on the surfaces of meteorites. For the first group, the relative amounts of the radioactive parent and radiogenic daughter are used as a measure of age. Age is determined for the second group by the amount of radioactive isotope remaining after the object is isolated from further intake—for example, by death of an organism participating in the carbon-oxygen cycle, or by trapping of the cosmogenic isotope in sediment or ice. Tree-ring dating (dendrochronology), which is based on the counting of annual rings, may also be used and provides a very precise measure of age of the last eight millennia. See also Dendrochronology; Lead isotopes (geochemistry); Nucleosynthesis; Radiocarbon dating.

There are also methods of establishing the relative sequence of events in time, most importantly, the use of unidirectional biologic evolution upon which the boundaries of the Phanerozoic time scale are based (5.5 × 10⁸ years to the present). The virtues of isotopic dating are its applicability to the full range of geologic time, including the Precambrian for which an adequate paleontologic time scale does not exist; better resolution of events during the Cenozoic (6.5 × 10⁷ years to present); and provision of the fourth physical dimension of astronomic time to quantify rates and energies involved in geologic processes. These isotopic chronometers have been used to measure the age of the Earth, Moon, and meteorites (4.5 × 10⁹ years), the age of the oldest datable rocks (3.7 × 10⁹ years), and many other significant geologic events such as the advance and retreat of continental glaciers. They have also been used to establish a Precambrian time scale, to calibrate the Phanerozoic time scale in solar years, and to provide a chronology for significant biologic, cultural, and environmental events related to the evolution of the human race. On a much shorter time scale, these methods have been used to determine rates of flow of water through aquifers and rates of material (aerosols) transport through the atmosphere. See also Amino acid dating; Dating methods; Geologic time scale; Rock age determination.