geochronology Determination of time intervals on a geologic scale, through either absolute or relative dating methods. Absolute dating methods involve the use of radioactive elements and knowledge of their rates of decay: this yields an actual age in years for a given rock or fossil. Relative dating involves the use of fossils or sediments to place events and rock sequences in order, and does not provide absolute dates. See also DATING METHODS; ABSOLUTE AGE; RELATIVE AGE; PLANKTONIC GEOCHRONOLOGY; and GEOCHRONOMETRY.

geochronometric scale(chronometric scale) A time-scale based on years bp (conventionally before 1950). Subdivisions on the scale are defined by particular units of duration (e.g. 10⁶ years, 10⁹ years) rather than reference points in actual rock successions. An example of such a subdivision is the placing of the boundary between the Archaean and the Proterozoic at 2500 Ma (i.e. 2500 × 10⁶ years) ago.

geochronometric scale (chronometric scale) A time-scale based on years BP (conventionally before 1950). Subdivisions on the scale are defined by particular units of duration (e.g. 10⁶ years, 10⁹ years) rather than reference points in actual rock successions. An example of such a subdivision is the placing of the boundary between the Archaean and the Proterozoic at 2500 Ma (i.e. 2500 × 10⁶ years) ago.

geochronometry The determination of the length of time intervals. Geochronometric resolutions for zonations based on different organisms may be calculated by dividing the time-span of a series by the number of zones and the intervals between zones. However, this will give only an approximate measure of time. See also DATING METHODS; and GEOCHRONOLOGY.

geochronology The determination of time intervals on a geologic scale, through either absolute or relative dating methods. Absolute dating methods involve the use of radioactive elements and knowledge of their rates of decay: this yields an actual age in years for a given rock or fossil. Relative dating involves the use of fossils or sediments to place events and rock sequences in order, and does not provide absolute dates.