geologic timescale

geologic timescale a chronological scale of earth's history used to measure the relative or absolute age of any part of geologic time. Of the numerous timescales, the most common is based on geologic time units, which divide time into eras, periods, and epochs. Each division is based on a specific set of geological or paleontological conditions that make it different from the other divisions, such as varying rock type or fossils within the strata. The largest unit is the eon; eons are subdivided into eras; eras into periods; and some, usually more recent periods, into epochs. In some timescales, epochs are further divided into ages. Each geologic time chart varies, depending on the latest findings dating rocks and fossils of that particular age, or on the country where the chart originated. For each unit and its subdivisions, which are listed in the table entitled Geologic Timescale , see separate articles.

geologic time-scale A twofold scale that subdivides all the time since the Earth first came into being into named units of abstract time, and subdivides all the rocks formed since the Earth came into being into the successions of rock formed during each particular interval of time. The branch of geology that deals with the age relations of rocks is known as chronostratigraphy. The concept of a geologic time-scale has been evolving for the last century and a half, commencing with a relative time-scale (mainly achieved through biostratigraphy), to which it has gradually become possible to assign dates which are, nonetheless, subject to constant revision and refinement. Since the first International Geological Congress in Paris in 1878, one of the main objectives of stratigraphers has been the production of a complete and globally accepted stratigraphic scale to provide a historical framework into which all rocks, anywhere in the world, can be fitted. Such a standard scale is still a long way off, but the names for geologic-time units and chronostratigraphic units down to the rank of period/system are in common use; many epoch/series and age/stage names are still regionally variable.

geologic time-scale A two-fold scale that subdivides all the time since the Earth first came into being into named units of abstract time, and subdivides all the rocks formed since the Earth came into being, into the successions of rock formed during each particular interval of time. The branch of geology that deals with the age relations of rocks is known as chronostratigraphy. The concept of a geologic time-scale has been evolving for the last century and a half, commencing with a relative time-scale (mainly achieved through biostratigraphy), to which it has gradually become possible to assign dates (see DATING METHODS) which are, nonetheless, subject to constant revision and refinement. Since the first International Geological Congress in Paris in 1878, one of the main objectives of stratigraphers has been the production of a complete and globally accepted stratigraphic scale to provide a historical framework into which all rocks, anywhere in the world, can be fitted (see STANDARD STRATIGRAPHIC SCALE; CHRONOSTRATIGRAPHIC SCALE; UNIFIED STRATIGRAPHIC SCALE). Such a standard scale is still a long way off, but the names for geologic-time units and chronostratigraphic units down to the rank of period/system are in common use; many epoch/series and age/stage names are still regionally variable. See Time Scales gives an outline geologic time-scale employing currently common names and dates (though they are not necessarily universally accepted).