geological time scale
The geological time scale, geological time scale or international chronostratigraphic table is the reference framework to represent the events of the history of the Earth and of life ordered chronologically. It establishes divisions and subdivisions of the rocks according to their relative age and the absolute time that has elapsed since the formation of the Earth to the present, in a double dimension: stratigraphic (superimposition of rocks) and chronological (course of time). These divisions are mainly based on the faunal changes observable in the fossil record and have been dated with some precision by radiometric methods. The scale compiles and unifies the results of work on historical geology carried out over several centuries by naturalists, geologists, paleontologists, and many other specialists. Since 1974, the formal elaboration of the scale has been carried out by the International Commission of Stratigraphy of the International Union of Geological Sciences and the changes, after some years of studies and deliberations by specific subcommissions, have to be ratified in world congresses.
The scale is composed of the combination of:
Chronostratigraphic units (floor, series, system, erathem, eonotem), which correspond to groups of rocks, stratified or not, formed during a given time interval. They are based on variations in the fossil (biostratigraphy) and stratigraphic (lithostratigraphy) records. They are the units with which the divisions of the standard chronostratigraphic scale have been established for the Phanerozoic (and the Ediacaran and the Cryogenic of the Precambrian). They serve as reference material support.
Geochronological units (age, epoch, period, era, eon), units of time equivalent one to one with the chronostratigraphic ones. They are the relative temporal reference of the scale for the Phanerozoic.
Geochronometric units, defined by absolute ages (time in millions of years). They are the units with which the divisions of the scale have been established for the Precambrian (except the Ediacaran and the Cryogenic). The absolute dates shown on the scale for the Phanerozoic and the Ediacaran are under review, and those that do not have a formalized lower limit stratotype are approximate, so they cannot be considered geochronometric units. The basic unit of the scale is the floor (and its equivalent age), normally defined by changes detected in the fossil record and, occasionally, supported by paleomagnetic changes (polarity inversions of the earth's magnetic field), lithological changes due to climatic changes, tectonic effects or rises or falls in sea level. The higher rank units reflect the most significant changes in past faunas inferred from the fossil record (Paleozoic or Mesozoic), lithological characteristics of the region where they were defined (Carboniferous, Triassic or Cretaceous) and more rarely paleoclimatic aspects (Cryogenic). Many names refer to the place where the reference stratigraphic successions were established or initially studied (Permian or Maastrichtian).
For certain subdivisions of the scale, “Lower” and “Upper” are used if referring to chronostratigraphic units (rock bodies) or “Early” and “Late” if referring to geochronological units (time). In both cases, the name of the corresponding unit of higher rank is added before it, as in Upper Triassic (series) and Late Triassic (epoch).
History and nomenclature of the time scale
In ancient Greece, Aristotle (384-322 BC) observed that fossil seashells in rocks resembled those found on beaches: he inferred that fossils in