The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronology (scientific branch of geology that aims to determine the age of rocks). It is used primarily by Earth scientists (including geologists, paleontologists, geophysicists, geochemists, and paleoclimatologists) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardized international units of geologic time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective[1] is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC)[2] that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units.[2]
While some regional terms are still in use,[3] the table of geologic time presented in this article conforms to the nomenclature, ages, and color codes set forth by the ICS as this is the standard, reference global geologic time scale – the International Geological Time Scale.[1][4]
See also: Age of Earth, History of Earth, and Geological history of Earth
The geologic time scale is a way of representing deep time based on events that have occurred throughout Earth's history, a time span of about 4.54 ± 0.05 Ga (4.54 billion years).[5] It chronologically organizes strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the Cretaceous–Paleogene extinction event, marks the lower boundary of the Paleogene System/Period and thus the boundary between the Cretaceous and Paleogene Systems/Periods. For divisions prior to the Cryogenian, arbitrary numeric boundary definitions (Global Standard Stratigraphic Ages, GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record.[6][3]
Historically, regional geologic time scales were used[3] due to the litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardizing globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The ICC represents this ongoing effort.
The relative relationships of rocks for determining their chronostratigraphic positions use the overriding principles of:
Superposition – Newer rock beds will lie on top of older rock beds unless the succession has been overturned.
Horizontality – All rock layers were originally deposited horizontally.[note 1]
Lateral continuity – Originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.
Biologic succession (where applicable) – This states that each stratum in a succession contains a distinctive set of fossils. This allows for correlation of stratum even when the horizon between them is not continuous.
Cross-cutting relationships – A rock feature that cuts across another feature must be younger than the rock it cuts.
Inclusion – Small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming.
Relationships of unconformities – Geologic features representing periods of erosion or non-deposition, indicating non-continuous sediment deposition.