---
title: "Geologic time scale"
chunk: 2/7
source: "https://en.wikipedia.org/wiki/Geologic_time_scale"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:46:38.551417+00:00"
instance: "kb-cron"
---

=== Global Boundary Stratotype Section and Point (GSSP) ===
Historically, regional geologic time scales were used due to the litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. A Global Boundary Stratotype Section and Point (GSSP) defines the lower boundary of a stage as being at a precise point in a specific rock succession in a particular geographic location. These reference points are known informally as "golden" spikes. All the beds above the spike belong to one time interval and all those below it to another. This allows beds of a similar age around the world to be correlated with the strata that contain the golden spike. For example, the iridium anomaly produced by the Chicxulub asteroid impact marks the lower boundary of the Paleogene System and thus the boundary between the Cretaceous and Paleogene. Whilst the GSSP is defined at Oued Djerfane in Tunisia, strata containing the iridium anomaly are found worldwide.
The Proterozoic (apart from the Ediacaran), Archean and Hadean are subdivided by absolute ages (Global Standard Stratigraphic Ages) rather than geological features. Proposals have been made to better reconcile these divisions with the rock record.

== Divisions of geologic time ==

The standard international units of the geologic time scale are published by the International Commission on Stratigraphy on the International Chronostratigraphic Chart. However, regional terms are still in use in some areas. The numeric values on the International Chronostratigraphic Chart are represented by the unit Ma (megaannum, for 'million years'). For example, 201.4 ± 0.2  Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other SI prefix units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with the latter often represented in calibrated units (before present).
The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units:

An eon is the largest geochronologic time unit and is equivalent to a chronostratigraphic eonothem. There are four formally defined eons: the Hadean, Archean, Proterozoic and Phanerozoic.
An era is the second largest geochronologic time unit and is equivalent to a chronostratigraphic erathem. There are ten defined eras: the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean, Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic and Cenozoic, with none from the Hadean eon.
A period is equivalent to a chronostratigraphic system. There are 22 defined periods, with the current being the Quaternary period. As an exception, two subperiods are used for the Carboniferous Period.
An epoch is the second smallest geochronologic unit. It is equivalent to a chronostratigraphic series. There are 37 defined epochs and one informal one. The current epoch is the Holocene. There are also 11 subepochs which are all within the Neogene and Quaternary. The use of subepochs as formal units in international chronostratigraphy was ratified in 2022.
An age is the smallest hierarchical geochronologic unit. It is equivalent to a chronostratigraphic stage. There are 96 formal and five informal ages. The current age is the Meghalayan.
A chron is a non-hierarchical formal geochronology unit of unspecified rank and is equivalent to a chronostratigraphic chronozone. These correlate with magnetostratigraphic, lithostratigraphic, or biostratigraphic units as they are based on previously defined stratigraphic units or geologic features.

The subdivisions Early and Late are used as the geochronologic equivalents of the chronostratigraphic Lower and Upper, e.g., Early Triassic Period (geochronologic unit) is used in place of Lower Triassic System (chronostratigraphic unit).

== Naming of geologic time ==
The names of geologic time units are defined for chronostratigraphic units with the corresponding geochronologic unit sharing the same name with a change to the suffix (e.g. Phanerozoic Eonothem becomes the Phanerozoic Eon). Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (new life). Names of systems are diverse in origin, with some indicating chronologic position (e.g., Paleogene), while others are named for lithology (e.g., Cretaceous), geography (e.g., Permian), or are tribal (e.g., Ordovician) in origin. Most currently recognised series and subseries are named for their position within a system/series (early/middle/late); however, the International Commission on Stratigraphy advocates for all new series and subseries to be named for a geographic feature in the vicinity of its stratotype or type locality. The name of stages should also be derived from a geographic feature in the locality of its stratotype or type locality.
Informally, the time before the Cambrian is often referred to as the Precambrian or pre-Cambrian (Supereon).

== History of the geologic time scale ==