---
title: "Geologic time scale"
chunk: 5/7
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"
---

=== Modern international geological time scale ===
The establishment of the IUGS in 1961 and acceptance of the Commission on Stratigraphy (applied in 1965) to become a member commission of IUGS led to the founding of the ICS. One of the primary objectives of the ICS is "the establishment, publication and revision of the ICS International Chronostratigraphic Chart which is the standard, reference global Geological Time Scale to include the ratified Commission decisions".
Following on from Holmes, several A Geological Time Scale books were published in 1982, 1989, 2004, 2008, 2012, 2016, and 2020. However, since 2013, the ICS has taken responsibility for producing and distributing the ICC citing the commercial nature, independent creation, and lack of oversight by the ICS on the prior published GTS versions (GTS books prior to 2013) although these versions were published in close association with the ICS. Subsequent Geologic Time Scale books (2016 and 2020) are commercial publications with no oversight from the ICS, and do not entirely conform to the chart produced by the ICS. The ICS produced GTS charts are versioned (year/month) beginning at v2013/01. At least one new version is published each year incorporating any changes ratified by the ICS since the prior version.
The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

== Table of geologic time ==
The following table summarises the major events and characteristics of the divisions making up the geologic time scale of Earth. This table is arranged with the most recent geologic periods at the top, and the oldest at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time. As such, this table is not to scale and does not accurately represent the relative time-spans of each geochronologic unit. While the Phanerozoic Eon looks longer than the rest, it merely spans ~538.8 Ma (~11.8% of Earth's history), whilst the previous three eons collectively span ~4,028.2 Ma (~88.2% of Earth's history). This bias toward the most recent eon is in part due to the relative lack of information about events that occurred during the first three eons compared to the current eon (the Phanerozoic). The use of subseries/subepochs has been ratified by the ICS.
While some regional terms are still in use, the table of geologic time conforms to the nomenclature, ages, and colour codes set forth by the International Commission on Stratigraphy in the official International Chronostratigraphic Chart.

== Major proposed revisions to the ICC ==

=== Proposed Anthropocene Series/Epoch ===

First suggested in 2000, the Anthropocene is a proposed epoch/series for the most recent time in Earth's history. While still informal, it is a widely used term to denote the present geologic time interval, in which many conditions and processes on Earth are profoundly altered by human impact. The definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult.
In May 2019 the Anthropocene Working Group voted in favour of submitting a formal proposal to the ICS for the establishment of the Anthropocene Series/Epoch. The formal proposal was completed and submitted to the Subcommission on Quaternary Stratigraphy in late 2023 for a section in Crawford Lake, Ontario, with heightened Plutonium levels corresponding to 1952 CE. This proposal was rejected as a formal geologic epoch in early 2024, to be left instead as an "invaluable descriptor of human impact on the Earth system"

=== Proposals for revisions to pre-Cryogenian timeline ===

==== Shields et al. 2021 ====
The ICS Subcommission for Cryogenian Stratigraphy has outlined a template to improve the pre-Cryogenian geologic time scale based on the rock record to bring it in line with the post-Tonian geologic time scale. This work assessed the geologic history of the currently defined eons and eras of the Precambrian, and the proposals in the "Geological Time Scale" books 2004, 2012, and 2020. Their recommend revisions of the pre-Cryogenian geologic time scale were as below (changes from the current scale [v2023/09] are italicised). This suggestion was unanimously rejected by the International Subcommission for Precambrian Stratigraphy, based on scientific weaknesses.

Three divisions of the Archean instead of four by dropping Eoarchean, and revisions to their geochronometric definition, along with the repositioning of the Siderian into the latest Neoarchean, and a potential Kratian division in the Neoarchean.
Archean (4000–2450 Ma)
Paleoarchean (4000–3500 Ma)
Mesoarchean (3500–3000 Ma)
Neoarchean (3000–2450 Ma)
Kratian (no fixed time given, prior to the Siderian) – from Greek κράτος (krátos) 'strength'.
Siderian (?–2450 Ma) – moved from Proterozoic to end of Archean, no start time given, base of Paleoproterozoic defines the end of the Siderian
Refinement of geochronometric divisions of the Proterozoic, Paleoproterozoic, repositioning of the Statherian into the Mesoproterozoic, new Skourian period/system in the Paleoproterozoic, new Kleisian or Syndian period/system in the Neoproterozoic.
Paleoproterozoic (2450–1800 Ma)
Skourian (2450–2300 Ma) – from Greek σκουριά (skouriá) 'rust'.
Rhyacian (2300–2050 Ma)
Orosirian (2050–1800 Ma)
Mesoproterozoic (1800–1000 Ma)
Statherian (1800–1600 Ma)
Calymmian (1600–1400 Ma)
Ectasian (1400–1200 Ma)
Stenian (1200–1000 Ma)
Neoproterozoic (1000–538.8 Ma)
Kleisian or Syndian (1000–800 Ma) – respectively from Greek κλείσιμο (kleísimo) 'closure' and σύνδεση (sýndesi) 'connection'.
Tonian (800–720 Ma)
Cryogenian (720–635 Ma)
Ediacaran (635–538.8 Ma)
Proposed pre-Cambrian timeline (Shield et al. 2021, ICS working group on pre-Cryogenian chronostratigraphy), shown to scale:

ICC pre-Cambrian timeline (v2024/12, current as of January 2025), shown to scale: