What is the geologic time scale?

It is the standard framework that divides Earth's 4.5-billion-year history into named intervals based on the rock and fossil record. From largest to smallest these are eons, eras, periods, epochs and ages, defined by the International Commission on Stratigraphy.

Ma stands for mega-annum, or millions of years ago. So 66 Ma is 66 million years ago, the boundary between the Cretaceous period and the Cenozoic era. For more recent dates this tool uses ka, thousands of years ago.

Which divisions are the largest?

Eons are the largest division. The four eons are Hadean, Archean, Proterozoic and Phanerozoic. The Phanerozoic, the eon of visible life, divides into the Paleozoic, Mesozoic and Cenozoic eras, which in turn divide into periods and epochs.

When did the dinosaurs die out?

The non-avian dinosaurs went extinct at the Cretaceous–Paleogene boundary about 66 million years ago, marking the end of the Mesozoic era and the start of the Cenozoic. Entering 66 in the lookup lands right on that boundary.

Why do boundary ages change?

The International Commission on Stratigraphy revises boundary ages as radiometric dating and stratigraphic correlation improve. The figures here follow a recent published chart, but small adjustments appear in newer editions.

What is the Geologic Time Scale Reference?

ICS/IUGS geologic time scale reference from the Hadean eon to the Holocene epoch with boundary ages in millions of years, plus an age lookup that names the eon, era, period and epoch. It runs free in your browser on Gera Tools, with nothing uploaded.

Geologic Time Scale Reference

Name: Geologic Time Scale Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A framework for deep time

The geologic time scale organises Earth’s 4.5-billion-year history into a nested hierarchy of eons, eras, periods and epochs, each bounded by ages tied to the rock and fossil record. This reference lists the major divisions from the Hadean eon to the Holocene epoch with their boundary ages in millions of years, and a lookup that names the full chain of intervals containing any age you enter.

The hierarchy explained

Geologic time divides from the largest units down:

Eon  →  Era  →  Period  →  Epoch  →  Age

Eons are the broadest division, spanning hundreds of millions to over a billion years each. Earth’s four eons are:

Hadean (4,500–4,000 Ma): formation of Earth; no preserved crust; no life yet
Archean (4,000–2,500 Ma): first stable crust; earliest microbial life
Proterozoic (2,500–538.8 Ma): first complex cells; early multicellular life; two major glaciations
Phanerozoic (538.8 Ma–present): the eon of “visible life,” divided into three eras

The Phanerozoic eon, which contains all the fossil-bearing rock familiar from natural history museums, divides into:

Paleozoic (538.8–251.9 Ma): fish, amphibians, reptiles, ferns; ended by the Permian extinction
Mesozoic (251.9–66 Ma): dinosaurs, first mammals, first flowering plants
Cenozoic (66 Ma–present): mammals diversify; humans appear in the Quaternary

How it works

Each interval in the reference has an older and a younger boundary age in Ma (millions of years ago):

Eon    →  Era      →  Period       →  Epoch
Phanerozoic  Cenozoic  Quaternary   Holocene (0.0117 Ma – present)

The lookup walks down the ranks and, for the age you enter, finds the eon, era, period and epoch whose boundary band contains it. Entering 66 Ma resolves to the Phanerozoic eon, Cenozoic era, and Paleogene period — right at the Cretaceous–Paleogene boundary that ended the non-avian dinosaurs.

Key boundary ages and what happened at them

Some of the most studied boundaries in the time scale correspond to mass extinction events or major biological transitions:

Boundary	Age (Ma)	Event
Cretaceous–Paleogene	66	Non-avian dinosaur extinction; Chicxulub impactor
Permian–Triassic	251.9	Largest mass extinction; ~90% of marine species lost
Devonian–Carboniferous	358.9	Late Devonian extinction; forests appear
Ediacaran–Cambrian	538.8	Cambrian Explosion; most animal body plans appear
Archean–Proterozoic	2,500	Great Oxidation Event; first atmospheric oxygen

These boundaries are defined by the rock record — typically by the disappearance of specific fossil species or geochemical signatures — not by the events themselves.

Units of time: Ma and ka

Ma = mega-annum = millions of years ago
ka = kilo-annum = thousands of years ago

The Holocene epoch, which began at the end of the last ice age, started approximately 11,700 years ago — written as 0.0117 Ma or 11.7 ka. The most recent periods and epochs use ka for precision because their boundaries are close enough together that Ma would round them to the same value.

Boundary ages in this reference follow the ICS (International Commission on Stratigraphy) chronostratigraphic chart and are periodically revised as radiometric dating and stratigraphic correlation improve.