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.

Geologic Time Scale Reference

Email me this result

Get this tool's output sent to your inbox, plus one useful tool a week. No spam, unsubscribe any time.

A framework for deep time

The geologic time scale organises Earth’s 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.

How it works

Intervals nest from largest to smallest, and each is defined by 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 (younger, older] age band contains it. So 66 Ma resolves to the Phanerozoic eon, Cenozoic era and Paleogene period, right at the boundary that ended the age of dinosaurs.

Tips and notes

Ma = millions of years ago; ka = thousands of years ago.
The four eons are Hadean, Archean, Proterozoic and Phanerozoic.
The Phanerozoic (“visible life”) splits into Paleozoic, Mesozoic, Cenozoic.
The Cretaceous–Paleogene boundary at 66 Ma ended the non-avian dinosaurs.
Boundary ages follow the ICS chart and are revised as dating improves.