Earth's history is divided into eons, which are divided into eras, which are divided into periods etc. Consecutive time units differ in their geology and/or fossil species. But what events are the basis for dividing time into discrete bands?

Roughly speaking (forgive the approximation), the Hadean is the time before life, the Archean is the subsequent time before eukaryotes, the Proterozoic is the subsequent time before complex multicellular animals, and the Phanerozoic is the time since then. Yes, I know that the actual definitions are in terms of either what can be seen in the rocks or a certain number of Mya; and yes, I know the Ediacaran biota technically lived in the late Proterozoic; but, if I've understood correctly, the reason we separate these four parts of Earth's history is because of such distinctions. We can similarly divide the Phanerozoic into three eras based on the P-T and K-Pg mass extinctions. (However, the other three of the Big Five extinction events define the start of new periods rather than eras.)

Is there a more thorough list than this of the qualitative events and processes in Earth's history that motivate the specific ways we've defined eons, eras and periods? I realise there may be some arbitrariness in it, e.g. the fact that some mass extinctions "only" start a new period instead of a new era. However, I'll be satisfied with a summary of the key moments on which these divisions are based.

  • $\begingroup$ Probably more appropriate for HSM, since it's mostly an artifact of earlier geologists' trying to correlate rock layers. $\endgroup$
    – Spencer
    Mar 30, 2017 at 4:14
  • $\begingroup$ @Spencer I take your point, but even if a particular decision was made without understanding why there was a sudden banding in rocks, it may be possible with hindsight to say, "these rock changes happened for reason X". If anyone does migrate this question to HSM, I hope people there offer the most "up-to-date" division criteria, even if they also provide the original ones. $\endgroup$
    – J.G.
    Mar 30, 2017 at 7:44
  • 2
    $\begingroup$ I think this question is a good fit for Earth Science. $\endgroup$
    – gerrit
    Mar 30, 2017 at 10:42

1 Answer 1


The divisions in the geologic time scale have evolved over time. Its origins can be traced back to Nicolaus Steno in 1669 described two basic geologic principles.

  1. The first stated that sedimentary rocks are laid down in a horizontal manner.
  2. The second stated that younger rock units were deposited on top of older rock units.

Work by James Hutton, Charles Lyell and William Smith helped define "principle of uniformitarianism". This work led to definitions in the Mesozoic and Paleozoic eons in Europe and Great Britain.

The Mesozoic and Paleozoic eons helped define the extinction events reflected in the fossil record. It was these correlations that led to the discovery of the extinction events and not the other way around.

Radiometric dating based on the work of Clair Patterson added absolute ages to the time scale.

Over time, the process of grouping together or splitting apart rock units has grown more sophisticated as better dating techniques have been introduced and more data from mapped rock outcrops have been collected analysed.





  • $\begingroup$ I know this can be formulated as another question but how absolute are the radiometric dating mechanisms basing on accuracy? $\endgroup$ Mar 31, 2017 at 5:27
  • $\begingroup$ Accuracy with radiometric dating can be tricky. Solid lab technique, known relationship between isotopes, and the samples must meet the require of being a closed system (meaning no chemical input/output within the rock that would alter parent and daughter isotopes ratios. I think radiometric dating is useful but often viewed that it is 100% accurate. $\endgroup$ Mar 31, 2017 at 11:40

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