How is geological time divided into units?

Question

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.

Answer 1

But what events are the basis for dividing time into discrete bands?

You seem to have answered your own question here. Geology and/or fossil species distinguish consecutive time units. The geologic time scale organizes strata by observing changes in stratigraphy that correlate with major paleontological or geological events.

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?

Periods are subdivided into epochs, which are then divided into ages, which are further divided into chrons.

An epoch is the equivalent of a chronostratigraphic series. An age is the smallest geochronologic unit and is equal to a chronostratigraphic stage. An age, a non-hierarchical unit, is the equivalent of a chronostratigraphic chronozone.

However, I'll be satisfied with a summary of the key moments on which these divisions are based.

Hadean, 4600 Ma:

Formation of Earth (c. 4540 Ma), Formation of Moon (c. 4527 Ma)

Archean, 4000 Ma:

End of Late Heavy Bombardment; first life

Proterozoic, c. 2500 Ma:

First major rise in atmospheric oxygen, Huronian glaciation (c. 2300 Ma)

Phanerozoic, c. 538.8 Ma:

Cambrian explosion

First vertebrate land animals (c. 395 Ma)