The language of geology, like most fields of science, is initially quite opaque on first encounter.  But the terminology of geological periods and eras can be especially baffling.  Terms such as "Cambrian," "Ordovician," "Devonian," etc. have a history that is itself quite fascinating.  As might be deduced from those names, much of the terminology originated in the British Isles and reflects the regions where outcrops of rock formations were first studied.  Others are totally improbable, such as the Permian, named for the Perm region in Russia.

Students quickly learn the important terms and their subdivisions as part of the business of doing geology.  It is a relatively easy matter to learn some of the characteristics of the major geological eras and periods—the Cambrian was a time of creepy-crawly creatures, the Jurassic was a time of gigantic dinosaurs, the Cenozoic was the time when the mammals finally got their chance; but the finer subdivisions may vary from continent to continent.  The broadest division is the eon—for example the Phanerozoic: the time when life has occupied the planet.  The next division is the era—for example the Paleozoic: the time of primitive life.  (You will discern here a bit of the anthropocentric bias, where the entire history of the Earth was presumed to be directed toward forming its most magnificent creature, Man.)  The next division is the period—for example the Cambrian and Ordovician of the early Paleozoic: times when slithering and scurrying creatures first appeared.  Next is the epoch.  For the earlier phases of the Earth's history these are simple designations like "early." "middle," and "late."  In the Tertiary period, which preceded the present Quaternary period, the epochs become more ostentatious, particularly the "Paleocene," "Eocene," "Oligocene," "Miocene," and "Pliocene."  The time intervals became finer and finer as they got closer to the present, and we know more about them.  The Cenozoic was when the mammals were given their chance at evolving after the extinction of the dinosaurs.  By the Quaternary the mammals had almost attained their present forms, so there are two epochs: the "Pleistocene," or time of the ice ages, and the "Holocene," or recent times.  The epochs are further divided into ages, which are usually only of regional significance.

The divisions of periods and epochs originally came about because they were marked by discontinuities in the geologic and fossil records.  In some cases this is a consequence of physical processes, such as the separation of two continents, the formation of mountain ranges, or a change in sea level.  Most divisions are marked by discontinuities in the fossil records.  Many of those have since been found to be the result of Mass Extinction Events, in which huge numbers of life forms suddenly disappeared, to be replaced with something totally different ( See the time line for the history of the Earth.).  Where the fossil record showed everywhere a complete turnover of the forms of life, that was considered the transition from one era to another.  The two most spectacular turnovers were at the end of the Palezoic (or "early life") and the end of the Mesozoic (or "middle life").  The Cretaceous extinction event at the end of the Mesozoic seems to have been caused by the impact of a comet or asteroid with the Earth.  That comet completely annihilated the dinosaurs, and has caught the public imagination.  The Permian extinction event at the end of the Paleozoic wiped out some very interesting creatures, but it is not so popular; its cause remains puzzling.

The following table shows the important subdivisions, with approximate ages.  The ages in the left column are the onset times.  The relative onset times are important because they often mark an abrupt change in the environment or life forms.  The values of the cited ages aren't extremely important; they suggest mainly that the Earth is very old.  So it is wise not to be too finicky about the ages, because they tend to be revised occasionally, and may differ according to who is using them, eg. the American Geological Society or the Geological Society of London.  The ages have been taken from the most recent publications of the Geological Society of London.

While the term "Precambrian" is still in common use, it has more properly been divided into several eons, whose proper terminology is "Archaen" and "Proterozoic".

My apologies if the colors in the following chart do not entirely match the more-or-less standard colors of geological maps.  For comparison I have provided a chart of more-or-less standard geological map colors.

Printable chart
TIFF image of chart

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The big events of Earth's history

Climate models

A technical note on the determination of periodic cycles from long trains of data

References and further reading