December 4, 2019

Geologic Time

Geologists have a bunch of words for geologic time periods. Here is a quick guide, along with some ideas of what these mean in terms of years. These dates are almost certainly subject to revision and differences of opinion.

First some big divisions:

Phanerozoic -- 543 my to present
Precambrian -- 3800 my to 543 my
Hadean -- pre 3800 my
You almost never hear the term Hadean used. The current estimate for the age of the earth is 4600 my. The Precambrian is often used and is sometimes broken down as follows:
Proterozoic  2500 to 543 my
Archaen  -- 3800 to 2500 my
These can be broken up as follows, but rarely are:
Late Proterozoic -- 900 to 543 my
Middle Proterozoic -- 1600 to 900 my
Early Proterozoic -- 2500 to 1600 my
Late Archean -- 3000 to 2500 my
Middle Archean -- 3400 to 3000 my
Early Archean -- 4600 to 3400 my
You don't often hear the word "Phanerozoic" used as geologists tend to be much more specific. There are 3 big divisions of the Phanerozoic:
Cenozoic -- 65 my to present
Mesozoic -- 248 to 65 my
Paleozoic -- 543 my to 248 my
The Paleozoic gets broken up as follows:
Permian -- 290 to 248 my
Pennsylvanian -- 323 to 290 my
Mississippian -- 354 to 323 my
Devonian -- 417 to 354 my
Silurian -- 443 to 417 my
Ordovician -- 490 to 443 my
Cambrian -- 543 to 490 my
The Mesozoic gets broken up as follows:
Cretaceous -- 144 to 65 my
Jurassic -- 206 to 144 my
Triassic -- 248 to 206 my
The Cenozoic gets broken up as follows:
Holocene -- 0.01 my to present
Pleistocene -- 1.8 to 0.01 my
Pliocene -- 5.3 to 1.8 my
Miocene -- 23.7 to 5.3 my
Oligocene -- 33.7 to 23.7 my
Eocene -- 54.8 to 33.7 my
Paleocene -- 65 to 54.8 my
The Holocene and Pleistocene are referred to together as Quarternary.
The rest of the Cenozoic gets referred to as Tertiary.

Sometimes Miocene and Pliocene get lumped together as "Neogene", but this is rare.
Similarly, Paleocene, Eocene, and Oligocene rarely get lumped as "Paleogene".

There is always some individual eager to use some seldom used term in an effort to try to impress you.

Taken from page 14 of "Aerial Geology" by Mary Caperton Morton.


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Tom's Geology Info / tom@mmto.org