Chapter contents:

Geological time– 1. Relative period dating – 2. Absolute age dating– 3. Geology time range ←– 4. Geological maps

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The geological time scale. Image by Jonathan R. Hendricks.  This work is license is granted under a an innovative Commons Attribution-ShareAlike 4.0 worldwide License.

The geological time scale--shown over in a simplified form--is one of the crowning achievements of science in general and also geology in particular. It is a reference and also communication device for comparing rocks and also fossils from transparent the world and also is geology"s tantamount of the periodic table that the elements. Because that example, a paleontologist can speak to her colleague and say, "I just discovered an awesome new trilobite native the Devonian of new York" and her partner will instantly understand as soon as in geological time that trilobite lived.

Most that the borders on the geological time range correspond come the origination or die out of specific kinds that fossils. Knowing when significant groups of fossils very first appeared or go extinct is as such incredibly helpful for identify the eras of rocks in the field. Because that example, if you discover a rock through a trilobite fossil ~ above it, friend will automatically know that the absent is Paleozoic in period (541 Ma to 252 Ma) and also not older or younger; knowing the species of trilobite allows even higher precision.

This relates to a 3rd important principle of relative period dating (see ar 2.1 for the various other two): the rule of faunal succession. Faunal succession is the rule that various kinds the fossils characterize various intervals of time. This is because evolution and extinction are facts the nature.

The principle of faunal sequence was emerged by an English surveyor called William "Strata" blacksmith (1769-1839). Together he studied layers the rocks to identify where to build canals, that noticed that he uncovered the exact same ordering the fossil species from ar to place; Fossil A was constantly found below Fossil B, which in turn was always found listed below Fossil C, and also so on. Through documenting these sequences of fossils, Smith had the ability to temporally correlate rock layers (or, strata) from ar to place (in various other words, to develop that absent layers in two different places are indistinguishable in period based top top the reality that they incorporate the same species of fossils). Temporal correlation permitted Smith to construct the an initial geological map (see ar 2.4) of whole country.

Temporal correlation has actually made many world very, very rich by permitting them come predict the areas of beneficial geological resources such together fossil fuels. Much more generally, it has allowed us to reconstruct the geological history of planet by comparing rocks and also fossils from ar to place. This is critically important since no single place on earth preserves a finish geological history, or even a small fraction of it.

The geological time scale offers a an international summary of many small-scale temporal correlations of rock layers made at local and regional scales. The is based almost entirely upon mindful observations of the distribution of fossils in time and also space.

Learning the geology time scale

Because the its usefulness for communicating around events in Earth"s history, it is necessary that all students the geology, paleontology, and also evolutionary biology walk the geology time range to memory. This is most quickly done by first breaking the moment scale into its ingredient parts: eons, eras, periods, and epochs.


The eon is the broadest category of geological time. Earth"s background is identified by 4 eons; in bespeak from earliest to youngest, these room the Hadeon, Archean, Proterozoic, and Phanerozoic. Collectively, the Hadean, Archean, and Proterozoic are occasionally informally described as the "Precambrian." (The Cambrian period defines the beginning of the Phanerozoic eon; so, every rocks older 보다 the Cambrian space Precambrian in age.)

We live during the Phanerozoic, which method "visible life." This is the interval of geological time characterized by abundant, complicated fossilized remains. Gift the youngest eon the time, it is also really well stood for by absent at Earth"s surface (because the the rule of Superposition; see ar 1). Since of these 2 factors, most paleontologists and geologists study fossils and rocks native the Phanerozoic eon.

Do no let the time scale at the peak of this page provide you a false impression, however, around the temporal term of the Phanerozoic eon family member to the 3 older Precambrian eons. Note in the figure listed below the absolute periods of the boundaries separating every eon the time.


Left: the 4 eons of geological time. Right: the "Precambrian" eons (Hadean, Archean, and also Proterozoic) represent 88% of geological time. Image through Jonathan R. Hendricks.  This job-related is license is granted under a an innovative Commons Attribution-ShareAlike 4.0 worldwide License.

You have already learned the the planet is 4.54 billion years old. The Phanerozoic eon started 541 million years earlier (or, 0.541 billion year ago). Thus, the Phanerozoic eon to represent a paltry 12% that Earth"s history! Instead, most of Earth"s background is represented by the three Precambrian eons. This older eons tell the story of Earth"s beginning, life"s origin, and also the climb of complicated life.

The Hadean and also Archean are an overwhelming eons to study, however, because they are exposed in very limited places ~ above Earth"s surface. (Since they space the earliest eons, rocks that are Hadean and also Archean in age are often hidden far listed below younger rocks at Earth"s surface.) Proterozoic rocks--which span practically 2 billion years (42% of Earth"s history)--are much more accessible, but, till recently, have actually received considerably less fist from paleontologists than rocks indigenous the younger, fossil-rich Phanerozoic eon. That is slowly start to change, however, as an ext clues around the beginnings of complex life start to be revealed indigenous Proterozoic-aged rocks.


The three ages of the Phanerozoic eon. Photo by Jonathan R. Hendricks.  This job-related is license is granted under a creative Commons Attribution-ShareAlike 4.0 international License.

Most that our understanding of the fossil record comes from the three eras of the Phanerozoic eon. The Paleozoic ("old life") era is characterized by trilobites, the first four-limbed vertebrates, and the origin of soil plants. The Mesozoic ("middle life") era represents the "age the dinosaurs," though likewise is remarkable for the an initial appearances the mammals and also flowering plants. Finally, the Cenozoic ("new life") era is sometimes referred to as the "age the mammals" and is the era during which us live today.

As temporal clues of reference, the is precious memorizing the periods of the boundaries that separate the three periods of the Phanerozoic eon. Long prior to geologists knew this absolute period dates, castle realized that the boundaries represent necessary events in the background of life: mass extinctions. For example, countless fossils the are generally found in the youngest Paleozoic rocks are not discovered in overlying Mesozoic rocks. Similarly, dinosaur fossils uncovered in the youngest Mesozoic rocks are never ever again uncovered in the overlying Cenozoic rocks. Paleontologists and also geologists supplied these massive extinction events to define this (and other) limits within the Phanerozoic part of the geological time scale. It is as such no simultaneous that few of the significant boundaries coincide v mass die out events!

The older Archean and also Proterozoic eons are likewise divided into several eras. Because that example, the youngest era the the Proterozoic eon is called the Neoproterozoic. Because that the services of simplicity, these older ages are not consisted of on the time scale displayed at the top of this page; lock do, however, exist!


Just as eons space subdivided right into eras, ages are subdivided into units the time dubbed periods. The most famed of every geological durations is the Jurassic period of the Mesozoic era (the movie Jurassic Park, the course, has other to do with that).

The Paleozoic era is divided into 6 periods. From earliest to youngest, these room the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Note that in the unified States, the Carboniferous is separated into two separate periods: the Mississippian and the Pennsylvanian.

The seven periods of the Paleozoic era. Photo by Jonathan R. Hendricks.  This work is licensed under a an imaginative Commons Attribution-ShareAlike 4.0 global License.

The three periods of the Mesozoic era. Picture by Jonathan R. Hendricks.  This work-related is license is granted under a an innovative Commons Attribution-ShareAlike 4.0 worldwide License.

The three periods of the Cenozoic era. Photo by Jonathan R. Hendricks.  This work-related is licensed under a an imaginative Commons Attribution-ShareAlike 4.0 international License.

Epochs and also Ages

Periods of geology time space subdivided right into epochs. In turn, dates are split into even narrower devices of time dubbed ages. Because that the sake of simplicity, just the dates of the Paleogene, Neogene, and also Quaternary durations are presented on the time scale at the top of this page. The is crucial to note, however, that all of the durations of the Phanerozoic era are subdivided right into the epochs and ages.

The Paleogene duration is split into--from earliest to youngest--the Paleocene, Eocene, and also Oligocene epochs. The Neogene is split into the Miocene and Pliocene epochs. Finally, the Quaternary is separated into the Pleistocene and also Holocene epochs. Some geologists now think that--since people are having such a notable affect on the Earth and also its life--a new, youngest date should be included to the Quaternary: the Anthropocene. Over there is still considerable discussion in the geology community around whether this epoch should it is in added, and debate about what qualities should define its beginning.

Epochs the the Paleogene, Neogene, and Quaternary periods.Image by Jonathan R. Hendricks.  This work-related is licensed under a creative Commons Attribution-ShareAlike 4.0 international License.

The geology Time Scale

Now that you have actually learned about the hierarchical materials of the geology time scale--eons, eras, periods, and also epochs--consider again how all of these components fit together. Keep in mind that some borders (those that follow horizontal lines on the moment scale) are tantamount in age. Because that example, the Mesozoic-Cenozoic boundary is indistinguishable to the Cretaceous-Paleogene boundary (both have period of 66 Ma). Similarly, the Paleogene-Neogene boundary is indistinguishable to the Oligocene-Miocene boundary.

It is much much easier to memorize the time scale by an initial breaking that down into its ingredient parts. Plenty of geology student have emerged clever mnemonic tools to help remember every one of the names in the time scale and also a fast Google search will carry out many instances (which are frequently rather salty!) that you might find helpful as you memorize the time scale.

The geological time scale. Picture by Jonathan R. Hendricks.  This occupational is license is granted under a an imaginative Commons Attribution-ShareAlike 4.0 worldwide License.

The geological time range is a an outcome of numerous years of investigation and also remains really much a job-related in progress. Observe the geology time range below, which is indigenous a college-level geology textbook published over 130 years back by Le Conte (1885). What similarities does that share through the time scale above? What is different? 

Most of the names of time intervals on the contemporary geological time scale have actually been stable for many years. The absolute ages linked with the limits that different these time intervals continue to be sleek as new evidence becomes available. Because that example, the modern absolute age associated with the Proterozoic-Phanerozoic border (equivalent come the beginning of the Cambrian period) is 541 Ma. In 2009, this period was 542 Ma; in 1999, it was 543 Ma; and in 1983, it to be 570 Ma.

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A easily downloadable copy the the many up-to-date skilled version of the time scale--produced through the Geological culture of America may be downloaded here>. To compare it through the streamlined version the you have learned; friend will note that the is much, much more detailed. Most geologists know the epochs and ages of the time intervals they emphasis on in your research, however very couple of geologists understand the complete professional variation of the moment scale. Almost all geologists and paleontologists keep a copy next to their desks, however, to refer to whenever necessary.