Chapter contents:

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


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The geological time scale. Image by Jonathan R. Hendricks.  This work-related is licensed under a an imaginative Commons Attribution-ShareAlike 4.0 worldwide License.


The geological time scale--shown above in a streamlined form--is one of the crowning success of science in general and also geology in particular. It is a reference and also communication device for to compare rocks and fossils from transparent the world and also is geology"s indistinguishable of the regular table that the elements. For example, a paleontologist can speak to her colleague and say, "I just found an awesome new trilobite from the Devonian of new York" and her partner will automatically understand once in geology time the trilobite lived.

Most that the boundaries on the geology time scale correspond come the origination or extinct of certain kinds that fossils. Learning when significant groups the fossils very first appeared or walk extinct is therefore incredibly advantageous for determining the eras of rocks in the field. For example, if you discover a rock through a trilobite fossil upon it, friend will automatically know that the absent is Paleozoic in age (541 Ma come 252 Ma) and not larger or younger; knowing the types of trilobite enables even higher precision.

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

The rule of faunal sequence was developed by one English surveyor named William "Strata" smith (1769-1839). Together he learned layers the rocks to identify where to build canals, the noticed the he found the exact same ordering that fossil species from place to place; Fossil A was constantly found listed below Fossil B, which in turn was constantly found listed below Fossil C, and so on. Through documenting this sequences that fossils, Smith was able to temporally correlate absent layers (or, strata) from ar to location (in various other words, to create that rock layers in two different places are indistinguishable in age based ~ above the reality that they incorporate the same types of fossils). Temporal correlation enabled Smith to construct the first geological map (see section 2.4) of an entire country.

Temporal correlation has made many human being very, an extremely rich by allowing them to predict the locations of valuable geological resources such together fossil fuels. Much more generally, the has allowed us to reconstruct the geological background of planet by compare rocks and fossils from ar to place. This is critically important due to the fact that no single place on planet preserves a complete geological history, or also a small portion of it.

The geological time scale gives a worldwide summary of many small-scale temporal correlations of absent layers make at local and local scales. The is based practically entirely upon cautious observations that the distributions of fossils in time and also space.

Learning the geological time scale

Because the its usefulness because that communicating around events in Earth"s history, it is essential that every students of geology, paleontology, and evolutionary biology commit the geology time scale to memory. This is most easily done by first breaking the moment scale right into its ingredient parts: eons, eras, periods, and epochs.


Eons

The eon is the broadest category of geological time. Earth"s background is characterized by four eons; in stimulate from oldest to youngest, these are the Hadeon, Archean, Proterozoic, and also Phanerozoic. Collectively, the Hadean, Archean, and also Proterozoic are sometimes informally described as the "Precambrian." (The Cambrian period defines the beginning of the Phanerozoic eon; so, every rocks older than the Cambrian space Precambrian in age.)

We live throughout the Phanerozoic, which means "visible life." This is the expression of geology time identified by abundant, complicated fossilized remains. Being the youngest eon that time, it is also an extremely well stood for by absent at Earth"s surface (because the the rule of Superposition; see ar 1). Since of these two factors, most paleontologists and also geologists study fossils and rocks from the Phanerozoic eon.

Do no let the time range at the peak of this page offer you a false impression, however, around the temporal duration of the Phanerozoic eon family member to the three older Precambrian eons. Keep in mind in the figure listed below the absolute eras of the borders separating every eon the time.


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Left: the four eons of geology time. Right: the "Precambrian" eons (Hadean, Archean, and Proterozoic) stand for 88% of geological time. Image by Jonathan R. Hendricks.  This job-related is licensed under a an imaginative Commons Attribution-ShareAlike 4.0 worldwide License.


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

The Hadean and also Archean are daunting eons come study, however, due to the fact that they room exposed in very minimal places top top Earth"s surface. (Since they space the earliest eons, rocks that space Hadean and Archean in age are often buried far listed below younger rocks in ~ Earth"s surface.) Proterozoic rocks--which span practically 2 billion years (42% the 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. The is slowly beginning to change, however, as much more clues around the origins of complex life begin to it is in revealed from Proterozoic-aged rocks.


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The three periods of the Phanerozoic eon. Image by Jonathan R. Hendricks.  This occupational is licensed under a creative Commons Attribution-ShareAlike 4.0 worldwide License.


Most that our understanding of the fossil record originates from the three periods of the Phanerozoic eon. The Paleozoic ("old life") era is characterized by trilobites, the very first four-limbed vertebrates, and the beginning of land plants. The Mesozoic ("middle life") era to represent the "age the dinosaurs," though likewise is notable for the an initial appearances that mammals and flowering plants. Finally, the Cenozoic ("new life") era is sometimes called the "age that mammals" and is the era during which we live today.

As temporal points of reference, it is worth memorizing the periods of the limits that separate the three ages of the Phanerozoic eon. Long prior to geologists knew this absolute period dates, lock realized the the borders represent important events in the history of life: massive extinctions. For example, countless fossils that are commonly 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 geologists used these mass extinction events to define these (and other) borders within the Phanerozoic part of the geological time scale. That is thus no simultaneous that some of the significant boundaries coincide with mass die out events!

The enlarge Archean and also Proterozoic eons are likewise divided into several eras. For example, the youngest era that the Proterozoic eon is referred to as the Neoproterozoic. Because that the benefits of simplicity, these older eras are not contained on the moment scale shown at the height of this page; they do, however, exist!


Periods

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

The Paleozoic era is divided into six periods. From oldest to youngest, these are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and also Permian. Note that in the united States, the Carboniferous is divided into two different periods: the Mississippian and the Pennsylvanian.


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


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


The three periods of the Cenozoic era. Image by Jonathan R. Hendricks.  This job-related is licensed under a an innovative Commons Attribution-ShareAlike 4.0 worldwide License.


Epochs and Ages

Periods of geological time room subdivided right into epochs. In turn, dates are divided into also narrower systems of time referred to as ages. Because that the services of simplicity, just the dates of the Paleogene, Neogene, and Quaternary periods are displayed on the time scale at the height of this page. The is crucial to note, however, that every one of the periods of the Phanerozoic era space subdivided into the epochs and also ages.

The Paleogene period is separated into--from oldest to youngest--the Paleocene, Eocene, and Oligocene epochs. The Neogene is divided into the Miocene and Pliocene epochs. Finally, the Quaternary is divided into the Pleistocene and Holocene epochs. Part geologists currently think that--since humans are having such a notable affect on the Earth and its life--a new, youngest date should be added to the Quaternary: the Anthropocene. There is still considerable discussion in the geological community about whether this date should it is in added, and also debate about what characteristics should specify its beginning.


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


The geological Time Scale

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

It is much less complicated to memorize the time scale by very first breaking it down into its component parts. Countless geology student have emerged clever mnemonic tools to help remember every one of the names in the time scale and also a fast Google find will carry out many examples (which are often rather salty!) the you might find useful as friend memorize the time scale.


The geology time scale. Image by Jonathan R. Hendricks.  This work-related is licensed under a an innovative Commons Attribution-ShareAlike 4.0 worldwide License.


The geological time range is a result of hundreds of years that investigation and remains very much a occupational in progress. Observe the geological time range below, which is from a college-level geology textbook published over 130 years earlier by Le Conte (1885). What similarities does that share v the time scale above? What is different? 


Most that the surname of time intervals top top the modern-day geological time scale have actually been stable for many years. The pure ages connected with the limits that separate these time intervals proceed to be polished as brand-new evidence becomes available. For example, the contemporary absolute age associated with the Proterozoic-Phanerozoic boundary (equivalent to the beginning of the Cambrian period) is 541 Ma. In 2009, this period was 542 Ma; in 1999, it was 543 Ma; and also in 1983, it was 570 Ma.

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A easily downloadable copy of the most up-to-date professional version of the moment scale--produced by the Geological society of America might be downloaded here>. To compare it through the streamlined version the you have actually learned; friend will note that the is much, much more detailed. Most geologists understand the epochs and ages of the time intervals they emphasis on in your research, yet very few geologists understand the finish professional variation of the time scale. Virtually all geologists and paleontologists store a copy next to their desks, however, to describe whenever necessary.