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Roger Weller, geology instructor                               

Geologic Time

Molly Darlington

Physical Geology

Spring 2006


Once upon a Geological Time 


Where do scientists begin the story of Earth’s history?  One place to start is by looking at the age of the materials that make up the Earth itself.   When scientists are able to determine the age of rocks and fossils, they can then tell a more accurate story of the Earth’s history. 


When Geologists and other scientists refer to the age of the Earth, they are using one of two methods to determine this.  First, we must understand they are dating the rocks and other materials found on the Earth to determine this.  There are two basic types of dating methods geologists rely on, Relative Time Dating and Absolute Time Dating.


Relative Time Dating:  In Relative Time dating a specific age in years is not given.  This type of dating puts events in their correct chronological order based upon the sedimentary layer in which an object is found.  In the late 18th century and early 19th century, James Hutton developed the idea that the layers found in rock outcrops were representations of the story of time on the planet.  He concluded, the bottom most layers were deposited first, thus making them the oldest and subsequent layers were progressively younger in age.   Since there is no “set” rate of deposit for these layers, the actual length of time represented by these layers is a good guess.

Stratigraphy: is the science or study of layers or strata of rocks.  Sedimentary rocks build up layers from the oldest deposits, at the bottom with the newest at the uppermost layers.  Individual layers are called Stratum. Biostratigraphy is a specialized form of Stratigraphy, which uses plant and animal fossils to establish the different layers.  There are four basic principals of Stratigraphy, which help us understand Earth’s time line. 

The Principle of Original Horizontality: Sediments are layered in a horizontal fashion.  Non-horizontal layers have been folded or tilted after the original sedimentation.  This happens because of upheaval, earthquakes, or other large earth movements.


The Principle of Lateral Continuity: Because rock layers cover large areas of the Earth’s surface, scientists can relate layers in one location to another.  The correlations between one area of the Earth and another are largely based on fossil deposits.


The Principle of Superposition: As layers are formed, older layers are generally at the bottom of the sedimentation with younger layers being laid over the older ones.  There are many ways these sediments can be formed.  A volcanic ash buildup is one.  Lake or other water body sedimentation is another way these layers can be formed.         


The Principle of Faunal Succession: An English scientist, William Smith is accredited with the discovery of this idea.  Smith noticed that specific forms of life were fossilized in particular layers of rock, giving a time line story indicating when, in time, events occurred.  This progression of or evolution of life occurred in a vertical fashion up through the layers.  The same vertical changes in fossils occur in different places all over the earth. 

The major division of relative time can be and are expressed in a vertical chart, progressing in age from the oldest, at the base, with the youngest time found at the top of the chart.  The following chart shows this concept with a brief description of each age represented.


Absolute Time Dating: The age of a rock in years is called its absolute age.  The most common types are based on the rate of decay of naturally occurring radioactive elements.  The age of the material being dated is commonly expressed in a number of years.  When rocks are formed, small amounts of radioactive elements usually get included.  As time passes, the "parent" radioactive elements change at a regular rate into non‑radioactive "daughter" elements.  Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements is found in the rock.   Radiometric dating is another term used to talk about Absolute Time dating.

A chemical element consists of atoms with a specific number of protons in their nuclei, but different atomic weights owing to variations in the number of neutrons.  Atoms of the same element with differing atomic weights are called isotopes.  Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter).


Most radioactive isotopes have a rate of decay that is much too fast to be of any use in determining the geologic age of materials.  Carbon-14 dating is the best-known type of radiometric dating, but the half-life of carbon-14 is much too short to determine the age of things in terms of millions of years.  A few isotopes do decay at a, slow enough, rate to be able to date things into the millions of years range, and can be effectively used as geological clocks.


The table below indicates which parent isotopes decay slowest, what their half-lives are, and the daughter isotope, which results from the decay process.
















704 Million Years






1.25 Billion Years






4.5 Billion Years






14.0 Billion Years






35.9 Billion Years






48.8 Billion Years






106 Billion Years



The mathematical expression that relates radioactive decay to geologic time is called the age equation and is:


A typical “parent - daughter” combination used is Uranium to Lead decay.  The Uranium is trapped in the rock as it forms and over time as the Uranium decays, it is replaced by non-radioactive Lead.



Until the advent or discovery of Radiometric dating, a type of Absolute time dating, there was no way to verify the accuracy of the relative ages of materials found in the various layers of sedimentary rocks.  Rocks which have been dated utilizing one of the various methods of actual Time dating, can be incorporated into a relative time scale.   With this combination of the two types of dating methods, geologists can relatively, accurately determine the date of earth events and much of the time line associated with the evolution of life on the planet.


Geological time is traditionally divided into eons (Archaean or Archaeozoic, Proterozoic, and Phanerozoic in ascending chronological order), which in turn are subdivided into eras, periods, epochs, ages, and finally chrons.

The chart to the right is a composite chart utilizing both relative and absolute time dating techniques.  It is a more accurate chronological time dating method, than either of the two types of dating generally is on their own.


Links to other versions of this time scale are listed below:


Geologic Time Scale ‑ American

Geologic Time Scale ‑ European





All graphic references are sited and linked at the location of each Graphic. All other references are listed below.



Cochise Geology Home Page: Chapter 4‑Relative Geologic Time Scale and Stratigraphy by

Roger Weller‑out/ch04.htm


Cochise Geology Home Page: Chapter 5‑Numerical Dating of the Earth by Roger Weller‑out/ch05.htm


Exploring the Environment: Earth Floor


Geology and Geophysics, University of Calgary: Geological Time Scale


Smithsonian National Museum of Natural History: Geologic Time the Story of a Changing Earth


Tiscali.reference: Geological Time


University of California, Santa Barbara: Exercise 1‑2 Chronological Methods by George H. Michaels and Brian M. Fagan


USGS: Geologic Time