Geologic Time Scale – Eons, Eras, Periods, Epochs and Ages

The Geologic Time Scale is a chart showing all the geologic events that have happened on earth and assigns a time to them. It is like the calendar of the earth grouping major occurrences based on the time in history they happened.

What is the Geologic Time Scale?

The geologic time scale goes as far back as the earth’s beginning which is about 4.6 billion years ago, till the present day. Because the earth is very old, it has been subjected to several events. These events have made it what it is today. We can easily note the time these events and changes occurred in the earth using the geologic time scale.

Through this, we can also see all the changes that had occurred such as the emergence of specific species as well as when they became extinct. Tracing the progressive development of organisms with time is also possible from the geologic time scale. For instance, pieces of evidence of the first sign of life were dated to be about 3.5 billion years old. Life from this point on continued to evolve till more advanced multi-cellular organisms were recorded to have appeared around 580 million years ago.

This scale is used in every area of geology to ascertain the time and relationship of events across time. Although it is mostly used in geology, every area of the natural sciences contributed to its understanding and development.

The Divisions of the Geologic Time Scale

From the records and information provided by linking rock strata, the time scale was divided into specific time periods. This information could also be drawn from the remains of organisms found within the rocks to link which rocks are of the same age. These similarities in rock-drawn information allowed the time scale to the well divided.

The time scale is divided into eons, eras, periods, epoch, and ages. This can be likened to how our regular calendar is divided into months, weeks, and days. Unlike the years, months, and weeks we are familiar with, these times are not equally divided. Thus some periods are longer than some and some eras cover more time than others.


An Eon is the largest division on the time scale and it is not less than a billion years. There are four eons recognized on the geologic timescale: the Hadean Eon (which is the oldest), Archean Eon, Proterozoic Eon, and the Phanerozoic (which is most recent). Each of these eons had a significant difference in climate, the composition of the earth, and the life forms that lived then.

The Hadean, the Archean, and the Proterozoic eons are collectively known as the “Precambrian Period”. Ages from the Phanerozoic upward are called Cambrian ages.

The Hadean Eon

The Hadean Eon is the oldest time on the geologic time scale. This eon began with the formation of the earth about 4.6 billion years ago. During this time, the temperatures of the earth were high and no life could survive here. The name “Hadean” came as a result of the high temperature and incessant volcanic activities. It could be said the environment of the earth at this time looked “hellish”.

It was believed that the moon was formed during this period. There might have been the presence of early oceans and water bodies too.

The only rock unit identified in the Hadean Eon is found in Canada. Not much is available from this eon as it does not have a record of any life form.

The Archean Eon

This eon succeeds the Hadean eon and spans from 4.0-2.5 billion years ago. The first signs of life forms were observed in the sea during the Archean Eon. These were single-celled organisms. It could also be deduced that the climate during this time was not as hot as in the Hadean Eon but was warm.

The atmosphere during this time was not very clear as it was composed mainly of volcanic gases that had been emitted. Some of the volcanic and greenhouse gases include carbon dioxides, hydrogen sulfides, and sulfur dioxides. On the land, there were three continents; Ur, Vaalbara, and Kenorland.

There are several pieces of evidence for this eon from all over the world. These include; the remains of stromatolites from sandstones in Western Australia, and biogenic materials from graphite in Southwestern Greenland. The stromatolites were dated about 3.48 billion years ago while the graphite was dated about 3.7 billion years ago.

The Proterozoic Eon

Subsequent eons from the Proterozoic Eon had multicellular organisms. This eon spanned 2.5 billion years ago to 541 million years ago. The life forms continued to evolve and get advanced as the earth aged. There were periods within this eon where the earth’s temperature dropped significantly below zero. This resulted in the formation of the “snowball earth”.

Tectonic forces had always been shaping the earth from the beginning. During this eon, the landmass was broken into the continents Columbia, Rodinia, and Pannotia.

The Phanerozoic Eon

This eon is the youngest and is still running. It covers from about 541 million years ago till the present. It is well known for having records of complex and diversified life forms. Vertebrates such as some fishes, birds, reptiles, and mammals appeared here.

There was also the abundant diversification of plant life during this time. The land at this time was drifted by tectonic forces to form one supercontinent called Pangea. Much later within the eon, Pangea broke up into the seven continents we now have.

The Phanerozoic eon is most famous for having two ice ages. There were also records of mass extinctions during this time.


Eons are subdivided into Eras. There are 10 eras, the oldest being the Eoarchean Era that ranged from 4.0 billion to 3.6 billion years ago. Next is the Paleoarchean Era, the Mesoarchean Era, and the Neoarchean Era.

In the Proterozoic Eon, there are three eras which are, the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. There are three eras in the Phanerozoic Eon: the Paleozoic, the Mesozoic, and the Cenozoic era.

These eras like all geologic times are delineated by the specific rock types or fossil abundant here. Eras within the Phanerozoic eon are replete with fossil remains of organisms.

Paleoproterozoic Era

This era is the widest era on the geologic time scale. It is most famous for the increase in oxygen in the atmosphere. This resulted in the death of the anaerobic organisms that populated the earth. In their place, eukaryotes emerged. Another feature this era is known for is the stabilization of the continents.

Mesoproterozoic Era

The Mesoproterozoic witnessed the breaking up of the Columbia supercontinent that was around in the previous era. In its place came the continent Rodinia. This era spanned from 1,600 million years to 1,000 million years ago.

Stromatolites were on the rise within this era and many other sexually reproducing animals also emerged.

Neoproterozoic Era

This era began about 1,000 million years ago and ended about 541 million years ago. During this time, there was intense ice covering the earth. The glacial covering may have covered even the equator. There are also records of trace fossils within this era. The appearance of the trace fossils here marked the upper boundary and the beginning of the next era.

Paleozoic Era

The Paleozoic is the first era within the Phanerozoic Eon. It began about 541 million years ago and ended about 252 million years ago. Within the Phanerozoic, this era has the longest time frame. The earliest records of life on land were identified within this era.

There was intense primitive vegetation during this time. Many of which decomposed to become most of the coal beds in North America and Europe. The era ended with the Permian-Triassic Extinction. This extinction is recorded to be the biggest in the history of the earth.

Mesozoic Era

This era lasted from 252 to 66 million years ago. It is most notable for the rise of reptiles and conifers. These reptiles include the famous Dinosaurs that dominated much of the Triassic and Jurassic periods of this era.  Flowering plants and birds were also common during this time.

The Supercontinent Pangea began to rift and the continents began moving to the positions we have today.

Cenozoic Era

Beginning after the Cretaceous-Paleogene Extinction that wrapped up the Mesozoic Era 66 million years ago, this era continues to date. Mammals became the most dominant vertebrates after most of the dinosaurs had been wiped out.


For simplicity and precision, eras are subdivided into periods. Periods could range from ten to a hundred million years. There is more interest in the periods because, during these time spans, specific rocks were formed. The existence of organisms could be specifically placed into the exact periods they occurred.

There are 22 recognized periods on the geologic time scale. Both the Hadean and the Archean Eons do not have periods nor epochs. The Proterozoic Eon has 10 periods while the Phanerozoic Eon on the other hand has 12 recognized periods.

Igneous, sedimentary or metamorphic rocks formed within a period are known as systems. Some periods have different names. For instance, American geologists divide the Carboniferous Period into Mississippian and Pennsylvanian Periods. This period is the second in the Paleozoic era and is notable for being the beginning of the formation of most of the coal beds we have today.

The Triassic and the Jurassic Periods are the most famous (from the movie Jurassic Park) because of the abundant dinosaur that lived here. The Cretaceous Period that succeeded the Jurassic had an abundance of chalk from where it was named.


Epochs are divisions of periods on a time scale. The rock units deposited here are referred to as series. Because they cover smaller periods, they separate specific differences.

They are smaller time units and are commonly used for the Cenozoic Era and its periods such as the Quaternary Period. These epochs have abundant fossils that have been recovered hence, there is considerable understanding of these time units.

There are about 34 epochs identified in the geologic time scale with each spanning not more than tens of millions of years. Some epochs are also broken into ages.

How was the Geologic Time Scale formed?

Fossils of organisms in rocks were recovered in some rocks in Ancient Greece. Aristotle however, observed that the fossils found in these rocks are similar to those on beaches. This formed the basis of distinctly noting that the fossils found in the rocks were remains of organisms and not rock matters.

Much later, Nicholas Steno propounded the principle that sedimentary rocks were laid down horizontally in successions. He further asserted that each layer of the sediment indicated a time in history. This was called the law of superposition. The law of superposition explains that in any given rock assemblage the one below is older than the one above.

In the 18th century, there were attempts to group all the rocks found on earth. The most trending of such ideologies grouped the rocks into four types on the premise that they were all formed at the same time. These groupings were into Primary rocks, Secondary rocks, Tertiary rocks, and Quaternary rocks.

By the 19th century, similar fossils identified in different rock strata were used to group these rocks at the same time. This School of thought was championed by William Smith. He pushed that using fossil contents was a more accurate method of delineating age instead of the physical and chemical properties of the rocks. With this, no matter how distant the rock strata might be from each other. This idea was pioneered by geologists such as William Smith.

How were these dates and names on the Time Scale determined?

Applying Nicholas Steno’s law of superposition, sediments found on the surface are first attended to and compared with other types elsewhere. Using just this law, however, posed issues as the rocks originally laid down might have been disturbed, tilted, or even eroded.
Thus the wide use of fossils. Rock units with similar fossil contents can now be assigned the same age. Eras, Epochs, and Periods were delineated using the fossil they contained.
The dates were determined based on the start and end of specific events. for instance, the emergence of a kind of organism could be a result of a change in the climate. This signals the start of a change in the earth and this period could be signaled by the death of such organism. Because these markers or changes vary, some portions of the time scale are longer than some.

The dates in time scale are written in millions of years. A portion that is about 4.5 million years can be written as 4.5 Mya or 4.5 Ma. Where Mya and Ma both mean million years. For larger time duration in billions of years, those are written as Ga.

Most of the names like “Cambrian” were given by British geologists. Some other ages were named after the areas they were first identified such as “Jurassic” named after the Jura Mountains.

Major Earth Events as shown on the Geologic Time Scale

We have shown that the geologic time scale chronicles the events that shaped the earth to what we know it to be. Events can be delineated based on the time they occurred. Such events include;

The emergence of organisms and their extinctions

From the fossil collections and rock records used to draw the time scale, it is easy to mark out when an organism emerged and the point it became extinct. For example, the first appearance of life on the planet can be identified.

Also, it has been understood that animals such as the dinosaurs emerged during the Triassic Period and became extinct around the close of the Cretaceous Period. Talk of an easy way to keep track of all the first happenings!

The dominance of specific species and their abundance

Different species dominated at different times. This could be attributed mainly to the presence of the right conditions to support their existence. For instance, the carboniferous had a lot of plants that eventually formed most of the coal beds in Western Europe.

For many parts of the Mesozoic Era, large-sized animals such as dinosaurs and mammoths were more dominant. Now in the Quaternary Period, humans are dominant. This information can be justified by the abundance of the remains of these organisms in the sediments that were laid down during these times.

The climate at specific times

We can draw out the different climatic conditions our planet has had over its 4.6 billion years of existence. During the Hadean Eon, for instance, the climate was very hot and could not support life at all. The climate across the whole earth was really cold during the Neoproterozoic Era resulting in almost the whole earth being covered in ice.

The Carboniferous period had a large number of plants.  It is easy to say that the temperature and weather condition during this time were perfect encouraging the growth of the plants.

The geology per time

There were different types of rocks formed at different times. We can identify which rock is older when several rocks occur together. The characteristic feature of the earth at each point affected the type of minerals in as well as how the rock appeared.

For rocks that were formed during the Archean Eon, many of them have been subjected to intense heat and stress. These stresses had produced several deformations. This is in contrast to other younger rocks of the same type.

What is the Importance of the Geologic Time Scale?

Describing ancient environment

The fossil assemblages in each rock give an idea of the paleoenvironment. If the fossils belonging to marine organisms are found buried in a dry area, it can be assumed that such an area was once covered by water. Another example is if fossils belonging to tropical organisms are found in a temperate place, the paleoenvironment could be assumed to be tropic.

The time scale also serves to explain the climate within these ancient environments. With this, information as to what caused dastardly climatic changes can be known and avoided. This makes it an important tool in paleoenvironmental studies. We can thus tell the story of our planet from this chart.

Grouping rocks

Particular rock types are known to occur at specific periods. The time scale helps us know which rocks occur at the same time and which could have similar characteristics. With the understanding of what happened at different times on the earth, scientists can pin rocks to the time they occurred.

This understanding of the occurrence of rock at a specific time makes it easy to prospect for minerals. Petroleum companies already know there was no evidence of life in the Hadean or Archean Eons so they would not waste money prospecting within these rocks.

Identifying evolutionary trends.

By following a particular species down the years, it is easy to see how much evolution affected the organisms. Whether or not a species was able to adapt is seen through the geologic time scale. It is also possible to track the evolutionary trends of the earth’s climate and weather.

This makes the time scale an important tool for tracing the ancestry of organisms.


Time units are delineated with the aid of their fossil contents or properties.  Similar rocks from a different place with the same properties or fossil content can be said to be of the same age. Although using radiocarbon dating, these ages can be confirmed. Radiocarbon dating gives an absolute age of these rocks. This involves calculating the carbon isotopes in the rock sample.

The branch of geology specializing in studying this time scale using several tools such as rock occurrence and fossil contents is referred to as Geochronology.

Geologic time is quite different from the normal time. So, 10 or 20 years have very little geologic significance. It is not uncommon then to see millions of years written on the chart. These ages were then arranged from the oldest to the youngest and described on the chart. Although the assigned names are here to stay, the specific years they cover are still being reviewed.

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