Geologic Time Scale
The Key To Earth's History

The Geologic Time Scale Defined

The Geologic Time Scale is a tool for understanding how the Earth formed and came to be the way it is today. This is a simplified answer leaving much to be explained.

Geologic time takes in all that has happened on earth since the Archaen Eon about 4 billion years ago. Informally it goes back to the Hadean 4.6 billion years ago. The distinction here is because the time scale was created by geologists who study rock strata (layers of rock). There are no rock strata to study from the Hadean, so technically it is not part of the geologic time scale.

It is hard to wrap your mind around a time span this big. The geologic time scale is like a huge calendar to put events into perspective and to chunk down this tremendous amount of time into something more manageable. The geologic time scale does this by creating segments.

Geologic Intervals

Eons are the largest time segments. These are each broken down into Eras which in turn are divided into Periods. The periods are further divided into Epochs.

Graphic Representations of Geologic Time

Charts, Timelines, clocks, and other graphical representations of the Geologic time scale help us visualize the information and their relationships. Below is a simplified Geologic Time Chart.

The purpose of this chart is to help you easily find in-depth information on eons, eras, and periods of earth’s history. We think it is convenient and useful to see the time periods all laid out in chart form. As a reference tool, you can easily note the sequence of the various divisions and the length of each unit of time relative to each other.

This chart is not to scale. A Geologic Timeline to scale is a valuable tool but requires lots of space. There are many commercially available timelines that are scaled to represent all 4.5 billion years of earth's history. They tend to be over 10 feet long! We are currently working on our own version.

Just like the layers of the earth, the top divisions of the chart represent the most recent time. Moving down the chart, you go further and further back in time.

The names of each Eon, Era or Period are linked to pages that contain information on the geology, biology, and climate of that particular time. Simply click on the name of the time division for more detailed information about it.

Note: At present, information about individual epochs can be found within their respective periods.

Below the chart, you will find links to lesson plans and activities for kids related to the geologic time line.

Geologic Time Line Chart

Eon	Era	Period	Epoch	Old Periods
Phanerozoic Eon 541 mya to Present	Cenozoic Era 66 mya to Present	Quaternary 2.58 mya to Present	Holocene .012 to present	Quaternary 2.58 mya to Present
			Pleistocene 2.58 to .012 mya
		Neogene 23 to2.58 mya	Pliocene 5.3 to 2.58 mya	Tertiary 66 to 2.58 mya
			Miocene 23 to 5.3 mya
		Paleogene 66 to 23 mya	Oligocene 34 to 23 mya
			Eocene 56 to 34 mya
			Paleocene 66 to 56 mya
	Mesozoic Era 252 mya to 66 mya	Cretaceous 145 mya to 66 mya
		Jurassic 201 to 145 mya
		Triassic 252 to 201 mya
	Paleozoic Era 541 to 252 mya	Permian 299 to 252 mya
		Carboniferous 359 to 299 mya
		Devonian 419 to 359 mya
		Silurian 444 to 419 mya
		Ordovician 485 to 444 mya
		Cambrian 541 to 485 mya
Precambrian Time 4,600 to 541 mya		Ediacaran Period 635 to 541 mya
	Proterozoic Era 2,500 to 541 mya
	Archaean 4,000 to 2,500 mya
	Hadean 4,600 to 4,000 mya

The Basic Principles of The Geologic Time Scale

The most basic concept for the time scale is that new rock layers form on top of old layers.

Geologic time uses the Earth’s geologic composition (rock strata) to date the history of the planet. This is called stratigraphy.

This history dates back to more than 4.6 billion years ago…with the clock still running. That’s right, geologic time is still being recorded today.

Geologic Time and the Relationship to Strata Depth

The Earth is said to have formed some 4.6 billion years ago (give or take). To put this in context, that is 4,600 million, or 4,600,000,000.

The sequential order of events can be determined by looking at the layers. Depth plays a key role, with the layers at the bottom generally representing the oldest event. More recent events pile layers on top of the older ones.

Although the thickness of these layers can yield key insights into how long a particular geological event may have lasted,Relative age dating is primarily concerned with sequence and not the duration of events.

Fun Fact: Did you know that if you were to count to a billion, non-stop, no breaks, no sleep, it would take you 32 years! That’s right, 32 years of non-stop 24/7/365 counting. Crazy, right? What about counting to 4.6 billion? It would take you 147 years counting non-stop.

When Did Geologic Time Start?

Geologic time began at what is known today as the Archean Eon (4 to 2.5 billion years ago). Modern geological time (extending to today) often includes what is known as the Hadean Eon (an informal historical interval from 4.6 to 4 billion years ago).

Fun Fact: Earth didn’t just ‘magically’ appear one day. It took millions of years for the earth to form and achieve the substantial size it is today. The beginning of geological time generally accepts that the clock started when Earth achieved its current mass.

Dating Geologic Time

Geologists use two methods of dating rock strata:

Relative Age Dating

Absolute Age Dating

Relative age dating seeks to place significant geological events in the Earth’s history in sequential (chronological) order.

For example, major geological events may include a volcanic eruption, emergence of an ocean, duration of a desert area and more.

Geologists look to rock formations to reveal the timeline of such events. For example, a large layer of lava or ash, marine limestone or layer or rock. However, the presence of such formations is only relevant when they are spread over a significantly large area that can be visualized on a geologic map.

From these samples, the sequential order of events can be determined by looking at the layers. Depth plays a key role, with the first layer generally representing the oldest event. Although the thickness of these layers can yield key insights into how long a particular geological event may have lasted, Relative age dating is primarily concerned with sequence and not the duration of events.

Absolute Age Dating

Absolute age dating is a form of radiometric dating used to provide scientists with a specific and definite date for a unit of rock. This type of radiometric dating offers a realistic determination of the numeric age of a sample, providing a depth of information about the duration and subdivisions of geological time.

Noted for remarkable accuracy, advancements in this field allow geochemists to analyze small samples for trace elements with a precision never before achieved. Such advancements have shed much light on both the rates of biological as well as physical processes as they relate to the development of our planet.

The History of The Geologic Time Scale

In the 1700s, geologists began to use relative age dating principles in order to chart the chronological order of rocks found around the world.

How We Name The Divisions of Time
In the early 1800’s a system for naming geologic time periods was devised using four periods of geologic time. They were named using Latin root words, Primary, Secondary, Tertiary and Quaternary. That worked for the time but we know so much more now than was known in the 1800's. We no longer use this system. With the advent and advancement of radiometric age dating we are able to establish reliable and accurate dates for the Earth and it’s geological events.

The system we use today has evolved over time. It is not a static thing. It has changed more than a few times in recent history and It will change again as information is collected, analyzed, and debated.

Closing Thoughts on Geologic Time

Think of geologic time as the earth’s calendar of geological events over time. This calendar is divided into Eons, Eras, Periods, Epochs, and Ages (in order of descending duration).

The assignment of these geological units is based on stratigraphy (study and interpretation of rock strata over time). Fossils found within the strata provide key evidence and means of establishing a geologic timeline, revealing key information about the emergence and disappearance of many species.

These divisions are not randomly assigned but they roughly correspond to changes in the geology, geography, climate and other characteristics of the earth. For the most part they follow the evolution of life or the lack of it, on earth. Having said that, the changes in living things comes from changes in the state of the earth itself. The transformation may be brought about by some major geologic event like a collision with another traveler through the solar system like a comet or meteor. Prolonged volcanic episodes can also be the agent of change.

Check out some of the Educational Materials for sale on our sister site fossilicious.com.

Children's Books on Fossils and Geologic Time

Fossil Collections

Share this page:

What’s this?

Facebook Twitter

Enjoy this page? Please pay it forward. Here's how...

Would you prefer to share this page with others by linking to it?

Click on the HTML link code below.
Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.