Time and Geology

We spend a lot of time in geology talking about time.  Most of us understand time, and there are some great videos out there by a group called minute physics on this very topic.  But my discussion isn't about what you and I consider time, it is about geologic time.  We can all agree that the Earth is old.  Some of us may remember a more specific number, like 4.6 billion years old. But as an Earth Scientist, how do we record 4.6 billion years of information anyway?


In order to record billions of years of history, geologist have created the geologic time scale.  In other words, they have taken thousands of years and simplified them into one "period" of time.  Some of you probably remember these scales from Earth Science class.  A really good chart of the geologic time scale can be found here.  So what exactly do all those titles and boxes mean?

If it helps think of the geologic time scale as a state.  The state itself can be called a supereon.  This will cover the largest amount of time.  Within your state you may have several counties.  Each of these counties will be an eon.  Within each county you will have towns.  In this case towns represent eras.  Each town will contain streets, which represent the periods.  Each street contains houses, or epochs.  And finally each house contains rooms, or ages.

So now that you got a taste for the key terms in the geologic time scale, lets start putting some numbers to it.

Supereon: There is currently one named supereon: The PreCambrian
                  We are all living in the second (un-named) supereon.
The Eon: An eon covers 500 Million (500,000,000) years or more.
                 There are currently 4 eons on the geologic time scale.
The Era: An Era includes several 100 Million (900,000,000 - 500,000,000) years.
                There are currently 10 Eras on the geologic time scale.
Periods: On a scale of 100 Million (100,000,000 - 500,000,000) years.
Epoch: On a scale of 10 Million (10,000,000 - 90,000,000) years.
Ages: On a scale of 1 Million (1,000,000 - 9,000,000) years.

As you can tell by the time scale we are dealing with geologic time moves very slow.  Although we can relate it to years and decades, so little changes (geologically) that it is easier to create our own scale.  A scale that encompasses entire millennium in its shortest time frame.  The first of the homo sapiens are dated as far back as 200,000 years ago.  Meaning unless we adopt a new lower level in the time scale, human existence will hardly even register on it!

Ok, so you understand how large the scale is, and how young we are, but how do we define the Eon's and Epoch's and all that stuff?  The geologic time scale is determined by important events that occur on Earth. Lets relate this back to our fictional state. First let me give one added piece of information about your state.  It was developed with the best technology at that time, but never upgraded.

So, if you were an alien and you just landed on the Earth today, how would you define our fictional state.  You would look around at all the evidence.  You would note a section of houses that all look the same, and you would determine that those houses belong together in an epoch.  You could sub-divide those houses, by going inside.  The ones with color televisions would belong to an age that came before the houses with HD T.V.'s.

The alien could look at the building materials of our roads and determine which neighborhood or town came first.  A town that uses dirt roads belongs to Era: 'Dirt', while towns with paved roads belong to Era: 'Paved'.  As the alien continues to learn and group things he will eventually create something similar to our geologic time line.  

The smaller the detail of our city, the closer in age it would be.  These smaller changes could be significant from one another, but can still be grouped together under a large similarity.  So if all the houses are significant they will each have their own Ages.  But the neighborhood as a whole may be considered an Epoch.  This is how Earth Scientist have determined the Geologic Time Scale.  Each Age is marked by something important, but all the Ages in an Epoch share something different then all the Ages in the previous one.  As you continue up the scale, all the periods within on Eon are similar. Even though they may have differences among them, the Periods grouped are significantly different in one Eon to the next.

So by now I hope that you have a greater understanding of the Geologic Time Scale.  If there is anything you are still confused on please feel free to let me know in the comments below.