

To introduce students to the vastness of geologic time and the concept of scale.
Unraveling time and the Earth's biologic history are arguably geology's most important contributions to humanity. Yet it is very difficult for humans to appreciate time beyond that of one or two generations, much less hundreds, thousands, millions and billions of years. Perhaps we can only hope that students catch glimpses of our rich geologic heritage, particularly when most of our teaching is done in a classroom and not in a field setting. This exercise begins to make time more "three dimensional" and most importantly, students gain a better appreciation for geologic time and our Earth's history.
To better understand the concept of geologic time, have students produce a timescale metaphor to share with the class that is true to scale and reflects some of the important events in the history of the Earth (see list on the following page). Write an essay that: (1) discusses why you chose the metaphor you used; (2) shows your math calculations; and (3) discusses what you learned from this exercise including your perspective of where humans fit in the grand scheme of things. Have fun! Be creative! No metaphor is too silly, as long as your math is correct and your choice has meaning to you.
The method used to determine a metaphor value truetoscale will be similar for all metaphors. Units in the metaphor model can be in time, distance, volume, mass, etc. depending upon what type of metaphor you choose to work with. The general equation used to generate numbers in your metaphor which will be true to scale is:
Known age of past event
(years before present)
______________________________ Known age of the Earth (years before present) 
= 
UNKNOWN Time
scale metaphor equivalent ___________________________ Maximum measurement in metaphor 
For example, suppose your metaphor uses distance. Remember, the use of time, volume, or mass in your metaphor would be just dandy. Since we are using a distance metaphor as an example here, a football field with a length of 100 yards will do just fine. To find at what distance along the football field, for example, the "first oxygen" yard mark would be, you would set up the ratio shown below:
Known first oxygen
(2.01 x 109 years)
________________________________ Known age of the Earth (4.6 x 109 years ago) 
= 
UNKNOWN (first
oxygen on football field)
________________________________ 100 yards (Football Field Length) 
So taking the math one step further gives you:
(2.01 x 109 years)(100 yards) = (X yards)(4.6 x 109 years)
Solving the ratio (for X) will tell you that the "first oxygen" location on the football field would be 43.7 yards away from the goal line of your choice! Determining the location of the other important dates in the history of the Earth is up to you.
Some Important Dates in the History of the Earth

Discuss the age of the Earth's crust at different locations (i.e. ocean floor, continents, etc.) Is it older in some places and younger in others? What does this tell you about how the Earth's crust has changed over the last billion years? Discuss plate tectonics.
Invite a paleontologist to discuss our geologic history.
The time of human life on Earth is incredibly short when compared to Earth's vast history, yet during our short existence we have permanently altered our environment. Discuss the implications of the environmental crisis as it exists in the context of Earth history.
How old are the rocks in your state? Look at geologic maps to examine the history of your area.