Geologic Time Scale Analogy

Source: Ritger, S.D. and R.H. Cummins. 1991. Using student-created metaphors to comprehend geologic time. Journal of Geological Education. 9:9-11.


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 time-scale 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 explanation paper that: (1) discusses why you chose the metaphor you used; (2) shows your math calculations for all events; 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 true-to-scale 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
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

Years Ago



Origin of the Earth


Oldest Dated Crustal Rocks


Oldest Evidence for Life


First Oxygen Atmosphere/Ozone Layer Forms


Oldest Metazoan Fossils


Oldest Fossil Fish


First Land Plants


That important first step:  Amphibians Evolve


Huge Mass Extinction at End of Permian Period / Close of the Paleozoic Era


First Mammals


First Birds


Atlantic Ocean first opens


Angiosperms (Flowering Plants) on the Scene


Adaptive Radiation of Mammals
Dinosaurs Go Extinct
Close of the Mesozoic Era/ Beginning of the Cenozoic Era


San Andreas Fault became active and the Santa Ana Mountains began uplifting to make South Corona a more interesting place


Australopithecus afarensis (Lucy) fossils lived in Ethiopia


Pleistocene Ice Age begins


Age of Homo erectus fossils from Ethiopia


Homo sapiens appears in the fossil record


Last ice sheet retreats from northern United States


Columbus lands in New World

16, 17, or 18

You were born

Include main events from pp 84 (evolution of life), 88 (Pangea events) & 93 (5 past mass extinctions) not already in this list