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.

Purpose

To introduce students to the vastness of geologic time and the concept of scale.

Background

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.

Instructions

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.

Example:

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
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

Years Ago

Event

4,600,000,000

Origin of the Earth

3,900,000,000

Oldest Dated Crustal Rocks

3,800,000,000

Oldest Evidence for Life

2,000,000,000

First Oxygen Atmosphere/Ozone Layer Forms

900,000,000

Oldest Metazoan Fossils

510,000,000

Oldest Fossil Fish

458,000,000

First Land Plants

375,000,000

That important first step:  Amphibians Evolve

245,000,000

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

200,000,000

First Mammals

160,000,000

First Birds

145,000,000

Atlantic Ocean first opens

130,000,000

Angiosperms (Flowering Plants) on the Scene

65,000,000

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

9,000,000

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

3,400,000

Australopithecus afarensis (Lucy) fossils lived in Ethiopia

2,000,000

Pleistocene Ice Age begins

600,000

Age of Homo erectus fossils from Ethiopia

100,000

Homo sapiens appears in the fossil record

15,000

Last ice sheet retreats from northern United States

506

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