Geologic Time Scale Analogy

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R. Hays Cummins, Western Program, Miami University

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Portions of the following exercise are from the article: Ritger, S.D. and R.H. Cummins. 1991. Using student-created metaphors to comprehend geologic time. Journal of Geological Education. 9:9-11.

Abstract

We use this exercise to introduce students to the vastness of geologic time and to hope they better appreciate the concept of scale. Most students are amazed and shocked by this exercise. Students create a metaphor that is presented to the class. They also compose an essay on why they chose the metaphor they did & they discuss the impact this project had on their thinking. We use this exercise in our Natural Systems course: Patterns & Processes in Natural Systems and in Geology 499/599: Tropical Marine Ecology.


Background

We've all done it. Careful lectures are given on the historical development of a time scale, first with qualitative techniques and then "time stamped" using quantitative methods. We also have students memorize the time scale as part of an earnest effort to facillitate students' appreciation of geologic time. Yet, sometimes despite our best intentions, we fall short. Unravelling 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. While we feel it is very important for students to memorize the major components of the time scale, we take things one step further by having them create their own metaphors for geologic time. We have found that 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. When students are then brought into the field, they "see" more mystery in the rocks. This always makes field trips more fun!

The Exercise

To better understand the concept of geologic time, your next lab exercise is to produce a time-scale metaphor that is true to scale and reflects some of the important events in the history of the earth (see list below). The exercise requires that you produce a metaphor to share with the class and that you 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. Your insructors, however, appreciate unusual and distinctive efforts--creativity is rewarded. Any examples used in class, however, are off limits. Due: Week of 11/6-11/10

Some Important Dates (subject to revison!) in the History of the Earth:
Millions of Years Ago Event
4600 Origin of the Earth
3900 Oldest Dated Crustal Rocks
3800 Oldest Evidence for Life
2000 First Oxygen Atmosphere/Ozone Layer Forms
900 Oldest Metazoan Fossils
510 Oldest Fossil Fish
458 First Land Plants
440 Bedrock in Oxford Forms/Ohio near the equator
375 That important first step: Amphibians Evolve
245 Huge Mass Extinction at End of Permian Period/
Close of the Paleozoic Era
200 First Mammals
160 First Birds
145 Atlantic Ocean first opens
130 Angiosperms (Flowering Plants) on the Scene
65 Adaptive Radiation of Mammals/Dinosaurs Go Extinct/
Close of the Mesozoic Era/Beginning of the Cenozoic Era
3.4 New discoveries of (LUCY)Australopithecus afarensis fossils from Ethiopia--Males and females show sexual dimorphism
2 Pleistocene Ice Age begins/Light from the Andromeda galaxy seen today left Andromeda 2 x 106 years ago!
.600 Age of Homo erectus fossils from Ethiopia
.125 Oldest rocks in the Bahamas
.100 Homo sapiens appears in the fossil record
.015 Last ice sheet retreats from Ohio
.007 Grahams Harbor, San Salvador, Bahamas floods due to rising sea level after Ice sheets are reduced to modern day volume
.000503 Columbus lands in New World
?? Your Birthday

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 towork with. The general equation used to generate numbers in your metaphor which will be true to scale is:



For example, suppose your metaphor uses distance as its "guiding light." Remember, the use of time, volume, or mass in a metaphor would be just dandy. Since we are using a distance metaphor as an example here, a football field with a length of 100 yds will do just fine. To find where on the football field, let's say, the "first oxygen" yard mark would be, you would set up the ratio shown below:



X=?

The "first oxygen" location on the football field would be (X) yards away from the goal line of your choice! The rest is up to you.


I really liked this metaphor(it is Friday night) and I thought you'd like to see it.

From the Department of Geological Sciences, University of Saskatchewan


Another View of the Geologic Time Scale

EON        | ERA      | PERIOD              | EPOCH      | DATES  | AGE of       | Interesting Biological Events:
Phanerozoic| Cenozoic | Quaternary          | Holocene   | 0-2    | Mammals      | Humans
            |           |                      |Pleistocene|         |               |
-
            |           |Tertiary| Neogene  | Pliocene   | 2-5    |               |
-
            |           |          |           |Miocene    | 5-24   |               |
-
            |           |          |Paleogene| Oligocene  | 24-37  |               |
-
            |           |          |           |Eocene     | 37-58  |               |
-
            |           |          |           |Paleocene  | 58-66  |               | Extinction of dinosaurs
            |Mesozoic  |Cretaceous                        | 66-144 | Reptiles     | Flowering plants
            |           |Jurassic                          | 144-208|               | 1st birds/mammals
            |           |Triassic                          | 208-245|               | First Dinosaurs
            |Paleozoic |Permian                           | 245-286| Amphibians   | End of trilobites
            |           |Carboniferous| Pennsylvanian     | 286-320|               | First reptiles
            |           |               |Mississippian     | 320-360|               | Large primitive trees
            |           |Devonian                          | 360-408| Fishes       | First amphibians
            |           |Silurian                          | 408-438|               | First land plant fossils
            |           |Ordovician                        | 438-505| Invertebrates| First Fish
            |           |Cambrian                          | 505-570|               | 1st shells, trilobites dominant
Proterozoic | Also known as Precambrian                     | 570-2,500              | 1st Multicelled organisms
Archean    |                                                | 2,500-3,800            | 1st one-celled organisms
Hadean     |                                                | 3,800-4,600            | Approx age of oldest rocks 3,800
Note: Dates are in millions of years

Taken from: MODERN PHYSICAL GEOLOGY, Graham R. Thompson Ph.D., Jonathan Turk Ph.D., Saunders College Publishing and the University of Alaska, Department of Geology.

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