Roping Geologic Time

Randall M Richardson

University of Arizona, Department of Geosciences

Summary

After having talked about the geologic time scale, I ask for two volunteers from the class to hold a rope that is 50 feet long. I say that one end is the beginning of the Earth (4.6 billion years ago), and the other is today. I then give out 16 clothes pins and ask various students to put a cloths pin on the 'time line' at various 'geologic events'. Throughout the activity I have a quiz going on where the students calculate percentages of Earth History for major geologic events, and compare it to their own ages. On their time scale, the dinosaurs died only about two 'months' ago! The exercise is very effective at letting them get a sense of how long geologic time is, and how 'recently' some major geologic events happened when you consider a time scale that is the age of the earth.

Historical Geology | College Introductory, College Lower (13-14)

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Context

Audience:

undergraduate: entry level

Skills and concepts that students must have mastered:

Ability to calculate percentages.

How the activity is situated in the course:

Taught along with radiometric age dating and learning about sequencing geologic events.

Goals

Content/concepts goals for this activity:

Immensity of Geologic Time.
The order of major geologic events.
The 'bunching up' of geologic history in 'recent' geologic time.

Geologic skills:

Higher order thinking skills goals for this activity:

Other skills goals for this activity:

Quantitative skills, by comparing Earth history to their own ages.

Description of the activity/assignment

After having talked about the geologic time scale (Precambrian: prior to 570 Ma; Paleozoic: 570-245 Ma; Mesozoic: 245-65 Ma; Cenozoic: 65 Ma - Present), I ask for two volunteers from the class to hold a rope that is 50 feet long. I say that one end is the beginning of the Earth (4.6 billion years ago), and the other is today. I then give out 16 clothes pins and ask various students to put a cloths pin on the 'time line' at various 'geologic events'. For example, I ask them to put one where the dinosaurs died out (end of the Mesozoic). They almost invariably put it much too old (65 Ma is less than 2% of Earth history!). Then I ask them to put one on their birthday (they now laugh). Then I ask them to put one where we think hominoids (humans) evolved (~3-4 Ma), and they realize that we have not been here very long geologically. Then I ask them to put one at the end of the Precambrian, where life took off in terms of the numbers of species, etc. They are amazed that this only represents less than 15% of Earth history. Throughout the activity I have a quiz going on where the students calculate percentages of Earth History for major geologic events, and compare it to their own ages. On their time scale, the dinosaurs died only about two 'months' ago! The exercise is very effective at letting them get a sense of how long geologic time is, and how 'recently' some major geologic events happened when you consider a time scale that is the age of the earth.

Evaluation

Key is designing questions that address student understanding of the lenght of geologic history, and the 'recentness' of major events (extinction of the dinosaurs, etc.) in geologic history.

Same questions in exams on same material, in classes with and without activity, that address geologic time.