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CHAPTER 12
Geologic Clock
Materials: poster board, research of biological event dates, pencil, markers
Process Skills: Modeling
One way to understand the relative length of time in Earth’s history is to compare its age to a clock face.
Precambrian time goes from 12 noon to about 10:30 P.M. The time span from early human ancestors—
more than 5 million years ago—to Homo sapiens covers less than a second on our 12-hour clock!
Problem
How do different geologic time periods compare?
Procedure
1. Draw a large circle and mark the 12, 3, 6, and 9 positions of a clock face. Use the scale 1 hour = 400
million years ago, and label the four positions with the appropriate number of years, starting with 12
o’clock = 4800 million years ago. (Example: the three o’clock position = 3600 million years ago.)
2. Using the geological time scale, label Precambrian time and the three eras on your clock, along with
the approximate time frames in which they occurred.
Precambrian 4800mya-542mya
Paleozoic 542-251mya
Mesozoic 251-66mya
Cenozoic 66mya-present day
3. Label the events you researched in class, on your clock, in the appropriate positions/timeframes
according to the dates you found. Example: Earth formed 4.6bya would go near 12:00.
Analyze and Conclude
1. What do you notice about the time spent in the Precambrian Eon as compared to the three later Era’s?
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2. Calculate Using your scale of 1 hour = 400 million years, how many millions of years in the Earth’s
history would 1 minute represent?
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3. Most of life developed over how many hours on our geological time clock?
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4. Pick three events that occurred in order on the timeline and explain why each one had to have come
before the other. Ex: photosynthesis has to make oxygen this is why photosynthesis comes first.
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EVOLUTION 40
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