2. Make a copy of Master 2.3c, Earth History Events, and cut it into
strips along the dashed lines.
A. Introduction
Ask the students to tell you what they mean when they say that
something happened “a long time ago.” (Answers will range from a
few months to centuries and beyond.) Ask students if it was “a long
time ago” that dinosaurs became extinct, and our earliest human
ancestors first appeared. Then ask them to guess the order in which
these events occurred. Record their guesses without comment.
Emphasize that scientists seek proof of how long ago different events
occurred by studying things that record the passage of long periods of
time, such as the layers in rocks (strata) and index fossils. Index fossils
represent species that existed only during specific time periods, so their
presence is an index to the age of the rocks. Radiometric dating
techniques can also reveal how long ago rocks were formed. The
dating of events that occurred a long time ago and the sequence in
which they occurred are among the puzzles scientists must solve. We
are constantly adding to our knowledge of Earth history.
B. Lesson Development
1. Distribute copies of the Centuries Worksheet (Master 2.3a). Holding
up the copy you placed on top of the paper stack, tell the students that
the stack is 12 reams high, and that every single page in it stands for
the same length of time. Explain that every dot on the sheet stands for
one year. The first dot on the top line represents this year. Each dot
after that one is a previous year. The entire sheet contains 2,000 dots.
Ask the students to circle the dot representing the year in which they
were born.
2. Distribute copies of Master 2.3b, Selected Events in Human History.
With these sheets and their Centuries Worksheets in front of them,
have students place the number of each event on Master 2.3b on or
near the dot that represents its year.
3. Tell the class that geologic time calls for a different scale than
historical time. From now on, one dot equals one hundred years. Each
sheet of paper in the 12-ream stack now represents 200,000 years. The
farther down the stack a sheet is, the farther back in history the time it
represents. Ask the students to determine how far down the stack a
sheet representing one million years ago is located. (It will be five
sheets down.)
4. Ask students how many dots there would be in a ream of 500 sheets
of paper if each sheet had 2,000 dots. (1,000,000 dots). If each dot
represents 100 years, how many years would one ream of sheets
represent? (100 years x 1,000,000 dots per ream = 100,000,000 years.)
5. Distribute the strips of paper cut from Master 2.3c, having each
student choose one. Give students these directions:
a. Calculate how many years an inch of paper represents. Look at your
paper strip and decide if the event it names will fit within the span of
years represented by the reams of paper (1.2 billion years).
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