Comparing the magnitude of geologic time with spans of time in a person's lifetime is difficult for many students. In this activity, students use a long paper strip and a reasonable scale to represent visually all of geologic time, including significant events in the development of life on earth as well as recent human events. The investigation requires two class periods and is appropriate for grades 5 through 12. The activity is adapted with permission from the Earth Science Curriculum Project.13
This activity provides all students with an opportunity to develop understandings of the earth systems as described in the National Science Education Standards. Specifically, it introduces them to the following concepts:
A mathematical scale representing the length of geologic time.
The relationship of time between human events, events in earth's history, and the total age of the earth.
The formation of the sun, the earth, and the rest of the solar system from a nebular cloud of dust and gas 4.6 billion years ago.
The estimation of geologic time by observing rock sequences and using fossils to correlate the sequences at various locations. A current method of dating earth materials uses the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.
The ongoing evolution of the earth system resulting from interactions among the solid earth, the oceans, the atmosphere, and organisms.
The evidence for one-celled forms of life—the bacteria—extending back more than 3.5 billion years. The evolution of life caused dramatic changes in the composition of the earth's atmosphere, which did not originally contain oxygen.
Geologic time is largely subdivided on the basis of the evolution of life and on the amount and type of crustal activity that occurred in the past. Geologic time is ordered both relatively and absolutely. For relative dating the sequence in which rock strata formed is important; to explain the complete time scale for all of geologic history required correlating rock formations throughout the world. Fossils are important guides in this correlation as scientists assigned relative dates to the world's rocks according to a proposed sequence of life (fossil evidence).
Radiometric dating provides absolute ages for events in the earth's history. Radiometric dating techniques apply the decay rates of selected naturally radioactive isotopes to stable daughter isotopes to determine how long the unstable parent isotopes have been decaying. Fairly accurate dates have been determined for the events beginning in the Cambrian era; this comprises about 12 percent of the earth's history.
The following materials will be needed by each group of two students:
A paper strip, such as adding machine tape or shelf paper,
A meter stick,
Masking or cellophane tape.
If students use the scale suggested—1 millimeter to 1 million years and 1 meter to 1 billion years—a paper strip 5 meters in length efficiently accommodates the 4.5-billion-year time scale. Make copies of Student Investigation Sheet A on page 91 ("Approximate Ages of Events in Years Before the Present"). Students will use this page to conduct their investigation.
Engage Ask students how long a million years is. How would students count or measure a million of anything? Use this discussion to help students arrive at the question: How does a million years, or even the time since the last ice age, compare