Divisions of the Geologic Time Scale: A Lesson Plan

A Look at the Four Eras

The four eras that make up the geologic time scale are:

• Precambrian Era: This is the first era of the earth’s history and it is also the longest. This era includes events from the earth’s formation until about 640 million years ago. At the end of the Precambrian Era, new life forms began to appear on earth.
• Paleozoic Era: The 400 million years following the Precambrian Era make up the Paleozoic Era. During this time, plants and animals began to inhabit the earth. When his era ended, many kinds of organisms became extinct.
• Mesozoic Era: This era was dominated by the dinosaurs. Most of the earth had a warm and wet climate. The era came to an end when most of the dinosaurs and other life forms became extinct. Although the exact cause is unknown, some scientists attribute the extinction of the dinosaurs to a large asteroid or rock form space colliding with the earth.
• Cenozoic Era: Since the end of the Mesozoic Era, the earth has existed within the Cenozoic Era. This era is marked by a cooler and drier climate. The earth has experienced several ice ages during this era. Humans have lived on earth only for a small portion of this era. It dates back roughly 66 million year ago.

To help your students visualize this outline of earth’s history, download the copy of the Timeline of Earth’s History from the Media Gallery. It will demonstrate the length of each era in relation to the others.

Graphing the Geologic Time Scale

To help further the student’s understanding of the earth’s geologic timeline, they can make a circle graph to compare the eras and their lengths. Students will each need a copy of the Geologic Time Scale table that can be downloaded from the Media Gallery. This

project may be one that students can work on in pairs.

Using the Timeline of Earth History that was downloaded from the section above, the students will calculate the percentage of the total earth history that each era lasted. To solve, the students will divide the era length by the age of the earth, then multiply by 100. The following formula can be used: (era length ÷ 4,600) x 100. Students should record the percentages in their Geologic Time Scale Table. Answers for the table are:

• Precambrian: 4055 million years; 88%
• Paleozoic: 300 million years; 7%
• Mesozoic: 179 million years; 4%
• Cenozoic: 66 million years; 1%

Using the recorded percentages, have students draw and label a circle graph of the earth history in geologic time. They can then answer the following questions using their graph.

1. Which era occupies the smallest section of your circle graph? (Cenozoic) The largest? (Precambrian)
2. How will your circle graph change in ten million years? (The circle graph would not change much proportionally by adding 10 million years. The 10 million would be added to the Cenozoic Era, making this era a little larger and the others would be adjusted and become smaller) Build a second circle graph that represents the time scale in ten million years from now.

This lesson helps students visualize the earth’s history and see where the era we live in falls within the course of history.