1. The Geologic time scale
Chapter 7 lesson 4

2. What is the geologic time scale?
Geologists use the geologic time scale to show Earth’s history. It is a record of the geologic events and the evolution of life forms as shown by the fossil record.

3. Dividing geologic time
The divisions of geologic time scale depend on events in the history of life on Earth. Precambrian Time: The beginning – covers about 88% of Earth’s history- ended 542 million years ago. Few fossils survived this period. Eras: time between Precambrian Time and the present – 3 long units called ERAs (Paleozoic, Mesozoic and Cenozoic) Periods: Eras are divided into PERIODs

4. Questions:
Why is a time scale used instead of a calendar to represent Earth’s history?
A calendar would be hard to use because Earth’s history is so long.
When did geologic time begin?
About 4,600 million years ago
When does geologic time end?
It doesn’t…. We are in the Quaternary Period of the Cenozoic Era
How long is a period?
A. It doesn’t have an exact length.