1. The Geologic Time Scale The Geologic Time Scale is a record of life forms and geologic events in Earth’s history.
**Key Concept: Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

2. Dividing Geologic Time
Life and Geologic Time
Dividing Geologic Time
Sometimes it is possible to distinguish layers of rock that formed during a single year or season.
In other cases, thick stacks of rock that have no fossils provide little information that could help in subdividing geologic time.

3. The Geologic Time Scale
Earth has a very long history. Years and centuries are not very helpful for such a long history. So scientists use the geologic time scale for Earth’s history.
The geologic time scale is a record of how Earth and its life forms have changed through time. For example, the scale shows when life first appeared on Earth.
In the geologic time scale, time is divided into bigger blocks than years or centuries. The scale begins when Earth formed 4.6 billion years ago and goes to the present.

4. Divisions of Geologic Time
The geologic time scale is divided into eons, eras, periods, and epochs. Unlike divisions of time such as days or minutes, the divisions of the geologic time scale have no fixed lengths. Instead, they are based on changes or events recorded in rocks and fossils.

5. Divisions of Geologic Time (Look at a picture at bottom of your notes
The largest unit of time is an eon. An eon is an extremely long, indefinite period of time.
Earth’s 4.6-billion-year history is divided into Precambrian time and three eras: Paleozoic, Mesozoic, & Cenozoic.
Each era is subdivided into a number of periods. For example, the Paleozoic Era is divided into six periods. The Cambrian Period is important because it is the first period after Precambrian Time.
The periods of the Cenozoic, the most recent era, are further divided into epochs.

6. We live in the Cenozoic era!
Present day Earth is in the Cenozoic era and the Quaternary period in the Holocene epoch.
Geologic time has NOT ended!!!!!

8. Quaternary Period, and Holocene epoch.
Age of Man
Find the Cenozoic Era,
Quaternary Period, and Holocene epoch.
Write “Age of Man” beside it.

9. Precambrian Time at 4.6 Billion Years Ago
This period is MOST of Earth’s history. For nearly 4 billion years, during most of Precambrian time, no plants or animals existed. Multicellular organisms develop late in the Precambrian.

10. Paleozoic Era at 544 Million Years Ago
LIFE EXPLODES! At the beginning of the Paleozoic era, all life lived in the oceans. Early invertebrates developed and later reptiles became dominant on land. Early plant included simple mosses, ferns, and cone-bearing plants.

11. Mesozoic Era at 245 to 65 Million Years Ago
(Age of the Reptile/Dinosaurs
Dinosaurs lived along with the first mammals, birds, and flowering plants. Reptiles were dominant.

12. Cenozoic Era 65 mya to Present Day
The first humans appeared in the later part of the Cenozoic era, which continues today. The diversity of life forms increased. New mammals and birds appeared while other became extinct. Flowering plants became most common.

13. Review question #1
When does the geologic time scale begin? Circle the letter of the correct answer.
a. 4 billion years ago
b. 4.6 billion years ago
c. 544 million years ago

14. Review Question #2
Fill in the blanks in the diagram of the geologic time scale.

15. Review Question #3
Which part of the geologic time scale is the longest?
Circle the letter of the correct answer.
a. Precambrian Time
b. Paleozoic Era
c. Cenozoic Era

16. Review Question #3 Which era do we find humas?
Circle the letter of the correct answer.
a. Precambrian Time
b. Paleozoic Era
c. Cenozoic Era
d. Mesozoic Era

17. Review Question #3 Which era do we find dinosaurs?
Circle the letter of the correct answer.
a. Precambrian Time
b. Paleozoic Era
c. Cenozoic Era
d. Mesozoic Era

18. Review Question #4
Geologic time is divided into units based on ______.
a. geologic changes
b. fossils and rocks
c. types of life-forms living during certain periods
d. all of these