1. GEOLOGIC TIME SCALE

2. Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.
Geologic Time Scale: A record of the life forms and geologic events in Earth’s history.

3. Precambrian Time The beginning of geologic time.
Covers about 88% of Earth’s history.
Ended 544 million years ago.

4. After Precambrian time, the basic units of the geologic time scale are eras and periods.
Eras: One of the three long units of geological time between the Precambrian and the present. The three eras are the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.

5. Paleozoic Era
Paleozoic Era: Began about 544 million years ago and lasted for 300 million years. The word “paleo” means “ancient or early.”

7. Mesozoic Era
Mesozoic Era: The “Age of Reptiles.” Began about 245 million years ago and lasted about 180 million years. The word “meso” means “middle.”

8. Cenozoic Era
Cenozoic Era: Began about 66 million years ago and continues to the present. The word “ceno” means “recent” and “zoic” means “ “life.” Mammals became common during this time.

9. Eras are subdivided into units of geological time called periods.
Periods: One of the units of geological time into which geologists divide eras.