1. Aim: How can we analyze the geologic time scale?

2. Measuring Time The Earth - 4.6 billion years old
Geologic time is divided into sections
Each section shows a specific change in life or in the Earth’s surface

3. Giving Time a Name Largest sections are called “eons”
“Eons” are divided into “eras”
“Eras” are divided into “periods”
“Periods” are divided into “epochs”

4. What do you know? In Pairs
Are there any geologic time sections that you have heard of?
What are they?

5. How is Time Divided?
Major changes in Earth’s history mark the boundaries between the sections
Most sections have been divided because a major organism developed or went extinct.

6. Extinction Extinction = the death of every member of a species
Lack of food
Lack of space
Environmental changes
99.9% of all species that have lived on Earth have gone extinct

7. Mass Extinction
Mass extinction = the extinction of many species at the same time
It can be caused by gradual changes in climate or ocean currents (Uniformitarianism)
Or by catastrophic events, like an asteroid hitting the earth and blocking out the sun (Catastrophism)
Can you think of an example of a mass extinction?

8. Precambrian Time

9. Highlight your ESRT
Take out a highlighter and highlight the Precambrian line all the way across on pages 8 and 9

10. Precambrian Time
Time from the formation of the Earth (4.6 billion years ago) to 544 million years ago
The Earth was very different then
At first, there was no no oxygen or living things
Then the first organisms appeared in the oceans billion years ago
They were called Prokaryotes

11. Prokaryotes Only one cell and no nucleus
Cyanobacteria, were the first organisms on Earth
Through photosynthesis, they released oxygen
As oxygen began to build up, the ozone was formed
The ozone protects the Earth against the sun’s harmful rays (deadly to land organisms)
Now organisms could live on land too!

12. Eukaryotes
After another 1 billion years, more complex organisms developed
They were called Eukaryotes
Many cells with a nucleus
Evolved into organisms on Earth today

13. Paleozoic Era
Began 542 million years ago and ended 251 million years ago
Divided into 6 periods

14. Highlight your ESRT
Take out a highlighter and highlight the Paleozoic line all the way across on pages 8 and 9

15. Paleozoic Era
Many new life forms appeared during the 1st period, the Cambrian Period = “Cambrian Explosion”
For the first time on Earth, organisms had hard parts (shells, exoskeletons)
Some organisms that were alive during this period still exist today (ferns and salamanders) but they were giants!

16. Permian Extinction The largest mass extinction. 251 million years ago
end of the Paleozoic Era and the start of the Mesozoic Era
Scientists are not sure what caused this mass extinction (maybe climate change & volcanoes)
90% of ocean life and 78% of land life died
Reptiles and amphibians survived!

17. Mesozoic Era Began 251 million years ago
Surviving reptiles and amphibians evolved into many other species (like dinosaurs) = “Age of Reptiles”

18. Highlight your ESRT
Take out a highlighter and highlight the Mesozoic line all the way across on pages 8 and 9

19. Cretaceous-Tertiary Extinction
Another mass extinction happened 65.5 million years ago
Marked the end of the Mesozoic Era and the beginning of the Cenozoic Era
All of the dinosaurs and half of the other animals & plants went extinct
Scientists think an asteroid hit Earth, the dust clouds blocked out the sun, plants died, then herbivores, then carnivores.

20. Cenozoic Era
Began 65 million years ago and continues today

21. Highlight your ESRT
Take out a highlighter and highlight the Cenozoic line all the way across on pages 8 and 9

22. Cenozoic Era
Scientists know the most about this Era because the fossils are in the top rock layers and are easier to find
Land & climate has changed a lot
What big change will happen that will end this era & start the next?