1. Essential Question: How is the age of the Earth determined?
Geologic Time
Essential Question:
How is the age of the Earth determined?

2. Vocabulary Geologic Time Scale Law of Superposition
Eon, Epoch, Era, Period Intrusion
Cenozoic Ice Cores
Mesozoic Relative Age
Paleozoic Absolute Age
Precambrian Radioactive Dating
Index Fossil

3. Concept Geologic Time Events Major eras of history
How fossils/rocks are dated

5. Geologic Time Scale
Timeline that organizes major events in Earth’s history, from Earth’s formation to present day.
Not organized by number of years!
(Can be different amounts of time.)
For the first 1 billion years, there was no life on Earth.

6. Eon Longest unit of time on the scale.
Most fossils come from most recent eon, the Phanerozoic.

7. Era Next longest unit of time; divided up by mass extinctions.
Mass extinctions – when many species go extinct over short time period.
Shown in rocks when certain fossils disappear, and others begin.
Can be caused by a catastrophe.

8. Mass Extinctions Video
Please take notes on each extinction event in your Lab NB.
Name of Extinction: When? Why? 1. 2. 3. 4. 5.

9. Period 2nd shortest unit of time. Divided up by unique rock layers.
Associated with specific fossils.

10. Epoch
Shortest unit of time on the scale.

11. Precambrian
Most of Earth’s early history, about the first 3.5 billion years.
No life existed yet on early Earth, so there are few fossils from this time.
Most of the fossils found are single-celled and from the very end of the Precambrian.
The end of this time is known as the Cambrian Explosion, when many different life forms developed.

12. Paleozoic
Era when many life forms began to develop, including the first….
Fish, insects, plants, amphibians, and reptiles.

13. Mesozoic The era of the first….
Dinosaurs, mammals, birds, flowering plants.

14. Cenozoic Most recent era (65 mya to present day).
Rise of the mammals (became dominant species).
Dinosaurs extinct.
Humans appeared .

15. Relative Age Age of fossils compared with other fossils.
Fossils near surface are YOUNGER.
Fossils near bottom are OLDER.
Does NOT give an exact age in years.

16. Law of Superposition
Older layers of rock lie underneath younger layers of rock
Sedimentary – layers are deposited in order..
This means….
Older fossils/organisms are found on the bottom and newer fossils/organisms are found near the top!

18. Intrusion
When magma pushed up through rock layers and forms a new vertical layer – igneous rock.
It becomes the newest/youngest section of rock, even though it goes from bottom to top.

20. Ice Cores
Cylinder of ice removed from an ice sheet where layers of snow build up each year.
Contain evidence about Earth in the layers….
Bubbles trapped – atmospheric conditions.
Dust, volcanic ash, CO2 – changes in atmosphere.
Pollen trapped – what types of plants lived.

21. Absolute Age The actual age in years of a fossil or rock.
Absolute Age is found by Radioactive Dating.
Rock layers can change, so we can only estimate the actual age of Earth, which is around 4.5 billion years old.

22. Radioactive Dating
Measures age of fossil/rock by comparing its radioactive substance decay rate.
Two elements used are Uranium and Carbon-14. They decay at a rate scientists can measure, which can tell scientists the age of the fossils they are contained in.

24. Index Fossils Provide pretty accurate info about age of a rock layer.
Fossils that existed for a short time period that scientists have measured in years.
Can be compared to location of newly found fossils to estimate age of those fossils.