1. Fossil Dating

2. Relative and Absolute Dating

3. HOW DO WE KNOW THE AGE OF FOSSILS?
Scientists use 2 methods to determine the age of fossils:
1. Relative Dating
2. Absolute Dating

4. Relative and Absolute Dating

5. What is Dating?
When geologists date rocks, they are determining how long ago they formed.
Two ways to do this:
Relative Dating
Absolute Dating

6. Relative Dating
Determining how old something is compared to something else
Use words like “older” or “younger” instead of exact numbers

7. I. Relative Dating
Relative Dating is when you give the age of a rock or fossil compared to another rock or fossil.
Example: Rock A is OLDER than Rock B.
An actual age in years is not determined.

8. II. Rules of Relative Dating
Law of Superposition: When sedimentary rock layers are deposited, younger layers are on top of older deposits.

9. Rules of Relative Dating
2. Law of Original Horizontality: Sedimentary rock layers are deposited horizontally. If they are tilted, folded, or broken, it happened later.

10. Rules of Relative Dating
3. Law of Cross-Cutting Relationships: If an igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is YOUNGER than the rock it cuts through

11. Order these layers from oldest to youngest
Youngest layer
Oldest layer

13. Order the events from oldest to youngest
YOUNGEST LAYER/EVENT
Fault
Gray Layer
Multicolored layer
Spotted Layer
OLDEST LAYER/EVENT

14. Absolute Dating Determining how old something is
Use numbers (in millions of years, mya)
Only works for Igneous Rocks

15. Absolute Dating Determines the specific age of a fossil
Looks at chemical properties
2 types:
Carbon-14 (radiocarbon)
Potassium-Argon

16. Carbon-14 Dating Also known as Radiocarbon dating
Used to date organic substances
Scientists measure the radiocarbon in the fossil to determine its age
Can only date specimens up to about 60,000 years old

17. Potassium-Argon Dating
Scientists determine the age of the rock surrounding the fossil to determine the fossil’s age.
Used only for inorganic substances (rocks and minerals)
Scientists measure the amount of argon in the rock to determine its age
Dates rock 60,000 years old and older

18. How Absolute Dating Works
When magma/lava cools, radioactive elements are incorporated into the minerals
Examples:
-- Potassium 40
-- Uranium 235
Potassium 40 (to Argon 40)
Uranium 235 (to Lead 207)

19. How Absolute Dating Works
These elements begin to decay at a known rate starting when the rock cools
We can measure how much of the element is left
Tells us how much time has passed since the rock formed.

21. What can we learn from this?
 Geologist
Basalt, dated to 80 mya
 T-Rex Fossil
Sandstone
Basalt flow, dated to 100 mya
So when did this T-Rex live?
Between 100 and 80 mya
This may not seem very accurate, but compared to the 4,500 million years the earth has been around it gives us a lot more information than we had before

22. What can we learn from this?
Absolute Dating
Helps us determine the age of the earth
Helps us determine when specific events in the history of the earth happened (ex. Extinction of the dinosaurs)
Relative Dating
Can help us estimate the time span between major earthquakes, storms, tsunamis etc
Can help us determine the order that life forms developed on earth

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24. QUIZ

25. Which rock layer is the oldest? Youngest?

26. If you wanted to figure out the exact age of a rock, what technique would you use?
Relative Dating
Carbon-14 Dating
Law of Superposition
Potassium-Argon Dating

27. If you were looking to find the exact age of a fossil that is at least 1 million years old, what technique would you use?
a. Carbon-14 Dating
b. Potassium-Argon Dating
c. Relative Dating
d. You can’t; it is impossible

28. If you wanted to figure out the exact age of a fossil that is less than 40,000 years old, what technique would you use?
a. Carbon-14 Dating
b. Potassium-Argon Dating
c. Relative Dating
d. You can’t; it is impossible