Scientists use two methods to determine the age of fossils - relative dating and absolute dating. Relative dating involves determining if one fossil is older or younger than another fossil without an exact age. Absolute dating determines a specific numerical age by measuring properties like the amount of carbon-14 or potassium-argon in igneous rocks and fossils. Scientists can use carbon-14 dating on fossils less than 60,000 years old and potassium-argon dating on fossils and rocks over 60,000 years old.

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
1. 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
Fault
Gray Layer
Multicolored layer
Spotted Layer
OLDEST LAYER/EVENT
YOUNGEST 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

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?
 T-Rex Fossil
 Geologist
Basalt, dated to 80
mya
Basalt flow, dated
to 100 mya
Sandstone
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

24. QUIZ

25. Which rock layer is the
oldest? Youngest?
Oldest
Youngest

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