1. Relative / Absolute Dating
Fossils
and
Relative / Absolute Dating

2. THE FOSSIL RECORD

3. THE FOSSIL RECORD Importance
Provides the best proof of the history of life showing how extinct species have led to today’s species
“Lucy”, 3.2 MY
Archaeopteryx, 150 MY

4. Next:
Read and summarize Becoming a Fossil by Bill Bryson

5. NO Becoming a Fossil Formation of Fossils (Summary)
Organism is buried quickly in sediment under water (sometimes: ice, amber or tar)
Hard parts (bone& shell) are slowly replaced by minerals dissolved in water
Over millions of years, must maintain identifiable shape that someone will recognize as worth keeping
Will you ever find fossils in metamorphic or igneous rock? NO

6. Completeness of the Fossil Record?

7. The Story Fossils Tell What type of rock is it?
Provide an observation & an inference about this fossilized animal.
If this fossil was found in central Cumberland County, PA, how old is it? (Use the Geologic Map on next slide)

8. What type of rocks are most common in Bucks County?
What age fossils might you find in Bucks County?

9. Next
Complete the Mystery Fossil Activity

10. Mystery Fossil Activity
DIRECTIONS: 1. Every group member should help cut out the mystery fossil bones. 2. Work as a group to put together the skeleton of the animal. 3. Once you have agreed, glue the bones on a large piece of paper. 4. Work together to decide what the outside of the animal looked like. Was it covered with skin, fur, scales, something else? How large was it? What did it eat? 5. Once finished with re-creating the fossil – work as a group to complete the 10 Summary Questions.

11. Summary Questions
1. Did you make any assumptions at the beginning of the activity that slowed you down in putting the bones together? ______ Explain:
2. Did any of your group members resist the group’s ideas? _____ Explain:
3. Did any information from another group influence your construction? _____ If so, what?
4. What did your group you say about how and where this animal lived?
5. What did this animal eat?
6. How big was it?
7. Where did it live and when?
8. This animal was a(n) _____________________________________. We know this because:
9. How close to reality was your construction?
10. How is it possible for scientists to do studies about things that happened millions of years ago?

12. Mystery Revealed FACTS:
1. The Archaeopteryx may have eaten insects and small animals. It lived during the
Jurassic Period, about 150 million years ago.
2. Its name means “Ancient Wing.” It’s pronounced: ark-ee-OP-ter-iks
3. The Archaeopteryx seemed to be part-bird and part-dinosaur. Unlike modern-day birds,
it had teeth, three claws on each wing, and a long, bony tail. Like modern-day birds, it
had feathers and a very light body with hollow bones.
4. Length - 1 foot long from beak to tail
Wingspan feet
Weight - 11 to 18 ounces
5. This is the size of the Archaeopteryx
next to an adult male:
6. Archaeopteryx is one of the oldest-known birds.

14. RELATIVE DATING

15. WARM-UP

16. ANSWER

17. Uniformitarianism
 Earth’s geologic processes acted in the same manner and with essentially the same intensity in the past as they do in the present and that such uniformity is sufficient to account for all geologic change
MEANING- things that change earth’s geology today are the same things that have changed it for millions of years

18. TWO Ways to Date Fossils & Rocks
Relative Dating
Determines the sequence of events by comparing rock layers, but cannot tell how long go the events occurred.
Radiometric Dating
AKA – Absolute Dating
Precise dating by measuring radioactive decay of elements in rock
Radiometric Dating

19. Relative Dating Rules to Follow:
Law of Superposition - older rocks are found under younger rocks
Principle of Original Horizontality – sediment is deposited in flat, horizontal layers

20. Rules Continued
Principle of Cross-Cutting Relationships – A fault or igneous intrusion must be younger than the layers it cuts through.
Igneous Intrusions – solidified magma that is contained within another rock layer

22. Rules Continued Unconformities
A break in the rock record. Erosion removes rock before deposition continues.
Types:
Disconformity
Nonconformity
Angular Unconformity

23. Disconformity
Only with Sedimentary Rock - Erosion occurs before new sedimentary layers are formed

24. Nonconformity
Erosion occurs to metamorphic or igneous rocks and then younger sedimentary layers form on top

25. Angular Unconformity
New layers of sedimentary rock are deposited over older layers that have been tilted or folded and then eroded.

27. Practice – Which type of Unconformity?
Angular Unconformity
Disconformity
Nonconformity

28. Rules Continued
Inclusions – Pieces of rock contained within another rock layer. The rock layer containing the inclusions must be younger.

29. Practice – Place layers in order from youngest to oldest.

30. Answers

31. Practice
Youngest
______
Oldest

32. Practice
Youngest C B A D E
Oldest

33. PRACTICE

34. ANSWER

35. Correlation & Index Fossils
Correlation - Rock layers formed at the same time and by the same processes are compared at different locations to get a more complete picture of the rock record
Index Fossil – a fossil that is geographically widespread and abundant in the fossil record, but that existed for a limited span of time (so usually in one layer).
The presence of an index fossil in rock layers at different locations means that the layers are roughly the same age and likely formed in similar environments.

36. Correlation Example:

37. Radiometric Dating (Absolute Dating)
Radioactive Elements (or Isotopes): unstable atoms giving off radiation to become stable
protons & neutrons are given off as alpha, beta and gamma particles – this is called radioactive decay
Ex: Uranium-238 & Carbon-14
Radioactive dating: Radioactive decay (going from unstable to stable) occurs at a constant rate called a half life. Each radioactive element has its own half life.

38. Half Life - Radioactive Decay
Half life: the amount of time it takes for half the radioactive atoms in a substance to become stable.
Example: Red Dots are radioactive elements
Green Dots are stable elements

39. Examples of Radioactive Elements
Uranium-238 has a half-life of 4.5 billion yrs (becomes Lead)
Potassium-40 has a half-life of 1.3 billion yrs (becomes Argon-40)
Carbon-14 has a half-life of 5730 yrs (becomes Nitrogen) – AKA – Radiocarbon Dating – used to date once living material
Which would you use to date a wooly mammoth bone?
Which would you use to date igneous rock that may be the oldest sample on North America?

40. Carbon 14- It’s in You!!!! Carbon atom is in every living thing
Some are radioactive Carbon-14
When an organism dies Carbon-14 begins to decay to stable Nitrogen

41. Carbon-14 Rate of Decay
Every 5,730 years is the half life of Carbon-14

42. Example of Radiocarbon Dating
Scientists were able to determine the age of the Iceman.
5,300 years old
Used the decay of radioactive carbon

44. Practice What percentage of a radioactive element will be left after:
1 half-life ______ half-lives _____ half-lives ______
One isotope of Iodine has a half-life of 8 days. That is, after 8 days, ½ of all the atoms in a given sample of the isotope will have undergone radioactive decay. Using a 1 gram sample. How much will remain after 8, 16, and 32 days?
8 days ___________ 16 days ___________ 32 days __________