1. Earth’s History

2. Earth’s History Planet Earth is approximately 4.5 X 109 years old
Rocks of the crust provide clues to Earth’s past
By analyzing these clues we can infer events from the past

3. Earth’s History Principle of Uniformitarianism
Major assumption in geology
Events in the past occurred the same way that they are occurring today. Examples Include:
Weathering/erosion
Deposition
Volcanism
Plate tectonics

4. Geologic Time
Plate motions and mountain building events can be placed on the geologic time scale

5. Geologic Time
Geologists have divided Earth’s history into time units based on the the fossil record

6. Geologic Time A study of the fossil record shows
A great variety of plants, animals, and simpler life forms have lived on Earth in the past
That life forms have evolved through time
Most life forms of the geologic past have become extinct

8. Geologic Dating

9. There are two types of Geologic Dating
Absolute
Determines how many years old something is
Relative
Used to determine if one thing is younger or older than another

10. Tools used to determine
Absolute Age
Relative Age
Using radiometric dating
Law of superposition
Use of index fossils
Correlation of rock layers

11. Relative Dating

12. Law of Superposition
Sediments are laid down underwater in horizontal layers and form sedimentary rocks

13. Law of Superposition
In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest
Lower layers must be in place before younger rocks can be deposited on top of them
Exception: when something occurs to overturn layers

14. Grand Canyon

15. GRAND CANYON- LAW OF SUPERPOSITION
YOUNGEST ON TOP
OLDEST ON BOTTOM

16. Law of Superposition Rock layers are older than folds found in them
Layers were there before they were folded

17. Folds/Tilted

18. Law of Superposition Rock layers are older than faults found in them
This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault

19. Law of Superposition
Fossils are generally the same age as the rock layers in which they are found
Animal remains are deposited along with the sediments that will turn into sedimentary rocks

20. Law of Superposition
Igneous intrusions are younger than the rock that they cut through or flow out of

21. Igneous Intrusion - Cross Cutting

22. Unconformity
When a new rock layer is formed atop an eroded surface

24. Taconic Unconformity Upper Silurian Carbonates
April 17
Upper Silurian Carbonates
Tilted Ordovician Shales and Sandstones
We stopped along the side of the road where engineers had cut into the rock formations to make the road. The unconformity was clearly visible showing banded carbonate rock overlying tilted shales and sandstones.
unconformity
Taconic Unconformity
Allen

25. Practice: what happened here?
April 17
Practice: what happened here?
Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the next slide.
Allen

28. Correlation
Matching similar rock layers in different locations to see if they formed at the same time

29. f03_09_pg37
f03_09_pg37.jpg

31. 1. Which layers are the same?
2. Which layer is older E or F?
3. What is correct sequence of rock layers from oldest to youngest?
4. An unconformity is represented by the interface between which 2 layers?

32. Which fossil might be found in Devonian rock layers?

33. Volcanic Ash Falls
Can also be used to correlate rock layers over a large area
Ash is a good indicator because:
The ash from one explosion has distinct characteristics
ash can be deposited around the globe
The event occurs at one, geologically brief, time

34. April 17
Fossils
Naturally occurring preserved remains or impressions of living things
Generally only hard parts get preserved
Bones, teeth, shells
Hard parts are replaced by naturally occurring minerals
Allen

35. Eurypterus
NY State Fossil
Silurian index
fossil

36. Fossils Can be a mold (impression) Or a cast Ex: footprint
April 17
Fossils
Can be a mold (impression)
Ex: footprint
Or a cast
Ex: filled in footprint
Allen

38. Fossils (Cont) Other types Ice Tar Carbonaceous film Amber Imprints
Petrified

41. Fossils Why don’t fossils exist in most igneous or metamorphic rock?
April 17
Fossils
Generally found in sedimentary rock layers
Why don’t fossils exist in most igneous or metamorphic rock?
Allen

42. April 17
Fossils
The living organism that made the fossil lived during the time the rock layer was forming
i.e. when the sediment was being deposited
Allen

43. Fossils Fossils can provide information about ancient environments
Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…

44. Fossils A study of the fossil record shows:
April 17
Fossils
A study of the fossil record shows:
A great variety of plants, animals and simpler life forms have lived on Earth in the past
That life forms have evolved through time
Most life forms of the geologic past have become extinct
Allen

45. Index Fossils Index Fossils--used to date rock layers (strata)
Fossils from creatures that existed for a geologically short period of time
Ex. less than 2.0 x 107
Fossils from creatures that had a wide geographic distribution

46. Which letter would make a good index fossil?
Location A
Location B
Location C
Rock layer 1 W
W Z
Rock layer 2
W Z Y Z
Rock layer 3
W X X
X Z
Which letter would make a good index fossil?

47. Absolute Dating

48. When an unstable radioactive element changes into a stable element
Radioactive Decay
When an unstable radioactive element changes into a stable element

49. Example Unstable Radioactive Isotope Stable Decay Product
Carbon 14 (C14)
Nitrogen 14 (N14)
Uranium 238 (U238)
Lead 206 (Pb206)
Parent
Daughter

50. Radiometric Dating
Half life: The time required for 1/2 of a parent material to break down to daughter material

51. Another look

52. Radiometric Dating
The age of objects can be determined by measuring the relative amounts of radioisotope and decay products

53. Radiometric Dating
The half lives of radioisotopes vary depending upon the isotope
Examples:
C14 = 5,770 years
U238 = 4,500,000,000 years
Rb87 = 47,000,000,000 years

54. Carbon 14 is used to date biological remains
Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies
Uranium 238 is used to date rocks
Larger half life

55. Half Life Practice Problems

56. If half the K -40 of a rock has decayed into Ar-40, how old is the rock?
How many years would it take for 75 % of a sample of K-40 to disintegrate?

57. A sample that has ½ c-14 and ½ n-14?
How old are:
A sample that has ½ c-14 and ½ n-14?
A sample that has ¼ K-40 and ¾ Ar-40?
A sample that has ½ U-238 ½ Pb-206?

58. How old are:
A sample that has 1/8 Rb-87 and 7/8 Sr-87?
A sample that has ¼ C-14 and ¾ N-14?
A sample that has ¾ C-14 and ¼ N-14?

59. What fraction remains of:
April 17
A 50 billion year old sample has ____ Rb-87 and ____ Sr-87.
A 2.6 billion year old sample has____ K-40 and ____Ar-40.
Allen

60. Stranz should take a break ;)
This is so boring
Stranz should take a break ;)