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
February 19
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?
February 19
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. Fossils

35. February 19
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

36. February 19
Fossils
Can be a mold – a hollow area in sediment in the shape of an organism or part of an organism
Ex: imprint of a shell
Or a cast – a copy of the shape of an organism by minerals and sediment
Ex: filled in shell
Allen

38. Fossils (Cont)
Preserved remains – organisms trapped in tar, amber, or ice remain unchanged
Carbon film – a thin coating of carbon on rock from remains of an organism

39. Fossils (Cont) Petrified – minerals replace all or part of an organism
Trace fossils – footprints, animal trails, or other clues left behind by animals that have been filled with sediment and turned to rock

40. Fossils Why don’t fossils exist in most igneous or metamorphic rock?
February 19
Fossils
Generally found in sedimentary rock layers
Why don’t fossils exist in most igneous or metamorphic rock?
Allen

41. February 19
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

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

43. Fossils A study of the fossil record shows:
February 19
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 changed through time
Most life forms of the geologic past have become extinct
Allen

44. 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

45. 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?

46. Absolute Dating

47. What is Absolute Dating?
Determining the actual age, in numbers of years, of something (rock, object, event).
How do you do it?
Index Fossils
Radioactive isotopes
Atoms of the same element that have different # of neutrons

48. Radioactive Isotopes
Radioactive isotopes are isotopes that are unstable and break down into other stable isotopes by a process called radioactive decay.

49. Radioactive Decay Alpha and Beta particles are emitted during decay
The Beta particles are what is measured to determine the amount of decay

50. Radioactive Isotopes
The radioactive isotope is called the parent isotope, and the stable isotope formed by its breakdown is called the daughter isotope.

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

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

53. Another look

54. Radiometric Dating
The age of objects can be determined by measuring the relative amounts of radioisotope (parent) and decay products (daughter)

55. 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

56. 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

58. Half Life Practice Problems

59. 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?

60. 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?

61. 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?

62. What fraction remains of:
February 19
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

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