1. Uniformitarianism “The Present is the Key to the Past”
Laws of nature DON’T change with time
Past events explained & estimated by modern processes

2. Uniformitarianism

3. Original Horizontality
sedimentary rocks form in horizontal layers parallel to Earth’s surface and will remain that way unless disturbed
SILT
SAND
CLAY
SILT
CLAY
EARTH'S SURFACE

4. Horizontal Layers of the Grand Canyon

5. Can you infer who is older?
Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events
Can you infer who is older?
He is

6. 3 Ways to Determine
Relative Time………?
Coming Right Up!!

7. Law of Superposition Top = youngest layer
In undisturbed sedimentary rock or lava flows
In undisturbed sedimentary rock or lava flows:
Top = youngest layer
Bottom = oldest layer

8. Law of Cross-Cutting Relationships
Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts
youngest layer

9. Example of Cross-Cutting
Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens
Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens

10. Example of Cross-cutting
Examples of Cross-Cutting
Extrusion - igneous rock that forms when lava
flows on Earth’s surface and hardens

11. Example of Cross-cutting Example of Cross-cutting
Inclusion - pieces of older rock trapped within younger rock

12. Examples of Cross-Cutting
Faults, joints, tilts, and veins also follow the
law of cross--cutting relationships

13. Law of Included Fragments
Sediments are older than the cement
and rock they are part of
The pebble is older than the conglomerate rock it is found in.

14. However, there are exceptions to these laws…….

15. Exceptions to Law of Superposition
Rock layers can be overturned, older layers pushed on top of younger layers
THEREFORE, geologists use these clues to find the original position of rock layers…

16. Exceptions to Law of Superposition
Graded Bedding - coarse, heavy particles - bottom layer

17. Exceptions to Law of Superposition
Ripple-Marks - tops of ripple marks point

18. Exceptions to Law of Superposition
Cross-Beds - curved at bottom layer, cut off at the top

19. Unconformity Gaps or missing layers in the rock record due to erosion
Angular unconformity

20. Formation of an Unconformity

21. Formation of an Unconformity

22. Formation of an Unconformity

23. Formation of an Unconformity

24. Formation of an Unconformity

25. Picture of Unconformity

26. Absolute Time – finding an exact date for rocks or geologic events in YBP (years before present)
Born 2004 or
1 YBP
Born 1922 or
83 YBP

27. How do geologists measure
Absolute Time?
Radioactive Decay
2. Biological Clocks - Tree rings, coral growth rings, glacial lake layers of sediment

28. Before you measure ABSOLUTE TIME you must understand the following terms…..

29. ISOTOPES
- alternate forms of an element
- some are RADIOACTIVE

30. Radioactive isotope - an isotope that undergoes radioactive decay
Radioactivity
release of high-energy particles from unstable atoms, maybe cancer-causing
Radioactive isotope - an isotope that undergoes radioactive decay
- used to determine absolute age
Examples:
Uranium 238 and Carbon 14

31. Uranium - 238 Radioactive Isotope Radioactive Decay
radioactive isotopes give off radioactive particles until they become stable isotopes (new elements)
Example:
Uranium Radioactive Isotope
Radioactive Decay
Lead Stable Isotope

32. Half - Life Let’s look at the half - life of Uranium 238
Amount of time it takes 1/2 the atoms in a given sample to go through radioactive decay is called it’s
Half - Life
Let’s look at the half - life of Uranium 238

33. Pb206 - Stable decay product
U238 - Radioactive
Pb206 - Stable decay product
1/4
1 : 0
1/2 : 1/2
3/4
1/32
1/8
1/16
???
7/8
15/16
31/32

34. Since the Half - Life of Every Isotope is ….
1. Unique
2. Always the same time
3. Unaffected by:
Mass, Volume, Temperature & Pressure

35. We can use it to find:
THE
ABSOLUTE
AGE OF
ROCKS!!!

36. Half-Lives of Special Isotopes
Uranium Lead 206
4.5 x 109 years
(4,500,000,000 years)
Carbon Nitrogen 14
5.7 x 103 years
(5,700 years)

37. Unknown Radioactive Isotope. half - life = 3000 years
Unknown Radioactive Isotope half - life = 3000 years sample size = 200 g Let’s complete the table below.
Half Life
Remaining Unknown Radioactive Isotope (g)
Number of Years
200 1 2 3 4 5
3000
100 50
6000 25
9000
12.5
12,000
6.25
15,000

38. MASS OF UNKOWN (G) DECAY TIME (YEARS)
NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOWN
200
100
MASS OF UNKOWN (G)
0 Half - Lives
1 Half - Life
2 H - L 3 4
3000
6000
9000
12000
15000
DECAY TIME (YEARS)

39. Remaining Radioactive C14 (g)
Carbon half - life = 5,700 years sample size = 800 g Complete the following table.
Half Life
Remaining Radioactive C14 (g)
Number of Years
800 1 2 3 4 5
400
5,700
200
11,400
100
17,100 50
22,800 25
28,500

40. MASS OF C14 (G) DECAY TIME (YEARS)
NOW LET’S GRAPH THE HALF-LIFE OF CARBON 14
800
400
0 Half - Lives
MASS OF C14 (G)
1 Half - Life 2 3 4
5,700
11,400
17,100
22,800
28,500
DECAY TIME (YEARS)

41. Absolute Time vs. Relative Time
Most geologists use relative time because it’s
- less expensive
- easier to do

42. Absolute Dating & Geologic History

43. Fossils the remains of plants and animals that lived in the past
mainly formed in sedimentary rock

44. Fossils Form in Several Ways
1. Original remains - unchanged remains of a plant or animal
Ex. Dinosaur bones/teeth
2. Replaced remains - soft parts of original animal replaced by minerals
Ex. Petrified wood

45. Fossils Form in Several Ways
3. Mold - fossilized shell or bone dissolves and leaves a hollow depression in a rock
Ex. Ferns, leaves, or fish
4. Cast - new mineral material fills a mold
Ex. Shellfish

46. Fossils form in Several Ways
5. Trace fossils - fossil evidence of animal movement
Ex. Trails, footprints, burrows & borings

47. Evolution - process of change that produces new life forms over time
- fossils provide evidence of evolution

48. Natural Selection theory of evolution
best adapted organisms will survive in large numbers and pass on these adaptations to their offspring

49. Index Fossils Index fossils must: 1. Be recognizable 2. Widespread
- special fossils that give the relative age of the rocks that contain them
Index fossils must:
1. Be recognizable
2. Widespread
3. Exist for a short time

50. Key Bed
- single rock layer that is recognizable, widespread, and exists for a short time

51. Correlation - matching rock layers from one area to another
This can be done in several ways:
1. “Walking the outcrop”
2. Matching similar rock features
3. Index fossils

52. Correlating or matching index fossils from three outcrops.

53. Correlate the 3 rock sequences to make a single rock column for the geologic history of the region. (oldest on bottom)

54. Youngest
Oldest

55. Correlation means “to match” Geologists match all of the following:
No place on the earth has all the rocks formed through time. So geologists use correlation to make a single rock column that shows the entire history of the earth.
Correlation means “to match”
Geologists match all of the following:
rock types
rock sequences
ash layers from volcanic eruptions
index fossils

56. Page 8 & 9 of the Reference Tables.

57. Each black vertical bar represent the existence of a group of organisms.
Extinction
First appearance

61. MASS OF UNKNOWN (G)
DECAY TIME (YEARS)

62. MASS OF C14 (G)
DECAY TIME (YEARS)

63. # of protons + # of neutrons
Isotopes - different forms of the same element
- normal # of protons
- more than normal # of neutrons
# of protons + # of neutrons
# of protons
Normal Hydrogen
1 proton
0 neutrons
1 electron
Isotope of Hydrogen
1 proton
1 neutron
1 electron

64. Radioactive Isotope of Hydrogen
1 proton
2 neutrons
1 electron
Radioactive Hydrogen H-3