1. Dynamic Earth
Class 2
12 January 2006

2. Any Questions?

3. Uniformitarianism The present is the key to the past — James Hutton
Geologic processes that we see in operation today have worked much the same way over geologic time — however, rates and intensity of processes may have changed.

4. Early estimates of Earth’s age
1650: James Ussher -- Age: 6000 years (From reconstruction of the Bible)
By the mid-19th Century, the age of the Earth was still only an educated guess, but the general feeling was that the Earth was at least 500 million years old, and probably much older.

5. Radiometric Dating
Use of radioactive decay to determine the age of a rock
First proposed ~

6. Isotopes Different forms of the same element containing the same
number of protons, but varying
numbers of neutrons
i.e.:
235U, 238U 87Sr, 86Sr 14C, 12C

7. Naturally Occurring Isotopes of Carbon

8. Production and Decay of Radiocarbon

9. Beta Decay
Electron Capture
Alpha Decay

10. Radioactive Decay of Rubidium to Strontium

11. Radiometric Dating
Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay

12. Radioactivity and Absolute Time
Decay of
parent atoms
Growth of
daughter atoms

13. Radioactive Decay of Uranium 238 to Lead 206

14. Isotopic dating
Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters)
The rate at which this decay occurs is constant and known
Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding

15. Major Radioactive Elements Used in Isotopic Dating

16. Geologically useful decay schemes
Parent Daughter Half-life (years)
235U 207Pb 4.5 x 109
238U 206Pb 0.71 x 109
40K 40Ar 1.25 x 109
87Rb 87Sr 47 x 109
14C 14N 5730

17. Radiometric Dating
Shows that the earth is much older than people had previously suspected
Earth formed about 4.6 billion years ago

18. Relative vs Absolute Age
Usually geologists first establish relative ages then try to get absolute age dates
Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries

19. Principle of Superposition
In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom

20. Principle of Superposition
Sedimentary rocks are deposited in a layer-cake fashion:
Layer 4
Layer 3
Layer 2
Layer 1
Each layer is older than the one above and younger than the one below

21. Principle of Superposition
Youngest rocks
Oldest rocks
Principle of Superposition

22. Principle of Superposition
Although this is really obvious, it was not stated until 1669
This principle generally applies to volcanic rocks as well as sedimentary rocks

23. Principle of Original Horizontality
Layers of sediment are deposited in a horizontal or nearly horizontal position parallel to the Earth’s surface

24. Principles of original horizontality and superposition

27. Principle of Original Horizontality
Note that original horizontality is not strictly applicable to volcanic rocks because they are often deposited on slopes

28. Principle of Cross-cutting Relationships
Something (such as a dike or fault) that cuts across a layer must be younger than the layer

30. Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption

31. Unconformity Results from interruption of deposition
Represents a long period of time during which there either was no deposition, or earlier deposited material was eroded away

32. Sedimentation of Beds A-D Beneath the Sea

33. Uplift and Exposure of D to Erosion

34. Continued Erosion Removes D and Exposes C to Erosion

35. Subsidence and Sedimentation of E over C
Unconformity:
a buried surface of erosion

36. Formation of a Unconformity

37. Unconformity
Often not easy to recognize if the layers are all parallel
Much easier to recognize when there is a period of folding of the rocks before a period of erosion and renewed deposition

41. South rim of the Grand Canyon

42. South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian

43. South rim of the Grand Canyon
250 million years old
550 million years old
1.7 billion years old
Unconformity

44. The Great Unconformity of the Grand Canyon

45. Siccar Point, Scotland: Hutton’s Classic Unconformity -- Old Red Sandstone (~345 my) overlies rocks that are ~425 million years old

46. Siccar Point, Scotland
Buried and tilted erosional surface

47. Summary of Geologic Events in a small area

48. Relative Geologic Dating

50. These methods work well in small areas where we can see the relationships between rock layers.
What happens when we want to tell the relative ages of the strata on Oahu with respect to strata on Maui?
We have to figure out some way to correlate the layers of interest.

51. Correlation Process used to tie separated strata together
Based on matching physical features such as
Physical continuity - trace of rock unit
Similar rock types - marker beds, coal seams, rare minerals, odd color

52. Correlation
Within sedimentary layers there are often the remains of small animals (fossils)
Fossils are quite useful for correlating between two sections that are not laterally continuous

54. Principle of Lateral Continuity
Layered rocks are deposited in continuous contact

55. Correlation
Fossils have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age

56. Correlation
This technique is not very useful in Hawaii -- Why?
Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocks
Sometimes small amounts of sediment between layers of volcanic rock (such layers might have fossils), but most rocks in Hawaii do not have fossils

57. Radiometric dates provide absolute ages to the Geologic Column

58. Bracketing ages

59. Magnetostratigraphy Technique that works best in volcanic rocks
Time scale based on polarity reversal of Earth's magnetic field
Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks

60. Earth’s
Magnetic
Field

61. Magnetization of Magnetite

62. Lavas record magnetic reversals

63. Magnetic reversals over the past 20 million years

64. Magnetic time scale 0-700,000 -- Normal 700,000 - 2.5 my -- Reversed
> 2.5 my -- Normal
Ko`olau lavas mostly reversed in polarity, so they must be older than 700,000 yrs, but younger than 2.5 my
Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they must be older than 2.5 my

65. The Geologic time scale
Divisions in the worldwide stratigraphic column based on variations in preserved fossils
Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

66. The Geologic Column and Time Scale

67. Tuesday Geologic Time Scale (continued)
Homework #1 is due next Thursday, January 19