1. Absolute Dating & The Age of the Earth How do we know how old rocks are?
3.96 Billion Year Old Gneiss

2. Age of the Earth: Originally Based on Mythology
Buddhist Tradition: Infinite Age (Cyclic)
Han Chinese Tradition:
23 Million Year Cycle

3. Archbishop James Ussher (1654)
( )
4004 BC
October 23
9:00 AM

4. Most scientific attempts are based on principle that:
Requires:
Natural Process
Occurs at a Constant Rate
Leaves a Geologic Record
Age (Time) = Amount of Change Rate of Change

5. Georges Louis Leclerc Comte de Buffon (1779)
75,000 yrs
Cooling of Molten Ball

6. William Thomson, Lord Kelvin (1862)
( )
Million yrs
Cooling of Molten Ball

7. John Joly (1899)
90 Million yrs
Saltiness of the Oceans
( )

8. John Phillips (late 1800’s)
About Million yrs
Accumulation of Sedimentary Rocks

9. George Darwin (late 1800’s)
About 100 Million yrs
Evolution of the Moon
( )

10. The Discovery of Radioactivity (1896)
Antoine Henri Becquerel
Marie and Pierre Curie

11. Becquerel’s Mistake

12. Uraninite - Uranium Ore

13. Bertram Boltwood Authur Holmes 1904-1907: Dated first rocks:
250 million to 1.3 billion years
1921: Earth about 4 billion years old!

14. Radioactive Decay Parent Isotope --> Daughter Isotope + Decay Particle + Energy

15. Alpha Decay Daughter Isotope Atomic Number = -2 Atomic Weight = -4
Uranium > Thorium Alpha Particle + Energy

16. Beta Decay Daughter Isotope Atomic Number = +1 Atomic Weight = +0
Carbon-14 --> Nitrogen-14 + Beta Particle + Energy

17. Decay of Uranium-238 to Lead 206
Alpha Decay
Beta Decay

18. Half Life Time it takes for half of the parent isotope to decay into daughter isotope
Daughter Isotopes
Parent Isotopes

19. Radioactive Isotopes Used for Absolute Dating
parent daughter half life (years)
235U 207Pb 4.50 x 109
238U 206Pb 0.71 x 109
40K 40Ar 1.25 x 109
87Rb 87Sr 47.0 x 109
14C 14N 5,730

20. Dating & Radioactive Decay

21. Information Required for Radiometric Dating
Initial Parent Isotope Content
Half Life of Isotope
Current Parent Isotope Concentration
Closed System
Remember:
Age = Amount of Change Rate of Change

22. Mass Spectrometer

24. When does a system become “Closed”? (i.e., What are you dating?)
Cooling of Igneous Rock
When does a system become “Closed”? (i.e., What are you dating?)
Metamorphism
Death of Organic Material

25. Sedimentary Rocks: What are we dating?

29. Geologic Time Scale

30. Back to the Age of the Earth
Oldest Rocks on Earth (Acasta Gneiss, Northern Canada)
- about 3.96 Billion Years
Oldest Mineral Crystals on Earth
(Zircon, Jack Hills Conglomerate, Western Australia)
- about 4.4 Billion Years
Age of the Earth Billion Years

31. Carbonaceous Chondrite
Meteorites
Carbonaceous Chondrite
(Allende Meteorite)
Iron Meteorite

32. Meteorite Ages Type Number Method Age (Gyr))
Chondrites (CM, CV, H, L, LL, E) 13 Sm-Nd /- 0.76
Carbonaceous chondrites 4 Rb-Sr /- 0.34
Chondrites (undisturbed H, LL, E) 38 Rb-Sr /- 0.02
Chondrites (H, L, LL, E) 50 Rb-Sr /- 0.04
H Chondrites (undisturbed) 17 Rb-Sr /- 0.04
H Chondrites 15 Rb-Sr /- 0.06
L Chondrites (relatively undisturbed) 6 Rb-Sr /- 0.12
L Chondrites 5 Rb-Sr /- 0.12
LL Chondrites (undisturbed) 13 Rb-Sr /- 0.02
LL Chondrites 10 Rb-Sr /- 0.06
E Chondrites (undisturbed) 8 Rb-Sr /- 0.04
E Chondrites 8 Rb-Sr /- 0.13
Eucrites (polymict) 23 Rb-Sr /- 0.19
Eucrites Rb-Sr /- 0.30
Eucrites Lu-Hf /- 0.19
Diogenites 5 Rb-Sr /- 0.18
Iron (plus iron from St. Severin) 8 Re-Os /- 0.21
After Dalrymple (1991, p. 291); duplicate studies on identical meteorite types omitted.

33. Other Forms of Absolute Dating
Dedrochronology
Fission Tracks