1. Absolute Dating of
Rocks and Fossils

2. Early Efforts on Dating the Age of the Earth
James Ussher (mid 1600's)
4004 B.C.
Georges Buffon (mid 1700’s)
75,000 yr
Charles Walcott (1893)
75 m.y.
Lord Kelvin (late 1800's)
100 m.y.

3. II. Constancy of Natural Laws
Early Efforts on Dating the Age of the Earth
II. Constancy of Natural Laws
Catastrophism
Georges Cuvier (late 1700’s)
Uniformitarianism
James Hutton (late 1700's)

4. Radiometric Dating of Rocks
Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).
Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.

5. Absolute Dating of Rocks
The main method used to get the exact age of a rock is called radioactive or radiometric dating. This method studies the decay rates of radioactive isotopes in order to determine the age of the rock.
But First
What is an atom?
What is an isotope?

6. } ATOMS Protons Neutrons Nucleus Electrons Nucleus
Fundamental unit of matter made up of subatomic particles }
Protons (positive charge)
Nucleus
Neutrons (no electrical charge)
Electrons (negative charge)
Protons
Neutrons
Nucleus
Electrons

7. ISOTOPES Carbon-12 Carbon-13 Carbon-14 12C 13C 14C 6 6 6 98.9 % 1.1 %
< 0.1 %
6 protons
6 protons
6 protons
6 neutrons
7 neutrons
8 neutrons

8. Types of Radioactive Decay
protons
alpha particle 4H 2
neutrons
electrons
238U
234Th 92 90
ALPHA
DECAY
alpha decay
Parent Nucleus
Changes in atomic number
and atomic mass number
Daughter Nucleus
92 protons
90 protons
Atomic Number = - 2
146 neutrons
Atomic Mass Number = - 4
144 neutrons
Alpha decay occurs when the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a helium nucleus is emitted.

9. Types of Radioactive Decay
protons
beta particle
p +
neutrons
electron
electrons
137Ba
137Cs 56 55
BETA
DECAY
beta decay
Parent Nucleus
Daughter Nucleus
Changes in atomic number
and atomic mass number
55 protons
Atomic Number = + 1
56 protons
82 neutrons
Atomic Mass Number = 0
81 neutrons
Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted.

10. Types of Radioactive Decay
protons
neutrons
electrons
electron capture
204Po
204Bi 84 83
ELECTRON
CAPTURE
Changes in atomic number
and atomic mass number
Parent Nucleus
Daughter Nucleus
Atomic Number = - 1
84 protons
Atomic Mass Number = 0
83 protons
120 neutrons
120 neutrons
Electron capture also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron.

11. U Pb206 Series
This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

12. Radioactive Dating
During radioactive decay, the original material or “parent material” decays at a set rate into the new material or “daughter material”
As the number of parent material decreases, the number of daughter material increases.
The amount of time that it takes for exactly one-half of the parent to turn into daughter is known as a half-life.
For example – the half-life of Carbon-14 is 5,700 years. Carbon-14 is the parent material and Nitrogen-14 is the daughter material it decays into.
Therefore, this means that it will take 5,700 years for one-half of the Carbon-14 to turn into Nitrogen-14.

13. Radioactive Dating
So, why does Radioactive Dating allow geologist to get absolute dates?
The half-lives of radioactive isotopes are set in stone and DO NOT change.
For example, the half-life of Carbon-14 is always 5,700 years, no matter what.
A table of some common half-lives are given in the next slide.

14. Radioactive Decay Rates
Isotope
Disintegration
Half-Life
Iodine-131
131I
131Xe
8.02 days
Cobalt-60
60Co
60Ni
5.3 years
Carbon-14
14C
14N
5.7 x 103 years
40Ar
40K
1.3 x 109 years
Potassium-40
40Ca
4.5 x 109 years
Uranium-238
238U
206Pb

15. Half Life How can we tell age based on the number of parent isotopes?
Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.
U238
Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb
Po218

16. Half Life Parent Isotope (100) Daughter Isotope Half-Life = 10 seconds
Time (s)
Half-Life
Parent
Daughter
100 10 1 50 50 20 2 25 75 30 3
12.5
87.5 40 4
6.25
93.75 50 5
3.125
96.875

17. Start with a million parent atoms…
100 50 25
12.5
6.25
3.13
PARENT=1,000,000
DAUGHTER=0

18. 1 Half-Life
100 50 25
12.5
6.25
3.13 1
PARENT=500,000
DAUGHTER=500,000

19. 2 Half-Lives 1 2 PARENT=250,000 DAUGHTER=750,000 100 50 25 12.5 6.25
3.13 1 2
PARENT=250,000
DAUGHTER=750,000

20. 3 Half-Lives 1 2 3 PARENT=125,000 DAUGHTER=875,000 100 50 25 12.5 6.25
3.13 1 2 3
PARENT=125,000
DAUGHTER=875,000

21. 4 Half-Lives 1 2 3 4 PARENT=62,500 DAUGHTER=937,500 100 50 25 12.5
6.25
3.13 1 2 3 4
PARENT=62,500
DAUGHTER=937,500

22. 5 Half-Lives 1 2 3 4 5 PARENT=31,250 DAUGHTER=968,750 100 50 25 12.5
6.25
3.13 1 2 3 4 5
PARENT=31,250
DAUGHTER=968,750

23. 6 Half-Lives 1 2 3 4 5 6 PARENT=15,625 DAUGHTER=984,375 100 50 25 12.5
6.25
3.13 1 2 3 4 5 6
PARENT=15,625
DAUGHTER=984,375

24. Example #1
100 50 25
20%
12.5
6.25
3.13 1 2 3 4 5 6
2.3 half-lives
If a sample has 20% 14C, how old is the sample?
Number of parent isotopes
Half-Life 14C = 5,700 years
Number of daughter isotopes

25. Example #2
100 50 25
12.5 8%
6.25
3.13 1 2 3 4 5 6
3.6 half-lives
A sample is found to have 131I at a concentration of 8%.
How old is the sample?
Number of parent isotopes
Half-Life 131I = 8 days
Number of daughter isotopes

26. Let’s Try Some More… Parent 1 1/2 1/4 1/8 1/16 1/2 3/4 7/8 15/16
Daughter 1
1/2
1/4
1/8
1/16
1/2
1st half-life
3/4
2nd half-life
7/8
3rd half-life
15/16
4th half-life

27. Once Again C-14 0 % N-14 100% 50% 25% 12.5% 6.25% 50% 1st half-life
75%
2nd half-life
87.5%
3rd half-life
93.75%
4th half-life

28. Radioactive Dating
The smaller the half-life, the less useful the radioactive isotope is for dating really old stuff.
For example, Carbon-14 half life is only 5,700 years. Therefore, it can only accurately date rocks no older than about 100,000 years old.
Therefore, to date rocks that formed back at the beginning of the Earth, 4.6 billion years ago, we would use an isotope with a much larger half-life.
Since Uranium-238 half-life is 4.5 billion years, anything containing it has undergone 1 half-life since the Earth first formed!