Early Efforts on Dating the Age of the Earth

James Ussher (mid 1600's)

Georges Buffon (mid 1700’s)

Lord Kelvin (late 1800's)

75,000 yr

100 m.y.

Charles Walcott (1893)75 m.y.

4004 B.C.

II. Constancy of Natural Laws

Catastrophism

Georges Cuvier (late 1700’s)

Uniformitarianism

James Hutton (late 1700's)

Early Efforts on Dating the Age of the Earth

Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).

Radiometric Dating of Rocks

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.

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?

Fundamental unit of matter made up of subatomic particles

Electrons

Nucleus

ATOMS

NeutronsProtons

}NucleusProtons (positive charge)

Electrons (negative charge)

Neutrons (no electrical charge)

ISOTOPES

Carbon-12 Carbon-13 Carbon-14

98.9 %6 protons6 neutrons

1.1 %6 protons7 neutrons

< 0.1 %6 protons8 neutrons

12C6

13C6

14C6

Types of Radioactive Decay

alpha particle

Parent Nucleus Daughter Nucleus

alpha decay

Changes in atomic number and atomic mass number

Atomic Number = - 2

Atomic Mass Number = - 4

4H2

ALPHADECAY

238U 92

234Th 90

92 protons

146 neutrons90 protons

144 neutrons

protonsneutronselectrons

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.

Types of Radioactive Decay

beta particle

Parent Nucleus Daughter Nucleus

beta decay

Changes in atomic number and atomic mass number

Atomic Number = + 1

Atomic Mass Number = 0

BETADECAY

137Cs 55

137Ba 56

55 protons

82 neutrons56 protons

81 neutrons

electron

e -protonsneutronselectrons

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.

p +

Types of Radioactive Decay

electron

capture

Parent Nucleus Daughter NucleusAtomic Number = - 1

Atomic Mass Number = 0

ELECTRONCAPTURE

204Po 84

204Bi 83

84 protons

120 neutrons83 protons

120 neutrons

e -protonsneutronselectrons

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.

Changes in atomic number and atomic mass number

U238 Pb206

Series

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

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.

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.

Radioactive Decay RatesRadioactive

Isotope Disintegration Half-Life

Carbon-14 14C 14N 5.7 x 103 years

Uranium-238238U 206Pb 4.5 x 109 years

Potassium-4040K

40Ar40Ca

1.3 x 109 years

Iodine-131

Cobalt-60

8.02 days131I

60Co 5.3 years

131Xe

60Ni

Half Life

Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

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

Po218

Parent Isotope

Daughter Isotope

Half Life

Half-Life = 10 seconds

Half-LifeTime (s) Parent Daughter

0

1

2

3

4

5

0

10

20

30

40

50

100 0

50 50

25 75

12.5 87.5

6.25 93.75

3.125 96.875

(100)

PARENT=1,000,000DAUGHTER=0

Start with a million parent atoms…

100

50

25

12.5

6.253.13

PARENT=500,000DAUGHTER=500,000

1

1 Half-Life

100

50

25

12.5

6.253.13

PARENT=250,000DAUGHTER=750,000

1 2

2 Half-Lives

100

50

25

12.5

6.253.13

PARENT=125,000DAUGHTER=875,000

1 2 3

3 Half-Lives

100

50

25

12.5

6.253.13

PARENT=62,500DAUGHTER=937,500

1 2 3 4

100

50

25

12.5

6.25

4 Half-Lives

3.13

PARENT=31,250DAUGHTER=968,750

1 2 3 4 5

5 Half-Lives

100

50

25

12.5

6.253.13

PARENT=15,625DAUGHTER=984,375

1 2 3 4 5 6

6 Half-Lives

100

50

25

12.5

6.253.13

1 2 3 4 5 6Number of parent isotopes

Number of daughter isotopes

2.3 half-lives

20%

100

50

25

12.5

6.253.13

Example #1

If a sample has 20% 14C, how old is the sample?Half-Life 14C = 5,700 years

1 2 3 4 5 6Number of parent isotopes

Number of daughter isotopes

3.6 half-lives

8%

100

50

25

12.5

6.253.13

Example #2

A sample is found to have 131I at a concentration of 8%.How old is the sample?Half-Life 131I = 8 days

Let’s Try Some More…

Parent Daughter

1

1/2

1/4

1/8

1/16

0

1/2

3/4

7/8

15/16

1st half-life

2nd half-life

3rd half-life

4th half-life

C-14 N-14

100%

50%

25%

12.5%

6.25%

0 %

50%

75%

87.5%

93.75%

1st half-life

2nd half-life

3rd half-life

4th half-life

Once Again

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!