Fossils and evolution

Rates of deposition of sediments

Cooling of the Earth from a molten state

Radiometric Dating

Methods for Estimating the Age of the Earth

Radiometric Dating of Rocks

1896-1898

Radioactivity discovered by:

Henri Becquerel and Marie and Pierre Curie.

Phenomenon of Radioactivity

Some elements, such as uranium (U) and thorium (Th), are unstable:

They decay spontaneously.

Age of the Earth

Geologic Time Scale

The Atom

NaCl

Na = silvery metalCl = poisonous gas

NaCl = Salt

2

1

8 7 8 2

11 protons 17 protons

Examples of Atomic Numbers

Element Number of Protons

Na 11

Cl 17

C 6

N 7

O 8

Fe 26

U 92

Phenomenon of Radioactivity

Some elements, such as uranium (U) and thorium (Th), are unstable:

They decay spontaneously.

Uranium Nucleus

spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Alpha Particle

emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Uranium - Thorium Decay

U He + Th238

92

4

2

234

90

spontaneous

decay

“parent” “daughter product”

alpha particle =2 protons + 2 neutrons = positively charged ion of Helium

Thorium:90 protons + 144 neutrons

Beta Particle Emission

But, Th is also unstable, and it emits a beta particle …

234

90

Th + Pa234

90

234

91

Thorium - Protactinium Decay

beta particle

beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

Protactinium:91 protons + 143 neutrons

Title

beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

U PbSeries

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

U PbSeries

The rate at which this process occurs is measured in terms of the “half life”.

Half Life

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

The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:  

(1) As radioactive elements decay in the Earth, they heat up the surrounding rocks.

(2) Radiometric Dating

As radioactive elements decay in the Earth, they heat up the surrounding rocks.

So, there is a source of heat inside the Earth that Kelvin didn't know about.

Thus: While the Earth was cooling off, it was also being heated up (due to radioactive decay in its interior), and the Earth took much longer to cool than Kelvin thought.

Radiometric Dating

Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed.

This is an example of uniformitarianism.

The Principle of Uniformitarianism

The physical, chemical and biological laws that operate today have also operated in the geologic past.

To estimate the age of a rock:

D = amount of daughter product. P = amount of parent.  

For a particular radioactive element in a rock, determine the present ratio = D/P.

D

P

Rate of decay (from theory and measurement)

Make assumptions about original ratios (from theory of geochemistry).

As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified).

D

P

D

P

Title

Potassium - Argon Dating

Electron Capture

Title

Electron Capture

Potassium - Argon Dating

K/Ar clock begins when K bearing minerals crystallize in a rock.The new minerals will contain K40, but will not contain Ar40 (because Ar40 does not combine with other elements).

As time passes:K40 Ar40 (half-life = 1.3 billion years)

K40/Ar40 ratio can be measured to determine age

Uniformitarianism

Potassium - Argon Dating

K40/Ar40 ratio can be measured to determine age.

K40 Ar40 (half-life = 1.3 billion years)

K40/Ar40 > 1 less than 1.3 billion years oldK40/Ar40 = 1 1.3 billion years old

K40/Ar40 < 1 more than 1.3 billion years old

Half-Lives of Radioactive Isotpes

Examples of Very Old Rocks Found on Earth:

3.4 billion year old granites - South Africa.

3.7 billion year old granites - Greenland.

Oldest terrestrial rocks > 3.8 billion years.

Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

But, rocks on the Earth have been weathered, metamorphosed and recycled through a very dynamic planet.

Thus, the Earth's earliest rocks have been recycled, and we need to look beyond Earth to find evidence of when the Earth solidified.

Plate Tectonics

Rock Cycle

Mountain Building

and Erosion

Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

Similar Rock Structures

The Earth's earliest rocks have been weathered, metamorphosed and recycled through a very dynamic planet.

We need to look beyond Earth to find evidence of when the Earth solidified.