Goal:I will be able to explain what relative and absolute age are and how we determine them.I will be able to explain the Law of Superposition, including inclusions and exclusions.

Essential Question:What is radioactive dating and how is it used?

1. Which layer (with a letter) is the oldest and how do you know?

2. If G is an intrusion, what do you know about the relative ages of G and C?

3. What could have caused part of layer E to be missing?

RADIOACTIVE DATING

What is Radioactive Decay?

Uniformitarianism

Geological processes that occur today also occurred in the past

Without it, we couldn’t determine much about the past

What is Radioactive Decay?

Radioactive Decay

Half-Life

Process in which radioactive elements break down, releasing particles & energy

Atoms of 1 element break down to form another element

Rate of decay never changes

Time it takes for half of radioactive atoms to decay

What is a Half-Life?

Half-LifeThe half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay. What pattern do you see in the graph?

Half-Lives Almost always need to make a chart to

visualize

Original Rock

1 Half Life 2 Half Lives

3 Half Lives

4 Half Lives

What is Radioactive Dating?

Radioactive Dating

Radioactive elements occur in igneous rocks

Rate of decay tells absolute age of rocks

Longer Half-Lives allow dating of older items

Scientists: Determine how much of the

radioactive element is in rock Compare with how much of

the stable element it breaks down into there is

Calculate age of rock

See page 541 for an in-depth

discussion of this

process

What is Radioactive Dating?

Carbon-14 Dating

All plants and animals contain Carbon-14

Breaks down as organism decays

Short half-life (5,730 years) which allows it to be used for younger specimens

Can only be used on specimens containing carbon (like a real bone, not a dino “bone” that has been mineralized)

What is the Half Life of Carbon-14?

The Half life of Carbon is about 5,730 years.

What about rocks?

IGNEOUS rocks contain radioactive isotopes in a predictable ratio when they cool.

Many of these have very long half-lives (on the order of millions of years)

Layers of igneous deposits (ash, lava, etc.) can be used to bracket the age of fossils found in between them

Half LivesPercentage

of Radioactive Material in

Rock

Number of Half Lives

100% - 50% 1

50% - 25% 2

25% - 12.5 %

3

12.5% - 6.25%

4

6.25% - 3.126%

5

Element Brennan is found in an igneous rock and we are trying to determine the rocks age. We know Brennan’s half life is approximately 3,000 years and that the rock is currently 25% radioactive. How old is the rock?

75% decayed

94% decayed87.5% decayed50% decayed

50%Radio- active 25% r-

a 12.5% 6%

100% Radioactive

Igneous Rock

One Half Life Two Half Lives Three Half Lives Four Half Lives

Element Brennan is found in an igneous rock and we are trying to determine the rocks age. We know Brennan’s half life is approximately 3,000 years and that the rock is currently 6.25% radioactive. How old is the rock?

Element Brennan is found in an igneous rock and we are trying to determine the rocks age. We know Brennan’s half life is approximately 3,000 years and that the rock is currently 50% radioactive. How old is the rock?

Half-Lives

Last Practice Question:A rock contains a radioactive element

with a half life of 100 million years. Tests show that the element in the rock has gone through three half lives.

How old is the rock?

3 X 100 million = 300 million years old!

Relative vs. Absolute Dating

Ash layer

Lava layer

How do we determine Earth’s age?How do

we determine Earth’s age?

Radioactive dating of rocks on Earth and the moon Hard on Earth because rocks

have changed Oldest rock on Earth 3.8-4.28

billion years old – Not accurate

Rocks on moon haven’t changed and were formed at same time as Earth – 4.6 billion years old