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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?
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 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!
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