Transcript Radioactive dating

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.
2.
3.
Which layer (with a letter) is the oldest and how do you
know?
If G is an intrusion, what do you know about the relative
ages of G and C?
What could have caused part of layer E to be missing?
RADIOACTIVE DATING
What is Radioactive Decay?
Uniformitarianism
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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
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Half-Life
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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-Life
The 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
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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
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Radioactive elements occur in
igneous rocks
Rate of decay tells absolute age of
rocks
Longer Half-Lives allow dating of
older items
Scientists:
 Determine
See page 541
for an in-depth
discussion of
this process
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
What is Radioactive Dating?
Carbon-14
Dating
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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?
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The Half life of Carbon is about 5,730 years.
What about rocks?
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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 Lives
Percentage 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
50%
decayed
50%
Radioactive
One Half Life
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.
6.25%
50%
radioactive.
How old is the rock?
100% Radioactive
Igneous Rock
75% decayed
87.5% decayed
94% decayed
25% r-a
Two Half Lives
Three Half Lives
Four Half Lives
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?
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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