Transcript 8.3 Radioactive Dating

8.3 Radioactive Dating
7.4.d Students know that evidence from geologic layers
and radioactive dating indicates Earth is approximately
4.6 billion years old and that life on this planet has
existed for more than 3 billion years.
Radioactive Decay
Rocks are a form of
matter
 Atoms- tiny particles
that make up all matter
 Element- any
substance that cannot
be broken down into
simpler substances
(examples: carbon,
oxygen, iron, lead)
 Most elements are
stable- do not change
under normal
conditions
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Radioactive Decay
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Radioactive DecayOver time unstable
elements break down, or
decay, by releasing
particles and energythese unstable elements
are said to be radioactive
Key Concept- During the
process of radioactive
decay, the atoms of one
element break down to
form atoms of another
element.
Radioactive Decay
During radioactive
decay, a “parent” atom
releases energy and
particles as it changes
to a new kind of atom, a
“daughter” atom.
Radioactive Decay
Radioactive elements happen naturally in
igneous rocks
 Scientists use the rate of decay to
calculate the rock’s age
 “Birthday” for an igneous rock is when it
first hardens to become rock
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Radioactive Decay
As the radioactive element in the rock
decays it changes into another element
 Composition changes slowly over time
 Amount of radioactive element goes
down and amount of new element goes
up
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Radioactive Decay
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Rate of decay is constant for each radioactive element
Scientists can measure the rate of decay experimentally
Half-life- the time it takes for half of the radioactive
atoms to decay
Determining Absolute Ages
Key Concept- Radioactive dating is used to
determine the absolute ages of rocks.
Determining Absolute Ages
Table lists several common
radioactive elements and their halflives.
 Carbon-14 has a half-life of 5,730
years and is useful in dating plants
and animals that lived up to about
50,000 years ago.
 Potassium-40 decays to a stable
argon-40 and has a half-life of 1.3
billion years. It is useful in dating
most ancient rocks because of its
long half-life.
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Determining Absolute Ages
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In radioactive dating first
measure the amount of
radioactive element in a
rock (“parent” element)
Then measure the amount
of stable element that the
unstable radioactive
element decays into
(“daughter” element)
By calculating the ratio of
radioactive element to the
stable element you can
determine the age of the
rock
How Old is Earth?
Key Concept- Radioactive dating shows that the oldest moon
rocks are about 4.6 billion years old. Scientists infer that
Earth is only a little older than those moon rocks– roughly
4.6 billion years old.
 Earth’s ever changing
processes makes it
difficult for scientists to
determine Earth’s
absolute age
 Radioactive dating shows
that the oldest rocks
found on Earth are about
4.0 billion years old
 The oldest fossils of
living things are about
3.5 billion years
More on Radioactive Dating
Click
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activity about radioactive dating.
Video: Greatest Discoveries with Bill Nye: Rocks of Ages