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.

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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

Radioactive Decay
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Radioactive Decay-
Over 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
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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

Radioactive Decay
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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
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
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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.

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.
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Earth’s ever changing processes makes it difficult for scientists to determine Earth’s absolute age
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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
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Click the PHSchool.com button for an activity about radioactive dating.
Video: Greatest Discoveries with Bill Nye: Rocks of Ages