8.3 Radioactive Dating8.3 Radioactive Dating7.4.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 DecayRadioactive DecayRocks are a form

of matterAtoms- 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 DecayRadioactive DecayRadioactive Decay-

Over time unstable elements break down, or decay, by releasing particles and energy- these 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 DecayRadioactive Decay

During radioactive decay, a “parent” atom releases energy and particles as it changes to a new kind of atom, a “daughter” atom.

Radioactive DecayRadioactive DecayRadioactive elements happen

naturally in igneous rocksScientists 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 DecayRadioactive Decay

As the radioactive element in the rock decays it changes into another element

Composition changes slowly over timeAmount of radioactive element goes

down and amount of new element goes up

Radioactive DecayRadioactive DecayRate of decay is constant for each radioactive

elementScientists can measure the rate of decay

experimentallyHalf-life- the time it takes for half of the

radioactive atoms to decay

Determining Absolute Determining Absolute AgesAges

Key Concept- Radioactive dating is used to determine the absolute ages of rocks.

Determining Absolute Determining Absolute AgesAges

Table lists several common radioactive elements and their half-lives.

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.

Example is the decay

of Potassium-40 to

Argon-40

Determining Absolute Determining Absolute AgesAges 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?How Old is Earth?

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

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.

More on Radioactive More on Radioactive DatingDating

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Video: Greatest Discoveries with Bill Nye: Rocks of Ages