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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 ... – PowerPoint PPT presentation

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Title: 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

Radioactive Decay
  • Radioactive Decay- Over time unstable elements
    break down, or decay, by releasing particles and
    energy- these unstable elements are said to be
  • 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
  • Scientists use the rate of decay to calculate the
    rocks age
  • Birthday for an igneous rock is when it first
    hardens to become rock

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

Radioactive Decay
  • Rate of decay is constant for each radioactive
  • Scientists can measure the rate of decay
  • 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 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

Example is the decay of Potassium-40 to Argon-40
Determining Absolute Ages
  • 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.
  • Earths ever changing processes makes it
    difficult for scientists to determine Earths
    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 the PHSchool.com button for an activity
    about radioactive dating.

Video Greatest Discoveries with Bill Nye Rocks
of Ages
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