Absolute Dating with Radioactivity

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Absolute Dating with Radioactivity
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What is Radioactivity?

• When the forces binding protons and neutrons
  together are not strong enough, and the nuclei
  spontaneously break apart (decay)
• Isotopeselements with the same number of
  protons, but different numbers of neutrons

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Common Types of Radioactive Decay

• Alpha Particlesa particles
• Composed of 2 protons and 2 neutrons
• Emitted from the nucleus
• Mass number of the isotope is reduced by four
• The atomic number is decreased by two

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Common Types of Radioactive Decay

• Beta Particlesß particles
• Composed of one electron
• Comes from a neutron
• Increases protons by one
• Atomic number increases by one

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Common Types of Radioactive Decay

• Electron Capture
• An electron is captured by the nucleus
• It combines with a proton and forms a neutron
• Mass number remains unchanged
• One less proton atomic number decreases by one

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Half-Life

• The time required for one-half of the nuclei in a
  sample to decay
• A common way of expressing the rate of
  radioactive disintegration

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Radiometric Dating

• Radioactivity (unstable nuclei) is a reliable
  means of calculating the ages of rocks and
  minerals
• The rates of decay for many isotopes have been
  precisely measured
• A correct date is not possible unless the mineral
  remained a closed system during the entire period
  since its formation

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Carbon-14

• Half-life is 5730 years old
• Used to date recent events in history
• While an organism is alive, the proportion of
  carbon-14 to carbon-12 is constant
• When the organism dies, carbon-14 decays to
  nitrogen-14 by beta emission
• Scientists compare the proportions of carbon-14
  and carbon-12

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Importance of Radiometric Dating

• Have produced thousands of dates for events in
  Earths history
• Some rocks have been dated at more than three
  billion years
• Proven that geologic time is immense

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Difficulties in Dating the Geologic Time Scale

• Not all rocks can be dated radiometrically
• Most sedimentary rock cannot be accurately dated
  because the particles that formed it are much
  older than the rock in which they occur

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Difficulties in Dating the Geologic Time Scale

• Metamorphic rock is difficult to date because the
  age of a particular mineral in the rock is not
  necessarily the same as when the rock initially
  formed
• Use cross-cutting relationships