Finding Absolute Age

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Finding Absolute Age

• Radioactive Dating

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Radioactive Dating

• The nuclei of radioactive elements (isotopes) are
  unstable.
• They decay at a fixed rate
• e.g. Uranium (U), Plutonium (Pu)

Decay Products
Parent Nucleus
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Radioactive Dating (cont)

• Radioactive decay starts from the time a rock
  crystallizes
• Half-life time it takes for ½ the original
  nuclei to decay.

e.g.
U-235 K-40 C-14
713 m.y. ? decays to Pb-207 1.3 b.y. ? decays
to Ar-40 5730 y. ? decays to N-14
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Radioactive Dating (cont)

• For an isotope to be suitable for determining the
  radiometric age of a sample it must fulfill the
  following

1. Some of the isotope must have been in the
sample when it formed. 2. There must be no
contamination or loss of parent or decay
products. 3. Isotopes with short half-lives used
for dating young rocks. 4. Isotopes with long
half-lives used for dating old rocks.
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Radioactive Decay of an Unstable Isotope
1. What is the half-life of this isotope? 2.
Which element is it?
1.3 billion years K-40 (potassium-40)
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Finding Absolute Age of Sedimentary Rocks

• Radiometric age refers to the time since a
  mineral grain crystallized
• can be found for igneous and metamorphic rocks
  (crystals are formed)
• sedimentary rocks are made up of grains derived
  from the weathering of older rocks

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Assignment

• Classwork

Activity 12 F Age Dating of Sedimentary Rocks
Page 275-276 (Q. 1 4 only)