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Earth Systems 3209. Reference: Chapters 6, 8; Appendix A & B. Unit: 2 Historical Geology . Unit 2: Topic 2.4. Absolute Time and Radioactivity. Focus on . . . defining terminology related to radioactivity.

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slide1

Earth Systems 3209

Reference:

Chapters 6, 8; Appendix A & B

Unit: 2Historical Geology

slide2

Unit 2: Topic 2.4

Absolute Time and Radioactivity

Focus on . . .

  • defining terminology related to radioactivity.
  • describing the relationship between half-life, parent and daughter isotopes.

Text Reference:

Pages 228 - 235

terminology related to radioactivity
Terminology Related to Radioactivity
  • Isotope – variations of an element that have different mass numbers.
  • Radioactive Elements– are unstable in nature and give off radiation as they undergo radioactive decay to become stable.
  • Radioactive elements decay at constant rates and are thought to start decaying as soon as the rock has formed.
terminology related to radioactivity1
Terminology Related to Radioactivity

For example:

Uranium 238 – Lead 206(Common in Igneous Rock)

terminology related to radioactivity2
Terminology Related to Radioactivity
  • Radioactivity– process where unstable isotopes will emit energy to achieve stability.
  • The rate at which a radioactive element decays is called its half-life.
terminology related to radioactivity3
Terminology Related to Radioactivity
  • Half Life – the time it takes for one half of an unstable isotope to decay to form a stable isotope.
terminology related to radioactivity4
Terminology Related to Radioactivity
  • Parent Element – unstable radioactive material
  • Daughter Element – stable decayed end product
  • When both are added together it equals 100%.
  • The ratio of the amount of unstable, parent materialto the amount of stable, daughter materialcan be used to determine the absolute age of the rock.
terminology related to radioactivity5
Terminology Related to Radioactivity
  • Several different dating methods can be used to find the age of different rocks.
  • Some of these dating methods and corresponding half-lives are listed below:

1) Uranium-238decays to Lead-206 ! 4.5 Billion Years

2) Uranium-235decays to Lead-207 ! 713 Million Years

3)Potassium-40 decays to Argon-40 ! 1.31 Billion Years

4)Carbon-14 decays toNitrogen-14! 5730 years

5)Rubidium-87 decays toStrontium-87 !47 Billion Years

limitations to using radioactive dating
Limitations to Using Radioactive Dating
  • Addition(hydrothermal fluids) or loss(leaching) of parent or daughter material can give false ages.
  • Certain parent isotopes are only appropriate under certain Conditions. Example, C-14 dates once living organisms and some isotopes have to long or short of a half-live.
  • Metamorphism resets the radioactive clock.
  • Sedimentary rocks are formed from previously existing weathered and eroded rocks therefore gives different ages for different parts of the sample.
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Example 1:

Which radioactive isotope would be used to date the bones of an ancient mummy?

(A) Carbon -14 (B) Potassium - 40

(C) Rubidium - 87 (D) Uranium -238

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Example 2:

What is the half life in millions of years of the element represented in the graph below?

(A) 50

(B) 100

(C) 150

(D) 200

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Your Turn . . .

Take the time and complete the following questions . . .(Solutions to follow)

Questions:

Why is Carbon-14 notused to date rocks of the Paleozoic Era?

(A) Carbon-14 is only mildly radioactive.

(B) Few instruments can detect the presence of Carbon-14.

(C) Its half-life is too short.

(D) There was a lack of organic material in the Paleozoic.

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Solutions . . .

Questions:

Why is Carbon-14 notused to date rocks of the Paleozoic Era?

(A) Carbon-14 is only mildly radioactive.

(B) Few instruments can detect the presence of Carbon-14.

(C) Its half-life is too short.

(D) There was a lack of organic material in the Paleozoic.

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Summary . . .

Overview of Points covered:

  • Terminology associated with Radioactivity:
  • Isotope
  • Half-life
  • Radioactive Decay
  • Parent Element
  • Daughter Element

1) U-238decays to Pb-206 ! 4.5 Byrs

2) U-235decays to Pb-207 ! 713 Myrs

3)K-40 decays to Ar-40 ! 1.31 Byrs

4)C-14 decays toN-14 ! 5730 years

5)Rb-87 decays toSr-87 ! 47 Byrs