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Earth Systems 3209

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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Earth Systems 3209

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  1. Earth Systems 3209 Reference: Chapters 6, 8; Appendix A & B Unit: 2Historical Geology

  2. 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

  3. 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.

  4. Terminology Related to Radioactivity For example: Uranium 238 – Lead 206(Common in Igneous Rock)

  5. 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.

  6. Terminology Related to Radioactivity • Half Life – the time it takes for one half of an unstable isotope to decay to form a stable isotope.

  7. 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.

  8. 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

  9. 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.

  10. 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

  11. 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

  12. 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.

  13. 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.

  14. 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

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