Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Earth Systems 3209 Unit: 2 Historical Geology Reference: Chapters 6, 8; Appendix A & B

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Absolute Time and Radioactivity Unit 2: Topic 2.4 Focus on...  defining terminology related to radioactivity.  d  describing the relationship between half-life, parent and daughter isotopes. Text Reference: Pages

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Terminology Related to Radioactivity For example: Uranium 238 – Lead 206 (Common in Igneous Rock)

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Terminology Related to Radioactivity  Half Life – the time it takes for one half of an unstable isotope to decay to form a stable isotope.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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 material to the amount of stable, daughter material can be used to determine the absolute age of the rock.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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-238 decays to Lead-206 !4.5 Billion Years 5) Rubidium-87 decays to Strontium-87 !47 Billion Years 2) Uranium-235 decays to Lead-207 !713 Million Years 3) Potassium-40 decays to Argon-40 !1.31 Billion Years 4) Carbon-14 decays to Nitrogen-14 !5730 years

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador 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

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Your Turn... Take the time and complete the following questions... (Solutions to follow) Questions: Why is Carbon-14 not used 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.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Solutions... Questions: Why is Carbon-14 not used 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.

Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador Summary... Overview of Points covered: Terminology associated with Radioactivity:  Isotope  Half-life  Radioactive Decay  Parent Element  Daughter Element 1) U-238 decays to Pb-206 ! 4.5 Byrs 5) Rb-87 decays to Sr-87 ! 47 Byrs 2) U-235 decays to Pb-207 ! 713 Myrs 3) K-40 decays to Ar-40 ! 1.31 Byrs 4) C-14 decays to N-14 ! 5730 years