Chapter 16.3: Absolute Age Dating


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Grade 8 Integrated Science Chapter 16 Lesson 3 on absolute age dating of fossils. This lesson follows the last lesson about relative age dating. This chapter describes radiometric age dating with explanations of radioactive decay and half-life. There is also a short explanation of igneous, metamorphic, and sedimentary age dating. The goal is that students understand radioactive decay, half-life, and how this can be used to determine the age of carbon fossils and different types of rocks.

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Chapter 16.3: Absolute Age Dating

  1. 1. Absolute Age Dating Chapter 16 Lesson 3 p 582-589
  2. 2. Vocabulary • Absolute age (583) – the numerical age, in years, of a rock or object • Isotope (584) – atoms of the same element that have different numbers of neutrons • Radioactive decay (584) – the process by which an unstable element naturally changes into another element that is stable • Half-life (585) – the time required for half of the parent isotopes to decay into daughter isotopes
  3. 3. Absolute Age of Rocks • Absolute age means the numerical age, in years, of a rock or object. – What is your absolute age? – How is absolute age different from relative age? • Scientists have been able to determine the absolute ages of rocks and other objects only since the beginning of the twentieth century. – Once radioactivity had been discovered. – Radioactivity is the release of energy from unstable atoms
  4. 4. Atoms • You are all familiar with atoms. – What are the parts of an atom? – What determines the element of an atom? – What is in the nucleus of an atom? – What surrounds the nucleus?
  5. 5. Review of Isotopes • All atoms of a given element have the same number of protons – How many protons does a hydrogen atom have? • However, an element’s atoms can have different numbers of neutrons. • Atoms of the same element that have different numbers of neutrons are called isotopes. – We name isotopes with the element name and the number of particles (protons+neutrons) in its nucleus.
  6. 6. Radioactive Decay • Most isotopes are stable. – Stable isotopes do not change under normal conditions • Unstable isotopes are called radioactive isotopes. – Radioactive isotopes decay, or change, over time. – As they decay, they release energy and form new, stable atoms. • Radioactive decay is the process by which an unstable element naturally changes into another element that is stable.
  7. 7. Radioactive Decay • The unstable isotope that decays is called the parent isotope. • The new element that forms is called the daughter isotope. • In the figure, the atoms of an unstable isotope of hydrogen (parent) decay into atoms of a stable isotope of helium (daughter)
  8. 8. Half-Life • The rate of decay from parent isotopes into daughter isotopes is different for different radioactive elements. – Rate of decay is constant for a given isotope – This rate is measure in time units called half-lives • An isotope’s half-life is the time required for half of the parent isotopes to decay into daughter isotopes. – Half-lives of radioactive isotopes range from a few microseconds to billions of years.
  9. 9. • As time passes, more and more unstable parent isotopes decay and form stable daughter isotopes. • The means the ratio of parent and daughter isotopes is always changing. • When half the parent isotopes have decayed into daughter isotopes, the isotope has reached one half- life.
  10. 10. Half-lives Time Percentageofremaining parentatoms 0 1 2 3 4 100 50 25 12.5 6.25
  11. 11. • After one half-life, 50% of the isotopes are parents and 50% of the isotopes are daughters • After two half-lives, 50% of the remaining parent isotopes have decayed so that only a quarter of the original parent isotopes remain. • This process continues until nearly all parent isotopes have decayed into daughter isotopes.
  12. 12. Radiometric Ages • Because radioactive isotopes decay at a constant rate, they can be used like clocks to measure the age of the material that contains them. • In this process, called radiometric dating, scientists measure the amount of parent isotope and daughter isotope in a sample of material they want to date. – From this ratio, they can determine the material’s age.
  13. 13. Review • What is measured in radiometric dating? – The amount of the parent isotope and daughter isotope.
  14. 14. Radiocarbon Dating • One important radioactive isotope used for dating is an isotope of carbon called radiocarbon. – Radiocarbon is also known as carbon-14 or C-14. • How many protons and neutrons does C-14 have? – 6 protons and 8 neutrons • Radiocarbon forms in Earth’s upper atmosphere where it mixes with a stable carbon isotope called carbon-12 or C-12. • The ratio of the amount of C-14 and C-12 in the atmosphere is constant.
  15. 15. Radiocarbon Dating • All living things use carbon as they build and repair tissues • As long as an organism is alive, the ratio of C-14 to C-12 in its tissues is identical to the ratio in the atmosphere. • However, if an organism dies, it stops taking in C-14. – The C-14 present in the organism starts to decay to nitrogen-14 (N-14). – As the dead organism’s C-14 decays, the ratio of C-14 to C- 12 changes. • Scientists measure the ratio of C-14 to C-12 in the remains of the dead organism to determine how much time has passed since the organism died.
  16. 16. Radiocarbon Dating • The half-life of carbon-14 is 5,730 years. • That means radiocarbon dating is useful for measuring the age of remains of organisms that died up to about 60,000 years ago. • In remains older than this, there is not enough C-14 left to measure accurately.
  17. 17. Review • What two isotopes of carbon are present in our atmosphere? • Is the ratio of carbon isotopes in the atmosphere constant or changing? • C-14 decays into what isotope? • Should we expect more C-14 or N-14 in an organism that has been dead for 40,000 years? C-12 and C-14 The ratio of C-12 to C-14 is constant. C-14 decays into N-14. It should have more N-14 because I has been dead for longer than C-14’s half-life (5,730 yrs).
  18. 18. Dating Rocks • Radiocarbon dating is useful only for dating organic material – material from once-living organisms. – This material includes bones, wood, parchment, and charcoal. • Most rocks do not contain organic material. • Even most fossils are no longer organic. – Their living tissue has been replaced by rock-forming minerals. • So, for dating rocks, geologists use different kinds of radioactive isotopes.
  19. 19. Dating Igneous Rock • One of the most common isotopes used in radiometric dating is uranium-235 or U-235. • U-235 is often trapped in the minerals of igneous rocks that crystallize from hot, molten magma. • As soon as it is trapped in a mineral, U-235 decays into lead-207 or Pb-207. – What ratio would scientists use to determine how much time has passed since the mineral was formed? – Which isotope should there be more of it the rock is older than one half-life?
  20. 20. One half-life equals .704 billion years
  21. 21. Dating Sedimentary Rock • How does sedimentary rock form? – From sediment and a lot of pressure over a long time. • In order to be dated by radiometric means, that sediment that formed the rock must contain U- 235. – The grains of sedimentary rocks come from a variety of weathered rocks form different locations. • However, by measuring U-235 would scientist be getting the date that the sedimentary rock formed or the date that the grain of sediment formed?
  22. 22. Dating Sedimentary Rock • Radioactive isotopes within these grains generally record the ages of the grains – not when the sediment was deposited. • For this reason, sedimentary rock is not as easy to date as igneous rock
  23. 23. More radioactive isotopes used in radiometric dating • Which has the shortest half-life? • Which has the longest?
  24. 24. Different Types of Isotopes • The half-life of U-235 is 704 million years. – This makes it useful for dating rocks that are very old. • Many different isotopes are also used. – However, would isotopes with short half-lives be useful in dating old rocks? – Which isotope would be too small to measure, the parent or the daughter? • Geologists often use a combination of radioactive isotopes to measure the age of a rock to make it more accurate.
  25. 25. The Age of Earth • The oldest known rock formation dated by geologists using radiometric means is in Canada. • It is estimated to be between 4.03 and 4.28 billion years old. • However, individual crystals of the mineral zircon in igneous rocks in Australia have been dated at 4.4 billion years. • Radiometric dating of rocks from the Moon and meteorites indicate that Earth is 4.54 billion years old. – Scientists accept this age because evidence suggests that Earth, the Moon, and meteorites all formed at about the same time.
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