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.

Text of Chapter 16.3: Absolute Age Dating

  • 1.Absolute Age Dating Chapter 16 Lesson 3 p 582-589

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. 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. 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. 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 elements 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. 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. 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. 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 isotopes 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. 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. Half-lives Time Percentageofremaining parentatoms 0 1 2 3 4 100 50 25 12.5 6.25 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. 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 materials age. 13. Review What is measured in radiometric dating? The amount of the parent isotope and daughter isotope. 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 Earths 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. 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 organisms 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. 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. 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-14s half-life (5,730 yrs). 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. 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. One half-life equals .704 billion years 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. 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. More radioactive isotopes used in radiometric dating Which has the shortest half-life? Which has the longest? 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. 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.