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. *This outline follows the Chapter 16.3 presentation slides.
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 1 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 Absolute Age of Rocks Absolute age means the ______________________ age, in years, of a rock or object. o What is your absolute age? o 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 ___________________________ century. o Once ______________________________ had been discovered. o Radioactivity is the release of _________________________ from unstable atoms. Atoms You are all familiar with atoms. o What are the parts of an atom? o What determines the element of an atom? o What is in the nucleus of an atom? o What surrounds the nucleus? Review of Isotopes All atoms of a given element have the same number of _______________________. o How many protons does a hydrogen atom have? However, an elements atoms can have different numbers of_____________________. Atoms of the same element that have different numbers of neutrons are called isotopes. o We name isotopes with the element name and the number of particles (protons+neutrons) in its nucleus.
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 2 Radioactive Decay Most isotopes are________________________________. o Stable isotopes do not change under normal conditions Unstable isotopes are called _______________________________ isotopes. o Radioactive isotopes _______________________, or change, over time. o As they decay, they release energy and form _______________, stable atoms. Radioactive decay is the process by which an unstable element naturally changes into another element that is ________________________. The unstable isotope that decays is called the ______________________ isotope. The new element that forms is called the ____________________________ isotope. o In the figure, the atoms of an unstable isotope of hydrogen (parent) decay into atoms of a stable isotope of helium (__________________________) Half-Life The rate of decay from parent isotopes into daughter isotopes is different for _________________________ radioactive elements. o Rate of decay is constant for a given __________________________________ o This rate is measure in _______________________ units called half-lives
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 3 An isotopes half-life is the time required for ____________________ of the parent isotopes to decay into daughter isotopes. o Half-lives of radioactive isotopes range from a few microseconds to _______________________ of years. As time passes, more and more unstable parent isotopes decay and form _______________________ daughter isotopes. The means the ratio of parent and daughter isotopes is always ________________________. When ______________________ the parent isotopes have decayed into daughter isotopes, the isotope has reached one half-life. Half-lives Time Percentageofremaining parentatoms 0 1 2 3 4 100 50 25 12.5 6.25 After one half-life, _________% of the isotopes are parents and _________% of the isotopes are daughters After two half-lives, 50% of the remaining parent isotopes have decayed so that only ___________% of the original parent isotopes remain. This process continues until nearly all parent isotopes have decayed into daughter isotopes. Radiometric Ages Because radioactive isotopes decay at a constant rate, they can be used like _____________________ to measure the age of the material that contains them.
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 4 In this process, called _________________ ____________________, scientists measure the amount of parent isotope and daughter isotope in a sample of material they want to date. o From this ratio, they can determine the materials _________________. Review What is measured in radiometric dating? Radiocarbon Dating One important radioactive isotope used for dating is an isotope of carbon called _____________________________. o Radiocarbon is also known as carbon-14 or C-14. How many protons and neutrons does C-14 have? Radiocarbon forms in Earths upper atmosphere where it mixes with a stable carbon isotope called carbon-_______ or C-12. The ratio of the amount of C-14 and C-12 in the _________________________ is constant. All _____________________ 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 ______________________ to the ratio in the atmosphere. However, if an organism dies, it stops taking in C-______. o The C-14 present in the organism starts to decay to _________________-14 (N-14). o As the dead organisms C-14 decays, the ratio of C-14 to C-12 _____________________. 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. o The half-life of carbon-14 is _________________________ years. That means radiocarbon dating is useful for measuring the age of remains of organisms that died up to about _________________________ years ago. o In remains older than this, there is not enough _____________ left to measure accurately. 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?
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 5 Should we expect more C-14 or N-14 in an organism that has been dead for 40,000 years? Dating Rocks Radiocarbon dating is useful only for dating _____________________ material material from once-living organisms. o This material includes bones, _________________, parchment, and charcoal. Most rocks do not contain __________________________ material. Even most ________________________ are no longer organic. o Their living tissue has been replaced by rock-forming _______________________. So, for dating rocks, geologists use different kinds of radioactive isotopes. Dating Igneous Rock One of the most common isotopes used in radiometric dating is ___________________________ or U-235. U-235 is often trapped in the minerals of ____________________ rocks that crystallize from hot, molten magma. As soon as it is trapped in a mineral, U-235 decays into ____________________ or Pb-207. o What ratio would scientists use to determine how much time has passed since the mineral was formed? o Which isotope should there be more of it the rock is older than one half-life?
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 6 Draw a graph of U-235 decay. One half-life equals .704 billion years Dating Sedimentary Rock How does sedimentary rock form? In order to be dated by radiometric means, that ___________________ that formed the rock must contain U-235. o The grains of sedimentary rocks come from a variety of weathered rocks from different _________________________. 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? Radioactive isotopes within these grains generally record the ages of the ______________________ not when the sediment was deposited. For this reason, sedimentary rock is not as __________________ to date as igneous rock
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 7 More radioactive isotopes used in radiometric dating Which has the shortest half-life? _____________________________________________ Which has the longest? ____________________________________________________ Different Types of Isotopes The half-life of U-235 is ____________________________________ million years. o This makes it useful for dating rocks that are very _______________. Many different ______________________ are also used. o However, would isotopes with short half-lives be useful in dating old rocks? o Which isotope would be too small to measure, the parent or the daughter? Geologists often use a _________________________ of radioactive isotopes to measure the age of a rock to make it more accurate. The Age of Earth The oldest known rock formation dated by geologists using radiometric means is in _________________________. o It is estimated to be between _______________________________ billion years old. However, individual crystals of the mineral zircon in igneous rocks in Australia have been dated at ________________________ billion years. Radiometric dating of rocks from the Moon and meteorites indicate that Earth is ___________________ billion years old. o Scientists accept this age because evidence suggests that Earth, the ___________, and ___________________ all formed at about the same time.
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 8 *** Extra Figures***
Chapter 16 Lesson 3: Absolute Age Dating p582589 page 9