3 Absolute Dating: a Measure of Time

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CHAPTER 6 The Rock and Fossil Record SECTION 3 Absolute Dating: A Measure of Time

National Science BEFORE YOU READ Education Standards After you read this section, you should be able to answer ES 2b these questions: • How can geologists learn the exact age of a rock? • What is radiometric dating?

What Is Radioactive Decay? Geologists can use the methods of relative dating to STUDY TIP learn whether a rock is older or younger than another Learn New Words As you rock. However, they often also need to know exactly how read, underline words that old a rock is. Finding the exact age of an object is called you don’t understand. When you learn what they mean, absolute dating. One way to learn the age of a rock is to write the words and their use unstable atoms. deﬁ nitions in your notebook. All matter, including rock, is made of atoms. All atoms are made of three kinds of particles: protons, neutrons, and electrons. All of the atoms of an element, such as uranium, have the same number of protons. However, some atoms of an element have different numbers of neutrons. Atoms of an element that have different numbers of neutrons are called isotopes. READING CHECK Many isotopes are stable and are always in the same 1. Deﬁ ne What are isotopes? form. However, other isotopes are unstable and can break down into new isotopes of different elements. An unstable isotope is also called a radioactive isotope. Radioactive decay happens when a radioactive isotope breaks down into a new isotope.

Proton

Neutron Electron After radioactive decay, an isotope of a new element is left. The new TAKE A LOOK isotope is stable. This isotope is unstable, 2. Compare How is a or radioactive. radioactive isotope different from a stable isotope? Radioactive isotopes can decay in different ways. During one kind of radioactive decay, a neutron becomes a proton and an electron. The electron moves to a different part of the atom.

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RADIOMETRIC DATING A radioactive isotope is also called a parent isotope. Parent isotopes break down into daughter isotopes. Critical Thinking Because of radioactive decay, the amounts of parent and 3. Infer What happens to daughter isotopes in a rock are always changing. However, the amount of parent isotope they change at a constant, known rate. Therefore, scien- in a rock with time? What tists can learn the age of a rock by studying the amounts of happens to the amount of daughter isotope? parent and daughter isotopes in it. Radiometric dating is the process of determining the absolute age of a sample based on the ratio of parent isotope to daughter isotope. In order to use radiometric dating, you need to know the half-life of the parent isotope. The half-life of a radioactive isotope is how long it takes for half of a sample of the isotope to decay. For example, imagine that a parent isotope has a half-life of 10,000 years. A sample of this isotope has a mass of 12 mg. After 10,000 years, only one-half, or 6 mg, of the sample will be left.

YEARS YEARS 0ARENTISOTOPEMG 0ARENTISOTOPE 4HISSAMPLECONTAINS MG ?????? Math Focus MGOFAPARENT !FTERONEHALF LIFE ISOTOPE4HEISOTOPE OFTHEORIGINAL HASAHALF LIFEOF MASSOFPARENT 4. Calculate Fill in the blank YEARS ISOTOPEISLEFT lines in the ﬁ gure with the mass of parent isotope that is left at each step.

YEARS YEARS 0ARENTISOTOPE 0ARENTISOTOPE MG MG ??????? ???????????? !FTERTWOHALF LIVES !FTERTHREEHALF LIVES OROF THEORIGINALMASSOF OROFTHEORIGINALMASSOF PARENTISOTOPEISLEFT PARENTISOTOPEISLEFT

The half-lives of different isotopes can be very different. Some parent isotopes have half-lives of more than 4 billion years. Others have half-lives of only about 6,000 years. Very old rocks can be dated only if isotopes with long half-lives are used. Very young rocks can be dated only if isotopes with short half-lives are used. How do scientists know which isotope to use to find the age of a rock? They use information about the relative age of the rock to guess about how old the rock is. Then, they find its age, using an isotope that is useful for dating rocks of that age.

SECTION 3 Absolute Dating: A Measure of Time continued

What Isotopes Can Be Used for Radiometric Dating? Remember that different parent isotopes have different half-lives. Each parent isotope can be used to date rocks of different ages.

POTASSIUM-ARGON METHOD Potassium-40 is one isotope that is often used in radiometric dating. It has a half-life of 1.3 billion years. It decays to produce the daughter isotope argon-40. Scientists usually use the potassium-argon method to date rocks that are older than about 1 million years. READING CHECK URANIUM-LEAD METHOD 5. Explain Using relative dating, a scientist learns that Uranium-238 is also used for radiometric dating. It a rock is about 50,000 years has a half-life of 4.5 billion years. It decays to produce old. Can the scientist use the lead-206. Scientists use the uranium-lead method to date potassium-argon method to ﬁ nd the exact age of this rocks that are older than about 10 million years. rock? Explain your answer. RUBIDIUM-STRONTIUM METHOD Rubidium-87 is also used for radiometric dating. It has a half-life of about 48 billion years. It decays to produce the daughter isotope strontium-87. The half-life of rubidium-87 is very long. Therefore, this method is only useful for dating rocks older than about 10 million years.

CARBON-14 METHOD Carbon-14 is a radioactive isotope of the element carbon. Carbon-14, along with the other isotopes of carbon, com- bines with oxygen to form the gas carbon dioxide. Plants use carbon dioxide to make food. Therefore, living plants are always taking in small amounts of carbon-14. Animals that eat plants also take in carbon-14 from the plants. READING CHECK When a plant or animal dies, it stops taking in carbon-14. 6. Describe How do animals The carbon-14 already in its body starts to decay to produce take in carbon-14? nitrogen-14. Carbon-14 has a short half-life: only 5,730 years. Therefore, this method can be used to date the remains of organisms that died in the last 50,000 years.

Parent isotope Daughter isotope Half-life Potassium-40 TAKE A LOOK Uranium-238 7. Identify Fill in the spaces in the chart to show the Rubidium-87 features of different parent Carbon-14 isotopes.

Section 3 Review NSES ES 2b SECTION VOCABULARY absolute dating any method of measuring the radioactive decay the process in which a age of an event or object in years radioactive isotope tends to break down into a half-life the time required for half of a sample stable isotope of the same element or another of a radioactive isotope to break down by element radioactive decay to form a daughter isotope radiometric dating a method of determining isotope an atom that has the same number of the absolute age of an object by comparing the protons (or the same atomic number) as other relative percentages of a radioactive (parent) atoms of the same element do but that has a isotope and a stable (daughter) isotope different number of neutrons (and thus a different atomic mass)

1. Describe How is radioactive decay related to radiometric dating?

2. Calculate A parent isotope has a half-life of 1 million years. If a rock contained 20 mg of the parent isotope when it formed, how much parent isotope would be left after 2 million years? Show your work.

3. List What are two radioactive isotopes that are useful for dating rocks that are older than 10 million years?

4. Apply Concepts A geologist uses relative dating methods to guess that a rock is between 1 million and 5 million years old. What is one radioactive isotope the geologist can use to learn the exact age of the rock? Explain your answer.

5. Infer Why can’t geologists use the carbon-14 method to date igneous rocks? Why can’t they use the carbon-14 method to date dinosaur bones?

8. Possible answers: There was no deposition 2. After 1 million years, there would be (1/2) happening at that time; there was a lot of (20 mg) 10 mg of parent isotope remaining. erosion happening at that time. After 2 million years, (1/2) (1/2) (20 mg) 9. erosion, nondeposition 5 mg of parent isotope would remain. 10. a place where part of a sequence of parallel 3. uranium-238, rubidium-87 rocks is missing 4. Potassium-40, because it can be used to date 11. Rocks are pushed up and eroded. Later, sedi- rocks that are older than about 1 million years. ment is deposited on top of the eroded rock. 5. C-14 dating can be used only on the remains 12. The rock layers below an angular unconfor- of living organisms. Igneous rocks do not mity are tilted and may be any kind of rock. contain these remains. C-14 dating can be used only on remains that are less than Review 50,000 years old. Dinosaur bones are older than this. 1. An unconformity can form if no sediment is deposited for a long time. An unconformity can also form if layers of rock are eroded away. SECTION 4 LOOKING AT FOSSILS 2. an angular unconformity 1. Body fossils are fossilized parts of an organism. Trace fossils are signs that an 3. The youngest rock layers are at the top, and the oldest rock layers are at the bottom. organism once existed. shells, teeth, bones 4. to interpret rock sequences and to identify 2. rock layers 3. when minerals replace an organism’s tissue 5. A sequence of rock layers is uplifted and 4. They are not made of parts of an organism, eroded. Then, sediment is deposited on the but they show that an organism once existed. exposed rock layers. After a while, the sedi- 5. an impression left in sediment ment turns into rock. 6. the kinds of organisms that lived in the past; how the environment has changed; how SECTION 3 ABSOLUTE DATING: A organisms have changed MEASURE OF TIME 7. Organisms that had hard parts or lived in 1. atoms of an element with different numbers certain environments were more likely to be of neutrons fossilized when they died. 2. Radioactive isotopes can break down. Stable 8. They compare fossils. They also compare isotopes do not break down. fossils to living organisms. 3. The amount of parent isotope decreases, and 9. Answers include: according to their age, by the amount of daughter isotope increases. absolute and relative dating methods 4. 8 mg 10. their shells 5. No, because K-Ar dating can be used only for rocks that are older than about 1 million years. Review 6. by eating plants 1. tracks, burrows, coprolites 7. 2. Only a small fraction of the organisms that Parent isotope Daughter Half-life have existed in Earth’s history have been isotope fossilized. Many fossils have not yet been Potassium-40 argon-40 1.3 billion years discovered. Uranium-238 lead-206 4.5 billion years 3. A beetle, because fossils in amber are made Rubidium-87 strontium-87 48 billion years when an organism gets stuck in tree sap. Carbon-14 nitrogen-14 5,730 years Smaller organisms that can be found on trees are more likely than other organisms Review to become fossils in amber. 1. Radiometric dating uses known rates of The climate was probably much warmer radioactive decay to determine the age of a 4. when the plant was alive. rock sample. 5. It must be common throughout the world. It must have existed for a relatively short geo- logic time. It must be easy to identify.