6 SECTION 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

BEFORE YOU READ

After you read this section, you should be able to answer these questions:

? How can geologists learn the exact age of a rock? ? What is radiometric dating?

National Science Education Standards

ES 2b

What Is Radioactive Decay?

Geologists can use the methods of relative dating to learn whether a rock is older or younger than another rock. However, they often also need to know exactly how old a rock is. Finding the exact age of an object is called absolute dating. One way to learn the age of a rock is to use unstable atoms.

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.

Many isotopes are stable and are always in the same 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

This isotope is unstable, or radioactive.

Electron

After radioactive decay, an isotope of a new element is left. The new isotope is stable.

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.

STUDY TIP Learn New Words As you read, underline words that you don't understand. When you learn what they mean, write the words and their definitions in your notebook.

READING CHECK 1. Define What are isotopes?

TAKE A LOOK

2. Compare How is a radioactive isotope different from a stable isotope?

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The Rock and Fossil Record

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SECTION 3 Absolute Dating: A Measure of Time continued

Critical Thinking

3. Infer What happens to the amount of parent isotope in a rock with time? What happens to the amount of daughter isotope?

RADIOMETRIC DATING A radioactive isotope is also called a parent isotope.

Parent isotopes break down into daughter isotopes. Because of radioactive decay, the amounts of parent and daughter isotopes in a rock are always changing. However, they change at a constant, known rate. Therefore, scientists can learn the age of a rock by studying the amounts of 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.

Math Focus

4. Calculate Fill in the blank lines in the figure with the mass of parent isotope that is left at each step.

YEARS 0ARENTISOTOPEMG

4HISSAMPLECONTAINS MGOFAPARENT

ISOTOPE4HEISOTOPE HASAHALF LIFEOF YEARS

YEARS 0ARENTISOTOPE MG ?????? !FTERONEHALF LIFE OFTHEORIGINAL

MASSOFPARENT ISOTOPEISLEFT

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

YEARS 0ARENTISOTOPE MG

???????????? !FTERTHREEHALF LIVES  OROFTHEORIGINALMASSOF 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.

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The Rock and Fossil Record

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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.

URANIUM-LEAD METHOD Uranium-238 is also used for radiometric dating. It

has a half-life of 4.5 billion years. It decays to produce lead-206. Scientists use the uranium-lead method to date rocks that are older than about 10 million years.

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, combines 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.

When a plant or animal dies, it stops taking in carbon-14. The carbon-14 already in its body starts to decay to produce 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.

READING CHECK 5. Explain Using relative dating, a scientist learns that a rock is about 50,000 years old. Can the scientist use the potassium-argon method to find the exact age of this rock? Explain your answer.

READING CHECK 6. Describe How do animals take in carbon-14?

Parent isotope Potassium-40 Uranium-238 Rubidium-87 Carbon-14

Daughter isotope

Half-life

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TAKE A LOOK

7. Identify Fill in the spaces in the chart to show the features of different parent isotopes.

The Rock and Fossil Record

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Section 3 Review

Date

NSES ES 2b

SECTION VOCABULARY

absolute dating any method of measuring the age of an event or object in years

half-life the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope

isotope an atom that has the same number of protons (or the same atomic number) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)

radioactive decay the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element

radiometric dating a method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope

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?

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Earth Science Answer Key continued

8. Possible answers: There was no deposition happening at that time; there was a lot of erosion happening at that time.

9. erosion, nondeposition 10. a place where part of a sequence of parallel

rocks is missing

11. Rocks are pushed up and eroded. Later, sediment is deposited on top of the eroded rock.

12. The rock layers below an angular unconformity are tilted and may be any kind of rock.

Review 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.

2. an angular unconformity 3. The youngest rock layers are at the top, and

the oldest rock layers are at the bottom.

4. to interpret rock sequences and to identify rock layers

5. A sequence of rock layers is uplifted and eroded. Then, sediment is deposited on the exposed rock layers. After a while, the sediment turns into rock.

SECTION 3 ABSOLUTE DATING: A MEASURE OF TIME

1. atoms of an element with different numbers of neutrons

2. Radioactive isotopes can break down. Stable isotopes do not break down.

3. The amount of parent isotope decreases, and the amount of daughter isotope increases.

4. 8 mg

5. No, because K-Ar dating can be used only for rocks that are older than about 1 million years.

6. by eating plants

7. Parent isotope Daughter

isotope

Half-life

Potassium-40 argon-40

1.3 billion years

Uranium-238 lead-206

4.5 billion years

Rubidium-87 strontium-87 48 billion years

Carbon-14

nitrogen-14 5,730 years

Review

1. Radiometric dating uses known rates of radioactive decay to determine the age of a rock sample.

2. After 1 million years, there would be (1/2) (20 mg) 10 mg of parent isotope remaining. After 2 million years, (1/2) (1/2) (20 mg) 5 mg of parent isotope would remain.

3. uranium-238, rubidium-87

4. Potassium-40, because it can be used to date rocks that are older than about 1 million years.

5. C-14 dating can be used only on the remains of living organisms. Igneous rocks do not contain these remains. C-14 dating can be used only on remains that are less than 50,000 years old. Dinosaur bones are older than this.

SECTION 4 LOOKING AT FOSSILS 1. Body fossils are fossilized parts of an

organism. Trace fossils are signs that an organism once existed.

2. shells, teeth, bones

3. when minerals replace an organism's tissue

4. They are not made of parts of an organism, but they show that an organism once existed.

5. an impression left in sediment

6. the kinds of organisms that lived in the past; how the environment has changed; how organisms have changed

7. Organisms that had hard parts or lived in certain environments were more likely to be fossilized when they died.

8. They compare fossils. They also compare fossils to living organisms.

9. Answers include: according to their age, by absolute and relative dating methods

10. their shells

Review

1. tracks, burrows, coprolites

2. Only a small fraction of the organisms that have existed in Earth's history have been fossilized. Many fossils have not yet been discovered.

3. A beetle, because fossils in amber are made when an organism gets stuck in tree sap. Smaller organisms that can be found on trees are more likely than other organisms to become fossils in amber.

4. The climate was probably much warmer when the plant was alive.

5. It must be common throughout the world. It must have existed for a relatively short geologic time. It must be easy to identify.

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