Lesson 3 | Absolute-Age Dating



Name Date Class

LESSON 3

Absolute-Age Dating

Directions: On each line, write the term from the word bank that correctly replaces the underlined words in each

sentence. NOTE: You may need to change a term to its plural form.

absolute age half-life isotope mineral radioactive decay

1. Salt is a(n) naturally occurring, inorganic solid with a

definite chemical composition and an orderly

arrangement of atoms.

2. My friend’s youngest sibling is a toddler. Her age in

numbers is three years.

3. C-12 and C-14 are atoms with different numbers of

neutrons, but both are made of carbon.

4. For uranium-235, the length of time after which half of

the parent isotopes decay is 704 million years.

5. Elements that are used to determine the absolute age of

rocks are those elements that undergo a process in

which atoms of the element change into atoms of

another element that is stable.

Clues to Earth’s Past 47

Name Date Class

LESSON 3

Absolute-Age Dating

Directions: Label this diagram by writing the letter of the statement that explains each isotope on the lines

provided.

A. When the neutron decays, energy is released.

B. The extra neutron decays, and a stable element forms.

C. The extra neutron makes the atom unstable.

Directions: On the line before each statement, write T if the statement is true or F if the statement is false.

4. The absolute age of a rock is its numerical age.

5. Atoms of an element that have the same number of protons but different

numbers of neutrons are called isotopes.

6. In radioactive decay, a stable element changes to an unstable element.

7. The unstable isotope that decays is called the daughter isotope.

8. The half-life of an isotope is the time it takes to become 50 percent parent

isotope and 50 percent daughter isotope.

9. Radiocarbon dating is useful for dating organic material.

10. Radiometric dating is most useful for dating sedimentary rock.

Clues to Earth’s Past 51

Name Date Class

LESSON 3

Absolute-Age Dating

Key Concept How can radioactive decay be used to date rocks?

Directions: On each line, write the term from the word bank that correctly completes each sentence. Each

term is used only once.

atoms constant daughter energy

half-lives normal one-half parent

radioactive radioactive decay rates stable

1. Most isotopes are .

2. Under conditions, isotopes do not change.

3. Unstable isotopes are known as isotopes.

4. Unstable isotopes release when they break down and form

new stable .

5. The process by which an unstable element changes into another element that is stable

is called .

6. Radioactive isotopes decay at different .

7. For every given isotope, the rate of decay is .

8. Rate of decay is measured in .

9. Half-life for an isotope occurs when half of the parent isotopes have

become isotopes.

10. By two half-lives, 25 percent of the isotopes are isotopes.

11. By three half-lives, parent isotopes have changed to daughter isotopes by

another .

56 Clues to Earth’s Past

Name Date Class

LESSON 3

Absolute-Age Dating

Key Concept How can radioactive decay be used to date rocks?

Directions: On the line before each statement, write T if the statement is true or F if the statement is false.

1. Radioactive isotopes decay at a constant rate.

2. Radiometric dating involves measuring the difference between the amount

of parent isotope and daughter isotope.

3. The decay of radioactive isotopes is used to determine the size of the material

in which they are present.

4. One commonly used isotope for radioactive dating is hydrogen.

5. The ratio of C-14 to C-12 in the atmosphere is unpredictable.

6. Another name for radioactive carbon is C-14.

7. In carbon-14, there are six protons and eight neutrons in its nucleus.

8. Radiocarbons form when it mixes with C-8 in Earth’s upper atmosphere.

9. One element used by all living things to build tissue is carbon.

10. The ratio of C-14 to C-12 in the tissues of living organisms always changes.

11. The ratio of C-14 to C-12 in dead organisms stays the same.

12. One way to measure the passage of time is to determine the ratio of C-14 to

C-12 in dead organisms.

13. The half-life of uranium is 5,730 years.

14. Carbon is useful for dating remains that are more than 50,000 years old.

15. Radiometric dating must take place where the remains are discovered.

16. Radioactive dating uses an isotope of radon.

Clues to Earth’s Past 57

Name Date Class

40 minutes

Correlate Rocks Using Index Fossils

Imagine you are a geologist and you have been asked to correlate, or match, the rock

columns below in order to figure out the relative ages of the layers. Remember that

geologists can correlate rock layers in different ways. In this lab, use index fossils to

correlate and find the ages of the layers.

Question

How can index fossils be used to figure out the relative ages of Earth’s rocks?

Procedure

1. Carefully examine the three rock columns on this page.

Each rock layer can be labeled with a letter and a number. For example, the

second layer down in column A is layer A-2.

2. Look at the fossil key on the next page. It shows the time range during which each

organism or group of organisms lived on Earth.

Use the key to correlate the layers using only the fossils—not the types of rock.

You might want to correlate by drawing lines to connect the layers.

Clues to Earth’s Past 63

VName Date Class

Lab Tips

You might want to copy the rock layers in your Science Journal and correlate them by

drawing lines connecting the layers.

Analyze and Conclude

3. Differentiate Which fossils in the key seem to be index fossils? Explain your choices.

64 Clues to Earth’s Past

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4. Match Correlate layer A-2 to one layer in each of the

other two columns. Approximately how old are these

layers? How do you know?

5. Infer What is the age of layer B-4? Hint: It lies

between two index fossils.

6. Infer How old is the fault in column C?

7. Compare and Contrast How is correlating rocks using fossils different from

correlating rocks using types of rock?

8. The Big Idea How can fossils be used to figure out the relative ages of rocks?

Communicate Your Results

Choose a partner. One of you is a reporter and one is a geologist. Conduct an interview

about what kinds of fossils are best used to date rocks.

Clues to Earth’s Past 65

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Content Vocabulary

Content Practice A

Flowchart of Radioactive Decay

Stable helium-3

nucleus

3.

Stable helium-3

nucleus

2.

Unstable hydrogen-3

Nucleus

1.

Key Concept Builder

Key Concept Builder

Lab A

Lab A continued

Lab A continued

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