Absolute Ages of Rocks
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Printed: November 17, 2014
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C HAPTER
Chapter 1. Absolute Ages of Rocks
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Absolute Ages of Rocks
Lesson 11.3: True or False
Name___________________ Class______________ Date________
Write true if the statement is true or false if the statement is false.
_____ 1. Radioactivity was discovered in the late 1800s.
_____ 2. Each ring inside a tree trunk represents one decade of growth.
_____ 3. During beta decay, a radioactive isotope emits two protons and two neutrons.
_____ 4. Carbon-14 breaks down in the process of beta decay.
_____ 5. All radioactive isotopes decay at the same rate.
_____ 6. Potassium-argon dating has been used to date human fossils.
_____ 7. The earliest geologic time scale included dates for each of the major subdivisions of time.
_____ 8. Ice cores can reveal how concentrations of atmospheric gases changed over time.
_____ 9. As time passes, the number of parent isotopes in a radioactive material increases.
_____ 10. A radioactive isotope with a short half-life cannot be used to date very old rocks.
Lesson 11.3: Critical Reading
Name___________________ Class______________ Date________
Read this passage based on the text and answer the questions that follow.
Radiometric Dating
Radiometric dating is the process of using concentrations of radioactive isotopes and their daughter products to
estimate the ages of materials. It is a very useful tool for dating fossils and rocks. Different isotopes are used to date
materials of different ages because different isotopes have different rates of decay. Using more than one isotope also
helps scientists check the accuracy of the ages they calculate.
Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is
especially useful for determining the absolute ages of recent human fossils and living sites. Carbon-14 is the only
radioactive isotope of carbon, and it has a half-life of 5,730 years. A tiny and constant percentage of carbon in the
atmosphere is carbon-14. Plants take in carbon dioxide containing carbon-14—along with nonradioactive isotopes
of carbon—during photosynthesis. Animals consume this carbon when they eat plants or other animals that have
eaten plants. The carbon-14 in organisms constantly decays, but it is continuously replaced as long as an organism
is alive. After an organism dies, the carbon-14 it contains continues to decay, but no new carbon-14 is taken in to
replace it. Therefore, the carbon-14 content of the remains constantly declines. The remaining carbon-14 in organic
materials can be measured and used to estimate the amount of time that has passed since the organism died.
Different radioactive isotopes are more useful for estimating the ages of older materials. For example, potassium40 decays to argon-40 with a half-life of 1.26 billion years. Potassium-argon dating can be used to date materials
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between 100,000 and over a billion years old. Two uranium isotopes are also used for radiometric dating. Uranium238 decays to lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life
of 704 million years. Uranium-lead dating can be used to date materials between 1 million and 4.6 billion years old.
Questions
1. What is radiometric dating? Why are different isotopes used to date materials of different ages?
2. Explain how carbon-14 enters living things and how the carbon-14 content of organisms changes after they
die. Why is radiocarbon dating useful only for relatively young specimens?
3. Identify other radioisotopes that are used for radiometric dating and the ages of the materials they can date.
Lesson 11.3: Multiple Choice
Name___________________ Class______________ Date________
Circle the letter of the correct choice.
1. Information on past climates can be obtained from
a.
b.
c.
d.
tree rings.
ice cores.
varves.
all of the above
2. If you start with a given amount of a radioactive isotope, what proportion of the original amount will remain
after three half-lives?
a. [U+215B]
b. 14
c. [U+2153]
d. 12
3. Carbon-14 releases a beta particle when it decays to
a.
b.
c.
d.
carbon-13.
carbon-12.
nitrogen-13.
nitrogen-14.
4. Alpha decay of a parent isotope results in the formation of a(n)
a.
b.
c.
d.
daughter isotope.
large amount of energy.
alpha particle.
all of the above
5. Which of the following radioactive isotopes has the longest half-life?
a.
b.
c.
d.
carbon-14
potassium-40
uranium-238
uranium-235
6. Drawbacks of radiometric dating include which of the following?
a. It is not very useful for aging sedimentary rocks.
b. It cannot be used to estimate the ages of organic materials.
c. It cannot be used to age rocks that are more than 1 billion years old.
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Chapter 1. Absolute Ages of Rocks
d. all of the above
7. Earth’s age is now generally considered to be about
a.
b.
c.
d.
4.6 million years.
100 million years.
2.6 billion years.
4.6 billion years.
Lesson 11.3: Matching
Name___________________ Class______________ Date________
Match each definition with the correct term.
Definitions
_____ 1. rate of decay of a radioactive isotope
_____ 2. using concentrations of radioactive isotopes to estimate the age of a material
_____ 3. two protons and two neutrons
_____ 4. layered sediment at the bottom of a lake near a glacier
_____ 5. form of an atom that is unstable and spontaneously gains or loses particles
_____ 6. isotope formed during radioactive decay
_____ 7. tendency of certain atoms to decay into other atoms and emit energy
Terms
a. alpha particle
b. daughter product
c. radiometric dating
d. radioactivity
e. radioactive isotope
f. half-life
g. varve
Lesson 11.3: Fill in the Blank
Name___________________ Class______________ Date________
Fill in the blank with the appropriate term.
1.
2.
3.
4.
5.
The age of a rock or fossil in years is its __________ age.
A method of dating old wood based on tree growth is called ____________ dating.
The time it takes for half of a given amount of radioactive isotope to decay is its __________.
Scientists drill into ice sheets to obtain cross-sections of ice called __________.
__________ of elements inside Earth’s interior provides a steady source of heat.
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6. The type of radiometric dating called __________ dating is used to date organic materials from 100 to 50,000
years old.
7. Of the three isotopes of carbon, only the isotope named __________ is radioactive.
Lesson 11.3: Critical Writing
Name___________________ Class______________ Date________
Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.
How did the discovery of radioactivity contribute to our knowledge of Earth’s history?
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