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11.4: Lesson 11.3: Absolute Ages of Rocks

Difficulty Level: At Grade Created by: CK-12
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Key Concepts

  • Radioactive decay
  • Radiometric dating

Lesson Objectives

  • Describe radioactive decay.
  • Explain radiometric dating.

Lesson Vocabulary

  • absolute age: actual age of a rock or fossil in years
  • carbon-14 dating: method of radiometric dating based on the decay of the radioactive isotope of carbon known as carbon-14
  • half-life: rate of decay of a radioactive isotope, equal to the time it takes for one-half of an original amount of the isotope to decay
  • isotope: atom of an element with a different number of neutrons than other atoms of the same element
  • radioactive decay: breakdown of unstable elements into stable elements
  • radiometric dating: use of radioactive decay to estimate the absolute age of a fossil or rock

Teaching Strategies

Introducing the Lesson

Help students recall what they already know about atoms and subatomic particles before they start reading about isotopes and radioactive decay in this lesson.

Question: What is an atom?
Answer: An atom is the smallest particle of an element that has the element’s properties
Question: What is the nucleus of an atom?
Answer: The nucleus is a mass in the center of an atom.
Question: What particles are found inside the nucleus?
Answer: The nucleus contains positive protons and neutral neutrons.

Remind students that all the atoms of a given element have the same number of protons. However, atoms of the same element may differ in their number of neutrons. Tell students they will learn why this is important for determining the ages of fossils and rocks when they read this lesson.


Have teams of students do the interactive simulation game activity “Radioactive Speed Dating” at the URL below. The object of the game is to correctly estimate the age of various virtual fossils and rocks using the principles of radiometric dating. The activity is designed as an in-class competition between teams of students. An answer key is available by e-mail.


Differentiated Instruction

Assign the questions below for students to think about. Then pair English language learners with other students and ask them to share and discuss their answers to the questions.

  1. How do the absolute ages of rocks differ from their relative ages?
  2. What are isotopes?
  3. What is radioactive decay, and why does it happen?
  4. What is the half-life of a radioactive isotope?
  5. How is radioactive decay used to estimate absolute ages of rocks?


Challenge students to think of a way to model the concept of half- life. Tell them that the model may be a physical, conceptual, or a mathematical model. Invite the students to explain their model to the rest of the class.

Science Inquiry

Have students estimate the age of a tree by counting tree rings in a cross-section of a tree trunk (either an actual cross-section or a clear image of one). Explain how the darker, narrower rings represent times of slower growth (winters) and the lighter, wider rings represent times of faster growth (summers), so each light/dark pair represents one year of growth. Ask students whether the annual light/dark pairs are all the same width or if some are narrower than others. Have them explain what might cause variations in width from year to year and what they could learn about past environments from these observations.

Common Misconceptions

Students may think that carbon-14 dating can be applied to rocks or to materials of any age. Explain that carbon-14 dating can be used only to date organic remains, so it cannot be used to date rocks. While this makes carbon-14 dating ideal for dating fossils, it can be used only for very recent fossils. In fact it is limited to a maximum age of about 50,000 years, which covers just a tiny fraction of geologic time.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Earth Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.

Lesson Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.

Lesson Quiz

Check students’ mastery of the lesson with Lesson 11.3 Quiz in CK-12 Earth Science for Middle School Quizzes and Tests.

Points to Consider

Scientists estimate the ages of rock layers in order to better understand Earth’s history and the history of life. What do you already know about Earth’s history? For example, do you know how Earth formed?

How old is Earth? When did the planet first form? And when did life first appear?

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