According to the theory of plate tectonics, Earth's crust is composed of a number of individual plates that change shape and position over time. Geophysical evidence indicates that the face of Earth's surface has changed significantly since its initial formation and that the plates on which the continents are located are in constant motion. The movement of the plates is responsible for the formation of ocean basins, mountain ranges, islands, volcanoes, and earthquakes. Important concepts in the theory of plate tectonics include the following:
- The ocean floors are continually moving — spreading from the center, sinking at the edges, and being regenerated.
- Convection currents beneath the plates are responsible for plate movement.
- The source of energy responsible for generating the heat and convection currents that move the plates is most likely radioactivity deep in Earth's mantle.
In this lesson, students are introduced to the theory of plate tectonics and explore how the theory was developed and supported by evidence. Through class discussion, videos, and activities, students seek connections between tectonic activity and geologic features and investigate how the theory of plate tectonics evolved.
- Understand how Earth is dynamic and how moving plates form ocean basins, mountain ranges, islands, volcanoes, and earthquakes
- Identify the three general categories of plate boundaries recognized by scientists: convergent, divergent, and transform
- Understand how the theory of plate tectonics was developed and supported
Grade Level: 6-8
Two class periods
- Tectonic Plates, Earthquakes, and Volcanoes Flash Interactive
- Plate Tectonics: An Introduction QuickTime Video
- Mountain Maker, Earth Shaker Flash Interactive
- Plate Tectonics: The Scientist Behind the Theory QuickTime Video
- Plate Tectonics: Further Evidence QuickTime Video
- Plate Tectonics: Lake Mead, Nevada QuickTime Video
- Tectonic Plates and Plate Boundaries Flash Interactive
- Plate Tectonics: The Hawaiian Archipelago QuickTime Video
- World Map With Shorelines and Continental Shelf Boundaries PDF Image
- Two desks
- Two sheets of paper (longer is better)
- Two markers, different colors
- 10 small aluminum baking pans (disposable; loaf or pie shape work well)
- 10 candles
Before the Lesson
If possible, arrange computer access for all students to work in pairs. Make copies of the World Map With Shorelines and Continental Shelf Boundaries PDF Image to distribute to students. Just before class, have the first two media resources ready: Tectonic Plates, Earthquakes, and Volcanoes Flash Interactive and Plate Tectonics: An Introduction QuickTime Video.
Part I: Introduction to the Theory of Plate Tectonics
1. Discuss how Earth is physically changing and ask students for their ideas about why it changes. Write the term plate tectonics on the board and ask if anyone has heard of this theory. Record class comments on the board and save for later.
2. Show students the locations of earthquakes around the world using the Tectonic Plates, Earthquakes, and Volcanoes Flash Interactive. Switch to the display of volcanoes. Do not show the plate boundaries at this point. Then ask:
- What do you notice about the distribution of earthquakes?
- What do you notice about the distribution of volcanoes?
- Do you see any correlations or patterns?
- Can you think of a possible explanation for the patterns you see?
3. Show the Plate Tectonics: An Introduction QuickTime Video. After viewing the video, return to the Tectonic Plates, Earthquakes, and Volcanoes Flash Interactive and now show the overlay of all three views: earthquakes, volcanoes, and plate boundaries. Point out the Ring of Fire. Ask students to interpret why the active areas are located where they are and to relate their interpretations to their previous comments and possible explanations.
4. Allow time for students to further explore the subject individually or in pairs. Distribute copies of the World Map With Shorelines and Continental Shelf Boundaries PDF Image and have students cut out the continents (following the continental shelf lines) to see how they fit together. At the same time, have students work with the Mountain Maker, Earth Shaker Flash Interactive to learn about the different types of boundaries. During this activity, have students write a list of relevant vocabulary words in their science journal. Encourage students to write the definitions in their own words, but to use the textbooks/computer to verify them.
5. Show the Plate Tectonics: The Scientist Behind the Theory QuickTime Video. Ask:
- Why was Wegener's original idea about continental drift referred to as intuition and not science?
- What did Wegener find that he believed was evidence to support his theory?
- Why didn't others think that his findings constituted evidence?
Part II: Evidence for Plate Tectonics
6. Remind students about the scientific process and discuss the importance of evidence for a scientific theory. Show the Plate Tectonics: Further Evidence QuickTime Video and the Plate Tectonics: Lake Mead, Nevada QuickTime Video. Ask:
- How did the new information about the ocean floor support Wegener's theory?
- How do the rocks at Lake Mead support the theory of plate tectonics?
- What other evidence would help convince you that the theory of plate tectonics was real?
7. The discovery that the ocean floor has a massive ridge running down the middle, that the oldest rocks are farthest from the ridge, and that the banded rock has preserved a record of periodic magnetic reversals are key pieces of evidence for plate tectonics. Have students work in groups to devise a demonstration of how the Atlantic Ocean was formed by sea-floor spreading. You may want to help students get started by discussing ways to represent the spreading apart of the ocean floor and the resulting appearance of the new sea floor. After the students have shown their demos, you may wish to show this effective demonstration:
- Align two desks with their edges just touching — the gap is your mid-ocean ridge.
- Place two pieces of paper vertically into the gap between the desks. Leave just enough of the papers sticking out so that there is something to hold onto.
- Slowly pull the papers out from the gap, spreading the papers apart onto the desks as you go. Make sure that both papers are pulled at the same rate.
- Have a student use a marker to draw a stripe of color on both pieces of paper at the ridge. This stripe of color represents the new rock that is formed at the ridge. As you continue to pull the papers, have the student draw more stripes in alternating colors to represent subsequent time periods. Just make sure each new stripe extends on both sides of the ridge (on both pieces of paper).
- The result should be a mirror-image set of colored stripes, representing how the new ocean floor spreads from the ridge. The discovery of symmetrical rock formations on the Atlantic floor provided strong evidence for the sea-floor spreading theory - the identical patterns showed younger rock near the ridge and contained a record of Earth's changing magnetic polarity through time.
8. Check students' understanding of plate motions by showing the Tectonic Plates and Plate Boundaries Flash Interactive with the color-coded and defined boundaries. Have students indicate which direction the plates are moving at the different types of boundaries. What geologic features could be seen at each boundary?
9. Have students watch the Plate Tectonics: The Hawaiian Archipelago QuickTime Video. Discuss the following:
- What is a hot spot?
- What does it mean to say that a volcano is dormant?
10. Have students work in small groups to simulate the formation of the Hawaiʻian Islands. Distribute the following materials to each group:
- An aluminum pan with about an inch of cornstarch covering the bottom.
- A candle or some other heat source
Have students add small amounts of water to the pan until the cornstarch reaches a pasty consistency. Instruct students to hold one edge of the pan over the candle until bubbles form in the cornstarch. Then tell them to very slowly move the pan across the flame. As the pan moves, they should notice the row of bubbles that is created. Discuss the following:
- What does the candle represent?
- What does the pan represent?
- What does the cornstarch represent?
- What do the bubbles represent?
- The bubbles are similar to a volcano that forms over a hot spot. How do volcanoes form islands?
- Which islands are the oldest?
- Why aren't all of Hawaiʻi's volcanoes active?
11. Revisit the information about plate tectonics that you recorded on the board in Step 1, and update it as needed.
Check for Understanding
Have students discuss the following:
- Why do we call Earth an "active" planet? What does this mean?
- How does the theory of plate tectonics account for earthquakes? Volcanoes? Mountains?
- Hawaiʻi's hot spot doesn't display the typical relationship between volcanoes and plate boundaries, yet it does provide evidence of plate tectonics. How?
- Why was Wegener's theory of continental drift not accepted when he first proposed it? What evidence revealed to scientists that continents could be moving apart from each other?
The Digital Library for Earth System Education (www.dlese.org) offers access to additional resources on this topic.