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2. Plate Tectonics Table of Contents Drifting Continents
Sea-Floor Spreading
The Theory of Plate Tectonics
Forces in Earth’s Crust
Volcanoes and Plate Tectonics
Volcanic Eruptions

3. Drifting Continents Piecing It All Together
The coastlines of some continents seem to fit
together like a jigsaw puzzle.

4. Pangaea: The History of the Continents

5. Drifting Continents Pangaea and Continental Drift
Many types of evidence suggest that Earth’s landmasses were once joined together.

6. Sea-Floor Spreading Ocean Floors
Mid-ocean ridges rise from the sea floor like stitches on the seams of a baseball.

7. Sea-Floor Spreading
Mid-Ocean Ridges

8. Clues about Continental Drift

9. Sea-Floor Spreading Sea-Floor Spreading
Some mid-ocean ridges have a valley that runs along their center. Evidence shows that molten material erupts through this valley and then hardens to form the ocean floor.

10. Sea-Floor Spreading Subduction
Oceanic crust created along a mid-ocean ridge is destroyed at a deep-ocean trench. During the process of subduction, oceanic crust sinks down beneath the trench into the mantle.

11. Sea-Floor Spreading Deep-Ocean Trenches The deepest part of
the ocean is along the Mariana Trench. Several trenches in the Pacific Ocean are shown in yellow.

12. The Theory of Plate Tectonics
Slip-Sliding Away
In 30 million years, it may take an hour longer to fly from New York to London because the cities are moving slowly apart.

13. Plate Tectonics

14. The Theory of Plate Tectonics
Earth’s Plates
Plate boundaries divide the lithosphere into large plates.

15. The Theory of Plate Tectonics
Plate Motion
Since the breakup of Pangaea, the continents have taken about 200 million years to move to their present location.
200 Million Years Ago
Earth Today
115 Million Years Ago

16. Types of Boundaries

17. The Theory of Plate Tectonics
How do the plates move?
Divergent
Boundaries
Convergent
Boundaries
Transform
Boundaries

18. The Theory of Plate Tectonics
Earth's Changing Crust
As plates move, they produce mountains, volcanoes, and valleys as well as mid-ocean ridges and deep-ocean trenches. Use the terms from the list to label the diagram.

19. Forces in Earth’s Crust
Stress in Earth’s Crust
Stress can push, pull, or squeeze rock in Earth’s crust. Three kinds of stress can occur in the crust.

20. Forces in Earth’s Crust
Faults
The three main types of faults are defined by the direction in which rock moves along the fault.

21. Forces in Earth’s Crust
The low angle of a thrust fault allows rock in the hanging wall to be pushed great distances. For example, over millions of years, rock along the Lewis thrust fault in Glacier National Park has moved 80 kilometers.

22. Forces in Earth’s Crust
Anticlines and Synclines
Compression can cause folds in the crust. Two types of folding are anticlines, which arch up, and synclines, which dip down.

23. Forces in Earth’s Crust Forces in Earth’s Crust
Tension and Normal Faults
What are the hanging wall and the two footwalls in diagram A? What is the new position of the hanging wall after movement occurs in diagram B?

24. Forces in Earth’s Crust
Tension and Normal Faults
As tension forces pull the crust apart, two normal faults can form a fault-block mountain range.

25. Forces in Earth’s Crust
The Kaibab Plateau
Look at the sequence of drawings. In your own words, describe what happens in the last two diagrams.

26. Earthquakes

27. Volcanoes and Plate Tectonics
The Ring of Fire
The Ring of Fire is a belt of volcanoes that circles the Pacific Ocean. As with most of Earth’s volcanoes, these volcanoes form along boundaries
of tectonic plates.

28. Earthquakes

29. Volcanoes and Plate Tectonics
Volcanoes and Converging Boundaries
Volcanoes often form where two plates collide.

30. Volcanoes

31. Volcanoes and Plate Tectonics
Hot Spot
The Hawaiian Islands have formed one by one as the Pacific plate drifts slowly over a hot spot. This process has taken millions of years.

32. When Earth Erupts

33. Volcanic Eruptions Inside a Volcano
A volcano is made up of many different parts.

34. Volcanic Eruptions Magma Composition
Magma varies in composition. It is classified according to the amount of silica it contains. The less silica that the magma contains, the more easily it flows.

35. Volcanic Eruptions Cascade Volcanoes
The Cascade volcanoes have formed as the Juan de Fuca plate sinks beneath the North American plate.

36. Volcanic Eruptions Mt. Rainier
Mount Rainier is part of the Cascade volcanoes. All past eruptions of Mount Rainier have included ash and lava.

37. Volcanic Eruptions
Magma at Mount Rainier