1. Chapter 21 Objectives Identify the causes of earthquakes.
Section 2 Earthquakes and Volcanoes
Chapter 21
Objectives
Identify the causes of earthquakes.
Distinguish between primary, secondary, and surface waves in earthquakes.
Describe how earthquakes are measured and rated.
Explain how and where volcanoes occur.
Describe the different types of common volcanoes.

2. Section 2 Earthquakes and Volcanoes
Chapter 21
Bellringer
1. Imagine a corked bottle of soda pop that is standing in a pan of hot water. What do you think will happen as the soda pop heats up?
2. What happens when the pressure builds up in the soda pop?
3. Molten rock in Earth’s mantle is like the soda pop. What happens when pressure builds up in Earth’s mantle?

3. Chapter 21 What are Earthquakes?
Section 2 Earthquakes and Volcanoes
Chapter 21
What are Earthquakes?
Earthquakes occur at plate boundaries.
Earthquakes are vibrations resulting from rocks sliding past each other at a fault
Seismic waves are waves of energy released during in earthquake
Focus the area along a fault at which the first motion of an earthquake occurs
Epicenter the point on Earth’s surface directly above an earthquake’s focus

4. What are Earthquakes? continued
Section 2 Earthquakes and Volcanoes
Chapter 21
What are Earthquakes? continued
Energy from earthquakes is transferred by waves.
Earthquakes generate three types of waves:
Longitudinal waves
Transverse waves
Surface waves
Longitudinal waves travel by compressing and stretching crust, also called primary waves (P waves)
Transverse waves travel in an up and downward movement, also called secondary waves (S waves)
Surface waves seismic waves that can move only through solids, move in a rolling circular motion

5. Section 2 Earthquakes and Volcanoes
Chapter 21
Longitudinal Waves

6. Section 2 Earthquakes and Volcanoes
Chapter 21
Transverse Wave

7. Seismic Waves: Surface Waves
Section 2 Earthquakes and Volcanoes
Chapter 21
Seismic Waves: Surface Waves

8. What are Earthquakes? continued
Section 2 Earthquakes and Volcanoes
Chapter 21
What are Earthquakes? continued
Waves move through Earth and along its surface.
Both P waves and S waves spread out from the focus in all directions through the earth.
Surface waves move only on Earth’s surface.

9. Measuring Earthquakes
Section 2 Earthquakes and Volcanoes
Chapter 21
Measuring Earthquakes
Seismologists detect and measure earthquakes.
Seismology the study of earthquakes including their origin, propagation, energy, and prediction
Seismologists use sensitive equipment called seismographs to record data about earthquakes.

10. Seismographs and Mapping Earth’s Layers
Section 2 Earthquakes and Volcanoes
Chapter 21
Seismographs and Mapping Earth’s Layers

11. Measuring Earthquakes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Measuring Earthquakes, continued
Three seismograph stations are necessary to locate the epicenter of an earthquake.
There are more than 1000 seismograph stations across the world.
Because P waves travel faster, the difference between the arrival of P waves and the arrival of S waves allows scientists to calculate how far away the focus is.

12. Measuring Earthquakes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Measuring Earthquakes, continued
Geologists use seismographs to investigate Earth’s interior.
The way P and S waves travel through Earth’s interior help scientists make a model of Earth with layers of different densities.

13. Measuring Earthquakes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Measuring Earthquakes, continued
The Richter scale is a measure of the magnitude of earthquakes.
Richter scale a scale that expresses the magnitude of an earthquake
The intensity of an earthquake is measured by the modified Mercalli scale. Intensity depends on many factors.
Earthquakes that occur deeper below the Earth’s surface will not be as intense at the surface.
The hardness of the rock above and around an earthquake affects the intensity.

14. Section 2 Earthquakes and Volcanoes
Chapter 21
Richter Scale

15. Measuring Earthquakes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Measuring Earthquakes, continued
Scientists are trying to predict earthquakes.
Scientists are trying to measure changes in Earth’s crust that might signal an earthquake.
The ability to predict an earthquake could save thousands of lives in the future.

16. Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes
A volcano is any opening in Earth’s crust through which magma has reached Earth’s surface.
Vent an opening at the surface of Earth through which volcanic material passes
Volcanoes generally have one central vent, but they can also have several smaller vents.
Magma that reaches Earth’s surface is called lava.

17. Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes

18. Chapter 21 Volcanoes, continued Shield volcanoes have mild eruptions.
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Shield volcanoes have mild eruptions.
Lava from shield volcanoes is very fluid and forms a gently sloping mountain.
Shield volcanoes are some of the largest volcanoes.

19. Chapter 21 Volcanoes, continued Composite volcanoes have trapped gas.
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Composite volcanoes have trapped gas.
Composite volcanoes are made up of alternating layers of ash, cinders, and lava.
The lave is thicker than that of shield volcanoes.
Gases are trapped in the magma, causing eruptions that alternate between flows and explosive activity that produces cinders and ash.
Composite volcanoes are typically tall with steep sides.

20. Chapter 21 Volcanoes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Cinder cones are the most abundant volcano.
Cinder cones are the smallest and most common volcanoes.
Large amounts of gas are trapped in the magma, and violent eruptions of hot ash and lava occur.
Cinder cones tend to be active for only a short time and then become dormant.

21. Section 2 Earthquakes and Volcanoes
Chapter 21
Types of Volcanoes

22. Chapter 21 Volcanoes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Most volcanoes occur at convergent plate boundaries.
75% of the active volcanoes on Earth are located in an area known as the Ring of Fire.
The Ring of Fire is located along the edges of the Pacific ocean, where oceanic tectonic plates are colliding with continental plates.

23. Section 2 Earthquakes and Volcanoes
Chapter 21
Ring of Fire

24. Chapter 21 Volcanoes, continued
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Underwater volcanoes occur at divergent plate boundaries.
As plates move apart at divergent boundaries, magma rises to fill the gap.
This magma creates the volcanic mountains that form ocean ridges.
Iceland is a volcanic island on the Mid-Atlantic ridge that is growing outward in opposite directions.

25. Chapter 21 Volcanoes, continued Volcanoes occur at hot spots.
Section 2 Earthquakes and Volcanoes
Chapter 21
Volcanoes, continued
Volcanoes occur at hot spots.
Some volcanoes occur in the middle of plates.
Mantle plumes are mushroom shaped trails of hot rock that rise from deep inside the mantle, melt as they rise, and erupt from volcanoes at hot spots at the surface.
The plumes remain in the same place as the tectonic plate moves, creating a trail of volcanoes.
The Hawaiian Islands are an example of this type of volcanic activity.

26. Hot Spots and Mantle Plumes
Section 2 Earthquakes and Volcanoes
Chapter 21
Hot Spots and Mantle Plumes

27. Understanding Concepts
Chapter 21
Standardized Test Prep
Understanding Concepts
1. Which of these occurs where two tectonic plates move away from each other?
A. convergent boundary
B. divergent boundary
C. ocean trench
D. subduction zone

28. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
1. Which of these occurs where two tectonic plates move away from each other?
A. convergent boundary
B. divergent boundary
C. ocean trench
D. subduction zone

29. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
2. What causes earthquakes along the San Andreas fault in California?
F. subduction of the Pacific plate by the North American plate
G. collision between the Pacific plate and the North American plate
H. divergent movement of the Pacific plate and the North American plate
I. horizontal movement along the boundary of the Pacific plate and the North American plate

30. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
2. What causes earthquakes along the San Andreas fault in California?
F. subduction of the Pacific plate by the North American plate
G. collision between the Pacific plate and the North American plate
H. divergent movement of the Pacific plate and the North American plate
I. horizontal movement along the boundary of the Pacific plate and the North American plate

31. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
3. How can the absolute age of a layer of rock be determined?
A. by the principle of superposition
B. by the ratio of radioisotopes
C. by the amount of weathering that has shaped the rock
D. by analysis of the types of minerals that make up the rock

32. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
3. How can the absolute age of a layer of rock be determined?
A. by the principle of superposition
B. by the ratio of radioisotopes
C. by the amount of weathering that has shaped the rock
D. by analysis of the types of minerals that make up the rock

33. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
4. Which of the following is an example of chemical weathering of rock?
F. deposition
G. erosion
H. frost wedging
I. leaching

34. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
4. Which of the following is an example of chemical weathering of rock?
F. deposition
G. erosion
H. frost wedging
I. leaching

35. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
5. Both S waves and P waves travel from the site of an earthquake. How does the difference in the way these waves travel reveal information about the structure of Earth’s interior?

36. Understanding Concepts, continued
Chapter 21
Standardized Test Prep
Understanding Concepts, continued
5. Both S waves and P waves travel from the site of an earthquake. How does the difference in the way these waves travel reveal information about the structure of Earth’s interior?
Answer: S waves cannot pass through liquid. The fact that P waves can be detected on the opposite side of the planet, and S waves cannot, indicates a liquid core.

37. Chapter 21 Reading Skills Standardized Test Prep
In 1912 Alfred Wegener first proposed the theory that all of the continents formed when one giant continent broke apart. Wegener used the shape of the continents, the distribution of fossils, and similarity of rocks at different parts of the world as evidence.
Wegener’s Continental Drift theory was not immediately accepted by scientists. Some wondered about, but could not find, forces that would be strong enough to move such large masses of solid rock over great distances. In the middle of the 20th century, evidence from ocean floor exploration provided new evidence that continents move. The theory of plate tectonics, which explains how land masses move, not across the ocean floor, but across the Earth’s mantle, is now widely accepted.
6. Why do most scientists now accept the idea of moving continents, even though earlier scientists did not?

38. Reading Skills, continued
Chapter 21
Standardized Test Prep
Reading Skills, continued
6. [See previous slide for question.]
Answer: New observations provided evidence that was not available in These observations explain how continents can move and support the theory.

39. Interpreting Graphics
Chapter 21
Standardized Test Prep
Interpreting Graphics
7. What type of volcano is illustrated here?
A. cinder cone
B. composite
C. seamount
D. shield

40. Interpreting Graphics, continued
Chapter 21
Standardized Test Prep
Interpreting Graphics, continued
7. What type of volcano is illustrated here?
A. cinder cone
B. composite
C. seamount
D. shield