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Chapter 21
Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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?
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Longitudinal Waves
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Transverse Wave
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Seismic Waves: Surface Waves
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Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Seismographs and Mapping Earth’s Layers
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Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Richter Scale
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Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Volcanoes
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Section 2 Earthquakes and
Volcanoes
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.
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Section 2 Earthquakes and
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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.
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Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Types of Volcanoes
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Section 2 Earthquakes and
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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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Ring of Fire
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Section 2 Earthquakes and
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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.
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Section 2 Earthquakes and
Volcanoes
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.
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Chapter 21
Section 2 Earthquakes and
Volcanoes
Hot Spots and Mantle Plumes
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Chapter 21
Standardized Test Prep
Understanding Concepts
1. Which of these occurs where two tectonic plates
move away from each other?
A.
B.
C.
D.
convergent boundary
divergent boundary
ocean trench
subduction zone
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Chapter 21
Standardized Test Prep
Understanding Concepts, continued
1. Which of these occurs where two tectonic plates
move away from each other?
A.
B.
C.
D.
convergent boundary
divergent boundary
ocean trench
subduction zone
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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
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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
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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
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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
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Understanding Concepts, continued
4. Which of the following is an example of chemical
weathering of rock?
F.
G.
H.
I.
deposition
erosion
frost wedging
leaching
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Standardized Test Prep
Understanding Concepts, continued
4. Which of the following is an example of chemical
weathering of rock?
F.
G.
H.
I.
deposition
erosion
frost wedging
leaching
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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?
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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.
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Chapter 21
Standardized Test Prep
Reading Skills
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?
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Reading Skills, continued
6. [See previous slide for question.]
Answer: New observations provided evidence that was
not available in 1912. These observations explain
how continents can move and support the theory.
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Chapter 21
Standardized Test Prep
Interpreting Graphics
7. What type of volcano is illustrated here?
A. cinder cone
B. composite
C. seamount
D. shield
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Standardized Test Prep
Interpreting Graphics, continued
7. What type of volcano is illustrated here?
A. cinder cone
B. composite
C. seamount
D. shield
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