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Planet Earth
Section 2
Section 2: Earthquakes and Volcanoes
Preview
• Key Ideas
• Bellringer
• What are Earthquakes?
• Measuring Earthquakes
• Volcanoes
Planet Earth
Section 2
Key Ideas
〉 Where do most earthquakes occur?
〉 How do scientists learn about earthquakes and the
Earth’s interior?
〉 What is a volcano?
Planet Earth
Section 2
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?
Planet Earth
Section 2
What are Earthquakes?
〉 Where do most earthquakes occur?
〉 By looking at maps showing past seismic activity, one
can see that earthquakes occur mostly at the
boundaries of tectonic plates, where the plates move
with respect to each other.
Planet Earth
Section 2
What are Earthquakes? continued
• 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 an earthquake.
• focus: the location within Earth along a fault at which
the first motion of an earthquake occurs
• epicenter: the point on Earth’s surface directly above
an earthquake’s starting point, or focus
Planet Earth
Focus and Epicenter
Section 2
Planet Earth
Section 2
What are Earthquakes? continued
• Energy from earthquakes is transferred by waves.
– The energy released from an earthquake is measured
as shock waves.
– Earthquakes generate three types of waves:
• Longitudinal waves, also known as P waves
• Transverse waves, also known as S waves
• Surface waves
Planet Earth
Section 2
What are Earthquakes? continued
– Longitudinal waves travel by compressing and
stretching crust. They are also called primary waves
(P waves).
– Transverse waves travel in an up and downward
movement. They are also called secondary waves
(S waves).
Planet Earth
Section 2
Visual Concept: Longitudinal Waves
Planet Earth
Section 2
Visual Concept: Transverse Wave
Planet Earth
Section 2
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.
• surface wave: a seismic wave that travels along the
surface of a medium and has a stronger effect near the
surface of the medium than it has in the interior
Planet Earth
Section 2
Visual Concept: Seismic Waves: Surface
Waves
Planet Earth
Section 2
Measuring Earthquakes
〉 How do scientists learn about earthquakes and the
Earth’s interior?
〉 Because energy from earthquakes is transferred by
waves, scientists can measure the waves to learn about
earthquakes and about the interior of Earth through
which the waves travel.
Planet Earth
Section 2
Measuring Earthquakes, continued
• Seismologists detect and measure earthquakes.
– Seismologists use sensitive equipment called
seismographs to record data about earthquakes.
– Records of seismic activity are called seismograms.
• seismology: the study of earthquakes including their
origin, propagation, energy, and prediction
Planet Earth
Section 2
Visual Concept: Seismographs and Mapping
Earth’s Layers
Planet Earth
Section 2
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.
Planet Earth
Section 2
Measuring Earthquakes, continued
• Geologists use seismographs to investigate Earth’s
interior.
– The way P and S waves travel through Earth’s interior
helps scientists make a model of Earth with layers of
different densities.
– Scientists have used this information to develop a
model of Earth’s interior structure.
Planet Earth
Section 2
Measuring Earthquakes, continued
• The Richter scale is used to measure earthquakes.
– Although the Richter scale was used popularly for
much of the 20th century, scientists today more
often use other scales, such as the momentmagnitude scale.
• Richter scale: a scale that expresses the magnitude of
earthquakes
Planet Earth
Section 2
Measuring Earthquakes, continued
• Magnitude scales alone cannot predict how severe an
earthquake will be.
– Many factors, such as distance from the focus and
local construction techniques, affect how much
damage an earthquake will cause.
• 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.
Planet Earth
Section 2
Volcanoes
〉 What is a volcano?
〉 A volcano is any opening in Earth’s crust through which
magma has reached Earth’s surface.
• crust: an opening in the surface of Earth through which
volcanic material passes
Planet Earth
Section 2
Visual Concept: Magma and Vents
Planet Earth
Section 2
Volcanoes
• Shield volcanoes have mild eruptions.
– Shield volcanoes are some of the largest volcanoes.
• Composite volcanoes have trapped gas.
– Composite volcanoes are made of alternating layers
of ash, cinders, and lava.
– Composite volcanoes are usually very steep.
• Cinder cones are the most abundant volcanoes.
– Cinder cones are the smallest and most abundant
volcanoes.
Planet Earth
Volcanoes
Section 2
Planet Earth
Section 2
Visual Concept: Types of Volcanoes
Planet Earth
Section 2
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.
Planet Earth
Ring of Fire
Section 2
Planet Earth
Section 2
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.
Planet Earth
Section 2
Volcanoes, continued
• Volcanoes occur at hotspots.
– 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.
Planet Earth
Section 2
Visual Concept: Hot Spots and Mantle Plumes