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Earthquakes and Volcanoes
3
section ●
Earthquakes, Volcanoes, and Plate Tectonics
What You’ll Learn
■
different ways Earth’s
plates move
■ how plate motion
causes earthquakes and
volcanoes
■ where the energy for
plate motion comes
from
Study Coach
Think-Pair-Share Work
Before You Read
You have read about volcanoes and earthquakes. On the
lines below, list some ways they are alike.
Read to Learn
with a partner. As you read this
section, discuss what you already
know about the topic and what
you learn from the text.
Earth’s Moving Plates
C Organize Information
●
What is plate tectonics?
Construct a three-tab Foldable.
Under the tabs, write
information about divergent
boundaries, convergent
boundaries, and hot spots.
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Earthquakes and Volcanoes
Imagine you and your classmates are moving long tables
to get ready for a program in the cafeteria. As you move the
tables, two or three of them crash into each other. What
would happen if you kept pushing on them? For a while,
one or two of the tables might keep another table from
moving. But if enough force were used, the tables would
slide past each other. One table might even slide up on top
of the other.
The movement of the tables and the possible crashing
among them is like the movement of the lithosphere. The
lithosphere is Earth’s crust and part of the upper mantle.
Earth’s lithosphere is broken into separate sections, or plates.
As these plates move around slowly, they collide, move
apart, or slide past each other. These movements, or plate
tectonics, can cause vibrations known as earthquakes. These
movements also can create conditions that cause volcanoes
to form.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
chapter
R
DE FUCA
A
Pa
R AN
Ri
PLATE
e
OUTH
O
U
AM
AMERICAN
A
M
PLATE
Convergent plate boundaries
ANTARCTIC
PLATE
ANTARCTIC
Volcano locations
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Where Volcanoes Form
Most volcanoes form near plate boundaries. Look at the
map above. It shows the location of volcanoes and plate
boundaries. There is a link between volcanic activity and
plate tectonics. Perhaps the energy involved in plate
tectonics causes magma to form deep under Earth’s surface.
Plate movement often explains why volcanoes form in
certain areas.
Picture This
1.
Interpret With your
pencil trace areas where
there are many volcanoes.
What do you notice about
the location of volcanoes?
What happens when plates move apart?
Divergent plate boundaries are boundaries where tectonic
plates move apart. On the map above, notice the arrows
pointing in opposite directions at divergent plate
boundaries. As the plates separate, rifts form. A rift is a long
crack that forms between tectonic plates at plate boundaries.
Rifts act like passageways for magma to flow through. Most
of the lava that flows onto Earth’s surface comes through
rift zones. Fissure eruptions often occur along rift zones.
These eruptions form lava that cools and hardens as basalt.
Basalt is the most common type of rock in Earth’s crust.
Reading Essentials
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What are convergent plate boundaries?
Plates move together at convergent plate boundaries.
When plates move toward each other and collide, the more
dense plate sinks under the less dense plate. This movement
can cause a volcano to form.
When one plate sinks under another plate, basalt and
sediment move down into the mantle. Water from the
sediment and altered basalt lowers the melting point of the
rock in the area. Heat in the mantle causes part of the
sinking plate to melt. The mantle over it also starts melting.
This melted material is then forced upward. Volcanoes form
in this way all around the Pacific Ocean where the Pacific
Plate collides with other plates.
Explain What happens
to plates that sink?
Picture This
3.
Determine Circle the
newest island. Put a box
around the island that
probably formed first.
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Earthquakes and Volcanoes
What is a hot spot?
Some volcanoes do not form along plate boundaries. For
example, the Hawaiian Islands are in the middle of the
Pacific Plate. Scientists suggest this is because the islands are
over a hot spot. A hot spot is a large body of magma that
has been forced upward through Earth’s mantle and crust.
When magma breaks through Earth’s crust, a volcano forms.
As the figure below shows, new islands are formed as the
Pacific Plate moves over the hot spot.
Volcanoes usually form at rift zones, subduction zones
(where one plate sinks under another), or over hot spots. In
these areas, magma deep inside Earth is forced upward
toward the surface. When magma breaks through, it flows
out as lava. The lava either piles up into layers or forms a
volcanic cone.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2.
Moving Plates Cause Earthquakes
Place two notebooks on your desk with the page edges
facing each other. Then push them together slowly. The
individual sheets of paper will start to bend upward from
the stress. If you keep pushing the notebooks, one will slip
past the other suddenly. This sudden movement is like an
earthquake.
Imagine what would happen if tectonic plates were moving
like notebooks. What do you think would happen if the
plates collided and then stopped moving? Pressure would
build up. Both plates would start to bend upward like the
sheets of paper did. In time, the plates may pass their elastic
limit. The breaking and elastic rebounding would release
energy in the form of vibrations felt as earthquakes.
Where do earthquakes occur?
4.
Recall What happens
when two tectonic plates
collide then stop moving?
Most earthquakes occur along plate boundaries. Many
earthquakes occur in the same places where volcanoes form.
This is because the movement of the plates that causes
volcanoes also can cause earthquakes. The Pacific Ring of
Fire is a belt of earthquakes and volcanoes all around the
Pacific Ocean.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How are seismic waves helpful?
By studying seismic waves, scientists have learned about
plate tectonics and Earth’s major layers. Seismic waves pass
through different materials in different ways. This helped
scientists discover the asthenoshere, the partially molten
layer that Earth’s plates float on.
What is driving Earth’s plates?
Energy causes Earth’s plates to move. There are several
theories about where this energy comes from. One idea is
that the energy comes from Earth’s core. Heat from the core
makes the rock in the mantle hotter. The hotter magma
rises towards the surface. As it cools, the magma gets denser
and then sinks back down towards the core. This cycle of
heating, rising, cooling, and sinking is a convection current.
Convection currents may be the reason Earth’s tectonic
plates move.
5.
Explain what happens to
magma in a convection
current.
Reading Essentials
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After You Read
Mini Glossary
hot spot: large body of magma that has been forced upward
from deep within Earth, which may cause volcanoes to
form in the middle of a plate
rift: long crack, fissure, or trough that forms between tectonic
plates moving apart at plate boundaries
1. Review the terms and their definitions in the Mini Glossary. Use the terms to explain
two reasons why volcanoes can occur.
2. Choose a sentence and write it in the correct box to show how a volcanic island forms.
Over time, lava flows build up into a volcanic mountain.
Magma breaks through Earth’s surface.
Lava cools on the bottom of the ocean.
1
Hot spot forms in Earth’s mantle.
2
3
5
Volcanic island is formed.
3. How did the strategy of sharing with your partner help you learn information from this
section?
End of
Section
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Earthquakes and Volcanoes
Visit blue.msscience.com to access your textbook, interactive
games, and projects to help you learn more about earthquakes,
volcanoes, and plate tectonics.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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