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Chapter 8
Volcanoes
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Section 1 Why Volcanoes Form
Section 2 Types of Volcanoes
Section 3 Effects of Volcanic Eruptions
Concept Map
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Chapter 8
Section 1 Why Volcanoes Form
Bellringer
List observations of volcanoes you have seen in person
or on television.
Write your observations in your Science Journal.
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Chapter 8
Section 1 Why Volcanoes Form
What You Will Learn
• Most volcanoes are located at or near tectonic plate
boundaries.
• Volcanoes form at divergent boundaries, convergent
boundaries, and hot spots.
• The temperature, pressure, and fluid content of rock
play roles in the formation of magma.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form
• A volcano is a vent or fissure in Earth’s surface
through which melted rock and gases pass.
• An estimated 1,500 volcanoes have been active
above sea level during the past 10,000 years.
• Many more volcanoes have been active beneath the
ocean.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• The map in the next slide shows the locations of
some of the world’s most active volcanoes.
• The map also shows the boundaries between
tectonic plates.
• A large number of volcanoes lie directly on tectonic
plate boundaries.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• The plate boundaries that surround the Pacific Ocean
have so many volcanoes that the area is called the
Ring of Fire.
• At tectonic plate boundaries, several processes
cause rock to melt at lower-crustal or upper-mantle
depths.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• Molten rock is called magma.
• Because magma is less dense than the solid rock
surrounding it, magma travels up toward the surface.
• When magma reaches the surface, it erupts to form a
volcano.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• Volcanoes can form at divergent boundaries,
convergent boundaries, or hot spots.
Divergent Boundaries
• As tectonic plates pull away from each other at a
divergent boundary, a set of deep vertical fractures
called fissures form.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
Divergent Boundaries
• Molten rock flows through these fissures onto the
ocean floor.
• The molten rock also forms submarine volcanoes.
• Underwater mountain chains known as mid-ocean
ridges are common at divergent boundaries.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• Most volcanic activity on Earth happens at mid-ocean
ridges.
• Most divergent boundaries are underwater.
• However, Iceland is an island that is being pulled
apart by a mid-ocean ridge.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
Convergent Boundaries
• At a convergent boundary, two plates collide.
• The denser plate slides under the other plate.
• As the denser plate bends, a deep depression known
as a trench forms.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• At a trench, one plate
moves downward into
the mantle.
• The process in which
one plate moves
beneath another is
called subduction
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• As the plate moves farther downward into Earth’s
mantle, the rock is subjected to greater heat and
pressure.
• As a result, the plate releases fluids, which causes
surrounding rock to melt.
• Magma then moves upward through cracks in the
Earth to form a volcano.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
Hot Spots
• Hot spots are volcanically active places that are not
located at tectonic plate boundaries.
• Hot spots lie directly above columns of hot rock that
rise through Earth’s mantle.
• These columns are called mantle plumes.
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Chapter 8
Section 1 Why Volcanoes Form
Where Volcanoes Form, continued
• Mantle plumes are stationary.
• As a tectonic plate moves over a mantle plume, rising
magma causes a chain of volcanic islands to form.
• The Hawaiian Islands formed as the Pacific Plate
passed over a mantle plume.
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Chapter 8
Volcanoes
Hot Spots and Mantle Plumes
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms
• Magma forms in the deeper parts of Earth’s crust and
in the uppermost parts of the mantle.
• In these locations, temperature and pressure are very
high.
• Changes in temperature and pressure cause magma
to form.
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Chapter 8
Section 1 Why Volcanoes Form
• Rock melts when its temperature increases or when
the pressure on the rock decreases.
• Water can lower the melting temperature of rock and
cause the rock to melt.
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms, continued
Increasing Temperature
• As a tectonic plate moves downward into the mantle,
the plate is exposed to greater temperatures at
depth.
• This increase in temperature may cause minerals in
the rock to melt.
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms, continued
• Not all minerals in the rock melt at the same time.
• Different minerals have different melting
temperatures.
• Minerals that have low melting temperatures melt
before minerals that have high melting temperatures.
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms, continued
Decreasing Pressure
• Magma can form when pressure on rock decreases.
• In Earth’s mantle, the pressure on rock is so great that
the rock cannot expand.
• Expansion is important in the formation of magma,
because magma takes up more space than solid rock
does.
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms, continued
• At divergent boundaries and hot spots, hot mantle
rock rises.
• At a shallower depth, the pressure on the rock
decreases.
• The decrease in pressure allows the hot rock to
expand and melt.
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Chapter 8
Section 1 Why Volcanoes Form
How Magma Forms, continued
Adding Fluids
• Oceanic lithosphere is composed of sediments and
volcanic rocks that contain water and other fluids.
• When oceanic lithosphere moves downward into the
mantle, the fluids contact surrounding rock.
• When fluids enter the hot mantle rock, the melting
temperature of the rock decreases.
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Chapter 8
Section 2 Types of Volcanoes
Bellringer
Look through the section. Then, write definitions for
each of the following words:
composite volcano
shield volcano
cinder cone volcano
Write your answers in your Science Journal. Review
and, if necessary, revise your definitions after reading
this section.
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Chapter 8
Section 2 Types of Volcanoes
What You Will Learn
• Nonexplosive eruptions of basaltic magma occur at
divergent boundaries.
• Shield volcanoes that form from enormous volumes
of basaltic magma occur at hot spots.
• Explosive eruptions of silica-rich magma occur at
convergent boundaries.
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Chapter 8
Section 2 Types of Volcanoes
Types of Volcanoes
• The process of magma formation is different at each
type of plate boundary.
• Therefore, the composition of magma differs in each
tectonic setting.
• Tectonic settings determine the types of volcanoes
that form and the types of eruptions that take place.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries
• At a divergent boundary, the lithosphere becomes
thinner as two plates pull away from each other.
• A set of deep cracks form in an area called a rift
zone.
• Hot mantle rock rises to fill these cracks.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• As the rock rises, a decrease in pressure causes hot
mantle rock to melt and form magma.
• The magma that reaches Earth’s surface is called
lava.
• Lava that flows at divergent boundaries forms from
melted mantle rock.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• This lava is rich in the elements iron and magnesium.
It is relatively poor in silica.
• Because of its composition, lava from mantle rock
cools to form dark-colored rock.
• The term mafic describes magma, lava, and rocks
that are rich in iron and magnesium.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• Because it is low in silica, mafic lava is runny and not
sticky.
• This type of lava generally produces nonexplosive
eruptions.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• Mid-ocean ridges are underwater mountain chains
that form where two tectonic plates are moving apart.
• As the plates move apart, magma from the mantle
rises to fill cracks that form in the crust.
• Some of the magma erupts as basaltic lava on the
ocean floor.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• The magma and lava cool to become part of the
oceanic lithosphere.
• As the plates continue to move, older oceanic
lithosphere moves away from the mid-ocean ridge.
• New cracks form, and new lithosphere forms in the
rift zone.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• The process in which new sea floor forms as older
sea floor is pulled apart is called sea-floor spreading.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• The mid-ocean ridge called the Mid-Atlantic Ridge is
unusually active.
• This activity has built part of the ridge into a large
island known as Iceland.
• Long linear cracks called fissures have formed where
the Atlantic and Eurasian plates are moving apart.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Divergent Boundaries, continued
• Basaltic magma rises to Earth’s surface through
these fissures and erupts nonexplosively.
• Icelandic volcanoes, such as Krafla, are often
associated with large, connected fissure systems.
• Lava erupts frequently through these fissures. As a
result, Iceland is continually getting bigger.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots
• A hot spot forms in a tectonic plate over a mantle
plume.
• Mantle plumes are columns of hot, solid rock that rise
through the mantle by convection.
• Plumes are thought to originate at the boundary
between the mantle and the outer core.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• When the top of a mantle plume reaches the base of
the lithosphere, the mantle rock spreads out and
“pools” under the lithosphere.
• Because pressure on the rock is low at this shallow
depth, the rock melts.
• Large volumes of magma are released onto the
ocean floor.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• Continuous eruptions may produce a volcanic cone.
• As the plate continues to move over the mantle
plume, a chain of volcanoes may form.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• Because lava at hot spots comes from the mantle, it
is mafic and fluid.
• Most eruptions at hot spots are nonexplosive.
• The type of rock that forms from this lava depends on
the temperature, gas content, flow rate, and slope of
the lava flow.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
Shield Volcanoes
• Shield volcanoes usually form at hot spots.
• Shield volcanoes form from layers of lava left by
many nonexplosive eruptions.
• The lava is very runny, so it spreads out over a wide
area.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• Over time, the layers of lava create a volcanic
mountain that has gently sloping sides.
• The sides of shield volcanoes are not very steep, but
the volcanoes can be very large.
• The Hawaiian shield volcano, Mauna Kea, is the
tallest mountain on Earth.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
Parts of a Volcano
• Most volcanoes share a specific set of features.
• The magma that feeds the eruptions pools deep
underground in a structure called a magma chamber.
• At Earth’s surface, lava is released through openings
called vents.
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Chapter 8
Volcanoes
Magma and Vents
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• Before erupting as lava, magma rises from the
magma chamber to Earth’s surface through cracks in
the crust.
• This movement of magma causes small earthquakes
that can be used to predict an eruption.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• Lava may erupt from a central summit crater of a
shield volcano.
• Lava may also erupt from fissures along the sides of
the shield volcano.
• After erupting from a vent, the fluid lavas move
downslope in lava flows.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Hot Spots, continued
• A lava flow is a long river of molten rock.
• Often the flow will cool and solidify on top while lava
in the interior continues to flow.
• Flowing lava in the interior travels through long,
pipelike structures known as lava tubes.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries
• At a convergent boundary, a plate that contains
oceanic lithosphere may descend into the mantle
beneath another plate.
• The descending lithosphere contains water.
• As the lithosphere descends into the mantle,
temperature and pressure increase.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• The subducting lithosphere releases water into the
surrounding mantle and overlying crust.
• The water lowers the temperature of the rock, and
the rock melts.
• The magma that forms rises through the crust and
erupts. These eruptions form a chain of volcanoes
parallel to the plate boundary.
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Chapter 8
Volcanoes
Volcano Formation at Convergent Boundaries
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• Magmas at convergent boundaries are composed of
melted mantle rock and melted crustal rock.
• Therefore, fluid mafic lava and lava rich in silica and
feldspar form at these boundaries.
• Lavas rich in silica and feldspar cool to form lightcolored rocks.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• The term felsic is used to describe magma, lava, and
rocks that are rich in silica and feldspars.
• Silica-rich magma tends to trap water and gas
bubbles.
• This causes enormous gas pressure to develop
within the magma.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• As the gas-filled magma rises to Earth’s surface,
pressure is rapidly released.
• This change in pressure causes a powerful explosive
eruption.
• Pyroclastic materials are released during the
eruption.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• Pyroclastic material forms when magma explodes
from a volcano and solidifies in the air.
• Pyroclastic material also forms when powerful
eruptions shatter existing rock.
• Four types of pyroclastic material include volcanic
bombs, lapilli, volcanic ash, and volcanic blocks.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• Pyroclastic flows are produced when a volcano ejects
enormous amounts of hot ash, dust, and toxic gases.
• This glowing cloud of pyroclastic material can race
down the slope of a volcano at speeds of more than
200 km/h.
• This speed is faster than the speed of most
hurricane-force winds.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• The temperature at the center of a pyroclastic flow
can exceed 700°C.
• At this high temperature, a pyroclastic flow burns
everything in its path.
• Extreme winds and temperatures make pyroclastic
flows the most dangerous of all volcanic phenomena.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
Cinder Cone Volcanoes
• Cinder cone volcanoes are the smallest type of
volcano.
• They generally reach heights of no more than 300 m.
• Cinder cone volcanoes are made of pyroclastic
material.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• Cinder cone volcanoes most often form from
moderately explosive eruptions.
• They have steep sides and a wide summit crater.
• Unlike other types of volcanoes, cinder cone
volcanoes usually erupt only once in their lifetime.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
Composite Volcanoes
• Composite volcanoes are also called
stratovolcanoes.
• They form from both explosive eruptions of
pyroclastic material and quieter flows of lava.
• This combination of eruptions forms alternating layers
of pyroclastic material and lava.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
• Composite volcanoes have a broad base and sides
that get steeper toward the crater.
• These volcanoes may generate many eruptions.
However, eruptions may occur at intervals of
hundreds of years or more.
• Mount Fuji in Japan is a famous composite volcano.
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Chapter 8
Section 2 Types of Volcanoes
Volcanoes at Convergent Boundaries,
continued
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Bellringer
Create a labeled drawing based on what you think
happens when a volcano erupts. Then, write a
hypothesis about how the volcanic eruption may
affect the surrounding areas.
Write your answers in your Science Journal.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
What You Will Learn
• Volcanic eruptions can cause the loss of human life
and the devastation of wildlife habitats.
• Volcanic eruptions can cause the average global
temperature of Earth to decrease.
• Volcanic eruptions provide benefits to humans and to
the environment.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions
• Volcanic explosions can have local and global
effects.
• In April 1815, Tambora volcano in Indonesia erupted
explosively.
• The pyroclastic flows and falling debris killed about
10,000 people in the area.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
• High in the atmosphere, ash and gas spread around
Earth.
• The average global temperature decreased by as
much as 3°C for one to two years.
• The lower temperature caused crop failures and
starvation, particularly in New England and Europe.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
• The effects of lower temperatures led to the deaths of
about 82,000 people.
• Therefore, an estimated total of 92,000 people
around the world lost their lives as a result of the
Tambora eruption.
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Chapter 8
Volcanoes
Effects of Volcanoes on Earth
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
Local Effects of Volcanic Eruptions
• Volcanic eruptions can cause loss of human life and
loss of wildlife habitat.
• The blast from an explosive eruption can knock down
trees, destroy buildings, and kill humans and animals.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
• Pyroclastic flows can burn everything in their path.
• Hot volcanic materials can melt the snowcap on a
mountain, causing devastating floods.
• Volcanic ash can mix with water to form fast-moving
mudflows called lahars.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
• The weight of falling ash can collapse structures,
bury crops, and damage engines.
• Volcanic ash can also cause respiratory problems in
humans.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
Global Effects of Volcanic Eruptions
• Large volcanic eruptions can affect Earth’s climate for
several years.
• During large eruptions, ash and sulfur-rich gases can
be pushed into the stratosphere.
• As the ash and gases spread around the planet, they
absorb and scatter sunlight.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Negative Effects of Volcanic Eruptions,
continued
• Enough sunlight may be absorbed or scattered to
lower the average global temperature of Earth.
• In 1991, after the eruption of Mount Pinatubo in the
Philippines, the amount of sunlight to reach Earth’s
surface decreased by 2-4%.
• This decrease caused the average global
temperature of Earth to decrease by several tenths of
a degree for several years.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions
Volcanic Soils
• Volcanic soils are some of the most fertile soils on
Earth.
• Volcanic rocks are made of minerals that contain
elements important to plant growth.
• When volcanic rocks break down, they form soils that
contain many nutrients that plants can use.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
Geothermal Energy
• Magma heats the rocks that surround it.
• These rocks often hold water that also becomes
heated.
• This heated water, called geothermal water, may
reach temperatures of hundreds of degrees Celcius.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• Geothermal water contains large amounts of heat
energy.
• This energy can be tapped by drilling wells to reach
the hot water.
• Water can also be pumped through heated rocks to
obtain energy.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• Geothermal water can be used to drive turbines that
generate electricity.
• It can also be used to heat homes, grow crops, or
keep roads free of ice.
• In Reykjavik, Iceland, 85% of all homes are heated
with geothermal water.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• Volcanic rocks are often used in construction.
• As early as 300 BCE, Romans made concrete from
volcanic ash and lime.
• This material was used to build the Colosseum in
Rome in 80 CE. The strength of this material has
allowed the Colosseum to stand for nearly two
thousand years.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• As recently as the 20th century, volcanic ash was
used to make concrete for dams in the United States.
• Today, basalt and pumice are often used in the
construction of roads and bridges, and the production
of concrete.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• Volcanic rocks have many other uses.
• Volcanic ash absorbs moisture, so it is used in cat
litter.
• Because pumice is abrasive, it is used in facial
scrubs, soaps, cleaners, and polishes.
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Chapter 8
Section 3 Effects of Volcanic
Eruptions
Benefits of Volcanic Eruptions, continued
• Pumice is added to soil to allow air and water to
circulate more easily through the soil.
• Because metals in pumice are not water soluble,
pumice is used alone or with silica sand to filter
drinking water.
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Chapter 8
Volcanoes
Concept Map
Use the terms below to complete the concept map
on the next slide.
eruptions
shield volcanoes
cinder cone volcanoes
composite volcanoes
lava
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Chapter 8
Volcanoes
Concept Map
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Chapter 8
Volcanoes
Concept Map
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