Essential Questions
• How do plate tectonics influence the formation of
volcanoes?
• Where are the major zones of volcanism?
• What are the parts of a volcano?
• How do volcanic landforms differ?
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Volcanoes
Vocabulary
Review
• convergent
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volcanism
hot spot
flood basalt
fissure
conduit
vent
crater
caldera
shield volcano
cinder cone
composite volcano
Volcanoes
• Volcanism describes all the processes associated with
the discharge of magma, hot fluids, ash and gases.
Zones of Volcanism
• Most volcanoes form at plate boundaries. The majority
form at convergent boundaries and divergent
boundaries.
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Volcanoes
Convergent volcanism
• In an oceanic-continental subduction zone, the denser
oceanic plate slides under the continental plate into
the hot mantle. Parts of the mantle above the
subducting plate melt and magma rises, eventually
leading to the formation of a volcano.
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Volcanoes
Convergent volcanism
• Most volcanoes located on land result from oceaniccontinental subduction. These volcanoes are
characterized by explosive eruptions.
Santiaguito Dome, Guatemala
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Volcanoes
Two major belts
• The volcanoes associated with convergent plate
boundaries form two major belts. The larger belt, the
Circum-Pacific Belt, is also called the Pacific Ring of
Fire. The outline of the belt corresponds to the outline
of the Pacific Plate.
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Volcanoes
Two major belts
• The smaller belt is the Mediterranean Belt. Its general
outlines correspond to the boundaries between the
Eurasian, African, and Arabian plates.
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Volcanoes
Divergent volcanism
• Eruptions at divergent boundaries tend to be
nonexplosive. At the divergent boundary on the ocean
floor, eruptions often form huge piles of lava called
pillow lava.
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Volcanoes
Hot spots
• Some volcanoes form far from plate boundaries over hot
spots.
• A hot spot is an unusually hot area in Earth’s mantle
where high-temperature plumes of mantle material rise
toward the surface.
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Volcanoes
Hot spots
• The Hawaiian islands are located over a plume of
magma. The hot spot formed by the magma plume
remains stationary while the Pacific Plate slowly moves
northwest.
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Volcanoes
Hot spots
• The volcanoes on the oldest
Hawaiian island, Kauai, are
inactive because the island
no longer sits above the
stationary hot spot. The
world’s most active volcano,
Kilauea, on the Big Island of
Hawaii, is currently located
over the hot spot.
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Volcanoes
Hot spots
• Chains of volcanoes that form over stationary hot spots
provide information about plate motions. The rate and
direction of plate motion can be calculated from the
positions of these volcanoes.
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Volcanoes
Hot spots
• The Hawaiian islands are at
one end of the HawaiianEmperor volcanic chain.
The oldest seamount, Meiji,
is at the other end of the
chain and is about 80
million years old.
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Volcanoes
Hot spots
• Flood basalts form when lava
flows out of long cracks in
Earth’s crust. These cracks are
called fissures.
• The Columbia River basalts,
located in the northwestern
United States, were formed
this way.
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Volcanoes
Hot spots
• About 65 mya in India, a huge flood basalt eruption
created an enormous plateau called the Deccan Traps.
The volume of basalt in the Deccan Traps is estimated
to be about 512,000 km3.
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Volcanoes
Anatomy of a Volcano
• Lava reaches the surface by
traveling through a tubelike
structure called a conduit.
• The lava then emerges
through an opening called a
vent.
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Volcanoes
Anatomy of a Volcano
• Over time, layers of solidified
lava can accumulate to form a
mountain known as a volcano.
At the top of a volcano, around
the vent, is a bowl-shaped
depression called a crater.
• Volcanic craters are usually
less than 1 km in diameter.
Larger depressions, called
calderas, can be up to 100 km
in diameter.
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Volcanoes
Types of Volcanoes
• The appearance of a volcano depends on two factors:
the type of material that forms the volcano and the
type of eruptions that occur.
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Volcanoes
Shield volcanoes
• A shield volcano is a mountain with broad, gently
sloping sides and a nearly circular base.
• Shield volcanoes form when layers of lava accumulate
during nonexplosive eruptions. They are the largest type
of volcano.
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Volcanoes
Cinder cones
• When eruptions eject small pieces of lava into the air,
cinder cones form as this material, called tephra, falls
back to Earth and piles up around the vent.
• Cinder cones have steep sides and are the smallest
type of volcano.
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Volcanoes
Composite volcanoes
• Composite volcanoes are formed of layers of ash and
hardened chunks of lava from violent eruptions
alternating with layers of lava that oozed downslope
before solidifying.
• These volcanoes are generally cone-shaped with
concave slopes.
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Volcanoes
Review
Essential Questions
• How do plate tectonics influence the formation of volcanoes?
• Where are the major zones of volcanism?
• What are the parts of a volcano?
• How do volcanic landforms differ?
Vocabulary
•
•
•
•
volcanism
hot spot
flood basalt
fissure
Copyright © McGraw-Hill Education
•
•
•
•
conduit
vent
crater
caldera
• shield volcano
• cinder cone
• composite volcano
Volcanoes