Volcanoes &
Plate Tectonics
CH 6 Prentice Hall
p.178-181
What is a Volcano?
• A weak spot in the crust where molten
material, or magma comes to the surface.
– Volcanic Activity is a constructive force that adds
new rock to existing land or forms new islands.
Volcano Menu
Magma / Lava
• Magma- a mixture of
rock forming
substances, gases
and water vapor
from the mantle.
• Lava- Magma that
reaches the surface.
– Cools and forms solid
rock.
Volcano
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Magma Rises
• Magma is a liquidtherefore it is less
dense than surrounding
solid material.
– Flows upward into any
cracks in the rocks
above.
– Rises until it reaches the
surface or becomes
trapped beneath layers
or rocks.
Volcano
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Volcano Erupts
• Dissolved gases trapped in magma are under
tremendous pressure.
• The gasses dissolved in magma rush out,
carrying magma with them.
Volcano
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Crater
• Lava collects in the crater, the bowl-shaped
area that forms around the volcano’s vent.
Volcano
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Vent
• The point on the surface where magma and
gas leave the volcanoes pipe.
Volcano
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Pipe
• A narrow, almost vertical crack in the crust
through which magma rises to the surface.
Volcano
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Magma Chamber
• A large underground pocket of magma that
forms from magma rising to the surface.
Volcano
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Lava flow
• The river of lava that pours down a volcano
over the land.
Volcano
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Stages of a Volcano
• Active:
– A live volcano that is erupting or shows signs
that it may erupt in the future.
• Dormant:
– A sleeping volcano. It may awake in the future
and become active.
• Extinct:
– A dead volcano. It is unlikely to erupt again.
Location of Volcanoes
• Most occur along diverging boundaries such
as the mid-ocean ridge or in subduction
zones.
Location of Volcanoes
• About 600 active volcanoes on land, more lie
beneath the sea.
• Occur in belts that extend across continents
and oceans.
• Ring of Fire-formed by the many volcanoes that rim the
Pacific Ocean
Divergent Boundaries
• Form along the midocean ridge.
– Ridge a long underwater
rift valley.
– Lava pours out of the
cracks in the ocean floor.
• Iceland- is a volcano on
the mid-ocean ridge that
has reached the surface.
Convergent Boundaries
• Plates collide
– Denser plate subducts towards the mantle.
• Forms a deep-ocean trench
• The crust melts and forms magma that rises.
• Rising magma erupts through a volcano on the surface.
Convergent Boundaries
• Ocean/Ocean
– Denser ocean plate subducts
• Forms a deep sea trench
• Forms an island arc of
volcanoes.
– Japan
– New Zealand
– Aleutians
Convergent Boundaries
• Continent to Ocean
– Dense ocean plate
subducts
• Forms a deep sea trench
• Forms volcanoes on the
land.
– Mt. St Helens
– Andes Mountains-S.
America.
Hot Spot Volcanoes
• An area where magma from deep
within the mantle melts through the
crust like a blow torch.
• Lie in the middle of plates.
– Hawaii- formed in the middle of the
Pacific Ocean plate.
– Yellowstone- formed under the
continent.
• The plate travels over the hot spot.
– Created a chain of islands to form
Hawaii
Hot Spot Volcanoes