18.3 Volcanoes
When magma reaches Earth’s surface it is called lava.
Objectives
• Describe the major parts of a volcano.
• Compare and contrast shield, cinder-cone, and
composite volcanoes.
• Contrast the volcanism that occurs at plate boundaries.
• Explain the relationship between volcanism and hot spots.
Vocabulary
–
–
–
–
vent
crater
caldera
shield volcano
–
–
–
–
–
cinder-cone volcano
composite volcano
tephra
pyroclastic flow
hot spot
• A crater is a
bowl-shaped
depression at
the top of a
volcano that is
connected to
the magma
chamber by a
vent.
Tephra are
rock fragments
thrown into the
air during a
volcanic
eruption.
• A vent is
where lava
erupts
through an
opening in the
crust.
Calderas
• Calderas are large depressions that can
form when the summit or the side of a
volcano collapses into the magma chamber.
Volcanoes
Three main types:
Shield
Composite
Cinder Cone
Shield
• Very fluid lava- has Iron (Fe) and
Magnesium (Mg).
• Lava flows great distances
• Mild eruptions
• Gentle sloping
• Some of the largest volcanoes in the world
Shield Volcano
Mauna Loa - Hawaii
Composite
• Made up of alternating layers of ash,
cinders and lava
• Lava rich in silica (Si)
• Lava thick
• Eruptions can be explosive
• Steep slopes
Composite
Mt Fuji
Japan
Cinder cone
•
•
•
•
•
•
Smallest volcano type
Most abundant type
Lots of gas trapped in magma
Very explosive eruptions
Active for only a short time
Has bowl shaped crater in the center
Cinder Cones
Crater Lake
Lava Flow
Where do volcanoes occur?
– 80% found at convergent boundaries.
– 15% found at divergent boundaries.
– 5% found far from plate boundaries.
Where do volcanoes occur?
Mediterranean
Belt.
Pacific Ring
of
Fire.
Ring
of
Fire
Hot Spots
– Hot spots are unusually hot regions of
Earth’s mantle where high-temperature
plumes of mantle material rise toward the
surface.
– A plume does not move laterally, which results
in a trail of progressively older volcanoes that
formed as a plate moved over a hot spot.
Hot Spot
Plumes
Pyroclastic
Flows – A pyroclastic flow is
Sakura-jima
a cloud of volcanic
gas, dust, and other
tephra traveling at
speeds of nearly 200
km/h. The center of a
pyroclastic flow can
exceed 700°C.
Soufriere Hills
Mt. Saint Hellens
Pinatubo Ash Deposit
Pyroclastic deposits
• Over 100 pyroclastic fall deposits in
Oshima on the Izu Islands
Section Assessment
1. Match the following terms with their
definitions.
A. an opening in the crust from
A vent
___
C crater
___
D tephra
___
E pyroclastic flow
___
B
___caldera
which lava flows
B. depression caused by a
collapsed magma chamber
C. a bowl-shaped depression
around an opening in the crust
D. volcanic materials that are
thrown into the air during a
volcanic eruption
E. a cloud of rapidly moving,
extremely hot volcanic material
Section Assessment
2. How can chains of volcanoes that form
over a hot spot track plate movement?
The hot spot is in a fixed location. All of the
volcanoes in the chain were over the hot spot
when they formed. The volcanoes’ movement
and the direction of the chain’s alignment
indicates the movement of the plate.
Section Assessment
3. Identify whether the following statements
are true or false.
true Many cinder-cone volcanoes are less
______
than 500 m high.
false Volcanism is more common along divergent
______
boundaries as compared to convergent
boundaries.
false
______ The Mediterranean Belt is also known as the
“Ring of Fire”.
true
______ It can be 700ºC in the center of a pyroclastic flow.
If Yellowstone Erupted
Miscellaneous Info
• Underwater volcano is called a seamountthey look a lot like composite volcanoes.
• Most volcanoes occur at plate boundariesconvergent plate boundaries.
• Volcanoes can occur in the middle of
tectonic plates- these are called hotspots.
Ex. Hawaiian Islands