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Volcanoes and Other
Igneous Activity
Chapter 7 (page 207)
Volcanic eruptions
Factors that determine the violence of an
eruption
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Composition of the magma (silica content)
Temperature of the magma (hot or cool)
Dissolved gases in the magma (volatiles)
Viscosity of magma (Viscosity is a measure of
a material's resistance to flow; it is determined
by the amount of silica content in the magma)
Volcanic eruptions
Factors affecting magma viscosity
• Temperature (hotter magmas are less viscous)
• Chemical composition (silica content)
- High silica – high viscosity (e.g., rhyolitic lava)
- Low silica – more fluid (e.g., basaltic lava)
• Dissolved gases (volatiles)
- Mainly water vapor and carbon dioxide
- Gases expand near the surface → explosiveness
Controls of Volcanic eruptions
 Mafic lava → low silica →low viscosity (more fluid) →
hotter → less gases → less explosive → flatter cones
Example: Hawaiian volcanoes, ocean floor volcanoes
 Felsic andesitic lava → high silica → high viscosity
(stiffer) → cooler→ more gases → more explosive →
steeper cones
Example: Mt St. Helen (cascade volcanoes), subduction
zone volcanoes
Types of Hawaiian lava flows
Types of lava
• Pahoehoe lava (gas filled, resembles braids in ropes)
• Aa lava (gas free, rough, jagged blocks)
Go to Youtube to watch videos
A Pahoehoe lava flow
A typical aa flow
A size comparison of the three
types of volcanoes
A volcanic bomb
Bomb is approximately 10 cm long
Types of Volcanoes
 Shield volcano
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Broad, slightly domed
Primarily made of basaltic (fluid) lava
Generally large size
Associated with ocean floor magmatism
e.g., Mauna Loa in Hawaii
Types of Volcanoes
Cinder cone
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Built from ejected lava fragments (cinder)
Steep slope angle
Rather small size
Frequently occur in groups
Associated with subduction zones
Sunset Crater – a cinder cone
near Flagstaff, Arizona
Types of Volcanoes
Composite cone (or stratovolcano)
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Large size
Steep slope
Interbedded lava flows and pyroclastic material
Most violent type of eruptions
Associated with subduction zones
Most are adjacent to the Pacific Ocean (e.g.,
Cascade, Andes volcanoes)
A composite volcano
(stratovolcano)
Features of Volcanoes
General features
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Conduit, or pipe
Crater
Vent
Crater
Fumaroles
Caldera
Crater Lake
What’s in a Volcano’s arsenal?
• Pyroclastic flow (very hot
ash)
• Lahar (deadly mud flow)
• Crater Lake’s deadly gases
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Mt. St. Helens – a typical
composite volcano
Mt. Hood, Oregon
Mt. St. Helens Pyroclastic flow
A lahar along the Toutle River
near Mt. St. Helens
Formation of a caldera
Crater Lake in Oregon
Formation of a volcanic neck
Plate Tectonics and Igneous
Activity
Volcanism at Divergent plate margins:
• Type of magma: Basaltic
• Example: Mid Ocean ridge
Volcanism at Convergent plate margins
• Type of magma: Andesitic/felsic
• Example Island arcs; Andes Mountains
Plate Tectonics and Igneous
Activity
Intraplate Volcanism (Hot Spot) not a plate
margin
• Form over a stationary pocket of magma;
type of lava: Basaltic
• Form a chain of volcanoes Example: Hawaiian
volcanic chain
• The only active volcano is over the hot spot
Locations of some of Earth’s
major volcanoes
Igneous Bodies
Are bodies formed by the
solidification of magma or lava
Intrusive igneous structures
exposed by erosion
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Plutons
Type – I: tabular sheet like bodies
 Concordant bodies (parallel):
• Sills (tabular)
• Laccoliths (irregular)
 Discordant bodies (cut through):
• dikes
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Plutons
Type – II: Massive irregular bodies:
 Batholiths
Stocks
A sill in the Salt River
Canyon, Arizona
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A batholith exposed by erosion
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End of Chapter 7
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Clicker test, do not talk, please.
1-Which Magma is more viscous?
a.
b.
c.
d.
Basaltic
Felsic/Andesitic
Ultramafic
None of these
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2-which magma traps more gases?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure
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3-Which Magma is Hotter?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic /Andesitic
None of these
I am not sure
37
4-Which magma produces more
violent eruptions?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure
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5-Magma viscosity is a function
of…
a.
b.
c.
d.
e.
Temperature
Silica content
Gases content
All of these
b only
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6-Which volcano is over a hot spot?
a.
b.
c.
d.
e.
Mt St Helens (Washington State)
Mt Rainier (Washington State)
Mauna Loa (Hawaii)
All of these
a and c only
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7-Which one is a Shield Volcano?
a.
b.
c.
d.
e.
Mt St Helens (Washington State)
Mt Rainier (Washington State)
Mauna Loa (Hawaii)
All of these
a and c only
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8-Which one is a Composite cone?
a.
b.
c.
d.
e.
Mt St Helens (Washington State)
Sunset Crater (Arizona)
Mauna Loa (Hawaii)
All of these
a and c only
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9-Which volcano type produces
lava + pyroclastic flows
a.
b.
c.
d.
e.
Mt St Helens (Washington State)
Kula (Hawaii)
Mauna Loa (Hawaii)
All of these
a and c only
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10-Volcanoes with steep cones have
a.
b.
c.
d.
Fluid lava
Viscous lava
No relation whatsoever
I am not sure
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11-Volcanoes at divergent boundary
erupt mainly…lava
a.
b.
c.
d.
Felsic to Andesitic
Mafic
None of these
I am not sure
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12-Volcanoes at convergent margins
produce mainly…lava
a.
b.
c.
d.
Felsic to Andesitic
Mafic
None of these
No relation whatsoever
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13-Which volcanoes are more
explosive?
a.
b.
c.
d.
e.
Shield volcanoes
Composite cones
Cinder cones
None of these
I am not sure
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14-Volcano explosiveness is
influenced by …
a.
b.
c.
d.
e.
Viscosity
Temperature
Amount of gases
All of the above
A and c only
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15-A Pahoehoe lava is
a.
b.
c.
d.
e.
Gases-free with jagged surface lava
Wrinkled, ropy, and gaseous lava
Explosive ash and fragments flow
None of these
I am not sure
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