Volcanoes and Other
Igneous Activity
Chapter 7 (page 207)
Flash back question
What’s the difference between
magma and lava?
Volcanoes…
They come in all sizes and shapes,
and they have a mind of their own.
There are volcanoes and
supervolcanoes.
Doomsday?
“If Yellowstone
volcano erupts, twothirds of America will
be left uninhabitable”
Super volcanoes or
Doomdaysters
Flash Back Concept Question
How would you identify a volcanic
rock?
What makes volcanoes tick?
Factors that determine the violence of an
eruption
• 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 a function of
all the above factors.
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
A size comparison of the three
types of volcanoes
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
Basaltic Lava flow
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
Harden Pahoehoe lava
Flash back concept question
How to recognize a volcanic rock
formed from a gas-rich lava?
A typical aa flow
Harden aa lava
Flash back Concept Question
How does a hot spot volcano form?
Hot spot formation
Hot spot volcanoes formation
Hot Spots
Yellowstone Hot Spot
Yellowstone Hot spot
Super volcanoes or
Doomdaysters
Types of Volcanoes
Cinder cone
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Built from ejected lava fragments (cinder)
Steep slope angle
Rather small size
Produces mainly andesitic/felsic lava
Frequently occur in groups
Associated with subduction zones
Sunset Crater – a cinder cone
near Flagstaff, Arizona
A volcanic bomb
Bomb is approximately 10 cm long
Types of Volcanoes
Composite cone (or stratovolcano)
Volcanoes going postal!
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Large size
Steep slope
Interbedded lava flows and pyroclastic material
Most violent type of eruptions
Produces mainly andesitic/felsic lava
Associated with subduction zones
Most are adjacent to the Pacific Ocean (e.g.,
Cascade, Andes volcanoes)
Pyroclastic eruption
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
37
Mt. St. Helens – a typical
composite volcano
Mt. St. Helens Pyroclastic flow
A lahar along the Toutle River
near Mt. St. Helens
Mt. Hood, Oregon
Nevado del Ruiz tragedy
Lake Nyos tragedy
Caldera formation
Crater Lake, Oregon
Crater Lake, Nicaragua
Arenal Volcano, CR
Tobagon Hot Springs, Costa Rica
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
Movies to Watch
Dante’s Peak
http://www.imdb.com/title/tt011892
8/?ref_=nv_sr_1
Pompeii
http://www.imdb.com/title/tt19210
64/?ref_=nv_sr_1
Igneous Bodies
Are bodies formed by the
solidification of magma or lava
Intrusive igneous structures
exposed by erosion
56
Plutons
Type – I: tabular sheet like bodies
 Concordant bodies (parallel):
• Sills (tabular)
• Laccoliths (irregular)
 Discordant bodies (cut through):
• dikes
57
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
61
Test your knowledge of
volcanoes.
1-Which Magma is more viscous?
a.
b.
c.
d.
Basaltic
Felsic/Andesitic
Ultramafic
None of these
63
2-which magma traps more gases?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure
64
3-Which Magma is Hotter?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic /Andesitic
None of these
I am not sure
65
4-Which magma produces more
violent eruptions?
a.
b.
c.
d.
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure
66
5-Magma viscosity is a function
of…
a.
b.
c.
d.
e.
Temperature
Silica content
Gases content
All of these
b only
67
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
68
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
69
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
70
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
71
10-Volcanoes with steep cones have
a.
b.
c.
d.
Fluid lava
Viscous lava
No relation whatsoever
I am not sure
72
11-Volcanoes at divergent boundary
erupt mainly…lava
a.
b.
c.
d.
Felsic to Andesitic
Mafic
None of these
I am not sure
73
12-Volcanoes at convergent margins
produce mainly…lava
a.
b.
c.
d.
Felsic to Andesitic
Mafic
None of these
No relation whatsoever
74
13-Which volcanoes are more
explosive?
a.
b.
c.
d.
e.
Shield volcanoes
Composite cones
Cinder cones
None of these
I am not sure
75
14-Volcano explosiveness is
influenced by …
a.
b.
c.
d.
e.
Viscosity
Temperature
Amount of gases
All of the above
A and c only
76
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
77