volcanoes and other igneous activity chapter 7 (page 207)
TRANSCRIPT
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Volcanoes and Other Igneous Activity
Chapter 7 (page 207)
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Volcanic eruptions
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 determined by the amount of silica content in the magma)
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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
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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
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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
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A Pahoehoe lava flow
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A typical aa flow
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A size comparison of the three types of volcanoes
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A volcanic bomb
Bomb is approximately 10 cm long
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Types of Volcanoes
Shield volcano
• Broad, slightly domed
• Primarily made of basaltic (fluid) lava
• Generally large size
• Associated with ocean floor magmatism
• e.g., Mauna Loa in Hawaii
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Types of Volcanoes
Cinder cone
• Built from ejected lava fragments (cinder)
• Steep slope angle
• Rather small size
• Frequently occur in groups
• Associated with subduction zones
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Sunset Crater – a cinder cone near Flagstaff, Arizona
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Types of Volcanoes
Composite cone (or stratovolcano)
• 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)
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A composite volcano (stratovolcano)
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Features of Volcanoes
General features • Conduit, or pipe• Crater • Vent• Crater• Fumaroles• Caldera• Crater Lake
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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
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Mt. Hood, Oregon
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Mt. St. Helens Pyroclastic flow
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A lahar along the Toutle River near Mt. St. Helens
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Formation of a caldera
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Crater Lake in Oregon
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Formation of a volcanic neck
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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
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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
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Locations of some of Earth’s major volcanoes
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Igneous Bodies
Are bodies formed by the solidification of magma or lava
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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:
BatholithsStocks
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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.
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1-Which Magma is more viscous?
a. Basaltic
b. Felsic/Andesitic
c. Ultramafic
d. None of these
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2-which magma traps more gases?
a. Mafic/Ultramafic
b. Felsic/Andesitic
c. None of these
d. I am not sure
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3-Which Magma is Hotter?
a. Mafic/Ultramafic
b. Felsic /Andesitic
c. None of these
d. I am not sure
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4-Which magma produces more violent eruptions?
a. Mafic/Ultramafic
b. Felsic/Andesitic
c. None of these
d. I am not sure
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5-Magma viscosity is a function of…
a. Temperature
b. Silica content
c. Gases content
d. All of these
e. b only
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6-Which volcano is over a hot spot?
a. Mt St Helens (Washington State)
b. Mt Rainier (Washington State)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
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7-Which one is a Shield Volcano?
a. Mt St Helens (Washington State)
b. Mt Rainier (Washington State)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
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8-Which one is a Composite cone?
a. Mt St Helens (Washington State)
b. Sunset Crater (Arizona)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
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9-Which volcano type produces lava + pyroclastic flows
a. Mt St Helens (Washington State)
b. Kula (Hawaii)
c. Mauna Loa (Hawaii)
d. All of these
e. a and c only
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10-Volcanoes with steep cones have
a. Fluid lava
b. Viscous lava
c. No relation whatsoever
d. I am not sure
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11-Volcanoes at divergent boundary erupt mainly…lava
a. Felsic to Andesitic
b. Mafic
c. None of these
d. I am not sure
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12-Volcanoes at convergent margins produce mainly…lava
a. Felsic to Andesitic
b. Mafic
c. None of these
d. No relation whatsoever
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13-Which volcanoes are more explosive?
a. Shield volcanoes
b. Composite cones
c. Cinder cones
d. None of these
e. I am not sure
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14-Volcano explosiveness is influenced by …
a. Viscosity
b. Temperature
c. Amount of gases
d. All of the above
e. A and c only
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15-A Pahoehoe lava is
a. Gases-free with jagged surface lava
b. Wrinkled, ropy, and gaseous lava
c. Explosive ash and fragments flow
d. None of these
e. I am not sure