GEOS 112 Lecture Topics 22003

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GEOS 112 Lecture Topics 2/20/03
Magmas, Igneous Rocks, and Volcanism

• Volcano Monitoring and Prediction
• Origin of Magmas
• Magmatic Processes at Plate Boundaries

Read Chapter 3-4 (Volcanism and Igneous
Rocks) Exam 1 Monsay (2/24) (weeks 1-5)
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Volcanic Hazards
Lava Flows
Volcanic Mud Flows (lahars)
Pyroclastic Flows
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Pyroclastic Flows (nuée ardente)

• Hot (up to 900C), dense mixtures of gas, ash,
  and volcanic debris
• Travel for 10s of km down canyons and stream
  valleys at speeds of up to 300 km/hr

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Lahars

• Dense mixtures of ash, volcanic debris, and water
• Travel for 10s of km down canyons and stream
  valleys at speeds of up to 100 km/hr

2 Ways to Make Lahars
Eruption melts glacial ice
Torrential rain mixes with ash
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Volcanic Hazard Maps

• based on geologic mapping of volcanic deposits
• obtain recurrence interval of large eruptions
• predict probable path of future pyroclastic flows
  and lahars

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Volcano Monitoring Techniques
All techniques seek to monitor the movement of
magma into the base of the volcano
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Volcanic Prediction
Short-term prediction (lt24 hours) is possible!!!
(1991 Mt. Pinatubo)

• Volcanoes usually (but not always!!) give some
  warnings
• Increased frequency and size of EQs under the
  volcano
• Rapid inflation of the volcano flanks
• Rapid increase in the amount of SO2 emitted

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Tilt
of EQs
SO2 Emission
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GEOS 112 Lecture Topics 2/20/03
Magmas, Igneous Rocks, and Volcanism

• Volcano Monitoring and Prediction
• Origin of Magmas
• Magmatic Processes at Plate Boundaries

Read Chapter 3-4 (Volcanism and Igneous
Rocks) Exam 1 Monsay (2/24) (weeks 1-5)
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Magmatism and Tectonics
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Magma Types Based on Chemistry
50 SiO2
60 SiO2
70 SiO2
gabbro/basalt
diorite/andesite
granite/rhyolite
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Physical Properties of Different Magma Types
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Processes That Change Magma Compositions

• Primary Process ?
• Partial Melting of Different Materials (e.g.
  mantle, oceanic crust, continental crust, etc.)
• Secondary Processes
• Crystallization
• Assimilation/Magma Mixing

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Partial Melting of Different Starting Materials
Peridotite (mantle rock) ? Basaltic Magma
Basaltic (oceanic crust) ? Andesitic Magma
Andesitic (continental crust) ? Rhyolitic Magma
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Magmatism and Tectonics
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Divergent Boundaries and Mantle Hotspots
Decompression melting of mantle (peridotite) to
produce basaltic magmas
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Decompression Melting
(just at melting temp)
(200C below melting temp)
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Magmatism and Tectonics
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Subduction Zone Convergent Boundaries
Flux melting of mantle above subducting plate
produces basaltic magmas
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Flux Melting at a Subduction Zone
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Addition of Fluids
Lowers melting T
Causes partial melting
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Origin of Andesitic Magmas
Modification of a Basaltic Parent
1. Crystallization in Magma Chambers
2. Assimilation of Continental Crust
3. Magma Mixing (basalt rhyolite andesite)
Origin of Granitic/Rhyolitic Magmas
Partial Melting of Continental Crust
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Crystallization in a Magma Chamber
magma composition progressively changes as
crystal are removed from the magma.
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Crystallization
Remaining Magma is Andesitic
Basaltic Magma
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Assimilation
Continental Crust
Blocks of continental crust fall into basaltic
magma and dissolve. basaltic magma assimilated
blocks andesitic magma
Hot, basaltic magma
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Origin of Andesitic Magmas
Modification of a Basaltic Parent
1. Crystallization in Magma Chambers
2. Assimilation of Continental Crust
3. Magma Mixing (basalt rhyolite andesite)
Origin of Granitic/Rhyolitic Magmas
Partial Melting of Continental Crust