aapg talk

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VOLCANOES
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Melting the Mantle
1. Increase Temperature 2. Decrease
Pressure 3. Alter Composition (to
decrease melting temperature)
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Where Does Melting Occur?
1. Mid Ocean Ridges (decompression) 2.
Hotspots (high temperature decompression) 3.
Subduction zones (add water, decrease

melting temperature)
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Volcanic Products
Magma result of melting rocks Lava magma that
flows out over the surface Pyroclastic
rocks Rock fragments resulting from
explosive volcanism, deposited by sedimentary
processes.
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Pu'u O'o, Hawaii
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Fire Fountain
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Controls on Eruptive Style

• 1. Viscosity
• Depends on SiO2 content
• Mafic magmas
• poor in SiO2, rich in Mg, Fe
• low viscosity
• Felsic magmas
• rich in SiO2, poor in Mg, Fe
• high viscosity

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Controls on Eruptive Style

• 2. Volatiles
• H2O and CO2
• Dry mantle rocks generate mafic magma
• Wet mantle rocks generate felsic magma
• Volatiles decrease magma viscosity when
  dissolved

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Eruption Style

1. Fissure eruption 2. Shield volcano 3.
Pillow basalts 4. Domes Cones 5.
Strato-volcano
Subaerial eruption, low viscosity magma
Submarine eruption, low viscosity magma

Subaerial eruption, high viscosity magma
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Fissure eruption Krafla, Iceland
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Hexagonal cooling joints in basalt Devil's
Postpile, California
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Hexagonal cooling joints in basalt, e.g. Giants
Causeway, Antrim
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Sunset Crater, Arizona
Cinders
Bonito Flow
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Cerro Negro, Nicaragua 1947 eruption
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