Igneous

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Igneous Rocks II
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Origin of Magmas

• Solid rock is at equilibrium with its surrounding
• Changes in the surroundings cause
• solid rock magma melting
• Raising T
• Lowering P
• Changing composition

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• Lowering P
• Mantle convection moves deep mantle rocks upwards
• Raising T
• Hot mafic magma intrudes into the crust
• Changing composition
• Adding small
• amounts of water

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How do rocks melt? Heat Geothermal Gradient
Temperature of most Silicic magmas
600-800oC -about the base of crust
Temperature of Mafic Magmas 1100-1200oC -upper
mantle conditions
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Magma Differentiation

• Magmas, and the resulting igneous rocks, show a
  wide range of compositions
• Source Rock
• variations cause major and minor variations in
  the magma
• Magma Mixing
• Assimilation

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• Partial Melting
• Individual minerals in source rock melt at
  different T
• Magma is enriched in many elements, especially
  SiO2

How do we know this? Norman Bowen
1887-1956 -Experimental Petrologist -developed
Bowens Reaction Series
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Bowens reaction series
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• Fractional Crystallization
• Individual minerals precipitate from magma at
  different T
• Magma is enriched in many elements, especially
  SiO2
• Magma may migrate buoyancy

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Bowens reaction series
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Plate Tectonic Setting of Igneous Rocks

• Divergent Plate Boundaries
• Partial melting of mantle produces basaltic magma
• Convergent Plate Boundaries
• Subduction produces partial melting of basalt,
  sediments, parts of mantle
• Andesitic and rhyolitic magma
• Ascending magma assimilates lower crustal
  material

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Plate Tectonic Setting of Igneous Rocks

• Mantle Plumes
• Partial melting of plumes of mantle material
• Basaltic magma is produced
• Rising magma produce
• Intraplate island chains
• Flood basalt Columbia River Basalts

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