Geol 11 Module 6A

• Magma


  Hot (600-1200°C) partially molten rock beneath the surface of the earth

  
• Lava


  Magma that reaches the surface of the earth through a vent
• Components of magma


  • Liquid components (silicates, carbonates, sulfides)
  • Solid components (minerals, rock fragments)
  • Dissolved gases (water vapor, CO2, SO2, HCl, HF)

  
• Classification of magma based on silica content

  • Felsic/Silicic/Acidic (>63% SiO2)
  • Intermediate (52-63% SiO2)
  • Mafic/Basic (45-52% SiO2)
  • Ultramafic/Ultrabasic (<45% SiO2)

  
• Viscosity


  The property of substances to resist flow
• Higher temperature

  Lower viscosity
• Higher SiO2 content

  Higher viscosity
• More H2O


  Lower viscosity
• Properties of different magmas


  • Ultramafic - hottest, densest
  • Felsic - least dense

  
• Factors affecting magma formation


  1. Increase in temperature

  2. Decrease in pressure

  3. Addition of volatiles

  
• Geothermal gradient is not sufficient to melt dry rocks

  
• Increase in geothermal gradient


  Can lead to melting of rocks and production of new magmas

  
• Decompression (drop in confining pressure)


  Can cause melting

  
• Addition of volatiles (e.g. water)


  Can cause rocks to melt at lower temperature

  
• Magmatic differentiation


  1. Fractional crystallization

  2. Partial melting

  3. Assimilation

  4. Magma mixing

  
• Fractional crystallization


  Process of crystal formation and its removal from the magma by gravity

  
• Crystallization of ferromagnesian minerals (e.g. olivine)


  Magma becomes more silica-rich and felsic in composition

  
• Partial melting


  Only certain parts of the rock will melt

  
• Partial melting of silica-rich minerals


  Resulting magma is typically more felsic in composition

  
• Rocks do not melt at a single temperature, but instead melt over a range of temperatures depending on their mineral composition

  
• Sudden increase in temperature beneath the surface


  Crystals formed at lower temperatures will start to melt

  
• Molten materials


  Become magma which separates from the rock due to its buoyancy as a function of temperature

  
• Silica rich minerals such as quartz have lower melting point, so the resulting magma is typically more felsic in composition

  
• Low degrees of partial melting form more siliceous magmas, while higher degrees of partial melting can generate more mafic compositions

  
• Melting a mafic source results in a felsic to intermediate magma, while melting ultramafic (peridotite) rocks from the mantle should form basaltic magmas

  
• Partial melting experiment


  1. Heating to 50°C for 30 minutes, only the wax has melted

  2. Heating to 120°C for 60 minutes, much of the plastic has melted and the two liquids have mixed

  3. Liquid separated from solids and allowed to cool to make a "pretend rock" with a different overall composition

  
• Magma Mixing or Mingling


  Two or more magmas with different compositions could mix with each other to produce compositions intermediate between the end members
• Assimilation


  When a molten body moves up through an existing basement rock, it assimilates rock (melts and incorporates elements from the surrounding rock), changing the magma composition

  
• Parts of the country rock are not completely melted, leaving behind inclusions inside the cooled intruding rock called xenoliths and xenocrysts
• Bowen's Reaction Series


  Describes the crystallization sequence of minerals as basaltic rocks are melted and gradually cooled

  
• Bowen's Reaction Series

  • Olivine
  • Pyroxene
  • Amphibole
  • Biotite
  • Plagioclase
  • K-feldspar
  • Muscovite
  • Quartz

  
• Olivine forms at the highest temperature (1200-1300°C), then combines with silica to form pyroxene as temperature drops
• In the continuous series, only the mineral plagioclase develops but the amount of calcium and sodium in its composition changes

  
• A rock dominated by olivine (and possibly few pyroxene) comes from an ultramafic source, while a mafic rock is dominated by pyroxene and Ca-rich plagioclase with significant amount of amphibole, biotite and possibly olivine

  
• Extrusive (volcanic) rocks


  Molten rocks that solidified at the surface (e.g. basalt, andesite, rhyolite)

  
• Pyroclastic rocks


  Rocks formed from the explosive eruption of volcanoes (e.g. tuff, ignimbrite)

  
• Intrusive (plutonic) rocks


  Rocks formed at depth (e.g. gabbro, diorite, granite)

  
• Intrusive Igneous Landforms


  • Stock
  • Batholith
  • Dike/Dyke
  • Sill
  • Laccolith
  • Lopolith
• Aphanitic texture


  Very fine grained; mineral grains are not visible to naked eye (usually less than 1 mm grain size)
• Phaneritic texture


  Coarse-grained; mineral grains visible to naked eye (usually 1-10 mm)