How Magma Forms

1
How Magma Forms

• Magma - Molten rock beneath the surface
• Lava - Molten rock on the surface

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Where Magma Forms
3
Heat Transfer Mechanisms

• Radiation - Emission of EM radiation
• Advection - Occurs at fissures
• Conduction - Transfer of energy molecule to
  molecule
• Convection - movement of material due to
  temperature differences

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Earths Energy Budget

• Solar radiation 50,000 times greater than all
  other energy sources primarily affects the
  atmosphere and oceans, but can cause changes in
  the solid earth through erosion and other
  surficial events.
• Radioactive decay 238U, 235U, 232Th, 40K, and
  87Rb all have t1/2 that gt109 years and thus
  continue to produce significant heat in the
  interior this may equal 50 to 100 of the total
  heat production for the Earth. Extinct
  short-lived radioactive elements such as 26Al
  were important during the very early Earth.
• Tidal Heating Earth-Sun-Moon interaction much
  smaller than radioactive decay.

5
Earths Energy Budget

• Primordial Heat Also known as accretionary heat
  conversion of kinetic energy of accumulating
  planetismals to heat. (An estimated 20,000
  meteorites strike the Earth every year
• Core Formation Initial heating from short-lived
  radioisotopes and accretionary heat caused
  widespread interior melting (Magma Ocean) and
  additional heat was released when Fe sank toward
  the center and formed the core.

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Heat Transfer
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Geothermal gradient The rate of change in
temperature with depth.20-30C/km in orogenic
belts 7C/km in trenches
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Crustal Geothermal Gradients
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Heat flowWarm near ridgesCold over cratons
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Cause of Crustal Melting
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Plagioclase Feldspar Water saturated vs. Dry
Solid
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Bowens Reaction Series
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Mafic Magma

• The term is derived from using the MA from
  magnesium and the FIC from the Latin word for
  iron
• Mafic magmas are usually produced at spreading
  centers from material which is newly
  differentiated from the upper mantle. Common
  mafic rocks include basalt and gabbro.
• Common minerals include
• olivine, pyroxene, amphibole, biotite mica, and
  the plagioclase feldspars.
• Consist of heavier elements - denser crust

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Felsic MagmaForm from melting of continental
crust

• The term is derived from using the FEL from
  Feldspar and the SIC from Silica
• Common felsic minerals include
• quartz, muscovite mica, and the orthoclase or
  potassium feldspars
• Felsic magma contain relatively high quantities
  of sodium, aluminum, and potassium and are
  composed of more than 65 silica.
• Rocks formed from felsic magma include granite,
  granodiorite, dacite, and rhyolite. All are
  light in color because of the dominance of
  quartz, potassium and sodium feldspars, and
  plagioclase feldspar minerals

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Intermediate Magma

• Formed by
• Subduction zone magmas - mixing of oceanic and
  continental crusts
• Mixing of mafic and felsic magmas - a high
  temperature mafic magma enters the crust and
  melts the crust, eventually mixing the two
  magmas.

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Characteristics of Intermediate Magma

• Contain plagioclase feldspar and some
  ferromagnesian minerals but little quartz or
  K-feldspar.
• Example rocks with this composition are diorite
  and andesite.
• Characteristic of subduction zone volcanoes