Metamorphism

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Metamorphism

• Changes in Rock Composition or Texture
• Due to Heat, Pressure and Action of Fluids

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We Do Not Live at Normal Conditions

• By the standards of Earths interior, we live in
  a frozen vacuum
• Things that look abnormal to us are normal
  behavior for materials
• Solids can flow
• Solids can react chemically with each other
• A given material can have several different
  atomic structures

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Chemical Changes in Rocks

• Weathering
• At Surface
• Diagenesis
• Sedimentary Rocks

• Metamorphism
• Starts about 200 C
• Outside range of normal near-surface conditions

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Where Does the Heat Come from?

• Uranium and Thorium ? Other Elements Lead
  Radiation (about 90)
• Potassium-40 ? Calcium-40 or Argon-40 Radiation
  (about 10)
• Possibly some primordial heat (controversial)

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Where Does the Pressure Come from?

• Air Pressure 14 P.s.i. (1 Atmosphere or 1 Bar
  100 kPa)
• Pressure Beneath 10 Meters (33 Ft.) Of Water 1
  Atm. 1 Bar
• Same Pressure Beneath 3.5 m (10 ft.) Of Rock
• Pressure in Deepest Part of Ocean 1000 Bars
• Pressure under One Mile of Rock 500 Bars
• 1000 Bars (2 Mi. or 3.5 km Of Rock) 1 Kilobar
  (Kb.) 100 MPa

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Pressure Units

• 1 Newton (N) gravitational attraction on 100
  grams
• One Newton 7 Fig Newtons
• 1 Pascal (Pa) 1 N/square meter
• 1 Pascal pressure under thin cardboard or 0.1
  mm water
• A Thin section exerts about 1 Pa pressure on the
  slide
• One bar 1 atm 100 kPa
• 1 MPa 10 bars
• 1 GPa 10 kilobars

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Types of Metamorphism

• Contact
• Around Intrusions or Local Heat Sources
• Regional
• Usually Accompanied by Deformation
• Cataclasis
• Shearing and fault zones
• Shock
• Ultra-high pressures and rapid deformation

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What Happens During Metamorphism

• Minerals React to Form New Minerals
• 2SiO2 CaMg(CO3) 2 CaMgSi2O6 2CO2
• Quartz Dolomite Pyroxene
• Minerals Change Form
• Al2SiO5 Al2SiO5
• Andalusite Kyanite

• New Materials Are Added (Metasomatism)
• CaMg(SiO2)2 2CO2 CaMg(CO3)2 2SiO2
• Pyroxene CO2 Dolomite Quartz
• Minerals in Solution Ore Bodies
• Recrystallization
• Cataclasis
• Shock Metamorphism

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Polymorphism

• Al2SiO5
• Andalusite
• Kyanite
• Sillimanite
• Ice - 6 high pressure forms
• Diamond - Graphite

• Calcite - Aragonite
• Quartz -
• - Tridymite - Cristobalite (increasing
  temperature)
• - Coesite - Stishovite (increasing pressure)

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Why Don't Rocks "De-metamorphose"?

• Reactions Can't Reverse Because Ingredients Lost
• 2AlSi2O5(OH) Al2SiO5 3SiO2 H2O
• Clay Mineral Andalusite Quartz Water
  (Lost)
• An example of carbonate metamorphism
• CaMg(CO3) 2 2SiO2 CaMgSi2O6 2CO2
• Dolomite Quartz Pyroxene CO2 (Lost)
• Reactions "Freeze"
• Sometimes it Does Happen if Fluids Present
• Retrograde Metamorphism
• On the surface we call it weathering

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Foliation

• Foliation is a sheetlike structure that forms
  when rocks are deformed.

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Foliation

• Flattening
• Grain Rotation
• Solution
• New Minerals
• Partial Melting
• Shearing

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Foliation

• In every case, the foliation is
• In the direction of least resistance
• at right angles to the direction of greatest
  compression.

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Metamorphic Grade -

• Degree to Which the Rock Has Changed Composition
• Can Often See Original Bedding
• Can Sometimes Even See Deformed Fossils
• At High Grades, Rocks Can Often Lose All Trace of
  Their Original Appearance

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Major Metamorphic Rock Types
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Major Metamorphic Rock Types
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Major Metamorphic Rock Types
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Major Metamorphic Rock Types
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Major Metamorphic Rock Types
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What About Other Rocks?
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Metamorphic Facies
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Metamorphic Map
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Mantle Rocks
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Metamorphism and Plate Tectonics