Metamorphic Rocks

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Metamorphic Rocks
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Change While Still Solid!!

• Changes in
• Pressure
• Temperature
• Composition

• Cause changes in
• Mineralogy
• Crystal size
• Rock Texture
• Ex Firing clay to make ceramic

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Molecular Connections

• All atoms move (as do the bonds between them)
  down to -273 C
• More energy, more movement rubber bands

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Composition (mineral) changes

• Talc ? Anthophyllite, Quartz, and H2O

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From Rock to Rock?

• Can change in texture
• Or crystal size

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How Crystals Grow In Solids

• Must steal ions from existing grains SLOW
• Unlike fluids (magma), cant get rid of unwanted
  ions
• Faster along crystal boundaries, or where water
  or another fluid is present

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A Range of Environments

• Geothermal gradient change in T with depth
• Average 30C/km
• Major deviations from common pressure/temperature
  vs. depth gradients can cause metamorphism

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T P Increase with depth
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So

• Changes in the rock depend on the minerals
  present in the original rock
• Each mineral is only stable in a certain T, P,
  and chemical environment

Metamorphic Grade
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Metamorphic Grade
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Changes in Temperature (T)

• Every mineral is stable within a certain
  temperature range
• Some, like the micas, contain water, which boils
  off at high T, changing to a new mineral
• Over a long time, if heat is elevated but below
  melting point, bonds link in new 3D structures
• Crystals will grow larger because bonds are
  relaxed

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T is the Great Destroyer

• Both are composed of the mineral calcite (CaCO3)

• Growth of minerals in high T destroys fossils

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Contact Metamorphism

• Highest grade changes closest to magma
• Diminishes with distance
• Randomly oriented
• crystals

Hot igneous rock (magma)
Note these rocks are unaffected
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Pressure (P)

• Every mineral is stable within a certain pressure
  range
• Burial metamorphism
• Progressive build-up of sediment causes an
  increase in T and P with depth
• As P increases, denser crystals form
• Common along East, Gulf Coasts (deep)

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If P is uniform

• Lithostatic pressure due to
  overburden
• Results in reduction in crystal size
• Randomly oriented crystals form

Uniform Stress
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If P in non-uniform

• New platy minerals (ex mica!) grow perpendicular
  to the direction of pressure
• Old minerals rotate perpendicular

Parent
Post-metamorphosis
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Granite
Gneiss (in this case, metamorphosed granite)
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Foliation
Weakly foliated

• Set of flat or wavy parallel planes
• As the grade increases, so does the distinctness
  of the foliation

Strongly foliated (note increase in grain size
due to elevated T)
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Regional Metamorphism

• Most common and widespread type
• Occurs in areas where Earths crust is squeezed
  (Himalayas ? India and Asia)
• Usually associated with volcanoes/plutons

Metamorphism ends where melting begins
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Increasing grade of metamorphism of volcanic rock
under directed pressure and elevated T.
Reduction in grain size, growth of elongated
crystals perpendicular to stress direction,
minerals segregate into layers. High T eventually
grows large crystals.
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Metamorphism Involving Fluids

• Hot water moves along grain boundaries and then
  in cracks opened by water P
• Derived from water bound in minerals
• Contains lots of dissolved ions
• When it moves through a rock body, its ions often
  exchanged with ions/atoms in rocks
• Most rocks and magma have small quantities of
  metal ions (gold, silver, copper, lead, zinc,
  uranium)
• Water can collect these ions and concentrate them
• All precious metal deposits form this way
• Randomly oriented crystals

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Black Smokers and Seafloor Metamorphism
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Metamorphic Facies

• Different kinds of metamorphic rocks are formed
  from parent rocks of different composition at
  same metamorphic grade
• Your parents mean a lot
• Determine the kind of elements available

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Metamorphic Facies

• 2. Different kinds of metamorphic rocks are
  formed under different grades of metamorphism
  from parent rocks of the same composition

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How do Rocks Reach the Surface?

• Only radiation grows legs so
• If changes in P, T, and composition can cause
  metamorphism, then slow changes to shallow depths
  would be problematic
• Rapid uplift and exposure but how?
• Evidence suggests that rapid erosion drives rocks
  to the surface