Metamorphism and Metamorphic Rocks

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Metamorphism and Metamorphic Rocks
A. Metamorphic rocks Form at conditions between
igneous and sedimentary rocks
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Metamorphism and Metamorphic Rocks
B. Metamorphism Heat, pressure, and chemical
reactions deep within the Earth alter the mineral
content and/or structure of preexisting rock
without melting it Any rock (igneous,
sedimentary, metamorphic) can be
metamorphosed Some atomic bonds are broken so
atoms and ions can rearrange themselves into new
minerals. If all the atomic bonds break, the
rock would melt.
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Metamorphism and Metamorphic Rocks
B. Metamorphism Derived from older rocks through
solid-state crystallization.  Does not involve
a significant melt phase. The lack of melting
distinguishes igneous from metamorphic
processes. 
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Metamorphism and Metamorphic Rocks

• B. Metamorphism
• Less stable minerals will become more stable
  minerals.
• Clay transforms into mica
• 2. Relatively stable minerals (e.g., quartz) will
  recrystallize or change structure.
• Cant witness metamorphism on the Earths
  surface.
• Occurs under all the rocks we see at the surface,
  we see metamorphic rocks when the area is
  uplifted and eroded.

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I. Factors controlling metamorphism A. Heat
most important factor this drives chemical
reactions Subject rock to a change in
temperature 1. Bury Rocks Geothermal gradient
75?F/mile (average 25C/km) So bury rocks deep
enough (either by tectonic movement or deep
burial under sediments) and they will
metamorphose Deepest mine 2.5 miles deeprock
is hot enough to burn human skin Deepest well
7.7 miles, temperature is 473?F (212?F is boiling)
Metamorphism and Metamorphic Rocks
2. Burial and geothermal gradient
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Metamorphism and Metamorphic Rocks
I. Factors controlling metamorphism A. Heat
most important factor this drives chemical
reactions Geothermal gradient 75?F/mile
(average 25C/km) What would be the temperature
at 5 km depth within the interior of North
America?  Every point within the Earth we could
measure or calculate a T value.  If nothing
perturbed our simple system, then we could easily
predict how T changed with depth.  However, it
not that simple. 
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Metamorphism and Metamorphic Rocks
I. Factors controlling metamorphism A. Heat
most important factor this drives chemical
reactions Geothermal gradient 75?F/mile
(average 25C/km) Temperature gradient in
volcanic arcs and mid-ocean ridges is
significantly greater than in the interiors of
continental areas.  Volcanic regions - the
geothermal gradient can reach 50C/km or more. 
The geothermal gradient in regions oceanward of
arcs is only about 10C/km. 
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Metamorphism and Metamorphic Rocks
I. Factors controlling metamorphism B.
Pressure 1. Confining pressure (lithostatic
pressure) Deep in the Earth Equal pressure from
all directions Rock will be compressed into a
smaller volume (no change in rock shape) Ions
migrate within minerals from high pressure to low
pressure regions
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Metamorphism and Metamorphic Rocks

• I. Factors controlling metamorphism
• B. Pressure
• Confining pressure (lithostatic pressure)
• Pressures are given in kilobars (kbars) or
  MPascals (MPa)
• For every 3 km increase in depth pressures
  increases by 1 kbar. 
• The continental interiors, the geobarometric (or
  geopressure) gradient is 0.33 kbar/km.  
• What would be the pressure at 5 km depth?

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Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks

• I. Factors controlling metamorphism
• B. Pressure
• 1. Confining pressure (lithostatic pressure)
• Deep in the Earth
• Equal pressure from all directions
• Rock will be compressed into a smaller volume (no
  change in rock shape)
• Ions migrate within minerals from high pressure
  to low pressure

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• I. Factors controlling metamorphism
• B. Pressure
• 2. Directed pressure
• Pressure is greater in one direction that in
  others
• Occurs at plate boundaries
• Changes the shape of a rock
• Flattens a rock in the direction of greatest
  pressure
• Minerals growing align perpendicular to the
  pressure ? foliation
• May also stretch, bend, or fold existing features

Metamorphism and Metamorphic Rocks
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• I. Factors controlling metamorphism
• C. Circulating Fluids
• Predominantly water with dissolved ions
• Increases potential for metamorphic reactions
• Aids in migration of unbonded atoms and ions
  ions move easily in fluids
• Water sources percolated from Earths surface
  trapped in subducting slab between sediment
  grains or cracks
• Release from water rich minerals (amphiboles,
  clays)
• Carry in new ions to the rock and carry away ions
  so they can change a rocks overall chemistry

Metamorphism and Metamorphic Rocks
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I. Types of Metamorphism heat, pressure, and
fluids interact differently in different
geological settings to produce different
metamorphic rocks A. Contact Metamorphism Solid
rock near magma may be baked by heat High temp,
low pressure (no foliation) Rocks not good
conductors of heat, so area of metamorphism is
localized Extent depends on size of intrusion
Small dike or sill ? centimeters to meters or
Large batholith ? several kilometers
Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks
II. Types of Metamorphism Heat, pressure, and
fluids interact differently in different
geological settings to produce different
metamorphic rocks A. Contact Metamorphism Solid
rock near magma may be baked by heat High
temperature, but low pressures. Near cooling
magma (igneous intrusions)
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II. Types of Metamorphism A. Contact
Metamorphism
Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks
II. Types of Metamorphism A. Contact
Metamorphism
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Metamorphism and Metamorphic Rocks
II. Types of Metamorphism B. Regional
Metamorphism Alters rock for thousands of square
kilometers 1. Continental Collision Zones (rocks
once at surface get buried) Directed pressure ?
foliation 2. Burial--Sediment gets buried in
deep sedimentary basins Confining pressure ? no
foliation Louisiana, Mississippi River delta,
bottom deposits are 12 km down This is high
temperature and high pressure
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II. Types of Metamorphism B. Regional
Metamorphism
Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks
II. Types of Metamorphism C. Subduction Zone
Blueschist metamorphism Low temperature, high
pressure In a subduction zone environment What
is the geothermal gradient Low or High?
5-10C/km
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II. Types of Metamorphism D. Hydrothermal
Metamorphism chemical alteration of preexisting
rocks by hot water Usually happens at divergent
plate boundariesbeneath ocean floors, where
seawater percolates down, warms up by magma and
alters rock High temperature, low
pressure Similar tectonic setting to contact
metamorphism
Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks
High pressure and medium pressure metamorphic
rocks in the Caledonides of western Norway. 
These rocks all have broadly basaltic
compositions, and were metamorphosed in the
Paleozoic.  The high pressure rocks were
brought rapidly enough to the surface so that
these high pressure rocks scarcely have any
evidence of retrograde metamorphism.
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Metamorphism and Metamorphic Rocks
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Metamorphism and Metamorphic Rocks