EASC 116 - Ch. 7 Metamorphic Rx
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Transcript of EASC 116 - Ch. 7 Metamorphic Rx
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Metamorphic Rocks
Chapter 7
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Metamorphic Rx
“meta” = change
“morphos” = shape/form
Parent rock = pre-existing rock that is subject to metamorphism
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Metamorphic Rx
Form by alteration of parent rock due to:
- increase in pressure
- increase in temperature
- exposure to chemically active fluid
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Metamorphism
Occurs within Earth’s crust
- minerals change until they are ________ under the new conditions
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Metamorphic Conditions
1) Increasing pressure (stress)
- occurs for two reasons:
a) deep burial
b) mountain building
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Rx reaction to pressure
Behave differently depending upon temperature
Shallow depths = cooler temp
- rx are brittle and tend to break
Ex: fault breccia
Called “cataclastic metamorphism”
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Rx reaction to pressure
Deep burial = higher temp’s
- rx behave plastically and fold under pressure (Fig. 7.1)
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Types of pressure
a) Differential stress
- applied unequally from different directions
- produces foliated texture
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Foliated texture
Foliation = parallel alignment of minerals w/in a rock
- creates a layered appearance
- associated w/mountain building
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Types of pressure
b) Confining pressure
- applied equally from all sides
- associated with deep burial
- nonfoliated texture
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Metamorphic Conditions
2) Increasing temperature
NOTE: Temperature cannot exceed the melting point of all minerals in rock
Why?
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Increasing temperature
- occurs for two reasons:
a) deep burial
- geothermal gradient
- increases 30o C per km
b) magma intrusion
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Increasing temperature
Changes to parent rock:
a) Compositional change
- unstable minerals either melt or rearrange into new minerals
- partial melting: remaining rock is metamorphic rock
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Compositional Change
Ex: Shale Slate
(clay minerals) (microscopic
micas)
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Increasing temperature
b) Textural Change
- partial melting allows minerals to recrystallize
- produces nonfoliated texture
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Textural Change
Ex: Sandstone Quartzite
(quartz) (quartz)
Note: The composition is still the same
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Metamorphic Conditions
3) Chemically active fluid
- superheated water with ions in solution
- “hydrothermal solution”
- occurs near magma chambers
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“Microscopic Water”
Water in crystalline structure
- necessary for atoms to rearrange themselves
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Sources of Water
a) Groundwater
b) Magma chambers
c) Crystalline structure of hydrous minerals
Ex: Gypsum = CaSO4 2H2O
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Metamorphic Rock Classification
Foliated textures
- degrees of increasing metamorphic intensity
(temperature, pressure)
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Foliated textures
Parent rock = shaleLow degree Slate
(microscopic micas)Low degree Phyllite
(light reflects from larger micas)
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Foliated textures
Low degree Phyllite
Medium degree Schist
(visible micas)
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Foliated textures
Low degree Slate
Medium degree Schist
High degree Gneiss
(black & white layers)
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Transitional change
Migmatite
- rock w/both igneous & metamorphic characteristics
- represents high degree of metamorphism (Fig. 7.23)
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Foliated textures
Most schists have muscovite & biotite = mica schists
Accessory minerals w/micas indicate degree of metamorphism w/in schist range
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Accessory minerals
Referred to as “index minerals”
Ex: chlorite mica schist
Low grade mm (~200oC)
Ex: garnet mica schist
Intermediate grade mm
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Nonfoliated textures
Composition controlled by parent rock
Parent Rock Meta. Rock
Sandstone Quartzite
Limestone Marble
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Regional metamorphism
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Index minerals
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Regional Metamorphism
Foliated rocks are often folded
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One rock is the raw material for another