Powerpoint Presentation Physical Geology, 10/e

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Metamorphism, Metamorphic Rocks, and Hydrothermal
RocksChapter 7
Metamorphic gneiss from Greenland, 3.7 Ba
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Metamorphic Rocks

• Metamorphic rocks common in the old, stable cores
  of continents, known as cratons

Oldest stable continental core Seismically
fast Geothermally cold Buoyant low density
(chemically depleted)
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Factors Controlling Metamorphic Rock
Characteristics

• Texture and mineral content of metamorphic rocks
  depend on
• Parent rock composition
• Temperature and pressure
• Effects of tectonic forces
• Effects of fluids, such as water

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Characteristics of Metamorphic Rock

• Tectonic forces
• Often lead to forces that are not equal in all
  directions (differential stress)
• Compressive stress causes flattening
  perpendicular to stress
  
• Shearing causes flattening by sliding parallel to
  stress
• Planar rock texture of aligned minerals produced
  by differential stress is known as foliation

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Deformation of Metamorphic Rock

• Fluids
• Hot water (as vapor) is most important,
• Rising temperature causes water to be released
  from unstable minerals
• Time
• Metamorphism, particularly from high pressures,
  may take millions of years
• Longer times allow newly stable minerals to grow
  larger and increase foliation

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Metamorphic Rock Classification

• Classification based on rock texture
• Foliated (layered) vs. non-foliated (non-layered)
• Foliated rocks named based on type of foliation
  (slaty, schistose, gneissic)
• Non-foliated rocks named based on composition

limestone
marble
granite
Foliated gneiss
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Types of Metamorphism

• Contact metamorphism
• High temperature
  
• Produces non-foliated rocks
• Rocks come in contact with magma bodies intruding
  cooler country rock

marble
quartzite
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Types of Metamorphism

• Regional metamorphism
• High pressure
• Results in rocks with foliated textures
• Can deform in mountain ranges
• May occur over wide temperature range

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Types of Metamorphism

• Regional metamorphism
• Higher pressure and temperature will produce
  increased metamorphic grade
  
• Prograde metamorphism of shale produces
• slate
• phyllite
• schist
• gneiss

phyllite
schist
gneiss
slate
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Types of Metamorphism

• Partial melting during metamorphism produces
  migmatites
• Migmatites exhibit both intrusive igneous and
  foliated metamorphic textures
• Shock metamorphism is produced by rapid
  application of extreme pressure
• Meteor impacts produce this
• Shocked rocks are found around and beneath impact
  craters

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Plate Tectonics and Metamorphism

• Regional metamorphism associated with convergent
  plate boundaries
• Pressure proportional to depth
• Temperature varies laterally at convergent
  boundaries
• Isotherms bow down in sinking oceanic plate and
  bow up where magma rises

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Hydrothermal Processes

• Rocks precipitated from or altered by hot water
  are referred to as hydrothermal
• Common at spreading centers (under water)
  
• Hydrothermal processes add water for metamorphic
  reactions
• Formation of hydrothermal rocks
• Water passes through rocks and precipitates new
  minerals on walls of cracks and in pore spaces
• Metallic ore deposits often form this way (veins)