Chapter 9 (part I): Metamorphic Rocks

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Chapter 9 (part I) Metamorphic Rocks

• Study Help for Chapter 9
• Definition of metamorphism, its causes, and the
  agents of metamorphism.
• Textures of metamorphic rocks, rock cleavage,
  foliation. The most common metamorphic rocks and
  what they are formed from (parent rock).
• Understand the main types/environments of
  metamorphism, which are the most/least common,
  where do they occur? How are they related to
  plate tectonics?
• Know the slate ? gneiss progression of
  metamorphism
• Progressive regional metamorphism and index
  minerals (Figure Story 9.7)

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Chapter 9 (part I) Metamorphic Rocks

• Introduction
• Metamorphic environments
• Agents of metamorphism

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1) Introduction
Recall
3 fundamental rock types

• Igneous
• Sedimentary
• Metamorphic

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1) Introduction
Recall
3 fundamental rock types

• Igneous
• Sedimentary
• Metamorphic

crystallization
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1) Introduction
Recall
3 fundamental rock types

• Igneous
• Sedimentary
• Metamorphic

crystallization
deposition
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1) Introduction
Recall
3 fundamental rock types
metamorphism
Lit. change form
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2) Metamorphic environments
Contact metamorphism Hydrothermal
metamorphism Regional metamorphism
Fig. 9.3
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3) Agents of metamorphism
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3) Agents of metamorphism

• Heat
• The most important agent
• Recrystallization results in new, stable minerals
• Two sources of heat
• Contact metamorphism rocks are baked by heat
  from magma
• An increase in temperature with depth due to the
  geothermal gradient (20-30 C per km in upper
  crust)

See Fig. 9.1
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3) Agents of metamorphism

• Pressure (stress)
• Increases with depth (300-400 bar/km)
• Confining pressure applies forces equally in all
  directions
• Rocks may also be subjected to differential
  stress which is unequal in different directions

See Fig. 9.1
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Origin of pressure in metamorphism
3) Agents of metamorphism
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3) Agents of metamorphism

• Pressure/Stress

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3) Agents of metamorphism

• Pressure/Stress

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3) Agents of metamorphism

• Pressure/Stress

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3) Agents of metamorphism

• Pressure/Stress

near surface deformation brittle
shearing force
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3) Agents of metamorphism

• Pressure/Stress

Deep crust/mantle deformation ductile
shearing force
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3) Agents of metamorphism

• Chemically active fluids hydrothermal fluids
• Mainly water with other volatile components
• Enhances migration of ions
• Aids in recrystallization of existing minerals
• Where is the water from from?
• Squeezed out of pore spaces of sedimentary rocks
• Squeezed out of fractures in igneous rocks
• Hydrated minerals such as clays and micas

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4) How metamorphism alters rocks
3) Agents of metamorphism

• Hydrothermal fluids cause mineralogical changes

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3) Agents of metamorphism

• The importance of parent rock
• Most metamorphic rocks have the same overall
  chemical composition as the parent rock from
  which they formed
• Mineral makeup determines, to a large extent, the
  degree to which each metamorphic agent will cause
  change
• e.g., in shale, clays recrystallize to micas, but
  quartz and feldspars remain largely unaltered

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3) Agents of metamorphism
Important parent rock mineralogy
Example magma intrudes rock host rock
alteration quartz sandstone
not much limestone
lots