Chapter 7 Metamorphism and Metamorphic Rocks

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Chapter 7 Metamorphism and Metamorphic Rocks

• Metamorphic rock is formed when existing rocks
  change due to subjection to pressure and or
  temperature
• Any rock can undergo metamorphism
• Metamorphism is the process by which heat,
  pressure, and chemical reactions deep within the
  earth alter the mineral content and or structure
  of existing rock without melting it down

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METAMORPHISM METAMORPHIC ROCKS

• NORMAL DEPTHS FOR METAMORPHISM gt10-KM FROM
  SURFACE (BELOW SEDIMENTARY DEPTHS ABOVE IGNEOUS
  DEPTHS)
• METAMORPHISM MINERAL CONTENT STRUCTURE OF
  SOLID ROCK ALTERED BY HEAT, PRESSURE CHEMICALLY
  ACTIVE FLUIDS DOESNT MELT
• CONDITIONS PROMOTING METAMORPHISM
• ROCK STABILITY IS RELATED TO FORMATION
  ENVIRONMENT
• METAMORPHISM BREAKS SOME BONDS, BUT DOES NOT
  INCLUDE MELTING

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Sedi., meta., igne. rocks
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• Metamorphic Rocks in North America

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CONDITIONS-CONTD

• HEAT MOST IMPORTANT FACTOR
• GEOTHERMAL GRADIENT (20-30 DEG C/KM-200 DEG C TO
  METAMORPHOSE ROCKS)
• SOURCES OF HEAT FOR METAMORPHISM
• CONDUCTION FROM DEEP INTERIOR
• RISING MAGMA
• RADIOACTIVE ISOTOPES
• FRICTION BETWEEN ROCK BODIES
• PRESSURE

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Pressure-contd.

• Lithostatic Confining pressure in all
  directions, rock becomes compressed
• Directed Pressure Acts in one principal plane
• Results of Pressure
• Compaction Smaller Denser
• Ion migration of unbonded ions (recrystallization)
  
• Foliation Preferred alignment of minerals
  -Subjected to direct pressure
• Units of Pressure 1 Bar 1 Atm. Pressure 1.02
  kg/cm2 14.7 lbs/inch2 105 Pascal

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Circulating Fluids

• Often necessary for chemical reactions
  (Facilitates movement of ions and atoms)
• Sources of Water
• Surface Infiltration
• Pores in Sedimentary Rocks
• Cracks on Subducting Plates
• Magmas
• Decomposing Minerals
• ALL METAMORPHIC ROCKS ARE CATEGORIZED AS
  FOLIATED OR NON FOLIATED

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PARALLEL CLEAVAGE
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MINERAL GRAINS
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MINERAL GRAINS-CONTD.
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Deep burial of rocks
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What Drives Metamorphism

• Heat
• Accelerate pace of chemical reactions
• Pressure
• Lithostatic (confining)- rock becomes smaller and
  denser
• Directed- minerals become aligned- Foliation
• Circulating Fluids
• Ions in water- change mineral composition
• Parent Rocks
• Original rocks composition will affect the
  outcome of metamorphism

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CONFINING PRESSURE
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DIRECTED PRESSURE
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TYPES OF METAMORPHISM

• Contact
• Heat is the dominant factor
• Area affected generally smaller than regional
  metarmorphism
• Regional are two types with extensive coverage
• Burial- occurs in deep sedimentary basins- no
  plate tectonics involved
• Dynamothermal- occurs where converging plates
  squeeze a rock caught between them
• Others
• Hydrothermal- involves hot water from magma
• Fault-zone- rocks grinding past one another
• Shock- meteorites strike
• Pyrometamorphism- lightning

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CONTACT METAMORPHISM
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DYNAMOTHERMAL METAM.
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HYDROTHER. METAM-CONTD
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Hydrothermal Metamorphism
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METAMORPHIC ROCK TYPES

• Foliated- based on type of foliation
• Slate- fine grain
• Phyllite- fine grain with sheen
• Schist- has split appearance
• Gneiss- layers/bands of minerals
• Non-foliated- based on mineral composition
• Marble
• Quartzite
• Hornsfel
• Mixed Rock
• Migmatite- indicates partial melting

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Quartz-Quartzite
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Quartz-Sandstone
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Metamorphism Temperature Pressure

• Information about degree to which a metamorphic
  rock differs from its parent material
• Metamorphic Grade-
• low (200-400) slate
• high (500-800) gneiss
• Index minerals/metamorphic Zones are used to
  determine metamorphic condition of temperature
  and pressure
• Chlorite, muscovite-low grade (low P/T)
• Garnet, staurolite- intermediate
• Sillianite- high grade (high P/T)

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Common Metamorphic Rocks

• FOLIATED ROCKS DERIVED FROM SHALE OR MUDSTONES
• SLATE (FINE GRAINED)
• SLATY CLEVAGE
• SLATE COLORS (RED IRON OXIDE GREEN CHLORITE
  PURPLE MANGANESE OXIDES BLACK CARBON)
• PHYLLITE FINE-GRAINED
• SCHIST COARSE GRAINED
• GNEISS COARSE GRAINED

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Relationships that produce crystallization
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Three main metamorphic environments
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Relationships that produce metamorphic rocks
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Metamorphic Surface Rocks
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Rock Slide of 1959
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St. Francis Dam