Metamorphic Rocks

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Metamorphic Rocks

• Metamorphic change of shape or form
  metamorphic rocks are formed by the alteration of
  other rocks by heat, pressure, and fluid
  alteration, while in a solid state.
• heat increases rates of chemical reactions that
  produce different minerals. Heat sources include
  intrusive magma bodies or burial during
  subduction
• pressure leads to recrystallization and
  formation of smaller, denser minerals that are
  stable under high pressure and temperatures.
• fluid activity superheated water present in pore
  spaces of rocks may contain ions in solution, and
  enhance the rate of chemical reactions

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3 types of metamorphism

• 1) Contact metamorphism takes place where magma
  contacts country rock, includes heat and fluid
  activity effects contact aureoles form in
  concentric bands around contact zone
• 2) Dynamic metamorphism Associated with fault
  zones, where high differential pressure occur.
  Results in mylonites, thin bands of hard, dense,
  fine-grained rocks.
• 3) Regional metamorphism Occurs over a large
  area, usually caused by tremendous pressures,
  temperatures, and deformation deep within the
  crust, example convergent plate margins.

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Index minerals

• Minerals that form only at specific temperature
  and pressure ranges are known as index minerals.
  These minerals allow geologists to identify low,
  intermediate, and high grade metamorphic zones
  (metamorphic facies).
• The types of index minerals formed reflect the
  original rocks, example at different
  temperatures and pressures clay-rich shales may
  form kyanite, sillimanite, or andalusite, all
  with the same chemical formula (AL2SiO5), but
  each with a different molecular geometry and
  crystal habit.

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Classification of metamorphic rocks

• Metamorphic rocks are divided into two groups
  foliated or non-foliated.
• Foliated rocks have mineral grains arranged in a
  parallel or layered fashion. Foliated leaf
  like, as in foliage. Foliated rocks include
• Slate fine-grained, fissile or slaty cleavage,
  formed from shale., color generally uniform
• Phyllite similar to slate, but coarser grained,
  generally has a glossy or lustrous sheen
• Schist Contains platy or elongate minerals, with
  clearly visible grains
• Gneiss Characterized by streaks or bands of
  light and dark minerals.

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Non-foliated metamorphic rocks

• These have no platy minerals and exhibit a
  granular texture. Include
• Marble metamorphosed limestone or dolostone. May
  range in grain size from fine- to
  coarsely-granular. Quartzite recrystallized
  sandstone, sugary in appearance.
• Hornfels non-foliated, fine-grained rock formed
  from non-fissile mudrocks
• Greenstone includes any dark, mafic,
  green-to-black rock containing chlorite, epidote
  or hornblende.