Metamorphism intro


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Metamorphism intro

  1. 1. Introduction to Metamorphism I. G. Kenyon
  2. 2. Metamorphism is just one part of the Rock Cycle
  3. 3. Definition of Metamorphism Meta means ‘change’, morph means ‘form’ A change in the form of pre-existing rocks Igneous, Sedimentary or Metamorphic Change is caused by an increase in temperature, an increase in pressure or acombination of an increase in temperature and an increase in pressure
  4. 4. The Limits of Metamorphism 1 Lower limit for temperature is 200° C, belowthis diagenesis and lithification are taking place and are classified as sedimentary processesUpper limit for temperature is 800°C, above thisrocks begin to melt and igneous processes occur
  5. 5. The Limits of Metamorphism 2There is no lower limit for pressure but minimal metamorphism takes place at depths of <2kmNo upper limit for pressure, but metamorphismis unlikely to occur beyond depths of 45 - 50 km as the rocks will have melted at this depth
  6. 6. Types of Pressure that causes Metamorphism Confining Pressure Directed Pressure The effect of the mass of The effect of tectonic forces the overlying rocks. It acts operating at convergentequally in all directions and (destructive) plate margins increases with depth
  7. 7. Relationship between Metamorphism, Temperature and Pressure Temperature / °CPressure /MPa Depth /km
  8. 8. Metamorphism - Effects Destruction/distortion of fossils Destruction of sedimentary structures Hardening of the rock Colour changes to the rock Growth of new metamorphic minerals Alignment of minerals in the rock MetamorphismShale Schist
  9. 9. Metamorphic Grade The extent to which the pre-existing rocks have been changed in form/alteredShale-Country Rock Slate-Low Schist-Medium Gneiss-High Low Grade – slight alteration Medium Grade – significant alteration High Grade – extensive/total alteration
  10. 10. Metamorphic Changes 1 Are assumed to be isochemical Bulk chemical composition of parent rock and new metamorphic rock are the same Both contain the same % Si, Al, O, Na etc.Only loss from the system is water as hydrous clayminerals are dehydrated by a rise in temperature
  11. 11. Metamorphic Changes 2Recrystallisation takes place in the solid state duringmetamorphism, there is no melting of rock involved Spherical grains of calcite Angular crystals of calcite Metamorphism Oolitic Limestone MarbleAnalogy - snow recrystallises to ice without melting
  12. 12. Types of MetamorphismContact or Thermal Metamorphism – changes due to an increase in temperature alone Dynamic Metamorphism – changes due to an increase in pressure aloneRegional Metamorphism – changes due to an increase in both temperature and pressure
  13. 13. Contact Metamorphism Associated with major igneous intrusions suchas plutons and batholiths The volume of countryrock altered by heat fromthe intrusion is known asthe metamorphic aureole Sandy Shale Recrystallisation occurs with a random orientation of minerals Sandstone Granite (non-foliated)
  14. 14. Dynamic Metamorphism Associated with majorSan Andreas Fault fault planes such as the San Andreas Fault and the Moine Thrust Relatively localised or small scale limited to a few metres either side of the fault plane Rocks are crushed and ground into angular fragments under intense shear pressure
  15. 15. Regional Metamorphism Associated with destructive plate margins and mountain building processes Occurs on a large (regional) scale and involves thousands of km³ of rock Subducted ocean floor sediments undergo regional metamorphism from 5 km to 50 km depths Regional Regional rocks have a foliatedMetamorphism texture with minerals being aligned parallel to each other
  16. 16. Susceptibility of Rocks to Metamorphism Argillaceous sediments such as clay, mudstone and shale undergo the greatest mineralogical changeRich in clay minerals such as chlorite, illite and kaolinite (china clay) which are only stable at low temperatures and pressures at or very close to the Earth’s surface Clay minerals are chemically complex and a range of mineral types can form by recrystallization under different temperature and pressure conditions
  17. 17. Metamorphic Rocks Identification Guide
  18. 18. Schist Happens!