Metamorphism intro
- 1. Introduction to Metamorphism I. G. Kenyon
- 2. Metamorphism is just one part of the Rock Cycle
- 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 a combination of an increase in temperature and an increase in pressure
- 4. The Limits of Metamorphism 1 Lower limit for temperature is 200° C, below this diagenesis and lithification are taking place and are classified as sedimentary processes Upper limit for temperature is 800°C, above this rocks begin to melt and igneous processes occur
- 5. The Limits of Metamorphism 2 There is no lower limit for pressure but minimal metamorphism takes place at depths of <2km No upper limit for pressure, but metamorphism is unlikely to occur beyond depths of 45 - 50 km as the rocks will have melted at this depth
- 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 convergent equally in all directions and (destructive) plate margins increases with depth
- 7. Relationship between Metamorphism, Temperature and Pressure Temperature / °C Pressure /MPa Depth /km
- 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 Metamorphism Shale Schist
- 9. Metamorphic Grade The extent to which the pre-existing rocks have been changed in form/altered Shale-Country Rock Slate-Low Schist-Medium Gneiss-High Low Grade – slight alteration Medium Grade – significant alteration High Grade – extensive/total alteration
- 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 clay minerals are dehydrated by a rise in temperature
- 11. Metamorphic Changes 2 Recrystallisation takes place in the solid state during metamorphism, there is no melting of rock involved Spherical grains of calcite Angular crystals of calcite Metamorphism Oolitic Limestone Marble Analogy - snow recrystallises to ice without melting
- 12. Types of Metamorphism Contact or Thermal Metamorphism – changes due to an increase in temperature alone Dynamic Metamorphism – changes due to an increase in pressure alone Regional Metamorphism – changes due to an increase in both temperature and pressure
- 13. Contact Metamorphism Associated with major igneous intrusions such as plutons and batholiths The volume of country rock altered by heat from the intrusion is known as the metamorphic aureole Sandy Shale Recrystallisation occurs with a random orientation of minerals Sandstone Granite (non-foliated)
- 14. Dynamic Metamorphism Associated with major San 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. 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 foliated Metamorphism texture with minerals being aligned parallel to each other
- 16. Susceptibility of Rocks to Metamorphism Argillaceous sediments such as clay, mudstone and shale undergo the greatest mineralogical change Rich 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. Metamorphic Rocks Identification Guide
- 18. Schist Happens!