Metamorphism and Metamorphic Rocks

1. Metamorphism and Metamorphic Rocks

2. Daily Question Regional Metamorphic rocks Igneous Rocks Correctly place the following statements in the Venn Diagram. • Occurs at divergent boundaries • Occurs at convergent boundaries • Forms as a result of melting (magma) • Temperature is an agent • Pressure is an agent • Generates foliated rocks • Forms as a result of being near an intrusion of magma • Found in mountain belts • May have been originally been a metamorphic rock • Form at temperatures above 200 oC • May underlie several adjacent states Contact Metamorphic Rocks

3. Definitions Metamorphism – mineralogical, chemical, and physical changes that occur in solid rocks. Occurs at depths greater than that of lithification (Diagenesis). Solid state recrystallization – changes that occur without the rock melting (rocks that melt are igneous).

4. Factors influencing Metamorphism • Temperature • Pressure • Fluid • Time Agents of Metamorphism

6. Factors influencing Metamorphism Temperature • below about 150oC, most minerals are stable (little or no metamorphism) • above 150oC, reaction rate increases as temperature increases, new minerals begin to form • above 600oC, some minerals begin to melt (transition to igneous rocks) • Temperature a function of depth within the Earth and the presence of a magma

7. Factors influencing Metamorphism Pressure • as pressure increases, pore spaces reduced and density increases, pore fluids are expelled • Minerals recrystallize and begin to realign Differential stress • pressure is greater in one direction than in another

8. Temperature, Pressure and Rock Type

10. Factors influencing Metamorphism Pore fluids (water and carbon dioxide) • as pressure increases, pore fluids are expelled • Pore fluids increase the rate of metamorphic reactions by: • storing ions involved in reactions • moving ions from one place to another Chemical Fluids from Magmas • Carry ions from the melt • Fluids increase the temperature in the rock

11. Veins Solutions are deposited in fractures and joints

12. Factors influencing Metamorphism Time • solid state recrystallization is a slow process • in general, size of minerals increases with time

13. Types of Metamorphism • Burial Metamorphism 2. Contact Metamorphism 3. Regional Metamorphism

14. Metamorphism & Plate Tectonics

15. Types of Metamorphism Burial Metamorphism • most common type, occurs where crust is greater than 5 km thick Relatively low temperature and pressure (low grade metamorphism) Maximum stress vertical, foliation parallel to ground surface

17. Types of Metamorphism Contact metamorphism • Occurs in rocks around a magma body • High temperature (heat from magma) “bakes” the rock around the magma creating an alternation • High temperature & fluid primary agents, little to no pressure (occurs at shallow depths in the crust)

18. Contact Metamorphism

19. Contact Metamorphism

20. Types of Metamorphism Regional metamorphism • Large scale – large volume of rock is affected • Associated with convergent plate margins and mountain building • Folding and faulting increase thickness of the crust • Occurs over a range of temperatures and pressures • Fluids are also present • Low grade to high grade metamorphism • Will have zones of contact metamorphism

21. Regional Metamorphism

22. Metamorphic Grade

24. Low Grade Metamorphism Example: Slate • Rocks become more dense and compact • forms at low temperature and pressure • microscopic crystals • dull luster • clay and mica minerals • foliated

25. Low Grade - Slate

26. Metamorphic Grade

27. Intermediate Grade Metamorphism Example: Phyllite • intermediate temperature and pressure • small crystals • shiny luster • mostly mica minerals • foliated

28. Intermediate Grade – Phyllite

29. Metamorphic Grade

30. High Grade Metamorphism Example: Schist • high temperature and pressure • large crystals • mica-rich • foliated

31. High Grade - Schist

32. Metamorphic Grade

33. High Grade Metamorphism Example: Gneiss • high temperature and pressure • large crystals • mica-poor • foliated

34. High Grade - Gneiss

35. Rock Cycle

36. Regional Revisited Slate Phyllite Schist, Gneiss Magma

37. Metamorphic Facies Different minerals form at different temperatures and pressures Group of stable minerals define a facies