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Metamorphic Rocks

Metamorphic Rocks. How a little bit of heat and pressure transform rocks!. How a little bit of heat & pressure transform rocks!. The Rock Cycle. Metamorphism (Increased T & P) Partial Melting Magma. Metamorphism  Metamorphic Rocks. It’s Morphin ’ Time!.

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Metamorphic Rocks

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  1. Metamorphic Rocks How a little bit of heat and pressure transform rocks! How a little bit of heat & pressure transform rocks!

  2. The Rock Cycle Metamorphism (Increased T & P) Partial Melting Magma

  3. MetamorphismMetamorphic Rocks

  4. It’s Morphin’ Time!

  5. Metamorphic means “to change form.”

  6. Definition • Re-crystallization of minerals while still solid into minerals that are stable at different temperatures and pressures

  7. From bread to toast!

  8. MetamorphismMetamorphic Rocks Why do the minerals recrystallize? • To reach chemical equilibrium!!!

  9. When you add pressure something’s got to give!

  10. The minerals have to get to… Chemical equilibrium!

  11. Evidence of Metamorphism Shale For example… Sedimentary Rocks stable at normal T & P… Bend under pressure! Gneiss

  12. Differential pressure “squashes” rock and included features 2. Flattened Pebbles Evidence of Metamorphism Conglomerate Metaconglomerate

  13. Foliation • Rocks with planar texture have foliation

  14. Foliation

  15. Evidence of Metamorphism • 3.Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure Quartz Sandstone Quartzite

  16. Where do we find heat and pressure?!

  17. Evidence of Metamorphism • 4. New mineral assemblages Example: Shale: Clay minerals (some quartz)  Metamorphism (Mid-grade) • Forms Schist: Mica, Feldspar and other silicate minerals 

  18. Recipe of Metamorphism • 1.Parent Rock • Even though minerals will change most elements are provided by parent rock, except water and some dissolved ions Shale Schist

  19. Characteristic Metamorphic Minerals sillimanite kyanite staurolite garnet andalusite For more images: http://skywalker.cochise.edu/wellerr/mineral/

  20. Tracing changes in pressure and temperature conditions that a rock experienced … • … gives insight into plate tectonic settings!!

  21. B. Recipe of Metamorphism Temp. (ºC) 0 C 600 1200 A B • 2.Heat • Geothermal gradient: more pressure = more heat!! • Minerals stable at lower temperatures converted to • minerals stable at higher temperatures • Solid state chemical reactions are accelerated A B C

  22. B. Recipe of Metamorphism Temp. (ºC) 0 C 600 1200 A B • 2.Heat • Geothermal gradient: more pressure = more heat!! • Minerals stable at lower temperatures converted to • minerals stable at higher temperatures • Solid state chemical reactions are accelerated A B C

  23. Ingredients of Metamorphism • 4.Addition or removal of fluids (and elements) • Water (and other fluids) within rocks and minerals • Moving during metamorphism • Accelerates solid-state chemical reactions and • May change rock composition • 5.Time  Millions of years!

  24. Metamorphic Facies • The minerals in a rock are clues to the (pressure and temperature) history of the rock • Facies = a set of metamorphic conditions • Each facies is characteristic of its • Tectonic environment • Source rocks in the environment (like shale)

  25. To change form Other rocks are exposed to high heat & pressure

  26. Summary • Evidence of metamorphism • Deformed layers, flattened pebbles, crystalline texture, & new mineral assemblages • The ingredients of metamorphism • Parent rock, heat, pressure, fluids, & time • Classifying metamorphic rocks • Low grade --> high grade & facies • Metamorphism of shale (& other parent rocks!) • Shale --> Gneiss, Sandstone --> Quartzite, Limestone --> Marble • Metamorphic minerals • Muscovite and Garnet = Index minerals

  27. Metamorphic Rock Types • There are two types of metamorphic rocks. • Each is classified according to its composition and texture. • Foliated • Nonfoliated

  28. How do we identify meta. rocks? Foliated Non-foliated

  29. Foliated Metamorphic Rocks • FOLIATED-the minerals have been flattened and pushed down into parallel layers. The bands in foliated metamorphic rock look like pages in a book.

  30. Foliation

  31. Compressive Stress Shear Stress Causes Foliation

  32. Differential pressure “squashes” rock and included features 2. Flattened Pebbles Evidence of Metamorphism Conglomerate Metaconglomerate

  33. Metamorphic Grade • Intensity of Metamorphism: • High grade (High P & T) • Low grade (Low P & T)

  34. Metamorphism of Shale not found in nature Fig. Story 9.7b

  35. Slaty foliation- Rock splits into flat, thin layers.

  36. Schistose foliation- • minerals have been squashed into flakes of mica.

  37. A Piece of Schist

  38. Metamorphically speaking… Schist happens

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