Metamorphism and Metamorphic Rocks

1. Metamorphism and Metamorphic Rocks

3. Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and igneous rxs

4. Metamorphism in Hollywood 2007 Heat and Stress Increasing

5. Why are the changes in Michael Jackson’s facial features such a perfect (albeit warped) analog for rock metamorphism? The physical composition of his face is unchanged (in metam. rock, bulk composition remains constant) B) The physical appearance of his face has changed (in metam. rock, mineral assemblages change) He will continue to change with new external conditions (increased heat and pressure will cause further change) D) All of the above

6. Hollywood’s analogue to metamorphism- Comparison of timescales Given Michaels rate of change over 25 years, how much faster is this than the time it takes to metamorphose rocks under added heat and pressure 1000 times faster 10,000 times faster 100,000 times faster 1000,000 times faster

7. Metamorphism • Metamorphism progresses from low to high grades • Rocks remain solid during metamorphism

8. Agents of metamorphism… • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially

9. Why does temperature increase with depth beneath the surface? A) heat given off from radioactive decay B) pressure greater beneath surface, heats things up C) remnant heat from planetary formation D) A & C E) A, B, & C

10. Origin of pressure in metamorphism

11. Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from • Different minerals, but made of the same stuff.

12. Metamorphism • Three types of metamorphic settings: • Contact metamorphism – from a rise in temperature within host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock

13. Contact metamorphism Produced mostly by local heat source

14. Hydrothermal metamorphism Requires hot circulation of fluids

15. Regional metamorphismin Mountain Belts Requires crustal thickening and shortening

16. Metamorphism and plate tectonics • Most regional metamorphism occurs along convergent plate boundaries • Compressional stresses deform plate edge • Occurs in major mountain belts: Alps, Himalayas, and Appalachians

17. Location of metamorphic zones in a subduction zone

18. Metamorphism and plate tectonics • Metamorphism at subduction zones • Cores of subduction zones contain linear belts ofmetamorphic rocks • High-P, low-T zones near trench • High-T, low-P zones in region of igneous activity (arc)

19. Location of metamorphic zones in a subduction zone

20. Metamorphic Environments • Index minerals and metamorphic grade • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

21. Index Minerals in metamorphic rocks

22. Metamorphic Environments • Metamorphic grade • A group of minerals that form in a particular P-T environment Zeolite (really low T,P; <200C) Greenschist (low T, P; 200-450C, 10-15 km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; 450-650C, 15-20 km) Granulite (super high T, P; >700C, >25km)

23. A certain type of metamorphic rock forms when the temperature equals 200 deg C. At what depth do these rocks form, if the geothermal gradient is 45 degC per km, and temperature at surface is 20 degC? • 2 km • 3 km • 4 km • 5 km • None of the above

24. A certain type of metamorphic rock forms when the temperature equals 200 deg C. At what depth do these rocks form, if the geothermal gradient is 45 degC per km, and temperature at surface is 20 degC? • 2 km • 3 km • 4 km • 5 km • None of the above

25. Metamorphic Environments in Subduction Zones

26. Greenschist Hand Sample Greenschist Thin Section

27. Mica Schist

28. Blueschist Amphibolite

29. Common metamorphic rocks • Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions

30. Sample of quartzite Thin section of quartzite

31. Flattening of quartz grains in quartzite

32. Sandstone to Quartzite Morph

33. Common metamorphic rocks • Nonfoliated rocks • Marble • Coarse, crystalline • Parent rock usually limestone • Composed of calcite crystals • Fabric can be random or oriented

34. Marble Marble

35. Marble (Random fabric = annealing; nonfoliated)

36. Change in metamorphic grade with depth

37. Shale to Slate to Schist Morph

38. Common metamorphic rocks • Foliated rocks • Slate • Very fine-grained • Excellent rock cleavage • Made by low-grade metamorphism of shale

39. Example of slate

40. Slate roof

41. Common metamorphic rocks • Foliated rocks • Phyllite • Grade of metamorphism between slate and schist • Made of small platy minerals • Glossy sheen with rock cleavage • Composed mainly of muscovite and/or chlorite

42. Phyllite (left) and Slate (right) lack visible mineral grains

43. Common metamorphic rocks • Foliated rocks • Schist • Medium- to coarse-grained • Comprised of platy minerals (micas) • The term schist describes the texture • To indicate composition, mineral names are used (such as mica schist)

44. Mica Schist - note well developed foliation

45. A mica garnet schist

46. Common metamorphic rocks • Foliated rocks • Gneiss • Medium- to coarse-grained • Banded appearance • High-grade metamorphism • Composed of light-colored feldspar layers with bands of dark mafic minerals

47. Gneiss displays bands of light and dark minerals

48. Diorite to Gneiss Morph (orthogneiss - from igneous protolith)

49. Question: How is the compressive stress oriented relative the rock below… Horizontally Vertically Diagonally None of above

50. What are metamorphic textures? • Texture refers to the size, shape, and arrangement of mineral grains within a rock • Foliation – planar arrangement of mineral grains within a rock