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Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company Grotzinger.

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Presentation on theme: "Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company Grotzinger."— Presentation transcript:

1 Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company Grotzinger Jordan

2 Chapter 6: Metamorphism: Alteration of Rocks by Temperature and Pressure

3 About Metamorphism Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic.Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. Metamorphic changes occur in the solid state, so there is no melting.Metamorphic changes occur in the solid state, so there is no melting.

4 Lecture Outline 1.Causes of metamorphism 2. Types of metamorphism 3. Metamorphic textures 4. Regional metamorphism and metamorphic grade 5. Plate tectonics and metamorphism

5 1. Causes of Metamorphism ● internal heat of Earth ● internal pressure of Earth ● fluid composition inside Earth

6 1. Causes of Metamorphism ● temperature increases with depth ● rate = 20º to 60ºC per km ● at 15 km depth: 450ºC

7 1. Causes of Metamorphism ● pressure and temperature increase with depth in all regions

8

9 1. Causes of Metamorphism

10 ● the role of temperature ● geothermal gradient ● shallow (20ºC / km) ● steep (50ºC / km)

11 1. Causes of Metamorphism ● the role of pressure (stress) ● confining pressure ● directed pressure

12 1. Causes of Metamorphism ● the role of pressure (stress) ● rate of increase = 0.3 to 0.4 kbar / km 0.3 to 0.4 kbar / km ● minerals are geobarometers

13 Thought questions for this chapter At what depths in the Earth do metamorphic rocks form? What happens if temperatures get too high? Why are there no metamorphic rocks formed under natural conditions of very low pressure and temperature, as shown in Figure 6.1?

14 2. Types of Metamorphism ● the role of fluids ● metasomatism ● accelerated chemical reactions reactions

15 Depth, km 0 35 75 Asthenosphere Continental mantle lithosphere Continental crust Oceanic crust Oceanic lithosphere 2. Types of Metamorphism

16 Depth, km 0 35 75 Asthenosphere Continental mantle lithosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism

17 Depth, km 0 35 75 Asthenosphere Continental mantle lithosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism Regionalmetamorphism

18 Depth, km 0 35 75 Asthenosphere Continental mantle lithosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism Regionalmetamorphism High-pressuremetamorphism

19 Depth, km 0 35 75 Asthenosphere Continental mantle lithosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism Regionalmetamorphism High-pressuremetamorphism Contactmetamorphism

20 Depth, km 0 35 75 Asthenosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism Regionalmetamorphism High-pressuremetamorphism Contactmetamorphism Burialmetamorphism Continental mantle lithosphere

21 Depth, km 0 35 75 Asthenosphere Continental crust Oceanic crust Oceanic lithosphere Shockmetamorphism Regionalmetamorphism High-pressuremetamorphism Contactmetamorphism Burialmetamorphism Continental mantle lithosphere Water Seafloormetamorphism

22 Thought questions for this chapter Draw a sketch showing how seafloor metamorphism might take place.

23 3. Metamorphic Textures

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25 Staurolitecrystal Mica

26 Staurolitecrystal Mica Foliated rocks contain platy minerals that are aligned along a preferred orientation.

27 Mica Pyrite Staurolite Quartz Feldspar 3. Metamorphic Textures

28 Mica Pyrite Staurolite Quartz Feldspar Foliation is the result of compressive forces.

29 Mica Pyrite Staurolite Quartz Feldspar Foliation is the result of compressive forces. Mineral crystals become elongated perpendicular to the compressive force.

30 3. Metamorphic Textures ● preferred orientation of crystals ● slaty cleavage ● foliation

31 shale sandstonelayers 5 cm 3. Metamorphic Textures

32 shale sandstonelayers 5 cm The original bedding can be seen in the thin sandy layers.

33 shale sandstonelayers 5 cm The original bedding can be seen in the thin sandy layers. foliationplane originalbedding

34 shale sandstonelayers 5 cm The original bedding can be seen in the thin sandy layers. Regional metamorphism causes cleavage planes to develop. foliationplane original bedding

35 Low gradeIntermediate grade 3. Metamorphic Textures

36 SlatePhyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavageSchistosityBanding DiagenesisLow grade Intermediate grade High grade 3. Metamorphic Textures

37 DiagenesisLow grade Intermediate grade High grade SlatePhyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavageSchistosityBanding Foliated rocks are classified by the degree of cleavage, schistosity, and banding.

38 3. Metamorphic Textures ● classification of foliated rocks ● metamorphic grade ● crystal size ● type of foliation ● banding

39 3. Metamorphic Textures ● nomenclature of foliated rocks ● slate ● phyllite ● schist ● gneiss ● migmatite

40 Foliatedtexture:schist with garnet porphroblasts

41 3. Metamorphic Textures ● nomenclature of granoblastic (non-foliated) metamorphic rocks (non-foliated) metamorphic rocks ● hornfels ● quartzites ● marbles ● greenstones ● amphibolites ● granulites (granofels)

42 Granoblastic Texture

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44 Thought questions for this chapter How is slaty cleavage related to tectonic forces? What forces cause minerals to align with one another? Would you choose to rely on chemical composition or type of foliation to determine metamorphic grade? Why?

45 ● grades of metamorphism ● low ● intermediate ● high 4. Regional Metamorphism and Metamorphic Grade

46 4. Regional Metamorphism and Metamorphic Grade and Metamorphic Grade ● mineral isograds (zones of change) ● index minerals reflect pressure and temperature conditions and temperature conditions ● groups of 2 to 3 index minerals form an isograd form an isograd

47 Canada NY VT ME NH MA CT RI Isograds Key: Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Low grade Medium grade High grade 4. Regional Metamorphism

48 Canada NY VT ME NH MA CT RI Isograds Key: Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Low grade Medium grade High grade Index minerals define metamorphic zones.

49 Canada NY VT ME NH MA CT RI Isograds Key: Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Low grade Medium grade High grade Index minerals define metamorphic zones. Isograds can be used to plot the level or degree of metamorphism.

50 Pressure (kilobars) Temperature (°C) Depth (km) Low Grade Intermediate Grade High Grade Increasing metamorphic grade Slate Phyllite Schist Blueschist Gneiss Migmatite

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52

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54 Pressure (kilobars) Temperature (°C) Depth (km) 20 15 10 5 0 02004006008001000 40 35 30 25 20 15 10 5 0 Hornfels Eclogite Zeolite Blueschist Greenschist Amphibolite Granulite Partial melting begins Contact metamorphism Conditions beneath mountain belts Subduction zone 13.5 Metamorphic facies correspond to particular combinations of pressure and temperature... … and can be used to indicate specific tectonic environments.

55 Thought questions for this chapter You have mapped an area of metamorphic rocks, such as the region in Figure 6.9a, and have observed a series of metamorphic zones, marked by north-south isograds, running from sillimanite in the east to chlorite in the west. Were metamorphic temperatures higher in the east or west? Which kind of pluton would produce the highest grade of metamorphism, a granite intrusion 20 km deep or a gabbro intrusion at a depth of 5 km?

56 5. Plate Tectonics and Metamorphism ● metamorphism occurs in or near ● plate interiors ● divergent plate margins ● convergent plate margins ● transform plate margins

57 Low P, Low T High P, High T High P, High T Tectonic transport moves rocks through different pressure-temperature zones, …

58 Low P, Low T High P, High T High P, High T Tectonic transport moves rocks through different pressure-temperature zones, … …and then transports them back to the shallow crust or the surface.

59 5. Plate Tectonics and Metamorphism ● metamorphic pressure-temperature paths ● history of burial and exhumation ● prograde and retrograde paths

60 5. Plate Tectonics and Metamorphism

61 Pressure (kilobars) Temperature (°C) Depth (km) Low Grade Intermediate Grade High Grade Slate Phyllite Gneiss Schist RETROGRADE PATH PROGRADE PATH The garnet crystal initially grows in a schist but ends up growing in a gneiss.

62 Pressure (kilobars) Temperature (°C) Depth (km) RETROGRADE PATH PROGRADE PATH Low Grade High Grade RETROGRADE PATH PROGRADE PATH Peak metamorphism Low temperature– high pressure metamorphism within a subduction zone Continental crust Shelf sediment Shelf sediment Deep-ocean sediment Deep-ocean sediment Oceanic crust Oceanic crust Trench Mélange ophiolites Mélange ophiolites Prograde path Peak metamorphism Retrograde path Continental crust Continental crust

63 Temperature (°C) Depth (km) RETROGRADE PATH PROGRADE PATH Low Grade High Grade RETROGRADE PATH PROGRADE PATH Peak metamorphism Low temperature– high pressure metamorphism within a subduction zone Continental crust Shelf sediment Shelf sediment Deep-ocean sediment Deep-ocean sediment Oceanic crust Oceanic crust Trench Mélange ophiolites Mélange ophiolites Prograde path Peak metamorphism Retrograde path Continental crust Continental crust High temperature– high pressure metamorphism within a mountain belt Deformed and metamorphosed shallow- and deep- ocean sediments Deformed and metamorphosed shallow- and deep- ocean sediments Continental crust Continental crust Continental crust Multiple thrusts Multiple thrusts Prograde path Peak metamorphism Retrograde path Suture Pressure (kilobars)

64 5. Plate Tectonics and Metamorphism ● rapid erosion (exhumation) rates of mountain ranges show a of mountain ranges show a relationship between relationship between ● tectonics (orogeny) ● climate ● controls the flow of metamorphic rocks to the surface rocks to the surface

65 Thought questions for this chapter Draw a sketch showing how seafloor metamorphism might take place. Subduction zones are generally characterized by high- pressure  low temperature metamorphism. In contrast, continent-continent collision zones are marked by moderate pressure  high temperature metamorphism. Which type of plate boundary has a higher geothermal gradient? Explain.

66 AmphiboliteBlueschist Burial metamorphism Contact metamorphism EclogiteExhumation Foliated rock FoliationGneiss Granoblastic rock GranuliteGreenschistGreenstone High-pressure metamorphism Hornfels Key terms and concepts

67 MarbleMélange Metamorphic facies MetasomatismMigmatitePhyllitePorphroblastQuartzite Regional metamorphism Schist Seafloor metamorphism Shock metamorphism SlateStressSuture Key terms and concepts

68 Ultra-high pressure metamorphism Zeolite Key terms and concepts


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