2 Alteration of Rocks by Temperature and Pressure Chapter 6:Metamorphism:Alteration of Rocks by Temperature and Pressure
3 About MetamorphismChanges 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.
4 Lecture Outline Causes of metamorphism 2. Types of metamorphism 3. Metamorphic textures4. Regional metamorphism and metamorphic grade5. Plate tectonics and metamorphism
5 ● internal heat of Earth ● internal pressure of Earth 1. Causes of Metamorphism● internal heat of Earth● internal pressure of Earth● fluid composition inside Earth
6 ● temperature increases with depth ● rate = 20º to 60ºC per km 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
10 ● the role of temperature ● geothermal gradient ● shallow (20ºC / km) 1. Causes of Metamorphism● the role of temperature● geothermal gradient● shallow (20ºC / km)● steep (50ºC / km)
11 ● the role of pressure (stress) ● confining pressure 1. Causes of Metamorphism● the role of pressure (stress)● confining pressure● directed pressure
12 ● the role of pressure (stress) ● rate of increase = 1. Causes of Metamorphism● the role of pressure (stress)● rate of increase =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 ● accelerated chemical reactions 2. Types of Metamorphism● the role of fluids● metasomatism● accelerated chemicalreactions
15 2. Types of Metamorphism Depth, km Oceanic crust 35 Continental crust 35Continental crustOceaniclithosphere75Continental mantle lithosphereAsthenosphere
31 3. Metamorphic Texturesshalesandstonelayers5 cm
32 The original bedding can be seen in the thin sandy layers. shale sandstonelayers5 cm
33 The original bedding can be seen in the thin sandy layers. foliation planeshalesandstonelayersoriginalbedding5 cm
34 Regional metamorphism causes cleavage planes to develop. The original beddingcan be seen in thethin sandy layers.Regional metamorphismcauses cleavage planesto develop.foliationplaneshalesandstonelayersoriginalbedding5 cm
37 Foliated rocks are classified by the degree of cleavage, schistosity, and banding.DiagenesisLow gradeIntermediategradeHigh gradeSlatePhylliteSchist(abundantmicaceousminerals)Gneiss(fewermicaceousminerals)MigmatiteSlaty cleavageSchistosityBandingBanding
38 ● classification of foliated rocks ● metamorphic grade ● crystal size 3. Metamorphic Textures● classification of foliated rocks● metamorphic grade● crystal size● type of foliation● banding
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 4. Regional Metamorphism and Metamorphic Grade● grades of metamorphism● low● intermediate● high
46 4. Regional Metamorphism and Metamorphic Grade● mineral isograds (zones of change)● index minerals reflect pressureand temperature conditions● groups of 2 to 3 index mineralsform an isograd
48 Index minerals define metamorphic zones. Canada ME NY Isograds VT NH MAKey:NotmetamorphosedCTChlorite zoneLowgradeRIBiotite zoneGarnet zoneMediumgradeStaurolite zoneHigh gradeSillimanite zone
49 Index minerals define metamorphic zones. Isograds can be used to plot thelevel or degree ofmetamorphism.CanadaMENYIsogradsVTNHMAKey:NotmetamorphosedCTChlorite zoneLowgradeRIBiotite zoneGarnet zoneMediumgradeStaurolite zoneHigh gradeSillimanite zone
54 Metamorphic facies correspond to particular combinations of HornfelsContact metamorphismZeolite5Conditions beneathmountain belts510Blueschist15Partial melting beginsAmphiboliteGranuliteGreenschistPressure (kilobars)Depth (km)10202513.515Subduction zone30Eclogite3520402004006008001000Temperature (°C)Metamorphic facies correspondto particular combinations ofpressure and temperature...… and can be usedto indicate specifictectonic 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 ● metamorphism occurs in or near ● plate interiors 5. Plate Tectonics and Metamorphism● metamorphism occurs in or near● plate interiors● divergent plate margins● convergent plate margins● transform plate margins
57 Tectonic transport moves rocks through differentpressure-temperaturezones, …Low P,Low THigh P,High T
58 Tectonic transport moves rocks through differentpressure-temperaturezones, …Low P,Low THigh P,High T…and then transports themback to the shallow crust orthe surface.
59 ● metamorphic pressure-temperature paths 5. Plate Tectonics and Metamorphism● metamorphic pressure-temperature paths● history of burial and exhumation● prograde and retrograde paths
61 The garnet crystal initially grows in a schist but ends up growing in a gneiss.LowGradeSlateRETROGRADE PATHIntermediateGradePhyllitePROGRADE PATHDepth (km)Pressure (kilobars)SchistGneissHighGradeTemperature (°C)
62 Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATHRETROGRADE PATHPressure (kilobars)Depth (km)PROGRADE PATHHighGradePROGRADE PATHPeak metamorphismTemperature (°C)Low temperature–high pressure metamorphism within a subduction zoneDeep-oceansedimentTrenchContinentalcrustShelfsedimentOceaniccrustMélangeophiolitesContinentalcrustProgradepathPeakmetamorphismRetrogradepath
63 Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATHRETROGRADE PATHPressure (kilobars)Depth (km)PROGRADE PATHHighGradePROGRADE PATHPeak metamorphismTemperature (°C)Low temperature–high pressure metamorphismwithin a subduction zoneHigh temperature–high pressure metamorphismwithin a mountain beltSutureDeep-oceansedimentTrenchContinentalcrustDeformed andmetamorphosedshallow- and deep-ocean sedimentsShelfsedimentContinentalcrustOceaniccrustMultiplethrustsMélangeophiolitesContinentalcrustContinentalcrustProgradepathProgradepathPeakmetamorphismRetrogradepathPeakmetamorphismRetrogradepath
64 ● rapid erosion (exhumation) rates of mountain ranges show a 5. Plate Tectonics and Metamorphism● rapid erosion (exhumation) ratesof mountain ranges show arelationship between● tectonics (orogeny)● climate● controls the flow of metamorphicrocks 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-pressurelow temperature metamorphism. In contrast, continent-continent collision zones are marked by moderate pressurehigh temperature metamorphism. Which type of plate boundary has a higher geothermal gradient? Explain.