Chapter+6+-+Metamorphism - Grotzinger • Jordan...

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Unformatted text preview: Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company About Metamorphism About • Changes in heat, pressure, and the Changes chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. textures, • Metamorphic changes occur in the solid Metamorphic state, so there is no melting. 1. Causes of Metamorphism 1. ● internal heat of Earth internal ● internal pressure of Earth internal ● fluid composition inside Earth fluid 1. Causes of Metamorphism 1. ● temperature increases with depth temperature ● rate = 20º to 60ºC per km rate ● at 15 km depth: 450ºC at 1. Causes of Metamorphism 1. ● the role of temperature ● geothermal gradient geothermal ● shallow (20ºC / km) shallow ● steep (50ºC / km) steep 1. Causes of Metamorphism 1. ● the role of pressure (stress) the ● confining pressure confining ● directed pressure directed 1. Causes of Metamorphism 1. ● the role of pressure (stress) the ● rate of increase = 0.3 to 0.4 kbar / km 0.3 ● minerals are geobarometers minerals Thought questions for this chapter Thought At what depths in the Earth do metamorphic rocks form? From near the surface (contact) to the depths at which From rock melt rock What happens if temperatures get too high? What They melt (igneous process) Why are there no metamorphic rocks formed under Why natural conditions of very low pressure and temperature, as shown in Figure 6.1? (insufficient P and T to alter them) as 2. Types of Metamorphism 2. ● the role of fluids the ● metasomatism metasomatism ● accelerated chemical reactions reactions Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Types of Metamorphism Depth, km 0 35 75 Oceanic crust Con tine ntal cru Con st tine nta l ma ntle li As the nos phe re tho sph Oceanic lithosphere er e Water Burial metamorphism Seafloor metamorphism 3. Metamorphic Textures 3. 3. Metamorphic Textures Foliation is the result of compressive forces. Mineral crystals become Mineral elongated perpendicular elongated to the compressive force. Feldspar Quartz Mica Mica Pyrite Staurolite 3. Metamorphic Textures Low grade Intermediate grade 3. Metamorphic Textures 3. ● classification of foliated rocks classification ● metamorphic grade metamorphic ● crystal size crystal ● type of foliation type ● banding banding 3. Metamorphic Textures 3. ● nomenclature of foliated rocks nomenclature ● slate slate ● phyllite phyllite ● schist schist ● gneiss gneiss ● migmatite migmatite 3. Metamorphic Textures 3. ● nomenclature of granoblastic nomenclature (non-foliated or massive) metamorphic rocks metamorphic ● quartzites quartzites ● marbles marbles You only need to know the metamorphic rocks given to you in the word handout All of the foliated rocks but only Marble and Quartzite in the nonfoliated rocks Thought questions for this chapter Thought How is slaty cleavage related to tectonic forces? What How forces cause minerals to align with one another? forces Crystals are oriented normal to the direction of tectonic Crystals pressure. pressure. Would you choose to rely on chemical composition or Would type of foliation to determine metamorphic grade? Probably foliation (see slide 21) Probably Why? (see slide 21) Why? (see 4. Regional Metamorphism and Metamorphic Grade and ● mineral isograds (zones of change) mineral ● index minerals reflect pressure index and temperature conditions and ● groups of 2 to 3 index minerals groups form an isograd form Index minerals define Index metamorphic zones. metamorphic 3. Metamorphic Textures Isograds can be used to plot the level or degree of level metamorphism. metamorphism. Canada ME NY Isograds VT NH Key: MA CT RI Low grade Medium grade High grade Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Incr easi Pressure Pressure (kilobars) Slate ng m etam orph Intermediate Grade Phyllite ic gr ade High Grade Schist Blueschist Temperature (°C) Gneiss Migmatite Depth (km) Low Grade e 5 Subd zone 20 0 200 400 15 20 25 n uctio 15 Granulite t his 13.5 10 Amphibolite Greenschist sc 10 5 Partial meltin g begins ol it Contac t metam orphism Con diti mo unt ons b ain e belt neath s e Blu Pressure (kilobars) Ze 0 Hornfels Depth (km) 0 30 Eclogite 35 600 40 1000 800 Temperature (°C) Metamorphic facies correspond to particular combinations of pressure and temperature... … and can be used to indicate specific tectonic environments. tectonic 5. Plate Tectonics and Metamorphism 5. ● metamorphism occurs in or near metamorphism ● plate interiors plate ● divergent plate margins divergent ● convergent plate margins convergent ● transform plate margins transform Tectonic transport moves Tectonic rocks through different pressure-temperature zones, … zones, Low P, Low T High P, High T …and then transports them back to the shallow crust or back the surface. the 5. Plate Tectonics and Metamorphism 5. ● metamorphic pressure-temperature paths paths ● history of burial and exhumation history ● prograde and retrograde paths prograde RE TR OG PR OG RA DE PA TH High Grade RA DE PA TH Depth (km) TH PA TH DE RA PA OG DE TR RA RE OG PR Pressure (kilobars) Low Grade Peak metamorphism Temperature (°C) Low temperature– high pressure metamorphism within a subduction zone Continental crust Deep­ocean sediment Shelf sediment Oceanic crust High temperature– high pressure metamorphism within a mountain belt Suture Trench Mélange ophiolites Continental crust Multiple thrusts Deformed and metamorphosed shallow­ and deep­ ocean sediments Continental crust Continental crust Prograde path Peak metamorphism Retrograde path Prograde path Peak metamorphism Retrograde path 5. Plate Tectonics and Metamorphism 5. ● rapid erosion (exhumation) rates rapid of mountain ranges show a relationship between relationship ● tectonics (orogeny) tectonics ● climate climate ● controls the flow of metamorphic rocks to the surface rocks Thought questions for this chapter Thought Subduction zones are generally characterized by highpressure−low temperature metamorphism. In contrast, low continent-continent collision zones are marked by moderate pressure−high temperature metamorphism. Which type of pressure high plate boundary has a higher geothermal gradient? Explain. plate Sorry I misread the question for lecture You actually use The figure on slide 29! It is continental-continental collision The zone see below. zone Subduction zone gradient = 300 C/35 km is approximately Subduction 8.1 C/km 8.1 Continenal collision zone gradient = 920 C/28 km is a little Continenal less than 33 C/km less Key terms and concepts Burial metamorphism Burial Contact metamorphism Foliated rock Foliation Gneiss Granoblastic rock High-pressure metamorphism Key terms and concepts Marble Marble Metamorphic facies Metasomatism Migmatite Phyllite Porphroblast Quartzite Regional metamorphism Schist Seafloor metamorphism Shock metamorphism Slate Slate Stress Stress Suture ...
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This note was uploaded on 07/12/2011 for the course EAS 2600 taught by Professor Ingalls during the Summer '08 term at Georgia Institute of Technology.

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