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Metamorphism Changes in Rock Composition or Texture
Due to Heat, Pressure and Action of Fluids
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Take-Away Points Rocks change due to heat, pressure, and fluids
Heat in the earth is original heat plus heat from radioactive decay Pressure is simply from the weight of overlying rocks. Pressure = Depth What happens during metamorphism Different metamorphic conditions result in different rocks and minerals Metamorphic rocks reveal temperature and pressure conditions in the earth.
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We Do Not Live at “Normal” Conditions
By the standards of Earth’s interior, we live in a frozen vacuum Things that look “abnormal” to us are normal behavior for materials Solids can flow Solids can react chemically with each other A given material can have several different atomic structures
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Chemical Changes in Rocks
Weathering At Surface Diagenesis Sedimentary Rocks Metamorphism Starts about 200 C Outside range of normal near-surface conditions 1. Rocks change due to heat, pressure, and fluids
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Where Does the Heat Come from?
Some Heat is Probably Original Much heat is from Radioactive Decay Uranium and Thorium Other Elements + Lead + Radiation (about 90%) Potassium-40 Calcium-40 or Argon-40 + Radiation (about 10%) Typical temperature increase 25 C per kilometer near surface 1-2 C per kilometer in deep earth 2. Heat in the earth is original heat plus heat from radioactive decay
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Where Does the Pressure Come from?
Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar) Pressure Beneath 10 Meters (33 Ft.) Of Water = 1 Atm. = 1 Bar Same Pressure Beneath 3.5 M (10 Ft.) Of Rock Pressure in Deepest Part of Ocean = 1000 Bars Pressure under One Mile of Rock = 500 Bars 1000 Bars (2 Mi. or 3 km Of Rock) = 1 Kilobar (Kb.) 3. Pressure is simply from the weight of overlying rocks. Pressure = Depth
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4. What happens during metamorphism
Types of Metamorphism Contact Around Intrusions Shallow: 0-6 Km Low Pressure Local heat source Regional Wide Areas 5-20 Km, Sometimes 30+ High Pressure Usually Accompanied by Deformation and Mountain Building 4. What happens during metamorphism
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What Happens During Metamorphism
Minerals React to Form New Minerals 2SiO2 + CaMg(CO3) 2 == CaMgSi2O6 + 2CO2 Quartz + Dolomite == Pyroxene Minerals Change Form Al2SiO5 == Al2SiO5 Andalusite == Kyanite New Materials Are Added (Metasomatism) CaMg(SiO2)2 + 2CO2 == CaMg(CO3)2 + 2SiO2 Pyroxene + CO2 == Dolomite + Quartz Minerals in Solution == Ore Bodies Recrystallization 4. What happens during metamorphism
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Why Don't Rocks "De-metamorphose"?
Reactions Can't Reverse Because Ingredients Lost 2AlSi2O5(OH) == Al2SiO5 + 3SiO2 + H2O Clay Mineral == Andalusite + Quartz + Water (Lost) An example of carbonate metamorphism: CaMg(CO3) 2 + 2SiO2 == CaMgSi2O6 +2CO2 Dolomite + Quartz == Pyroxene + CO2 (Lost) Reactions "Freeze" Sometimes it Does Happen if Fluids Present Retrograde Metamorphism On the surface we call it weathering 4. What happens during metamorphism
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Metamorphic Grade - Degree to Which the Rock Has Changed Composition
Can Often See Original Bedding Can Sometimes Even See Deformed Fossils At High Grades, Rocks Can Often Lose All Trace of Their Original Appearance 5. Different metamorphic conditions result in different rocks and minerals
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Major Metamorphic Rock Types
Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Lignite Bituminous 500 Anthracite 300 600 Graphite Marble Slate Chlorite 700 Greenstone 800 Quartzite Phyllite Biotite 900 Schist Garnet 1000 Amphibolite Staurolite 1100 Gneiss Kyanite 1200 Sillimanite Melting Begins 5. Different metamorphic conditions result in different rocks and minerals
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Major Metamorphic Rock Types
Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Lignite Bituminous 500 Anthracite 300 600 Graphite Marble Slate Chlorite 700 Greenstone 800 Quartzite Phyllite Biotite 900 Schist Garnet 1000 Amphibolite Staurolite 1100 Gneiss Kyanite 1200 Sillimanite Melting Begins 5. Different metamorphic conditions result in different rocks and minerals
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Major Metamorphic Rock Types
Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Lignite Bituminous 500 Anthracite 300 600 Graphite Marble Slate Chlorite 700 Greenstone 800 Quartzite Phyllite Biotite 900 Schist Garnet 1000 Amphibolite Staurolite 1100 Gneiss Kyanite 1200 Sillimanite Melting Begins 5. Different metamorphic conditions result in different rocks and minerals
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Major Metamorphic Rock Types
Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Lignite Bituminous 500 Anthracite 300 600 Graphite Marble Slate Chlorite 700 Greenstone 800 Quartzite Phyllite Biotite 900 Schist Garnet 1000 Amphibolite Staurolite 1100 Gneiss Kyanite 1200 Sillimanite Melting Begins 5. Different metamorphic conditions result in different rocks and minerals
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Major Metamorphic Rock Types
Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Lignite Bituminous 500 Anthracite 300 600 Graphite Marble Slate Chlorite 700 Greenstone 800 Quartzite Phyllite Biotite 900 Schist Garnet 1000 Amphibolite Staurolite 1100 Gneiss Kyanite 1200 Sillimanite Melting Begins 5. Different metamorphic conditions result in different rocks and minerals
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What About Other Rocks?
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Polymorphism Al2SiO5 Andalusite Kyanite Sillimanite
Ice - 6 high pressure forms Diamond - Graphite Calcite - Aragonite Quartz - - Tridymite - Cristobalite (increasing temperature) - Coesite - Stishovite (increasing pressure) 6. Metamorphic rocks reveal temperature and pressure conditions in the earth.
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Metamorphic Facies Depth\Temp 300C 400C 500 C 600 C 700 C 800 C 5 km
Zeolite Contact Metamorphism - Andalusite forms 10 km - 3 kb Greenschist Chlorite, Biotite form Slate Greenstone Quartzite Marble Amphibolite Garnet, Staurolite, Kyanite form Schist Amphibolite Gneiss Granulite Sillimanite forms Muscovite breaks down to K-feldspar Partial Melting 15 km Blueschist 20 km - 6 kb 25 km 30 km - 9 kb 35 km 40 km - 12 kb Not Found Eclogite (Mantle) 6. Metamorphic rocks reveal temperature and pressure conditions in the earth.
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Mantle Rocks
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Take-Away Points Rocks change due to heat, pressure, and fluids
Heat in the earth is original heat plus heat from radioactive decay Pressure is simply from the weight of overlying rocks. Pressure = Depth What happens during metamorphism Different metamorphic conditions result in different rocks and minerals Metamorphic rocks reveal temperature and pressure conditions in the earth.
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