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Metamorphism •Changes in Rock Composition or Texture •Due to Heat, Pressure and Action of Fluids 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 Chemical Changes in Rocks Weathering • At Surface Diagenesis • Sedimentary Rocks Metamorphism • Starts about 200 C • Outside range of normal near-surface conditions Where Does the Heat Come from? Uranium and Thorium Other Elements + Lead + Radiation (about 90%) Potassium-40 Calcium-40 or Argon-40 + Radiation (about 10%) 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.) Types of Metamorphism Contact • Around Intrusions • Shallow: 0-6 Km • Low Pressure • Local Regional • Wide Areas • 5-20 Km, Sometimes 30+ • High Pressure • Usually Accompained by Deformation 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 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 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 Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble 700 800 500 1100 1200 700 Quartzite 900 1000 600 Greenstone Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble 700 800 500 1100 1200 700 Quartzite 900 1000 600 Greenstone Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble 700 800 500 1100 1200 700 Quartzite 900 1000 600 Greenstone Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble 700 800 500 1100 1200 700 Quartzite 900 1000 600 Greenstone Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble 700 800 500 1100 1200 700 Quartzite 900 1000 600 Greenstone Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins What About Other Rocks? Polymorphism Al2SiO5 • Andalusite • Kyanite • Sillimanite Ice - 6 high pressure forms Diamond - Graphite Calcite - Aragonite Quartz • - Tridymite Cristobalite (increasing temperature) • - Coesite - Stishovite (increasing pressure) Metamorphic Facies Depth\Temp 300C 400C 5 km Zeolite Contact Metamorphism - Andalusite forms 10 km - 3 kb Greenschist Blueschist Chlorite, Biotite form •Slate •Greenstone •Quartzite •Marble Amphibolite Garnet, Staurolite, Kyanite form •Schist •Amphibolite •Quartzite •Marble •Gneiss Not Found Eclogite (Mantle) 15 km 20 km - 6 kb 25 km 30 km - 9 kb 35 km 40 km - 12 kb 500 C 600 C 700 C 800 C Granulite Sillimanite forms Muscovite breaks down to Kfeldspar Partial Melting •Gneiss Mantle Rocks
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