Metamorphism
advertisement
Metamorphism •Changes in Rock Composition or Texture •Due to Heat, Pressure and Action of Fluids Take-Away Points 1. Rocks change due to heat, pressure, and fluids 2. Heat in the earth is original heat plus heat from radioactive decay 3. Pressure is simply from the weight of overlying rocks. Pressure = Depth 4. What happens during metamorphism 5. Different metamorphic conditions result in different rocks and minerals 6. Metamorphic rocks reveal temperature and pressure conditions in the earth. 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 nearsurface conditions 1. Rocks change due to heat, pressure, and fluids 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 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 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 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 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 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 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 5. Different metamorphic conditions result in different rocks and minerals 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 5. Different metamorphic conditions result in different rocks and minerals 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 5. Different metamorphic conditions result in different rocks and minerals 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 5. Different metamorphic conditions result in different rocks and minerals 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 5. Different metamorphic conditions result in different rocks and minerals 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) 6. Metamorphic rocks reveal temperature and pressure conditions in the earth. 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 6. Metamorphic rocks reveal temperature and pressure conditions in the earth. Mantle Rocks Take-Away Points 1. Rocks change due to heat, pressure, and fluids 2. Heat in the earth is original heat plus heat from radioactive decay 3. Pressure is simply from the weight of overlying rocks. Pressure = Depth 4. What happens during metamorphism 5. Different metamorphic conditions result in different rocks and minerals 6. Metamorphic rocks reveal temperature and pressure conditions in the earth.
