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%) Possibly some primordial heat (controversial)
Where Does the Pressure Come from? Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar = 100 kPa) 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 3.5 km Of Rock) = 1 Kilobar (Kb.) = 100 MPa
Pressure Units 1 Newton (N) = gravitational attraction on 100 grams One Newton = 7 Fig Newtons 1 Pascal (Pa) = 1 N/square meter 1 Pascal = pressure under thin cardboard or 0.1 mm water A Thin section exerts about 1 Pa pressure on the slide One bar = 1 atm = 100 kPa 1 MPa = 10 bars 1 GPa = 10 kilobars
Types of Metamorphism Contact Around Intrusions or Local Heat Sources Regional Usually Accompanied by Deformation Cataclasis Shearing and fault zones Shock Ultra-high pressures and rapid 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 Cataclasis Shock Metamorphism
Polymorphism Al2SiO5 Andalusite Kyanite Sillimanite Ice - 6 high pressure forms Diamond - Graphite Calcite - Aragonite Quartz - - Tridymite - Cristobalite (increasing temperature) - Coesite - Stishovite (increasing pressure)
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
Foliation Foliation is a sheetlike structure that forms when rocks are deformed.
Foliation Flattening Grain Rotation Solution New Minerals Partial Melting Shearing
Foliation In every case, the foliation is: In the direction of least resistance at right angles to the direction of greatest compression.
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 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
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
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
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
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
What About Other Rocks?
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)
Metamorphic Map
Mantle Rocks
Metamorphism and Plate Tectonics