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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
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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%) 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.
MetamorphicGrade - 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
