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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

Metamorphism

Changes in Rock Composition or Texture

Due to Heat, Pressure and Action of Fluids

we do not live at normal conditions
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
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
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
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
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
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
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
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
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

Foliation is a sheetlike structure that forms when rocks are deformed.

foliation12
Foliation
  • Flattening
  • Grain Rotation
  • Solution
  • New Minerals
  • Partial Melting
  • Shearing
foliation13
Foliation

In every case, the foliation is:

  • In the direction of least resistance
  • at right angles to the direction of greatest compression.
metamorphic grade
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