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 2SiO 2 + CaMg(CO 3 ) 2 == CaMgSi 2 O 6 + 2CO 2 Quartz + Dolomite == Pyroxene Minerals Change Form Al 2 SiO 5 == Al 2 SiO 5 Andalusite == Kyanite New Materials Are Added (Metasomatism) CaMg(SiO 2 ) 2 + 2CO 2 == CaMg(CO 3 ) 2 + 2SiO 2 Pyroxene + CO 2 == Dolomite + Quartz Minerals in Solution == Ore Bodies Recrystallization Cataclasis Shock Metamorphism

Polymorphism Al 2 SiO 5 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 – 2AlSi 2 O 5 (OH) == Al 2 SiO 5 + 3SiO 2 + H 2 O – Clay Mineral == Andalusite + Quartz + Water (Lost) An example of carbonate metamorphism: – CaMg(CO 3 ) 2 + 2SiO 2 == CaMgSi 2 O 6 +2CO 2 – Dolomite + Quartz == Pyroxene + CO 2 (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 a.Flattening b.Grain Rotation c.Solution d.New Minerals e.Partial Melting f.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 CTemp FCoalLimestoneSandstoneBasaltShale Index Minerals Lignite Bituminous 500Anthracite GraphiteMarbleSlateChlorite 700Greenstone 800QuartzitePhylliteBiotite SchistGarnet 1000AmphiboliteStaurolite Gneiss Kyanite 1200Sillimanite 700 Melting Begins

Major Metamorphic Rock Types Temp CTemp FCoalLimestoneSandstoneBasaltShale Index Minerals Lignite Bituminous 500Anthracite GraphiteMarbleSlateChlorite 700Greenstone 800QuartzitePhylliteBiotite SchistGarnet 1000AmphiboliteStaurolite Gneiss Kyanite 1200Sillimanite 700 Melting Begins

Major Metamorphic Rock Types Temp CTemp FCoalLimestoneSandstoneBasaltShale Index Minerals Lignite Bituminous 500Anthracite GraphiteMarbleSlateChlorite 700Greenstone 800QuartzitePhylliteBiotite SchistGarnet 1000AmphiboliteStaurolite Gneiss Kyanite 1200Sillimanite 700 Melting Begins

Major Metamorphic Rock Types Temp CTemp FCoalLimestoneSandstoneBasaltShale Index Minerals Lignite Bituminous 500Anthracite GraphiteMarbleSlateChlorite 700Greenstone 800QuartzitePhylliteBiotite SchistGarnet 1000AmphiboliteStaurolite Gneiss Kyanite 1200Sillimanite 700 Melting Begins

Major Metamorphic Rock Types Temp CTemp FCoalLimestoneSandstoneBasaltShale Index Minerals Lignite Bituminous 500Anthracite GraphiteMarbleSlateChlorite 700Greenstone 800QuartzitePhylliteBiotite SchistGarnet 1000AmphiboliteStaurolite Gneiss Kyanite 1200Sillimanite 700 Melting Begins

What About Other Rocks?

Metamorphic Facies Depth\Temp300C400C500 C600 C700 C800 C 5 kmZeoliteContact Metamorphism - Andalusite forms 10 km - 3 kbGreenschist Chlorite, Biotite form Slate Greenstone Quartzite Marble Amphibolite Garnet, Staurolite, Kyanite form Schist Amphibolite Quartzite Marble Gneiss Granulite Sillimanite forms Muscovite breaks down to K- feldspar Partial Melting Gneiss 15 kmBlueschist 20 km - 6 kb 25 km 30 km - 9 kb 35 km 40 km - 12 kbNot FoundEclogite (Mantle)

Metamorphic Map

Mantle Rocks

Metamorphism and Plate Tectonics