Metamorphism Changes in Rock Composition or Texture Due to Heat, Pressure and Action of Fluids.

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%)

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.)

Types of Metamorphism

Contact• Around Intrusions • Shallow: 0-6 Km • Low Pressure • Local

Regional• Wide Areas • 5-20 Km, Sometimes

30+ • High Pressure • Usually Accompained

by 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

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

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 ShaleIndex Minerals

Lignite Bituminous

500 Anthracite

300 600 Graphite Marble Slate Chlorite

700 Greenstone

800 Quartzite Phyllite Biotite

500 900 Schist Garnet

1000 Amphibolite Staurolite

600 1100Gneiss

Kyanite

1200 Sillimanite

700Melting Begins

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)

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 GreenschistChlorite, Biotite form

•Slate

•Greenstone

•Quartzite

•Marble

AmphiboliteGarnet, Staurolite, Kyanite form

•Schist

•Amphibolite

•Quartzite

•Marble

•Gneiss

GranuliteSillimanite formsMuscovite breaks down to K-feldsparPartial Melting

•Gneiss

15 km Blueschist

20 km - 6 kb

25 km

30 km - 9 kb

35 km

40 km - 12 kb Not Found Eclogite (Mantle)

Mantle Rocks

Metamorphism Changes in Rock Composition or Texture Due to Heat, Pressure and Action of Fluids.

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