Changes in rock composition or texture

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


Take-Away Points
1. Rocks change due to heat, pressure, and fluids
2. Heat in the earth is original heat plus heat from
radioactive decay
3. Pressure is simply from the weight of overlying
rocks. Pressure = Depth
4. What happens during metamorphism
5. Different metamorphic conditions result in
different rocks and minerals
6. Metamorphic rocks reveal temperature and
pressure conditions in the earth.


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 nearsurface conditions

1. Rocks change due to heat, pressure, and fluids


Where Does the Heat Come from?
• Some Heat is Probably Original
• Much heat is from Radioactive Decay
– Uranium and Thorium  Other Elements +
Lead + Radiation (about 90%)
– Potassium-40  Calcium-40 or Argon-40 +
Radiation (about 10%)

• Typical temperature increase
– 25 C per kilometer near surface
– 1-2 C per kilometer in deep earth
2. Heat in the earth is original heat plus heat from radioactive decay


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.)
3. Pressure is simply from the weight of overlying rocks. Pressure = Depth


Types of Metamorphism
Contact
• Around Intrusions
• Shallow: 0-6 Km
• Low Pressure
• Local heat source

Regional
• Wide Areas
• 5-20 Km, Sometimes
30+
• High Pressure
• Usually Accompanied
by Deformation and
Mountain Building

4. What happens during metamorphism

What Happens During
Metamorphism
Minerals React to Form New
Minerals
• 2SiO2 + CaMg(CO3) 2 ==
CaMgSi2O6 + 2CO2

New Materials Are Added
(Metasomatism)
• CaMg(SiO2)2 + 2CO2 ==
CaMg(CO3)2 + 2SiO2

• Quartz + Dolomite ==
Pyroxene
Minerals Change Form
• Al2SiO5 == Al2SiO5

• Pyroxene + CO2 ==
Dolomite + Quartz
• Minerals in Solution ==
Ore Bodies
Recrystallization

• Andalusite == Kyanite

4. What happens during metamorphism


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

4. What happens during metamorphism


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
5. Different metamorphic conditions result in different rocks and minerals


Major Metamorphic Rock Types
Temp C

Temp F

Coal

Limestone

Sandstone

Basalt

Shale

Index
Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite
Bituminous

300

500

Anthracite

600

Graphite

Marble

700
800
500

1100
1200

700

Quartzite

900
1000

600

Greenstone

Amphibolite

Staurolite
Gneiss

Kyanite
Sillimanite
Melting
Begins

5. Different metamorphic conditions result in different rocks and minerals



Major Metamorphic Rock Types
Temp C

Temp F

Coal

Limestone

Sandstone

Basalt

Shale

Index
Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite
Bituminous

300

500

Anthracite

600

Graphite

Marble

700
800
500

1100
1200

700

Quartzite

900
1000

600

Greenstone

Amphibolite

Staurolite
Gneiss

Kyanite
Sillimanite
Melting
Begins

5. Different metamorphic conditions result in different rocks and minerals



Major Metamorphic Rock Types

Temp C

Temp F

Coal

Limestone

Sandstone

Basalt

Shale

Index
Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite

Bituminous

300

500

Anthracite

600

Graphite

Marble

700
800
500

1100
1200

700

Quartzite

900
1000

600

Greenstone

Amphibolite

Staurolite
Gneiss

Kyanite
Sillimanite
Melting
Begins

5. Different metamorphic conditions result in different rocks and minerals



Major Metamorphic Rock Types
Temp C

Temp F

Coal

Limestone

Sandstone

Basalt

Shale

Index
Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite
Bituminous

300

500

Anthracite

600

Graphite

Marble

700
800
500

1100
1200

700

Quartzite

900
1000

600

Greenstone

Amphibolite

Staurolite
Gneiss

Kyanite
Sillimanite
Melting
Begins

5. Different metamorphic conditions result in different rocks and minerals

Major Metamorphic Rock Types
Temp C

Temp F

Coal

Limestone

Sandstone

Basalt

Shale

Index
Minerals

Slate

Chlorite

Phyllite

Biotite

Schist

Garnet

Lignite
Bituminous

300

500

Anthracite

600

Graphite

Marble

700
800
500

1100
1200

700

Quartzite

900

1000

600

Greenstone

Amphibolite

Staurolite
Gneiss

Kyanite
Sillimanite
Melting
Begins

5. Different metamorphic conditions result in different rocks and minerals



What About Other Rocks?


Polymorphism
Al2SiO5

Calcite - Aragonite
Quartz • Andalusite
• - Tridymite - Cristobalite
• Kyanite

(increasing
• Sillimanite
temperature)
Ice - 6 high pressure
•
Coesite
Stishovite
forms
(increasing pressure)
Diamond - Graphite
6. Metamorphic rocks reveal temperature and pressure conditions in the earth.


Metamorphic Facies
Depth\Temp

300C

400C

5 km

Zeolite

Contact Metamorphism - Andalusite forms

10 km - 3 kb

Greenschist
Blueschist Chlorite,

Biotite form
•Slate
•Greenstone
•Quartzite
•Marble

Amphibolite
Garnet,
Staurolite,
Kyanite form
•Schist
•Amphibolite
•Quartzite
•Marble
•Gneiss

Not Found

Eclogite (Mantle)

15 km
20 km - 6 kb
25 km
30 km - 9 kb
35 km
40 km - 12 kb

500 C

600 C

700 C

800 C

Granulite
Sillimanite forms
Muscovite
breaks down to
K-feldspar
Partial Melting
•Gneiss

6. Metamorphic rocks reveal temperature and pressure conditions in the earth.


Mantle Rocks


Take-Away Points
1. Rocks change due to heat, pressure, and fluids
2. Heat in the earth is original heat plus heat from
radioactive decay
3. Pressure is simply from the weight of overlying
rocks. Pressure = Depth
4. What happens during metamorphism
5. Different metamorphic conditions result in
different rocks and minerals
6. Metamorphic rocks reveal temperature and
pressure conditions in the earth.