Changes in rock composition or texture

Different metamorphic conditions result in different rocks and minerals 6.. Metamorphic rocks reveal temperature and pressure conditions in the earth... Major Metamorphic Rock TypesTemp

•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

surface 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

• High Pressure

• Usually Accompanied

by Deformation and Mountain Building

4 What happens during metamorphism

What Happens During

• Pyroxene + CO2 ==

Dolomite + Quartz

• Minerals in Solution ==

Ore Bodies Recrystallization

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

Lignite Bituminous

Major Metamorphic Rock Types

Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals

Lignite Bituminous

Major Metamorphic Rock Types

Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals

Lignite Bituminous

Major Metamorphic Rock Types

Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals

Lignite Bituminous

Major Metamorphic Rock Types

Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals

Lignite Bituminous

What About Other Rocks?

• - Tridymite - Cristobalite (increasing

temperature)

• - Coesite - Stishovite (increasing pressure)

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

breaks down to K-feldspar

40 km - 12 kb Not Found Eclogite (Mantle)

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