Metamorphism, Lectures 11&12 Flashcards
Metamorphic rocks
Result from transformation of other rocks beneath the Earths surface.
Example of a metamorphic rock
marble
Marble characteristics
relatively soft
things are made from it
CaCO3 - calcite crystals predominantly make it up
When does metamorphism occur?
Metamorphism, occurs when rocks are subjected to sufficient temperature, pressure and fluid activity to change the original mineral composition and texture, thus forming new rocks.
Which state do metamorphic transformations usually occur in?
solid
Type of metamorphic rock formed depends on
3
- the time the rock was subject to the agents of metamorphism
- agents of metamorphism
- parent rock (composition and texture)
What 2 sources of heat causes temperature increases as an agent of metamorphism
- magma
2. deep burial in crust
Agents of metamorphism
3
- temperature
- pressure
- fluidity
Geothermal gradient across the globe
varies
Geothermometers
metamorphic minerals
What is the increase of temperature with increasing depth called?
geothermal gradient
Continental plate extension
geothermal gradient?
steep geothermal gradient - i.e.: 50 degrees C km -1
Ancient continental plate
geothermal gradient?
shallow geothermal gradient, i.e.: 20 degrees C km-1
Isotherm
Layer of equal temperature
As depth inside the Earth increases, what happens to pressure?
it increases
Increased pressure effect on the rate of chemical reactions that produce new mineral assemblages?
increased pressure = increased rate
What happens to minerals which are stable at low pressures near the Earth’s surface when they are subject to increased pressure in the crust?
they become unstable and recrystallise into new minerals
Geobarometers
metamorphic minerals
What causes recrystallisation of minerals?
when rocks are buried and subjected to greater stress
Effect of uniform stress on minerals
stress is equal in all directions,
smaller - denser minerals
produces non-foliated metamorphic rocks
Effect of differential stress
stress isn’t equal in all directions,
distinctive metamorphic textures in which crystals are aligned
produces foliated metamorphic rocks
Example of foliated metamorphic rocks
rock clevage in slate
Fluids as metamorphic agents
enhance metamorphism by increasing the rate of chemical reactions
Process by which fluids change a rock’s composition
metasomatism
Metasomatism
process by which fluids change a rock composition by carrying chemical substances
Fluids
liquids and gases
Types of metamorphism
3
1/ regional
2/ contact
3/ dynamic
Regional metamorphism
- Metamorphism which occurs over broad areas of the crust (google)
- notes: affects huge areas/volumes
What is regional metamorphism a feature of?
convergent plate boundaries
cores of mountain chains produced by continent-continent collisions
What causes regional metamorphism?
3
- high temperatures,
- high pressures,
- extreme deformation in deep parts of the crust
What happens during regional metamorphism?
rocks are transported to depths in Earth’s crust, then uplifted, exposed and eroded at the surface
How are metamorphic zones characterised?
by particular index minerals
Isograd
the index minerals are joined up on a map, resulting in a line of equal metamorphic intensity
Polymorphs
when chemical substances form more than one crystal structure, different structures = polymorphs
Metamorphic facies
a group of rocks characterised by a specific mineral assemblage formed under the same broad P-T conditons
What do metamorphic facies indicate?
the plate tectonic processes responsible for metamorphism
Contact metamorphism
occurs when rock is intruded by a body of hot magma
How does contact metamorphism occur?
high temp. and low pressure metamorphism around an igneous intrusion, heats surrounding rock - country rock
Country rock
rock native to an area (google)
Hornfels
typical rock from a contact metamorphic environment
Dynamic metamorphisim
occurs along fault zones where rocks are subjected to high pressures
Dynamic metamorphism causes
high rates of strain and temp. in ductile areas
Mylonites
generated by dynamic metamorphism, hard, dense, fine-grained foliated rocks
