Metamorphism, Lectures 11&12

Question 1

Metamorphic rocks

Answer 1

Result from transformation of other rocks beneath the Earths surface.

Question 2

Example of a metamorphic rock

Answer 2

marble

Question 3

Marble characteristics

Answer 3

relatively soft
things are made from it
CaCO3 - calcite crystals predominantly make it up

Question 4

When does metamorphism occur?

Answer 4

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.

Question 5

Which state do metamorphic transformations usually occur in?

Answer 5

solid

Question 6

Type of metamorphic rock formed depends on

3

Answer 6

1. the time the rock was subject to the agents of metamorphism
2. agents of metamorphism
3. parent rock (composition and texture)

Question 7

What 2 sources of heat causes temperature increases as an agent of metamorphism

Answer 7

1. magma

2. deep burial in crust

Question 8

Agents of metamorphism

3

Answer 8

1. temperature
2. pressure
3. fluidity

Question 9

Geothermal gradient across the globe

Answer 9

varies

Question 10

Geothermometers

Answer 10

metamorphic minerals

Question 11

What is the increase of temperature with increasing depth called?

Answer 11

geothermal gradient

Question 12

Continental plate extension

geothermal gradient?

Answer 12

steep geothermal gradient - i.e.: 50 degrees C km -1

Question 13

Ancient continental plate

geothermal gradient?

Answer 13

shallow geothermal gradient, i.e.: 20 degrees C km-1

Question 14

Isotherm

Answer 14

Layer of equal temperature

Question 15

As depth inside the Earth increases, what happens to pressure?

Answer 15

it increases

Question 16

Increased pressure effect on the rate of chemical reactions that produce new mineral assemblages?

Answer 16

increased pressure = increased rate

Question 17

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?

Answer 17

they become unstable and recrystallise into new minerals

Question 18

Geobarometers

Answer 18

metamorphic minerals

Question 19

What causes recrystallisation of minerals?

Answer 19

when rocks are buried and subjected to greater stress

Question 20

Effect of uniform stress on minerals

Answer 20

stress is equal in all directions,
smaller - denser minerals
produces non-foliated metamorphic rocks

Question 21

Effect of differential stress

Answer 21

stress isn’t equal in all directions,
distinctive metamorphic textures in which crystals are aligned
produces foliated metamorphic rocks

Question 22

Example of foliated metamorphic rocks

Answer 22

rock clevage in slate

Question 23

Fluids as metamorphic agents

Answer 23

enhance metamorphism by increasing the rate of chemical reactions

Question 24

Process by which fluids change a rock’s composition

Answer 24

metasomatism

Question 25

Metasomatism

Answer 25

process by which fluids change a rock composition by carrying chemical substances

Question 26

Fluids

Answer 26

liquids and gases

Question 27

Types of metamorphism | 3

Answer 27

1/ regional 2/ contact 3/ dynamic

Question 28

Regional metamorphism

Answer 28

- Metamorphism which occurs over broad areas of the crust (google) - notes: affects huge areas/volumes

Question 29

What is regional metamorphism a feature of?

Answer 29

convergent plate boundaries | cores of mountain chains produced by continent-continent collisions

Question 30

What causes regional metamorphism? | 3

Answer 30

1. high temperatures, 2. high pressures, 3. extreme deformation in deep parts of the crust

Question 31

What happens during regional metamorphism?

Answer 31

rocks are transported to depths in Earth's crust, then uplifted, exposed and eroded at the surface

Question 32

How are metamorphic zones characterised?

Answer 32

by particular index minerals

Question 33

Isograd

Answer 33

the index minerals are joined up on a map, resulting in a line of equal metamorphic intensity

Question 34

Polymorphs

Answer 34

when chemical substances form more than one crystal structure, different structures = polymorphs

Question 35

Metamorphic facies

Answer 35

a group of rocks characterised by a specific mineral assemblage formed under the same broad P-T conditons

Question 36

What do metamorphic facies indicate?

Answer 36

the plate tectonic processes responsible for metamorphism

Question 37

Contact metamorphism

Answer 37

occurs when rock is intruded by a body of hot magma

Question 38

How does contact metamorphism occur?

Answer 38

high temp. and low pressure metamorphism around an igneous intrusion, heats surrounding rock - country rock

Question 39

Country rock

Answer 39

rock native to an area (google)

Question 40

Hornfels

Answer 40

typical rock from a contact metamorphic environment

Question 41

Dynamic metamorphisim

Answer 41

occurs along fault zones where rocks are subjected to high pressures

Question 42

Dynamic metamorphism causes

Answer 42

high rates of strain and temp. in ductile areas

Question 43

Mylonites

Answer 43

generated by dynamic metamorphism, hard, dense, fine-grained foliated rocks