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Geology, LEC 172 > Metamorphism, Lectures 11&12 > Flashcards

Metamorphism, Lectures 11&12 Flashcards

1
Q

Metamorphic rocks

A

Result from transformation of other rocks beneath the Earths surface.

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2
Q

Example of a metamorphic rock

A

marble

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3
Q

Marble characteristics

A

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

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4
Q

When does metamorphism occur?

A

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.

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5
Q

Which state do metamorphic transformations usually occur in?

A

solid

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6
Q

Type of metamorphic rock formed depends on

3

A
  1. the time the rock was subject to the agents of metamorphism
  2. agents of metamorphism
  3. parent rock (composition and texture)
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7
Q

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

A
  1. magma

2. deep burial in crust

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8
Q

Agents of metamorphism

3

A
  1. temperature
  2. pressure
  3. fluidity
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9
Q

Geothermal gradient across the globe

A

varies

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10
Q

Geothermometers

A

metamorphic minerals

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11
Q

What is the increase of temperature with increasing depth called?

A

geothermal gradient

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12
Q

Continental plate extension

geothermal gradient?

A

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

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13
Q

Ancient continental plate

geothermal gradient?

A

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

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14
Q

Isotherm

A

Layer of equal temperature

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15
Q

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

A

it increases

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16
Q

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

A

increased pressure = increased rate

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17
Q

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?

A

they become unstable and recrystallise into new minerals

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18
Q

Geobarometers

A

metamorphic minerals

19
Q

What causes recrystallisation of minerals?

A

when rocks are buried and subjected to greater stress

20
Q

Effect of uniform stress on minerals

A

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

21
Q

Effect of differential stress

A

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

22
Q

Example of foliated metamorphic rocks

A

rock clevage in slate

23
Q

Fluids as metamorphic agents

A

enhance metamorphism by increasing the rate of chemical reactions

24
Q

Process by which fluids change a rock’s composition

A

metasomatism

25
Q

Metasomatism

A

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

26
Q

Fluids

A

liquids and gases

27
Q

Types of metamorphism

3

A

1/ regional
2/ contact
3/ dynamic

28
Q

Regional metamorphism

A
  • Metamorphism which occurs over broad areas of the crust (google)
  • notes: affects huge areas/volumes
29
Q

What is regional metamorphism a feature of?

A

convergent plate boundaries

cores of mountain chains produced by continent-continent collisions

30
Q

What causes regional metamorphism?

3

A
  1. high temperatures,
  2. high pressures,
  3. extreme deformation in deep parts of the crust
31
Q

What happens during regional metamorphism?

A

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

32
Q

How are metamorphic zones characterised?

A

by particular index minerals

33
Q

Isograd

A

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

34
Q

Polymorphs

A

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

35
Q

Metamorphic facies

A

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

36
Q

What do metamorphic facies indicate?

A

the plate tectonic processes responsible for metamorphism

37
Q

Contact metamorphism

A

occurs when rock is intruded by a body of hot magma

38
Q

How does contact metamorphism occur?

A

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

39
Q

Country rock

A

rock native to an area (google)

40
Q

Hornfels

A

typical rock from a contact metamorphic environment

41
Q

Dynamic metamorphisim

A

occurs along fault zones where rocks are subjected to high pressures

42
Q

Dynamic metamorphism causes

A

high rates of strain and temp. in ductile areas

43
Q

Mylonites

A

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

Geology, LEC 172 (6 decks)

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