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BIOS1013 - Environmental Geoscience > Lecture 6 - Metamorphism and Structural Geology > Flashcards

Lecture 6 - Metamorphism and Structural Geology Flashcards

1
Q

What is metamorphism?

A

Rocks changing in response to changes in temperature and/or pressure.

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2
Q

What does the type of metamorphic rock produced depend on?

A

The chemical composition.

Temperature

Pressure

Stress.

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3
Q

How does chemical composition change the type of metamorphic rock produced?

A

The composition of the original rock controls the mineralogy of the metamorphosed rock.

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4
Q

What does the number of minerals in a metamorphic rock depend on?

A

The chemical composition of the parent material and the temperature and pressure of the metamorphism.

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5
Q

How can rocks be heated to form a metamorphic rock?

A

By burial or by nearby igneous intrusion.

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6
Q

What temperature does metamorphism occur at?

A

It starts at 200°C and ends when the rock melts at around 650 - 800°C.

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7
Q

Why does metamorphism end when the rock melts?

A

When the rock cools down an igneous rock forms.

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8
Q

What rocks does metamorphism primarily act on?

A

Sedimentary rocks.

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9
Q

What happens with increasing temperature during metamorphism?

A

Water is expelled from hydrous minerals (clay turns into mica which turns in amphiboles).

Carbon dioxide is expelled from carbonates.

Minerals become coarser.

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10
Q

What happens with increasing pressure during metamorphism?

A

Denser minerals are produced.

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11
Q

What are the two types of metamorphism?

A

Low-grade metamorphism.

High-grade metamorphism.

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12
Q

What are the conditions required for low grade metamorphism?

A

A temperature of 200°C to 500°C.

Relatively low pressures.

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13
Q

What are the conditions required for high-grade metamorphism?

A

A temperature of over 500°C.

Relatively high pressures.

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14
Q

What is differential stress?

A

Where the pressure is different in different directions.

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15
Q

What does differential stress result in?

A

The rock developing a planar fabric known as a foliation.

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16
Q

What rock texture is formed during low-grade metamorphism?

A

Foliation called slaty cleavage.

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17
Q

What rock texture is formed during high-grade metamorphism?

A

The grain size increases and the foliation formed is called a schistosity.

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18
Q

What happens to the minerals as the grade of metamorphism changes?

A

New minerals form and the percentage of new minerals increases.

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19
Q

What are three types of low-grade metamorphism?

A

Cataclastic metamorphism.

Burial metamorphism.

Contact metamorphism.

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20
Q

What is cataclastic metamorphism?

A

Mechanical deformation of the rock occurs with minor chemical recrystallisation.

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21
Q

What is burial metamorphism?

A

Increase in temperature with depth but only a small increase in pressure. There is only a minor chemical recrystallisation.

This is similar to the sedimentation process but this has a higher energy.

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22
Q

What is contact metamorphism?

A

Occurs when hot magma intrudes into cool rocks.

Mechanical deformation is minor or absent.

The rock adjacent to the intrusion becomes heated which develops of a metamorphic aureole.

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23
Q

What is a hornfels?

A

Contact metamorphic rock.

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24
Q

What is the maximum thickness of a metamorphic aureole?

A

100 metres.

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25
Q

What happens with distance from the intrusion during contact metamorphism?

A

Less heat is transferred further away, so the level of metamorphism decreases.

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26
Q

What caused the Permian extinction?

A

Huge areas of rock intrusions occurred which resulted in contact metamorphism of the rocks.

This caused huge quantities of haloalkanes to be released which destroyed the ozone layer.

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27
Q

What is a type of high-grade metamorphism?

A

Regional metamorphism.

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28
Q

What are the features of regional metamorphism?

A

It results from mountain building.

There is a pronounced differential stress which produces foliations in addition to new minerals.

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29
Q

What is a metamorphic facies?

A

A set of mineral assemblages in metamorphic rocks that are formed under similar pressures and temperatures.

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30
Q

What metamorphic facies are formed where there is high temperature but low pressure?

A

They occur in the vicinity of igneous intrusions.

This forms hornfels with turn into granulite.

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31
Q

What metamorphic facies are formed where there is high temperature and high pressure?

A

This is typical of most regional metamorphic rocks.

Zeolite forms into greenschist into amphibolite facies.

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32
Q

What metamorphic facies are formed where there is low temperature and high pressure?

A

This occurs in subduction zones undergoing metamorphism.

Zeolite turns into blueschist into eclogite facies.

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33
Q

What is the order of progression for shale and mudstone rocks undergoing metamorphism?

A

Slate

Phyllite

Schist

Gneiss

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34
Q

What are the components of slate?

A

Fine grained clays, muscovite and chlorite.

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35
Q

What are the components of phyllite?

A

Muscovite, chlorite and phyllites.

They have a characteristic sheen due to slightly coarser grains.

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36
Q

What are the components of schist?

A

Muscovite, biotite and other new minerals such as garnet and kyanite.

It is coarser than phyllite.

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37
Q

What are the components of gneiss?

A

Biotite and muscovite grains > 1 mm.

New minerals including garnet and sillimanite are present.

38
Q

What does melting of shale and mudstone rocks form?

A

Granite or migamatite.

39
Q

What is psammite?

A

A metamorphosed sandy sediment.

40
Q

What is quartzite?

A

A metamorphosed pure quartz sandstone.

41
Q

What is marble?

A

A metamorphosed limestone.

42
Q

Which igneous rocks change during metamorphism?

A

The igneous rocks lower in silica like basalt and gabbro.

43
Q

What is a metabasite?

A

A metamorphosed basalt.

44
Q

What is a metagabbro?

A

A metamorphosed gabbro.

45
Q

What are the features of metamorphosed igneous rocks?

A

They are composed mainly of amphiboles.

They are usually dark coloured such as black or greenish.

The changes are caused by the addition of water.

46
Q

What is the origin of rock deformation?

A

Tectonic forces continuously squeeze, stretch, bend, and break rock in the lithosphere.

The source of energy is the Earth’s heat, which is transformed into mechanical energy.

47
Q

What is stress?

A

An applied force or pressure.

48
Q

What is uniform stress?

A

A condition in which the stress is equal in all directions.

49
Q

What is differential stress?

A

Differential stress is stress that is not equal in all directions.

50
Q

What are three types of stress?

A

Tensional stress

Compressional stress

Shear stress

51
Q

What is tensional stress?

A

The rock is pulled apart which makes it elongate.

52
Q

What is compressional stress?

A

The rock is squeezed preferentially in a direction.

53
Q

What is shear stress?

A

Stress is not applied in equal directions throughout the rock.

54
Q

What is strain?

A

The deformation of a rock.

It is a measure of the change in length, shape or volume when a stress is applied.

55
Q

What are the three stages of deformation?

A

Elastic deformation.

Ductile deformation.

Fracture.

56
Q

What is elastic deformation?

A

Strain that is reversible.

The rock will return to its original shape if the stress is released.

57
Q

What is ductile deformation?

A

The strain is irreversible.

Rock that has undergone this deformation will remain deformed even if the stress is released.

58
Q

What is fracture?

A

The strain exceeds both limits of elastic and ductile deformation and the rock breaks.

59
Q

What is brittle deformation?

A

There is very little ductile deformation before the rock fractures.

60
Q

What is ductile deformation favoured by?

A

High temperatures.

Low strain rates.

High confining pressure.

61
Q

What is brittle deformation favoured by?

A

Low temperatures.

High strain rates.

Low confining pressure.

62
Q

What causes a rock to fracture?

A

The stress exceeds the strength of the bonds between the atoms.

The atoms either have to move to another place in the crystal lattice in order to relieve the stress or the bonds must break which causes a fracture.

63
Q

What evidence is there for deformation?

A

The Law of Original Horizontality means that the rocks were laid horizontally. If these rocks are tilted, deformation must have occurred.

64
Q

What is a fault?

A

A fracture along which visible displacement has occurred.

65
Q

What is a joint?

A

A fracture on which no observable movement has occurred.

66
Q

How are faults classified?

A

By the dip of the fault and the direction of relative movement.

67
Q

What is the foot-wall?

A

The side of the fault that is wider at the bottom than the top.

68
Q

What is the hanging-wall?

A

The side of the fault that is wider at the top than the bottom.

69
Q

What is a normal fault?

A

Vertical movement at the fault where the hanging-wall block moves downwards.

This is usually associated with tensional stress.

70
Q

What is a reverse fault?

A

Vertical movement at the fault where the hanging-wall block moves upwards.

This is associated with compressional stress.

71
Q

What is a strike-slip fault?

A

Horizontal movement around the joint caused by shear stress.

72
Q

How can a strike-slip fault be observed?

A

Surface landforms such as rivers may no longer be connected.

73
Q

What is an oblique-slip fault?

A

A fault where there is both horizontal and vertical movement caused by tensional and shear stress.

74
Q

What is a thrust fault?

A

Thrust faults are low-angle reverse faults which have a dip of less than 15°.

These faults are common in mountain chains.

75
Q

What happens during a thrust fault?

A

Older rock is thrust upon younger rock.

76
Q

What two features show evidence of movement?

A

Slickensides

Fault Breccia

77
Q

What is a slickenside?

A

A smoothly polished surface caused by frictional movement between rocks along a fault.

The surface is usually striated with slickenlines in the direction of movement.

78
Q

What is a fault breccia?

A

A tectonite formed by a localised zone of brittle deformation of rock.

79
Q

What are features of ductile deformation?

A

Folds.

80
Q

What is dip?

A

The angle in degrees between a horizontal plane and the inclined plane.

It is measured from the horizontal.

81
Q

What is strike?

A

The compass direction of the horizontal line formed by the intersection of a horizontal plane and an inclined plane.

82
Q

What is the anticline?

A

The arch structure in a fold.

83
Q

What is the syncline?

A

The trough structure in a fold.

84
Q

What is an open (symmetrical) fold?

A

Both limbs dip equally away from the axial plane.

This forms a symmetrical wave pattern.

85
Q

What is an isoclinal fold?

A

The limbs of the fold are parallel to each other.

86
Q

What is an asymmetrical fold?

A

One limb of the fold dips more steeply than the other which causes a slanted axial plane.

87
Q

What is an overturned fold?

A

Strata in one limb have been tilted beyond the vertical. Both limbs dip in the same direction but not at the same angle.

88
Q

What is a recumbent fold?

A

Axial planes are horizontal or nearly horizontal. Strata on the lower limb of anticline and upper limb of syncline are upside down.

89
Q

As the grade of metamorphism increases, what happens to the rock?

A

There is increased foliation.

Grain size increases.

90
Q

Why would gneiss make the best road stone?

A

The crystals are interlocking and so the rock has a greater strength.

91
Q

What happens to the minerals in a rock that has undergone contact metamorphism?

A

They have no particular alignment due to the lack of pressure.

BIOS1013 - Environmental Geoscience (8 decks)

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