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Earth structure and tectonics - 33740 > Principles of tectonics > Flashcards

Principles of tectonics Flashcards

1
Q

What are the names of the layers of the earth?

A

Lithosphere <1300c (solid)
Mantle 2000-4000 (solid)
Outer core 4000* (liquid)
Inner core 5000* (solid)

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2
Q

How is earths magnetic field produced?

A

By a magnetic dipole that is roughly parallel to earths spin axis (N-S pole)

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3
Q

Where is earths magnetic field generated?

A

Outer liquid core when rotating round earths spin axis based on geodynamo

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4
Q

What does the geomagnetic field act as?

A

A shield for the earth against dangerous particles projected from the sun (solar wind)

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5
Q

What is the shape of earths geomagnetic field distorted by?

A

The solar winds creating a tail behind earth

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6
Q

What is plate tectonics?

A

Describes the large scale motion of tectonic plates

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7
Q

How many major and minor tectonic plates are there?

A

7 major a d 8 minor

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8
Q

What state are tectonic plates in?

A

Solid and rigid

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9
Q

What are tectonic plates composed of?

A

Crust and upper mantle

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10
Q

What do tectonic plates coincide with?

A

The lithosphere (30-150km thick)

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11
Q

What do tectonic plates “float” on?

A

Thin layer (50-100km) zone of rheid mantle called asthenosphere

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12
Q

What happens at the exterior and interior of tectonic plates?

A

Deformation at exterior where plates interact but interior (CRATON) is undeformed and old

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13
Q

What is the name of the crust mantle boundary?

A

Moho

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14
Q

What is the depth of moho for oceanic crust?

A

~6km

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15
Q

What is the depth of moho for continental crust?

A

30-60km

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16
Q

What is the thickness of oceanic lithosphere like?

A

0-100km with it increasing further form spreading ridge

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17
Q

What is the thickness of continental lithosphere like?

A

100-150km

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18
Q

How do tectonic plates move?

A

High temperature allows solid state flow of mantle and is supported by lubricating zone (asthenosphere)

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19
Q

What is the rate of solid state flow of the mantle?

A

~1 to 5cm/yr

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20
Q

What is the reason for the different elevations of continental and oceanic crust?

A

Continental crust is less dense (more buoyant) so floats more on mantle

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21
Q

What is the density of the mantle?

A

3.3 g/cm*3

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22
Q

What are the characteristics of continental crust?

A

Thickeness (30-70 km)
Granodioritic mean composition (felsic)
Density of 2.7 g/cm3

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23
Q

What are the characteristics of oceanic crust?

A

Thickness (5-6 km)
Basaltic mean composition (mafic)
Density of 3.0 g/cm3

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24
Q

What does the Wilson cycle explain?

A

the reason continents are much older than oceans

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25
Q

What does the Wilson cycle describe?

A

The periodic opening and closing if oceans

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26
Q

When was continental drift proposed and by whom?

A

1915 by Alfred Wegener

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27
Q

What did continental drift propose?

A

All continents were once united into a supercontinent but they drifted apart on gloating ocean beds

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28
Q

What is evidence is there for continental drift?

A

Same fossils found across different continents (mesosaurus)
Continent shape fit together kike jigsaw

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29
Q

How is the mesosaurus evidence of continental drift?

A

Freshwater animal which fossil found in south America and southern Africa different climate at present and separated by saline water

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30
Q

What are alternatives to continental drift?

A

Land bridges
Expanding Earth

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31
Q

What is the land bridges idea?

A

fossil distribution explained by land bridges now sunken in oceans

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32
Q

What was the one problem with continental drift when it was proposed?

A

There was no how/ driving process

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33
Q

When was the Wadati-Benioff zone proposed?

A

1949

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34
Q

What is the Wadati-Benioff zone?

A

Specific location distribution of earthquakes at depth forming a planar surface dipping ~45* below continents to 670km

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35
Q

What depth and angle is the Wadati-benioff zone?

A

670km and 45*

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36
Q

When was the Apparent Polar Wander proposed?

A

1956

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37
Q

Who proposed Apparent Polar Wander?

A

Warren Carey

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38
Q

What does Apparent Polar wander show?

A

Continents have drifted in the past

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39
Q

What happens with magnetism and cooling lava?

A

Magnetite (magnetic mineral) aligns with magnetic field

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40
Q

How is APW (apparent polar wander) shown?

A

Rocks of different ages on a continent show change in position of earths pole

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41
Q

How does |APW show continents moving?

A

As the poles are fixed so continents must have moved

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42
Q

When was seafloor spreading proposed?

A

1960 by hess

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43
Q

What is the idea of seafloor spreading?

A

Oceans form and continuously grow at cracks (ridges) where magma rises, cools and forms new oceanic crust

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44
Q

What period were Marine Magnetic Anomalies (MMA) proposed?

A

1959-63

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45
Q

What are marine magnetic anomalies?

A

Anomalies in strength of the magnetic field which can be observed in the oceans

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46
Q

What is the pattern is how with magnetic fields on the sea floor?

A

Alternating pattern of stronger (normal polarity) and weaker (reverse polarity) parallel to mid-ocean ridges

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47
Q

Who linked seafloor spreading and marine magnetic anomalies?

A

Vine et al explained the mechanism combining Hess’ model with marine magnetic anomalies

48
Q

What happens to the age of rock as you move away form mid-ocean ridges?

A

Rocks get older

49
Q

What is the rate of crust consumed to created like?

A

Equal

50
Q

What are the 3 types of plate boundary?

A

Divergent
Convergent
Transform

51
Q

What happens at divergent boundaries?

A

Crust formed
Normal faulting

52
Q

What happens at convergent boundaries?

A

Crust consumed
Reverse/thrust faults

53
Q

What happens at transform boundaries?

A

Plate slides laterally
Strike-slip

54
Q

What is the structure of oceanic crust? (top to bottom)

A

Sediment
Pillow Basalt
Basalt dikes
Gabbro
Peridotite (upper mantle)

55
Q

How is the structure of oceanic curst known?

A

Geophysics and fragments of oceanic crust on land - OPHIOLITES

56
Q

What is the rate if spread at fast spreading ridges?

A

> 4cm/yr

57
Q

What are the characteristics of fast spreading ridges?

A

Sufficient melt production
Steady-state magma chamber
All 4 layers of oceanic crust are found
Not many faults present

58
Q

What is the rate of spreading at slow spreading ridges?

A

2-4 cm/yr

59
Q

What are the characteristics of slow spreading ridges?

A

Lower melt production
Small magma chamber
All layers present but thinner
Occurrence of normal faults

60
Q

What are slow spreading ridges generally?

A

Rift valleys with intermittently present magma chamber

61
Q

What is the rate of spreading for ultra-slow spreading ridges?

A

<2cm/yr

62
Q

What forms at ultra-slow spreading ridges?

A

Oceanic detachments

63
Q

What are ultra-slow ridges like?

A

Low angle normal faults with curved shape rooted below spreading ridge

64
Q

What is east Africa experiencing as a result of continental rifting?

A

tectonic extension, lithospheric break up and volcanism

65
Q

What happens to ridges with transform boundaries?

A

Will be offset

66
Q

What are fracture zones?

A

lateral continuation of transform faults, but unlike those are inactive

67
Q

How do fracture zones grow?

A

Laterally as long as the ridge remains active

68
Q

What can fracture zones indicate?

A

direction of plates in the past

69
Q

What crust boundaries will subduction occur at?

A

Ocean-Ocean (Tonga)
Ocean-Continent (Andes)

70
Q

What crust boundary will under-thrusting occur at?

A

Continent-Continent (Tibet-Himalaya)

71
Q

What is the slab with subduction?

A

portion of the down-going plate sank into the mantle

72
Q

What is the trench when subducting?

A

Depression where plate bends into subduction zone

73
Q

What is a volcanic arc?

A

100-150 km above slab where fluids are expelled causing overlying mantle to melt

74
Q

What is the forearc?

A

region located between the trench and the volcanic arc

75
Q

What is the accretionary wedge?

A

pile of sediments scraped off (bulldozed) subducting plate

76
Q

What is the back arc?

A

region located behind the volcanic arc where spreading/extension may occur

77
Q

What is slab “roll back”?

A

When the slab sinks faster then convergence between the plates causing the trench to retreat back

78
Q

What might slab roll back lead to?

A

Extension of overlying plate opening a back-arc basin

79
Q

Where doe volcanoes mainly form?

A

At convergent margins (subduction)

80
Q

What is the average distance of a volcanic arc trench?

A

150-200km

81
Q

Where do earthquakes occur?

A

At plate boundaries (most large at subduction zones)

82
Q

What process usually occurs at Continent-Continent boundaries?

A

Orogeny- mountain building

83
Q

What will indicate a diffuse plate boundary?

A

A broad seismicity distribution

84
Q

What are the main drivers of tectonic plates?

A

Slab pull
Ridge push

85
Q

What is slab pull?

A

weight of cool, dense subducting slab pulls plate along

86
Q

What is ridge push?

A

bit like the plate sliding downhill from the high ridge

87
Q

What triggers ridge push?

A

Elevation difference between the ridge and adjacent deeper seafloor

88
Q

What causes slab pull?

A

negative buoyancy of dense slab (vertical down) applied along inclined surface (subduction zone) drag the plate down producing horizontal motion towards trench

89
Q

What is the habitable zone Earth is found in called?

A

Goldilocks zone

90
Q

How can earths activity be described?

A

Alive due to heat/ temperature and processes

91
Q

What metals are in the outer core?

A

Nickle and iron

92
Q

Why does the outer core produce the magnetic field?

A

Motion of boiling material and planet spin generates magnetic field

93
Q

What would happen with no magnetic field?

A

The atmosphere would be blown away

94
Q

What does convection not do?

A

Produce cells which drive spreading of ridges

95
Q

What is a hypsometic curve?

A

% of topography above and below sea level

96
Q

What % area is below and above sea level?

A

65% below
35% above

97
Q

What is the oldest continental crust?

A

Gneiss 4000Ma

98
Q

What is the oldest oceanic crust?

A

270 Ma

99
Q

Why are oceanic rocks younger than continental?

A

As they subduct and are destroyed

100
Q

How often does earths magnetic field switch?

A

300,000 years

101
Q

Is crust created or destroyed at strike slip?

A

Conserved none created or destroyed

102
Q

What is the sediment from an ancient ocean like?

A

Sediment on top layer of ocean veyr defomred?

103
Q

What are sheeted dikes like?

A

Sheeted dikes intruded each other and split one another and are thus parallel to each other
2km thick

104
Q

What might gabbro’s like for oceanic structure?

A

Isotropic or layered around 2.5km

105
Q

What occurs to perodotitie?

A

Serpentinasation

106
Q

Why are normal faults present at constructive margins?

A

gravitational collapse

107
Q

Are there faults at fast spreading ridges?

A

No faults in fast spread as new material just produced

108
Q

Why are there faults at slow spreading ridges?

A

Two plates moving apart so not controlled by ridge upwell this creates faulting

109
Q

What is the rock structure like at ultra-slow spreading ridges?

A

Gabbro predominanlty
Thinner lithosphere

110
Q

What are oceanic detachments like?

A

Oceanic detachment like spoon shaped which dip to spreading axis

111
Q

What might be created as a result of the east African rift valley?

A

A new ocean within a few million years

112
Q

Is a fracture zone a fault?

A

No just a damaged area

113
Q

What happens to crust at a continent-continent boundary?

A

Vast thickening

114
Q

What controls distance of volcanic arc?

A

Inclination of subduction - shallower is further

115
Q

What leads to the creation of volcanoes at subduction?

A

Fluids lower melting point of material producing volcanoes to reduce pressure

Earth structure and tectonics - 33740 (7 decks)

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