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Atoms to volcanoes - 33739 > Structure of Earth and magmatic processes > Flashcards

Structure of Earth and magmatic processes Flashcards

1
Q

What are the steps of earth formation?

A

Planetesimals grow from collisions
Irregular proto-shape earth develops Interior heats becomes soft
Gravity re-shapes proto to sphere
Interior separates to core and mantle

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2
Q

What is the LHB?

A

a spike in asteroid bombardment 4 billion years ago

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3
Q

Who was one of the first to try and age earth?

A

Lord kelvin 1862

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4
Q

How did lord Kelvin try do date earth?

A

based on calculation of how long earth would take to cool if it began as a molten mass

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5
Q

What was lord Kelvins estimate for the age of the earth?

A

20-400 million years old (wrong by significant margin as actual= 4.5 billion)

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6
Q

What is the reason for why lord Kelvin might have been wrong in his ageing of earth?

A

He didn’t know of the existence of radioactive elements which decay and release energy affecting internal heat

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7
Q

What does temperature inside planets getting above the melting point of most minerals mean?

A

separation into more and less dense layers

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8
Q

What are the layers of earth?

A

Crust
Upper mantle
Lower mantle
Outer core
Inner core

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9
Q

How will silica and iron abundance be affected by earth layering?

A

Silica- most abundant at crust decreasing towards core
Iron- least abundant at crust increase with depth

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10
Q

What is chemical differentiation?

A

When different minerals have different compositions and densities allowing physical partitions to occur

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11
Q

How can we tell the interior of earth must be much denser than the crust?

A

Average earth density= 5.5g/cm3
Crust range-2.5-3g/cm3
Interior must be more dense to bring up average

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12
Q

How can seismology and seismic waves be used to probe earths interior?

A

measuring the time required for seismic waves to travel through the earth by different paths

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13
Q

What are the 2 main types of seismic wave?

A

Primary = P
Secondary = S

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14
Q

What type of movement are P waves?

A

Compressional

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15
Q

What is the speed of P waves?

A

Propagate fastest
6.5km/sec

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16
Q

What states of matter can P waves travel through?

A

Solid
Liquid

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17
Q

What type of movement are S waves?

A

Shear

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18
Q

What state of matter can S waves travel through?

A

Solid

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19
Q

What is the speed of S waves in the crust?

A

4km/sec

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20
Q

What can a shadow zone with no S waves show us after a quake? (opposite side to quake)

A

earth must have a liquid core

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21
Q

What is the crust?

A

less dense rigid layer of rock forming earths exterior

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22
Q

What is the base of the crust defined by?

A

Mohorovic discontinuity (Moho)

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23
Q

What happens to seismic velocity and why at the moho?

A

distinct increase due to more dense material

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24
Q

Does the moho remain constant throughout crust?

A

No it is variable

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25
Q

What are the characteristics of oceanic crust?

A

5-10km thick
Denser so faster seismic velocity
Subducts
Basaltic rock

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26
Q

What are the characteristics of continental crust?

A

20-90 km thick (thicker under mountain, thinner under continental shelf)
Not dense enough to subduct
Granitic rock

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27
Q

What can the crust be split into?

A

Oceanic
Continental

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28
Q

What is the upper mantle like?

A

denser than the crust but still silicate rich

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29
Q

What will the topmost part of the upper mantle be like?

A

rigid

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30
Q

What will the topmost part of the upper mantle and the crust form?

A

tectonic plates

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31
Q

What is the top rigid upper mantle called?

A

The lithosphere

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32
Q

How thick is the ocean lithosphere?

A

50-140km

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33
Q

How thick is the continental lithosphere?

A

40-280km

34
Q

What starts to happen when you travel below the lithosphere?

A

Temp and pressure increase allowing some flow

35
Q

Why is drilling into earth to see the layering not possible?

A

Complex and costly with deepest hole being 12km which doesn’t pass into deeper layers

36
Q

What is the ductile region in the upper mantle?

A

the asthenosphere

37
Q

What is the boundary between the lithosphere and asthenosphere?

A

1300*c isotherm

38
Q

What is the remaining upper mantle? (not lithosphere or asthenosphere)

A

mesosphere

39
Q

What happens to material as you go deeper into earth?

A

more dense less viscous

40
Q

What happens to the structure of rock as you travel through the mantle?

A

Changes twice as 410 and 660km resulting in abrupt density change

41
Q

What marks the boundary between the upper and lower mantle?

A

the 660km disconformity

42
Q

How deep does the lower mantle go down till

A

3000km

43
Q

What is at the base of the lower mantle?

A

200km thick heterogenous layer of anomalously low seismicity (D layer)

44
Q

What might the “D layer” at the base of the lower mantle be made of?

A

ancient subducted plates

45
Q

What is the core-mantle boundary a major transition between?

A

silicate mantle changes to the denser iron rich metallic iron core

46
Q

What form will the iron rich metallic alloy be in the outer core?

A

liquid as temperatures are 5000*c

47
Q

Why is the inner core solid even though its so hot?

A

due to the immense pressure exerted on it

48
Q

What is the composition of the crust?

A

Si/Al rich
Na/K/Ca rich

49
Q

What is the composition of the mantle?

A

Fe/Mg rich
Si/Al poor

50
Q

What is the composition of the core?

A

Nearly pure Fe/Ni

51
Q

What % of earths surface is active volcanism confined to?

A

<0.5%

52
Q

How much of active volcanism/magma is ejected at subduction zones?

A

20% - 6km*3/yr

53
Q

How much of active volcanism/magma is ejected at intraplate?

A

10%- 2.5km*3/yr

54
Q

How much of active volcanism/magma is ejected at mid-ocean ridges?

A

70%- 21km*3/yr

55
Q

How much of magma is ejected as lava?

A

10%

56
Q

Where does the other 90% of magma occur in the crust?

A

solidifies in crust as igneous intrusion

57
Q

How does the mantle flow?

A

as it is not solid or liquid it flows by convection

58
Q

What type of rocks is the mantle composed of?

A

Ultrabasic
Ultramafic

59
Q

What will basic magmas solidify into?

A

basaltic lava
gabbroic intrusions

60
Q

What happens to pressure with depth?

A

it increases due to the weight of the overlying material

61
Q

What happens to temperature with depth? (earth structure)

A

Temp increases due to transfer of heat from planet interior to surface with insulating properties of overlying material

62
Q

What are the 2 sources heat in earth?

A

Early accretion and differentiation
Heat from radioactive breakdown of unstable nuclides

63
Q

What proportion of earths heat comes from breakdown unstable nuclides?

A

75-90%

64
Q

How is heat transported through earth?

A

Conduction
Convection

65
Q

What is conduction?

A

transfer of kinetic energy from hotter atoms to adjacent cooler atoms

66
Q

Where does conduction occur in earths structure?

A

the rigid sections (lithosphere)

67
Q

What is convection?

A

movement of ductile material due to density differences caused by thermal or compositional variation

68
Q

Where does convection occur within earths internal structure?

A

outer core
mantle

69
Q

What is the geothermal gradient?

A

variation of temperature with increasing temperature and pressure

70
Q

How does the presence of thinner lithosphere affect convection?

A

allows convection heat transfer to shallower depths resulting in a higher geothermal gradient

71
Q

How is the geothermal gradient useful?

A

it graphically shows how and where the mantle undergoes partial melting to form magma

72
Q

What is solidus?

A

point at which rock begins to melt

73
Q

What is liquidus?

A

where the rock becomes 100% melted

74
Q

What happens when the geothermal gradient crosses solidus?

A

you get partial melting

75
Q

How does the geotherm cross the solidus?

A

changing the position of the geotherm (thinning lithosphere)
Changing the position of the solidus

76
Q

How does thinning lithosphere move the geotherm?

A

convecting asthenospheric mantle rises adiabatically to fill the gap

77
Q

What does adiabatically mean?

A

without losing heat

78
Q

How can the solidus location be moved?

A

addition of volatiles (water) lower solidus (and liquidus) to promote partial melting

79
Q

How do volatiles like water end up in the mantle?

A

subducting oceanic crust which is saturated with water

80
Q
A

Atoms to volcanoes - 33739 (16 decks)

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