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LEC.171 > Lecture 7: Cooling the Earth > Flashcards

Lecture 7: Cooling the Earth Flashcards

1
Q

What physical state was Earth in when it formed?

A

Molten

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2
Q

What generated heat when the Earth formed?

A

accretion
differentiation
radioactive decay of short-lived isotopes

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3
Q

What has happened to the short-lived isotopes present when Earth was created?

A

They’ve all disappeared now

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4
Q

How did radioactive decay create heat?

A

emits particles whose kinetic energy of is then transformed into heat energy

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5
Q

What is proof that the Earth’s interior is still hot?

A

volcanoes, hot springs, high temperatures in mine shafts, etc.

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6
Q

How much does temperature increase by per kilometre descended near the Earth’s surface

A

30 degrees

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7
Q

How is the temperature of the planet changing and why?

A

Slowly cooling because of the flow of heat from the interior to the surface

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8
Q

Is rate of heat flow constant across the Earth’s surface?

A

No

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9
Q

What is the total heat loss from the Earth’s interior?

A

10^21 J y^-1

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10
Q

How much smaller is the heat energy lost from the Earth’s interior than heat energy received from the sun?

A

5000 times - although the sun only heats the surface of the crust

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11
Q

Where does the energy that drives the Earth’s processes come from?

A

The sun

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12
Q

Where does the energy that drives the Earth’s internal processes and plate tectonics come from?

A

energy from within the Earth

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13
Q

What are the four main types of isotopes on Earth?

A

Uranium 238
Uranium 235
Thorium 232
Potassium 40

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14
Q

What is the half life of uranium 238

A

4.5 Ga - roughly the same age as the Earth

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15
Q

What is the half life of uranium 232

A

0.71 Ga

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16
Q

What is the half life of Thorium 232

A

13.9 Ga

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17
Q

What is the half life of Potassium 40

A

1.3 Ga

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18
Q

What type of mineral is Uranium?

A

A trace mineral

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19
Q

What is the concentration of uranium on the typical continental crust?

A

4 ppm

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20
Q

What is the concentration of uranium in the mantle peridotite

A

0.01 ppm

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21
Q

How can heat flow/be transferred?

A

Conduction
Convection
Radiation

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22
Q

For what depth range is convection the main heat flow mechanism?

A

100 to 6370 km

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23
Q

What is convection?

A

Hot material rising buoyantly

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24
Q

What is the main heat flow mechanism at less than 100km depth?

A

Mainly convection - the lithosphere it to rigid to convect

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25
Q

How is heat conducted by conduction?

A

Heat energy makes atom vibrate

Heat conducted by atoms hitting each others

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26
Q

What does the intensity of vibrations determine in conduction?

A

Temperature

27
Q

What happens when atoms vibrate in conduction?

A

Transfers vibrational energy from hotter to colder region

28
Q

How good are rocks as conductors?

A

Very poor

29
Q 
Continental crust is - 
Thicker or thinner
More complex in structure and composition or simpler (list types) 
Older or younger (give age)
than oceanic crust
A

Thicker
More complex in structure and composition (igneous, metamorphic and sedimentary rocks)
Older (Up to approx 4 Ga old)

30
Q

Why does continental crust act as an insulating layer?

A

Because it is thick, old and cold

31
Q

What does continental crust do to heat flow?

A

Reduces heat flow from the Earth’s deep interior

32
Q

What proportion of continental heat is generated in the Continental crust

A

Approx 2/3

33
Q

Why does the continental crust generate so much energy?

A

Contains high concentrations of long-lived radioactive isotopes

34
Q

`What does heat flow on the continents correlate with?

A

Regional geology

35
Q

What is heat flow like on areas of recent rifting/volcanism?

A

q>80 mW per metre squared

36
Q

What is heat flow like on areas of average continental crust?

A

q approx 65 mW per metre squared

37
Q

What is heat flow like on areas of old stable cratonic regions?

A

q approx 40 mW per metre squared

38
Q

What does heat flow correlate with in the ocean basins?

A

age of oceanic crust

39
Q

What is heat flow like on oceanic areas near MORs

A

q approx 100-300 mW per metre squared

40
Q

What is heat flow like on oceanic areas of intermediate age?

A

q approx 60 mW per metre squared

41
Q

What is heat flow like on oceanic areas of oldest seafloor?

A

q approx 30 mW per metre squared

42
Q

Where is heat flow highest in oceans?

A

Over ridges (hot lava erupted)

43
Q

Where is heat flow lowest in oceans?

A

At trenches (subduction carries cold rock into the mantle)

44
Q

Does heat flow still vary over different ridges?

A

Heat flow is higher at fast-spreading ridges such as the East Pacific Rise, than slow ones such as the Mid-Atlantic Ridge

45
Q

What % of oceanic crustal heat flow originates from within the continental crust?

A

7%

46
Q

Which typically has higher heat flow, oceans or continents?

A

Oceans

47
Q

When was hydrothermal circulation discovered?

A

1977

48
Q

What happens with hydrothermal circulation?

A

Hot water and minerals expelled at hydrothermal vents

49
Q

What % of the planet’s total surface heat flow does hydrothermal circulation account for?

A

25%

50
Q

Why must the mantle convect?

A

because conduction is too inefficient to have cooled the mantle sufficiently to be solid

51
Q

What speed does the mantle flow?

A

very slowly (like a very very viscous fluid) by solid state creep.

52
Q

What is the temperature at the base of the continental crust?

A

1000 degrees

53
Q

What is the temperature at the core-mantle boundary?

A

3500 degrees

54
Q

What is the temperature at the centre of the Earth?

A

5000 degrees

55
Q

How do we detect temperature variations in the mantle?

A

Seismic topography

56
Q

Why do subducted slabs show up as cold on tomography images?

A

Subducting oceanic lithosphere stays colder than the surrounding mantle for >200 Ma because its thermal conductivity is so low

57
Q

What is the average velocities range for mantle movement?

A

1.5 to 9.5 cm/y.

58
Q

How does slab pull work?

A

As subducting plates sink into the mantle due to negative buoyancy, they exert a gravitational pull on the plate at the surface.
Subducting lithosphere represents the downwelling process of mantle convection
The subducting plates provide the main driving force for plate tectonics.

59
Q

How does continental drag work?

A

Thick roots (keels) of continental lithosphere may act as brake to horizontal plate motions.

60
Q

What is the main driving force of the plates?

A

Subduction

61
Q

What is plate motion moderated by?

A

Continental keels and collisions

62
Q

For plate tectonics to work what must happen?

A

Descended lithospheric material must be recycled through the mantle and return to the crust as new lithosphere

63
Q

How does the Earth cool?

A

Transport of heat by conduction

Transport of heat by convection (more efficient)

LEC.171 (8 decks)

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