Lecture 7: Cooling the Earth

Question 1

What physical state was Earth in when it formed?

Answer 1

Molten

Question 2

What generated heat when the Earth formed?

Answer 2

accretion
differentiation
radioactive decay of short-lived isotopes

Question 3

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

Answer 3

They’ve all disappeared now

Question 4

How did radioactive decay create heat?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

30 degrees

Question 7

How is the temperature of the planet changing and why?

Answer 7

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

Question 8

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

Answer 8

No

Question 9

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

Answer 9

10^21 J y^-1

Question 10

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

Answer 10

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

Question 11

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

Answer 11

The sun

Question 12

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

Answer 12

energy from within the Earth

Question 13

What are the four main types of isotopes on Earth?

Answer 13

Uranium 238
Uranium 235
Thorium 232
Potassium 40

Question 14

What is the half life of uranium 238

Answer 14

4.5 Ga - roughly the same age as the Earth

Question 15

What is the half life of uranium 232

Answer 15

0.71 Ga

Question 16

What is the half life of Thorium 232

Answer 16

13.9 Ga

Question 17

What is the half life of Potassium 40

Answer 17

1.3 Ga

Question 18

What type of mineral is Uranium?

Answer 18

A trace mineral

Question 19

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

Answer 19

4 ppm

Question 20

What is the concentration of uranium in the mantle peridotite

Answer 20

0.01 ppm

Question 21

How can heat flow/be transferred?

Answer 21

Conduction
Convection
Radiation

Question 22

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

Answer 22

100 to 6370 km

Question 23

What is convection?

Answer 23

Hot material rising buoyantly

Question 24

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

Answer 24

Mainly convection - the lithosphere it to rigid to convect

Question 25

How is heat conducted by conduction?

Answer 25

Heat energy makes atom vibrate | Heat conducted by atoms hitting each others

Question 26

What does the intensity of vibrations determine in conduction?

Answer 26

Temperature

Question 27

What happens when atoms vibrate in conduction?

Answer 27

Transfers vibrational energy from hotter to colder region

Question 28

How good are rocks as conductors?

Answer 28

Very poor

Question 29

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

Answer 29

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

Question 30

Why does continental crust act as an insulating layer?

Answer 30

Because it is thick, old and cold

Question 31

What does continental crust do to heat flow?

Answer 31

Reduces heat flow from the Earth's deep interior

Question 32

What proportion of continental heat is generated in the Continental crust

Answer 32

Approx 2/3

Question 33

Why does the continental crust generate so much energy?

Answer 33

Contains high concentrations of long-lived radioactive isotopes

Question 34

`What does heat flow on the continents correlate with?

Answer 34

Regional geology

Question 35

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

Answer 35

q>80 mW per metre squared

Question 36

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

Answer 36

q approx 65 mW per metre squared

Question 37

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

Answer 37

q approx 40 mW per metre squared

Question 38

What does heat flow correlate with in the ocean basins?

Answer 38

age of oceanic crust

Question 39

What is heat flow like on oceanic areas near MORs

Answer 39

q approx 100-300 mW per metre squared

Question 40

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

Answer 40

q approx 60 mW per metre squared

Question 41

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

Answer 41

q approx 30 mW per metre squared

Question 42

Where is heat flow highest in oceans?

Answer 42

Over ridges (hot lava erupted)

Question 43

Where is heat flow lowest in oceans?

Answer 43

At trenches (subduction carries cold rock into the mantle)

Question 44

Does heat flow still vary over different ridges?

Answer 44

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

Question 45

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

Answer 45

7%

Question 46

Which typically has higher heat flow, oceans or continents?

Answer 46

Oceans

Question 47

When was hydrothermal circulation discovered?

Answer 47

1977

Question 48

What happens with hydrothermal circulation?

Answer 48

Hot water and minerals expelled at hydrothermal vents

Question 49

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

Answer 49

25%

Question 50

Why must the mantle convect?

Answer 50

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

Question 51

What speed does the mantle flow?

Answer 51

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

Question 52

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

Answer 52

1000 degrees

Question 53

What is the temperature at the core-mantle boundary?

Answer 53

3500 degrees

Question 54

What is the temperature at the centre of the Earth?

Answer 54

5000 degrees

Question 55

How do we detect temperature variations in the mantle?

Answer 55

Seismic topography

Question 56

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

Answer 56

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

Question 57

What is the average velocities range for mantle movement?

Answer 57

1.5 to 9.5 cm/y.

Question 58

How does slab pull work?

Answer 58

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.

Question 59

How does continental drag work?

Answer 59

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

Question 60

What is the main driving force of the plates?

Answer 60

Subduction

Question 61

What is plate motion moderated by?

Answer 61

Continental keels and collisions

Question 62

For plate tectonics to work what must happen?

Answer 62

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

Question 63

How does the Earth cool?

Answer 63

Transport of heat by conduction | Transport of heat by convection (more efficient)