Lecture 3: Earth’s Core and Geomagnetism

Question 1

When was the Core-Mantle Boundary discovered and by who?

Answer 1

Beno Gutenberg in 1914

Question 2

How was the Core-Mantle Boundary found?

Answer 2

via the interpretation of the shadow zone on a travel time graph of seismic waves

Question 3

What interval can we detect P & S waves from after an Earthquake?

Answer 3

0° and 105°

Question 4

What interval do P & S waves disappear from after an Earthquake?

Answer 4

105° to 142° (shadow zone)

Question 5

When do P waves reappear after an earthquake?

Answer 5

142° to 180°

Question 6

What does P waves reappearing later than expected after an earthquake tell us?

Answer 6

that they’ve slowed down as well as been refracted

Question 7

Do S waves reappear after an Earthquake?

Answer 7

No

Question 8

Why do S waves not reappear after an Earthquake?

Answer 8

Likely because S-waves cannot go into the outer core (because it’s a liquid)

Question 9

Where is the shadow zone in the interior of the Earth?

Answer 9

At the boundary between the solid mantle and liquid outer core

Question 10

What happens at the CMB to P waves and why?

Answer 10

sharply refract vertically downwards due to the vast different in temperature

Question 11

What happens at the CMB to S waves and why?

Answer 11

there is no refraction at CMB due to shear modulus being 0 → S-waves cannot be transmitted through liquid

Question 12

What happens to the speed of P & S waves inside the Earth?

Answer 12

Constantly increasing because temperature is increasing

Question 13

What happens to P-waves in the shadow zone?

Answer 13

They travel slower

Question 14

What is the shadow zone the boundary between?

Answer 14

The core and mantle

Question 15

When was the inner core discovered?

Answer 15

1936

Question 16

How was in the inner core discovered?

Answer 16

by identifying the PKIKP phase (the small amount of energy that appears) in the shadow zone

Question 17

What is the depth of the inner core?

Answer 17

5150km

Question 18

How does P-wave velocities change in the inner core?

Answer 18

rapidly increase

Question 19

What happens to S-waves in the inner core and what does this mean?

Answer 19

reappear transformed from P-waves, so the inner core must be solid

Question 20

How does P-wave speed change from the crust to the mantle?

Answer 20

Increases from 7-8km/s

Question 21

How does P-wave speed change in the mantle?

Answer 21

Increases from 8-13.5km/s

Question 22

How does P-wave speed change in the liquid outer core?

Answer 22

Drops to 8.5km/s (because of its liquid state)

Question 23

How does P-wave speed change in the solid inner core?

Answer 23

speeds up to 11.5 km/s.

Question 24

How does S-wave speed change from the crust to the mantle?

Answer 24

Gradually increases from 4.5km/s to 7.5km/s

Question 25

How does S-wave speed change when it hits the outer core?

Answer 25

Falls to 0 because it is liquid (SM is 0)

Question 26

How does S-wave speed change when it hits the inner core?

Answer 26

after some P waves are transformed into S waves upon reaching the inner core, S wave speed increases to 4 km/s.

Question 27

How does density change from the crust to the mantle?

Answer 27

Gradually increases

Question 28

How does density change upon reaching the outer core (and why)?

Answer 28

Rapidly increases due to its high iron composition

Question 29

How does density change upon reaching the inner core (and why)?

Answer 29

Continues increasing as the state changes to solid

Question 30

How does iron content change from mantle to outer core?

Answer 30

Increases drastically

Question 31

How does oxygen and silicon content change from mantle to outer core?

Answer 31

Drastically decreases

Question 32

How does magnesium content change from crust to mantle?

Answer 32

Increases

Question 33

How does aluminium content change from crust to mantle?

Answer 33

Decreases

Question 34

What is the core mostly composed of?

Answer 34

Iron

Question 35

What is the geochemical evidence for the core being made of iron?

Answer 35

The sun and meteorites contain a lot of iron, so Earth must have an abundance of iron (more than we know is in the crust and mantle)

Question 36

What are the two main types of meteorite (and what are they similar to)

Answer 36

``` Stony meteorites (similar to Earth’s mantle) Iron Meteorites (similar to Earth’s core?) ```

Question 37

What is the two main geophysical evidences for an iron core?

Answer 37

Seismology and Geomagnetism

Question 38

How much faster do Seismic waves travel through the inner core N-S than E-W and why?

Answer 38

4% - Recent experiments/modelling show that Fe atoms might line up along N-S axis as they crystallise increasing rigidity along that axis

Question 39

How is the inner core changing and why?

Answer 39

The inner core is growing slowly as the Earth cools as molten iron in outer core freezes and 'snows' onto surfact of inner core Releases latent heat and iron-depleted liquid

Question 40

What is the geomagnetic evidence for an iron earth core?

Answer 40

Earth has magnetic field generated by convection of electrically conducting liquid in outer core → iron is suitable for this

Question 41

What is the earth's magnetic field similar to (but why is it not the exact same and what does this mean)?

Answer 41

A bar magnet But not exactly as when an object is heated hot enough (like the core) it loses its permanent magnetism → something else must be at play?

Question 42

What degree is the Earth's magnetic field tilted from the earth's rotation axis?

Answer 42

10 degrees

Question 43

Is geographic north magnetic north?

Answer 43

No

Question 44

What is a dip meter?

Answer 44

a compass whose needle can move in vertical plane

Question 45

What is magnetic dip measured relative to?

Answer 45

Horizontal

Question 46

What does magnetic dip vary with?

Answer 46

Magnetic latitude

Question 47

Where is dip 0 degrees?

Answer 47

At the magnetic equator

Question 48

Where is dip 90 degrees?

Answer 48

At magnetic poles

Question 49

What does magnetic field strength vary with?

Answer 49

Latitude

Question 50

Where is magnetic field strength strongest/weakest?

Answer 50

Weakest at magnetic equator | Strongest at magnetic poles

Question 51

Why does the earth's tilt change overtime?

Answer 51

because of the movement of liquid within the core

Question 52

Can the earth's interior produce permanent magnetism?

Answer 52

The Earth’s interior is too hot for permanent magnetism to be possible.

Question 53

How does the earth generate a magnetic field?

Answer 53

The Earth’s interior is too hot for permanent magnetism to be possible. The geomagnetic field is therefore generated by electrically conducting metallic liquid in the outer core

Question 54

What is the earth's magnetic field likely affected by?

Answer 54

The Earth’s inner core and rotation, explaining why it is generally aligned with the Earth’s spin axis

Question 55

What does the earth's magnetic field occasionally do?

Answer 55

Reverse direction by 180 degrees

Question 56

Why does the earth's magnetic field sometimes reverse?

Answer 56

Due to change in pattern of convective fluid flow

Question 57

How long does the magnetic field reversal process take?

Answer 57

Around 4000 years