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LEC.171 > Lecture 3: Earth’s Core and Geomagnetism > Flashcards

Lecture 3: Earth’s Core and Geomagnetism Flashcards

1
Q

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

A

Beno Gutenberg in 1914

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2
Q

How was the Core-Mantle Boundary found?

A

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

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3
Q

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

A

0° and 105°

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4
Q

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

A

105° to 142° (shadow zone)

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5
Q

When do P waves reappear after an earthquake?

A

142° to 180°

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6
Q

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

A

that they’ve slowed down as well as been refracted

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7
Q

Do S waves reappear after an Earthquake?

A

No

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8
Q

Why do S waves not reappear after an Earthquake?

A

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

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9
Q

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

A

At the boundary between the solid mantle and liquid outer core

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10
Q

What happens at the CMB to P waves and why?

A

sharply refract vertically downwards due to the vast different in temperature

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11
Q

What happens at the CMB to S waves and why?

A

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

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12
Q

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

A

Constantly increasing because temperature is increasing

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13
Q

What happens to P-waves in the shadow zone?

A

They travel slower

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14
Q

What is the shadow zone the boundary between?

A

The core and mantle

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15
Q

When was the inner core discovered?

A

1936

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16
Q

How was in the inner core discovered?

A

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

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17
Q

What is the depth of the inner core?

A

5150km

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18
Q

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

A

rapidly increase

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19
Q

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

A

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

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20
Q

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

A

Increases from 7-8km/s

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21
Q

How does P-wave speed change in the mantle?

A

Increases from 8-13.5km/s

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22
Q

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

A

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

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23
Q

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

A

speeds up to 11.5 km/s.

24
Q

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

A

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

25
Q

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

A

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

26
Q

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

A

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

27
Q

How does density change from the crust to the mantle?

A

Gradually increases

28
Q

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

A

Rapidly increases due to its high iron composition

29
Q

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

A

Continues increasing as the state changes to solid

30
Q

How does iron content change from mantle to outer core?

A

Increases drastically

31
Q

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

A

Drastically decreases

32
Q

How does magnesium content change from crust to mantle?

A

Increases

33
Q

How does aluminium content change from crust to mantle?

A

Decreases

34
Q

What is the core mostly composed of?

A

Iron

35
Q

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

A

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)

36
Q

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

A 
Stony meteorites (similar to Earth’s mantle)
Iron Meteorites (similar to Earth’s core?)
37
Q

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

A

Seismology and Geomagnetism

38
Q

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

A

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

39
Q

How is the inner core changing and why?

A

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

40
Q

What is the geomagnetic evidence for an iron earth core?

A

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

41
Q

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

A

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?

42
Q

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

A

10 degrees

43
Q

Is geographic north magnetic north?

A

No

44
Q

What is a dip meter?

A

a compass whose needle can move in vertical plane

45
Q

What is magnetic dip measured relative to?

A

Horizontal

46
Q

What does magnetic dip vary with?

A

Magnetic latitude

47
Q

Where is dip 0 degrees?

A

At the magnetic equator

48
Q

Where is dip 90 degrees?

A

At magnetic poles

49
Q

What does magnetic field strength vary with?

A

Latitude

50
Q

Where is magnetic field strength strongest/weakest?

A

Weakest at magnetic equator

Strongest at magnetic poles

51
Q

Why does the earth’s tilt change overtime?

A

because of the movement of liquid within the core

52
Q

Can the earth’s interior produce permanent magnetism?

A

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

53
Q

How does the earth generate a magnetic field?

A

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

54
Q

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

A

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

55
Q

What does the earth’s magnetic field occasionally do?

A

Reverse direction by 180 degrees

56
Q

Why does the earth’s magnetic field sometimes reverse?

A

Due to change in pattern of convective fluid flow

57
Q

How long does the magnetic field reversal process take?

A

Around 4000 years

LEC.171 (8 decks)

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