Geomagnetic Field

Question 1

What does the geomagnetic field look like? (draw diagram as well)

Answer 1

To first order, the geomagnetic field looks like a dipole. (draw diagram with correct arrows)

Question 2

What is the axial pole approximation?

Answer 2

To first order, the geomagnetic field looks like a dipole, so to first order, a reasonable fit with the Earth’s magnetic field can be made by a magnetic dipole approximation.

Question 3

Where is the best fitting dipole aligned?

Answer 3

aligned ~11.5° with respect to the Earth’s spin axis (the geographic north-south axis)

Question 4

What is an inclination angle? DRAW IT

Answer 4

I = inclination angle = the angle between the magnetic field and the horizontal (See notes for picture)

Question 5

What is the declination angle? DRAW IT

Answer 5

The angle between North and H (magnetic north) DRAW IT

Question 6

What is the relation between magnetic latitude and inclination angle?

Answer 6

tan(I) = 2tanλ, where λ = magnetic latitude, I=inclination angle

Question 7

HOW TO CALCULATE PALEOALTITUDE

Question 8

What are the primary causes of magnetization in rocks?

Answer 8

At the atomic level, materials have a net magnetic moment which is due to a combination of the following three factors:

1) rotation of electrons in various shells around the nucleus
2) the spin of electrons
3) the number of electrons in each orbital shell

Question 9

What are the three main types of magnetization?

Answer 9

1) Diamagnetism
2) Paramagnetism
3) Ferromagnetism

Question 10

Define diamagnetism:

Answer 10

With diamagnetic materials, all electron shells are full so any magnetization from electron spin is cancelled out because the spin of any electron is cancelled out by its pair with opposite spin

Question 11

What contributes to magnetization of a diamagnetic material?

Answer 11

The only contribution to magnetization of diamagnetic materials is from e-‘s orbiting the nucleus.

In this case, we may have an e- orbiting a nucleus which acts like a dipole with magnetic moment m.

Question 12

What are the relative directions of the magnetic field and the magnetic moment (m) for a diamagnetic material?

Answer 12

Point in opposite directions

Question 13

What is the magnetic susceptibility of something diamagnetic? why?

Answer 13

very low, mutual cancellation of magnetization

Question 14

Define paramagnetism:

Answer 14

Happens when molecules contain an odd number of electrons, so total cancellation of spin moments in pairs is impossible.

Question 15

What is each paramagnetic atom equipped with?

Answer 15

equipped with only freely swiveling magnetic moment, m, which would align itself with the magnetic field.

Question 16

What is the relative directions of the magnetic field and m in a paramagnetic material?

Answer 16

Induced magnetization in the same direction as the external field

Question 17

How magnetic is something that is paramagnetic?

Answer 17

weakly magnetic material, but induced magnetization is in same direction

Question 18

Define ferromagnetism:

Answer 18

Have unpaired electrons, similar to paramagnetism, but, all of the spins tend to line up

Question 19

Why do all spins tend to line up with ferromagnetism?

Answer 19

(0quantum mechanical effects - in Fe, it is energetically favorable for the spins of adjacent Fe atoms to be parallel

Question 20

What happens if there is an external magnetic field with a ferromagnetic material?

Answer 20

All of the spins want to line up with that field

Question 21

How magnetic is something that is ferromagnetic?

Answer 21

Very stong

Question 22

What are the relative directions of the induced magnetization and external magnetic field for something that is ferromagnetic?

Answer 22

same direction

Question 23

What is a curie temperature?

Answer 23

The curie temperature is the cutoff temperature between paramagnetic and ferromagnetic behavior (in minerals that display ferromagnetism)

Question 24

What happens at temperatures greater than the curie temperature?

Answer 24

At temperatures greater than the Curie temperature, the thermal effects are the strongest and materials only show paramagnetic behavior

Question 25

What happens at temperatures lower than the Curie temperature?

Answer 25

Below the curie temperature, the mutual interactions that give rise to ferromagnetism take over (in minerals that display ferromagnetism)

Question 26

What is the curie temperature for magnetite?

Answer 26

580°C

Question 27

What is the curie temperature for hematite?

Answer 27

680°C

Question 28

What is the blocking temperature?

Answer 28

The temperature at some 10’s of degrees cooler than the curie temperature

Question 29

How do rocks acquire permanent magnetization?

Answer 29

Through a process called Thermoremanent magnetization

Question 30

What is thermoremanent magnetization?

Answer 30

At some 10’s of degrees cooler (the blocking temperature) than the Curie temperature reorientation of spin is impossible and crystals remain with their magnetic moments parallel to Earth’s magnetic field.

Question 31

What is a virtual paleomagnetic pole?

Answer 31

Where the location of the magnetic pole for this rock would have been (assuming a dipole field)

Question 32

What is apparent polar wander?

Answer 32

Looking at older and older rocks, from the same continent, the VGP positions tend to get further and further away from the geographic pole. But the continents moved, not poles

Question 33

TIME TO LEARN THE PLATES - List them here

Answer 33

Eurasian
Australian
Philippine
Juan de Fuca
Pacific
Cocos
North American
Nazca
Antarctic
Caribbean
South American
Scotia
Arabian
African
Indian

Question 34

Now, list the plates in alphabetical order

Answer 34

African
Antarctic
Arabian
Australian
Caribbean
Cocos
Eurasian
Indian
Juan De Fuca
Nazca
North American
Pacific
Philippine
Scotia
​South American

Question 35

Fill in the plates, yeah!

Question 36

First assumption of plate tectonics:

Answer 36

1) The generation of new plate material occurs by sea-floor spreading.

That is, new oceanic lithosphere is generated along the active mid-ocean ridges.

Question 37

Second assumption of plate tectonics:

Answer 37

The new ocean lithosphere, once created, forms part of a rigid plate.

this plate may or may not include continental material

Question 38

Third assumption of plate tectonics:

Answer 38

The earth’s surface area remains constant; therefore sea-floor spreading must be balanced by consumption of plate elsewhere.

Question 39

Fourth assumption of plate tectonics:

Answer 39

The lithospheric plates are capable of transmitting stresses over great horizontal distances without buckling

i.e., the relative motion between plates is taken up only along plate boundaries

Question 40

What theorem do you need to describe motions on the surface of a sphere?

Answer 40

Euler’s fixed point theorem

Question 41

Euler’s fixed point theorem:

Answer 41

Every displacement from one position to another on the surface of the Earth can be regarded as a rotation abut a suitably chosen axis passing through the center of the Earth.

Question 42

Equation when talking about plate motion on a sphere?

Answer 42

v=Rwsin(theta)

Question 43

What does the sin(theta) factor imply about the relative motion between plates on a sphere?

Answer 43

The sin(theta) factor implies that the relative motion between two adjacent plates changes with position along the plate boundary! This is the primary distinction between motion of plates on a sphere and in a flat earth