# Geomagnetism Flashcards

1
Q

What is the equation for the magnetic potential?

A

V(r) = m*r/4πr^3, where m is the magnetic dipole moment from S->N

2
Q

What is the equation for the magnetic field?

A

-μ0*∇V(r), where ∇ is in spherical polar coordinates

3
Q

What are the equations for the 3 components of spherical polar coordinates for the magnetic field?

A
```B(r) = 2*μ0*m*cosθ/4πr^3
B(θ) = μ0*m*sinθ/4πr^3
B(Ф) = 0```
4
Q

What are meridians?

A

Arcs of constant Ф in spherical polar coordinates

5
Q

What is geomagnetic field strength measured with?

A

A magnetometer.

6
Q

What does the magnetic field strength vary between at earths surface?

A

25 μT to 65 μT

7
Q

What is the geomagnetic latitude?

A

Latitude analogues to geographical latitude, but defined relative to the magnetic poles.

8
Q

What is the declination angle for geomagnetism?

A

Angle between the magnetic and geographical meridians.

9
Q

What is the inclination angle for geomagnetism?

A

Angle between magnetic field and a horizontal on the surface.

10
Q

What are 2 equations for the inclination angle?

A

tan(I) = B(r)/B(θ) = 2*cos(θ)/sin(θ)

tan(I) = 2sin(λ)/cos(λ) = 2tan(λ), where λ = 90 - θ

11
Q

What are 3 important types of magnetism? How do they differ?

A
• Ferromagnetism (All spins aligned, generating strong B-field up to curie temp)
• Ferrimagnetism (spins on multiple sublattices of a crysta, some spins antialigned)
• Antiferromagnet (spins are all antialigned, so no net B-field)
12
Q

What are the locations of the magnetic poles right now?

A

North: 193.710 degrees W, 86.395 degrees N

South: 136.419 degrees E, 64.245 degrees S

13
Q

What type of magnet is the solid iron core of the Earth?

A

A giant ferromagnet, or geodynamo.

14
Q

What is the curie temperature of iron, and what is the temperature of Earths core?

A

T(c) of iron = 1043 K

T(core) = 6000 K +/- 500 K

15
Q

What does the temperature of Earths core mean for its magnetism?

A
• Far above curie temp
• Is not a permanent magnet
• Field strengths are very low
16
Q

Why does the Earth create a magnetic field?

A

Fluid flow between inner core and outer core means movement of molten iron, which results in currents and generates magnetic fields.

17
Q

What is thermoremanent magnetism?

A
• Ferrimagnet cools through Tc in the presence of external geomagnetic field
• Internal magnetisation will align with that field
• Ferrimagnetic material can permanently record the geomagnetic field strength and direction at the time it was cooled below Tc
18
Q

What is a mid ocean ridge?

A

A constructive plate boundary where new oceanic crust is created from upwelling magma.

19
Q

What can we use a combination of to study mid ocean ridges?

A

Radiometric dating, plate tectonics and magnetism.

20
Q

What can the magnetic survey record information about for things like magnetite?

A

Past field strength, orientation, inclination and polarity

21
Q

What is palaeomagnetism?

A

Can traverse the ridge axis and measure the local B-field, and subtract what we expect to find from the geomagnetic field - gives us a reference field.

22
Q

What does palaeomagnetism do for us?

A

Gives a local field due to magnetic materials in the crust e.g. magnetite. Records field as material cooled through Tc.

23
Q

How can we look at the polarity of the B-field? What do we observe?

A

Look at rocks away from mid ocean ridge and obtain an accurate timeline of polarity reversals. Observe a stripe pattern away from the ridge in both directions.

24
Q

What is the equation for x, the stripe widths when looking at polarity?

A

x(i) = v0/2 * Δt(i), where v is the spreading rate of the ith ridge and Δti is the time since geomagnetic reversal.

25
Q

How can we infer the latitude at which point the rock cooled below Tc?

A

Use tan(I) = 2*tan(λ)

26
Q

What were polarity reversals and inclination significant evidence for historically?

A

Continental drift.

27
Q

What is topography?

A
• Low density material below ridge axis where hot, less dense, material upwells from mantle
• Gentle rise of sea bed up to ridge axis
• Trench on ridge axis due to spreading of plates
28
Q

What is a gravity survey?

A

Survey done by ship at sea level so that the free air correction dg(r) = 0

29
Q

What is the equation for the Bouguer Correction?

A

dg(B) = 2πGh*(ρ(rock)-ρ(water))

30
Q

What is the equation for the Bouguer anomaly?

A

g(B) = g(0) - g(λ) - dg(F) - dg(B)

31
Q

Why does the Bouguer anomaly become less positive near the centre of ridge?

A

Due to increased amount of low density material (buried low density magma chamber below ridge)

32
Q

What are 2 models of magma upwelling?

A
• Slow extrusions under water form pillow shaped structures.

- Violent volcanoes and island formation near fast extrusion e.g. Iceland

33
Q

How can we use seismology with tomography?

A

Seismic wave tomography of P- and S- waves with large arrays of seismometers on the ocean floor.

34
Q

What does seismic tomography give us?

A

Wave speed map in 3D, and therefore k(r), μ(r), ρ(r)

35
Q

What can we conclude from seismic tomography?

A
• Plates move apart at constructive boundary
• Mid-ocean ridges are fed by upwelling magma
• Gravity surveys and seismic tomography to confirm low density buried magma chamber