Geomagnetism Flashcards
What is the equation for the magnetic potential?
V(r) = m*r/4πr^3, where m is the magnetic dipole moment from S->N
What is the equation for the magnetic field?
-μ0*∇V(r), where ∇ is in spherical polar coordinates
What are the equations for the 3 components of spherical polar coordinates for the magnetic field?
B(r) = 2*μ0*m*cosθ/4πr^3 B(θ) = μ0*m*sinθ/4πr^3 B(Ф) = 0
What are meridians?
Arcs of constant Ф in spherical polar coordinates
What is geomagnetic field strength measured with?
A magnetometer.
What does the magnetic field strength vary between at earths surface?
25 μT to 65 μT
What is the geomagnetic latitude?
Latitude analogues to geographical latitude, but defined relative to the magnetic poles.
What is the declination angle for geomagnetism?
Angle between the magnetic and geographical meridians.
What is the inclination angle for geomagnetism?
Angle between magnetic field and a horizontal on the surface.
What are 2 equations for the inclination angle?
tan(I) = B(r)/B(θ) = 2*cos(θ)/sin(θ)
tan(I) = 2sin(λ)/cos(λ) = 2tan(λ), where λ = 90 - θ
What are 3 important types of magnetism? How do they differ?
- 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)
What are the locations of the magnetic poles right now?
North: 193.710 degrees W, 86.395 degrees N
South: 136.419 degrees E, 64.245 degrees S
What type of magnet is the solid iron core of the Earth?
A giant ferromagnet, or geodynamo.
What is the curie temperature of iron, and what is the temperature of Earths core?
T(c) of iron = 1043 K
T(core) = 6000 K +/- 500 K
What does the temperature of Earths core mean for its magnetism?
- Far above curie temp
- Is not a permanent magnet
- Field strengths are very low
Why does the Earth create a magnetic field?
Fluid flow between inner core and outer core means movement of molten iron, which results in currents and generates magnetic fields.
What is thermoremanent magnetism?
- 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
What is a mid ocean ridge?
A constructive plate boundary where new oceanic crust is created from upwelling magma.
What can we use a combination of to study mid ocean ridges?
Radiometric dating, plate tectonics and magnetism.
What can the magnetic survey record information about for things like magnetite?
Past field strength, orientation, inclination and polarity
What is palaeomagnetism?
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.
What does palaeomagnetism do for us?
Gives a local field due to magnetic materials in the crust e.g. magnetite. Records field as material cooled through Tc.
How can we look at the polarity of the B-field? What do we observe?
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.
What is the equation for x, the stripe widths when looking at polarity?
x(i) = v0/2 * Δt(i), where v is the spreading rate of the ith ridge and Δti is the time since geomagnetic reversal.
