Paleomagnetism and mineral magnetism Flashcards
How is Earth’s magnetic field made?
Radioactive heating. Outer core in a turbulent convection. The convection is driven by the upward motion of the light elements as the heavier elements freeze onto the inner core. The motion of electrical conducting iron in the presence of the small magnetic field from the core induces a big electric current. This in turn generates Earth’s magnetic field
What is declination?
Difference between true and magnetic north that the compass shows. 0 – 360 degrees.
What is inclination?
The dip from horizontal position that the magnetic field has at a specific location.
-90 or 90 degrees at the poles and zero degrees at the equator.
What is the formula between declination and inclination?
tan(magnetic inclination)=2 tan(magnetic latitude)
What is the external magnetic field that affects the earth?
A small effect due to solar wind which is a stream of particles from the sun. Interaction of solar wind with earth’s magnetic field happens in the magnetosphere.
What is secular variation?
The slow, irregular changes in the direction of Earth’s magnetic field. The magnetic north wanders. It averages out over thousands of years
What is paleomagnetism?
Old magnetism, natural remnent magnetisation, in rocks. It provides declination and inclination of where the rock was formed.Lava cool’s and becomes magnetised, it takes 100 000 years for several piles of lava to form so it cancels out the secular variation.
Is measured by taking samples of rock.
What is magnetic susceptibility?
A measurement of the induced magnetisation due to an external magnetic field. The degree to which a body becomes magnetized is determined by its magnetic susceptibility and is the fundamental parameter in magnetic prospecting.
The magnetic susceptibility is a unitless constant that is determined by the physical properties of the magnetic material
Why are there magnetic atoms?
It’s because of magnetic properties due to the spin of atom’s electrons
What is a magnetic domain?
Groups of atoms join so that their magnetic fields are in the same direction. Only tiny grains have all their atomic magnets aligned in the same direction. Above a certain size the magnetization spontaneously divides into small regions, called magnetic domains.
When the external field is removed, the imperfection caused by magnetic domains in different directions prevent the domains/walls to return to their earlier states, so the rock tends to retain the remnance.
What are the most common magnetic materials called?
Those materials are called ferromagnetic, ferrimagnetic. They have remnant magnetisation
What is the curie temperature?
The temperature above which certain materials lose their permanent magnetic field; the magnetism completely disappears. When the Curie temperature is reached the alignment collapses because the thermal energy has exceeded the energy of the magnetic interaction.
If the rock has not been reheated above it’s Curie temperature and lost its magnetization the magnetism is primary.
Which minerals have the greatest magnetic remnance?
Magnetite has the greatest remnance, followed by hematite
What is the köningsbergers ratio?
The ratio between induced and remanent magnetisation in rocks. Q=M(remanent)/M(induced)
What is the fold test?
It is a field test, with a folded structure, that gives information about the remnant magnetisation. The direction of magnetisation is the same if folding happened before magnetisation.
