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Paleomagnetism - The reversals take about 5 10 3 years with...

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Paleomagnetism materials show one of three types of magnetism: diamagnetism (quartz, salt, feldspar) - low susceptibilities paramagnetism (fayalite Fe 2 paramagnetism (fayalite Fe 2 SiO 4 , ferrosilite FeSiO 3 ) susceptibilities 100-200 times diamagnetic ferromagnetism (magnetite Fe 3 O 4 , Hematite Fe 2 O 3 ) This is remanent unless heated to the Curie temperature. At constant T if an applied field is removed the magnetisation which is left is referred to as the Isothermal Remanent Magnetisation (IRM). If the material is cooled below the Curie temperature in a field, the field it retains is known as the Thermodynamic Remanent Magnetisation (TRM). One can study of the Earth's magnetic field at the times rocks formed and calculate the direction of the field at the time of the cooling through the Curie Point, assuming a dipole field. You can then calcualte where the dipole axis was atthe time. We find the field has reversed several times:
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Unformatted text preview: The reversals take about 5 10 3 years, with the main reversals about every 10 6 years. One of the main uses of this has been to look at sea-floor spreading. If we look at the morphology of, say, the mid-Atlantic ridge, we see a complex fracture pattern following the line of the ridge: If we cross the ridge from A to B and measure the magnetic field directions we find a pattern mirrored about the ridge line: the pattern is tge line: the pattern is the sort of reversal pattern we saw above: The time-scale is known from the previous diagram,which tells us the rate of sea-floor spreading. This has been crucial in developing the theory of continental drift. Typical rates of sea-floor spreading are: near Vancouver c.2.9metres/yr; off Peru c. 4.5 metres/year; these are typical values....
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