Magnetic Earth

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Origins of the Earth's Magnetic Field

outer core is hot and molten and believed to be made of iron and nickel - temp is probably higher near the inner core boundary so convection currents will exist is outer core.

the convecting mass of molten iron will generate electricity. Generation of electricity induces magnetism, which generates more electricity and so on

the effect is called the 'self-exciting dynamo'

The outer core temp is well above the Curie point at which materials lose their magnetism, so the earths magnetism is not permanent and is constantly generated

Magnetism can increase after being faded in the opposite direction, evidence for reversals is found in rocks as remnant magnetism

Palaeomagnetism - ancient magnetism preserved in rocks

iron rich magnetic minerals in lavas align themselves with the earth's magnetic field and as they cool through the Curie point, retain this magnetism permanently. They act like 'frozen-compasses', showing the direction to the poles at the time of their formation. the age of the lavas and therefore the date of each magnetic reversal is known from radiometric dating and from fossils in interbedded sediments

aerial surveys show many reversals in the remnant magnetism in basalts of the ocean floor. Sensitive magnetometers show normal field strength over the zones of normal polarity, above zones of reversed polarity the measurements are weaker.

Magnetic Inclination

following lines of the earth's magnetic field, a freely suspended compass needle lies exactly vertical at the magnetic poles. at the magnetic equator, the needle is exaclty horizontal.

so the inclination of a rock of known age to find its latitude when formed

Apparent Polar Wandering Curves

evidence found suggesting magnetic poles have wandered

some geologists believe the continents moved, other think the poles moved

one discovery was that the path of apparent polar wandering measured in north amrica was different from the path in europe, rocks of the same age in n. america and europe suggested that the north pole was in two positions at the same time. therefore the continents must have moved.

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