The Earth’s magnetic field results partly from convection currents in the outer core and changes in the magnetic field may reflect changes in the pattern and strength of the convection currents in the outer core. Indeed, the rate of westward drift of the field indicates fluid flow at a rate of about 1 mm/s in the outer core.Ancient magnetism - PalaeomagnetismEvidence of the Earth’s magnetic field through geological time is preserved in rocks because when rocks are formed iron-containing minerals are lined up like tiny magnets. The direction and strength of the ancient magnetism is related to the direction and strength of the Earth’s magnetic field at the time when the rock was formed.Measurements of the inclination of the ancient magnetic field allows the position of the ancient magnetic pole to be found relative to the position of the rock.
If the magnetism of a rock sample gives a reading of zero inclination then the rock formed at 90( to the magnetic poles. If the inclination is 90( then the rock formed at a magnetic pole.Variation of inclination with latitudeThe study of ancient magnetism or palaeomagnetism has shown two notable features.1.
Pole positions from many ancient rocks do not coincide with the position of the present pole. This could be because the rock has stayed in the same position and the pole has moved or it could be because the rock has moved taking its ancient pole position with it and so giving the false impression that the pole has moved. Study of rocks of the same age from different continents often give different pole positions. This means that the continents have moved over the surface of the Earth because different pole positions cannot exist at the same time and a single pole position can only be obtained by moving the continents back to their original positions.
This means that finding pole positions tells us the latitude of the rock at the time of its formation.2. Secondly, palaeomagnetic studies have shown that some rocks have been magnetized in a direction opposite to that of the present magnetic field. This shows that the magnetic field has switched itself around from its present normal condition to a reversed condition Field reversals seem to take place fairly regularly.