As magma rises as lava and cools to form rocks, any magneticminerals in the rock (eg magnetite and haematite) align themselveswith the earth's magnetic field. When the rock is solidified thismagnetism is 'frozen' in the rock to form a weak, permenantmagnet.The direction in which they point can be measured by a cryogenicmagnetometer.This magnetic orientation can then be used to find where, inrelation to the rock, magnetic north was. This leads to differentrocks in continets showing different magnetic norths. As there isonly one the continent itself has moved. Different samples will betaken from throughout the continent and the magnetic orientation istested to find where magnetic north was. The position of magneticnorth is plotted on a map to show apparent polar wander.