compasses and navigation

Volcanic rock contains lots of grains of iron. While the rock is still molten, these little grains line up with the Earth's magnetic field. As the rock cools they are frozen into place. The only explanation for the grains lining up one way in one layer of rock and then in the opposite direction in the next layer of volcanic rock is that the direction of the magnetic field itself changed between the times the layers were deposited. The Earth's magnetic field must be changing.

Sometimes the north and south magnetic poles change around. The last time the Earth's magnetic field changed over was about 780,000 years ago.

The Earth’s core is made of molten metal such as iron and nickel. Iron and nickel are very good conductors of electricity and so electric currents can flow easily in them. As the Earth rotates large electric currents are built up and these produce the Earth’s magnetic field.