Rose Swale, of Southland Girls' High School, asks :-

What causes the earth's magnetic field to reverse?

Peter Koons, a geophysicist at Otago University, responded.

Most of the earth's magnetic field is generated by the rotation of the earth's core.

The core consists mostly of iron and at the extreme temperatures and pressures of the core is liquid in the outer part and solid in the centre, or inner, core where the pressure is highest. This liquid/solid nature of the core is responsible for making the earth's magnetic field so strong. It also makes the magnetic field extremely complicated.

The manner in which the earth's field is generated is only qualitatively understood, but is related to the interaction of magnetic and electric currents with convection currents in the outer liquid core. All of this iron core is an exceptionally good, rotating conductor and the rotation of a good conductor in the presence of electrical currents can produce a magnetic field which is called a dynamo.

Laboratory experiments on model dynamos show some interesting characteristics. The magnetic field generally has two poles that lie close to the poles of rotation, but that wander around the rotation poles over short time spans. The magnetic field can suddenly reverse. This gives rise to the reversals that you asked about. Because the magnetic field is caused by interaction of a number of processes, a very minor, undetectable change in any part of the rotation system can result in a reversal. This makes the pattern irregular and not predictable.

The irregular pattern is a help to people interested in working out the age of the earth because bits of the distinctive pattern are recognisable in the earth's crust whose magnetism formed as it solidified.