Question: How do you work out how old the magnetic field is?

Laura Roberts Artal answered on 17 Jun 2013:

Hi Sophie Helen!
That is a really good question :)! The work I’m doing at the moment is trying to answer that very question! I’ve not got the full answer yet, but here goes:

When a rock is formed, it is hot, as it slowly cools down it has minerals in it that act as tiny bar magnets and align themselves with the direction of the Earth’s magnetic field at the time (think of the Earth’s magnetic field as a giant bar magnet). To work out how old the magnetic field is, you need to test super old rocks (which is what I’m doing) to see if they hold a record of a magnetic field. If they do and you know the age of the rocks, then you know the age of the magnetic field.
The rocks I’ve tested are 350000000 years old and they have a record of the magnetic field, so the Earth’s magnetic field is at least that old. It could be even older, but I’ve not worked with older rocks!