Whatever the cause, the magnetic field of the Earth has changed through the eras of geologic time. In other words, the magnetic north pole hasn’t always been where it is today. Scientists know this from studying rocks. When rocks are heated, such as lava in a volcano, the iron atoms line up with Earth’s magnetic field. When the volcano erupts and the rock cools, the atoms are trapped in this aligned position (just like in the experiment described before where magnetism was induced by heating an iron nail).

Geologists can look at many different layers of rock and volcanic eruptions that happened over many thousands and millions of years and use the ways in which the atoms are lined up in them to track the changes in the location of Earth’s magnetic north pole. Earth’s poles have even reversed themselves more than 20 times during the last 5 million years.