The Earth is indeed a magnet, although it’s very weak. A good fridge magnet is over 200 times stronger. As a very crude approximation, you can imagine the Earth’s magnetic field to be that of a bar magnet at the centre of the Earth, which is roughly aligned with the axis the Earth spins around, but tilted by about 11 degrees. The points where the North-South axis of this magnet intersects the Earth’s surface are called the geomagnetic poles. Because of the tilt of the magnet’s axis, the geomagnetic poles are not in the same places as the geographic poles.
This diagram illustrates how the Earth’s magnetic field can be approximated by that of a bar magnet. The blue axis represents the Earth’s rotational axis with the geographic North and South poles at top and bottom. The pink line represents the orientation of the bar magnet with North and South geomagnetic poles.
The South pole of our imagined bar magnet points in the northerly direction on Earth, while its North pole points in a Southerly direction. This is why the geomagnetic pole in the Earth’s North actually corresponds to South in the magnetic sense, and the geomagnetic pole in the Earth’s South corresponds to North in the magnetic sense.
This is not yet all as far as the poles are concerned. Because the picture of the bar magnet is only an approximation, the North and South geomagnetic poles are also only approximations of the true poles of the Earth’s magnetic field. The true poles are the points on Earth where the magnetic field lines point vertically down. Unlike the geographic and geomagnetic poles, the true magnetic poles are not directly opposite each other. They also move around independently of each other. At the moment, it seems the magnetic pole in the North is moving in a north-westerly direction by about 45km a year. What’s more, geological records show that [the Earth’s magnetic field can even flip around]. The last time this happened was about 780,000 years ago.
Where does the magnetic field come from?
Nobody knows for sure, but the accepted theory is that the Earth acts like a dynamo. To understand how this works, first remember some physics you might have learnt at school. When you move a conducting material through a magnetic field, then this induces an electric current in the material. This process is known as electromagnetic induction and is also what happens in the kind of dynamo that is used to power bicycle lights. Conversely, an electric current can generate a magnetic field.
The Earth has a solid inner core which is roughly the size of the Moon but around as hot as the surface of the Sun. The inner core is surrounded by a liquid outer core mostly made up of molten iron, which can conduct electricity. Various processes that occur within these deep layers of the Earth cause the liquid outer core to be in constant motion (the liquid convects.
To understand dynamo theory imagine that in the beginning there was a magnetic field. Since the conducting liquid outer core was in motion within the magnetic field, electric currents were generated in the liquid. Because of the rotation of the Earth, such currents are organised in spiralling columns.
The currents in turn generated their own magnetic field, which amplified the original magnetic field and evolved into the kind of magnetic field we see today. Because the liquid outer core keeps convecting, currents keep being generated, so the magnetic field can sustain itself over extremely long periods of time. And because the motion of the liquid is complex and variable, so is the magnetic field.
