I was assuming the earth's magnetism is similar to a permanent magnet; But below a depth of somewhere near the Mohorovičić discontinuity (Moho) between earth's crust and mantle it should be above curie temperature. That depth is pretty shallow compared to earth's radius, something like 40km deep.
Is earth roughly like a very thin hollow magnet?
Or is the magnetism caused by some completely different mechanism?

  • $\begingroup$ I'm far from an expert in this matter, but I would expect Earth's magnetic field to be created by the dynamical flow of plasma going on in it's core. The mechanism for creating a magnetic field in an ordinary magnet comes from the alignment of spins in the magnet. However, one could also think of creating a magnetic field by having some charged particle going around in a loop (some plasma in this case). I think the latter case would be a better explanation of the origin or Earth's magnetic field. $\endgroup$ – VanillaSpinIce Apr 8 '14 at 20:05

See this explanation from here:

Right at the heart of the Earth is a solid inner core, two thirds of the size of the Moon and composed primarily of iron. At a hellish 5,700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid. Surrounding this is the outer core, a 2,000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid. Differences in temperature, pressure and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises. The Coriolis force, resulting from the Earth’s spin, also causes swirling whirlpools. This flow of liquid iron generates electric currents, which in turn produce magnetic fields. Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo. The spiralling caused by the Coriolis force means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce one vast magnetic field engulfing the planet.

This effect is actually called the dynamo effect. That is precisely why the Earth has a north and south pole and acts like a bar magnet.

Curie's temperature is not applicable at all here. That is just the temperature where a material with permanent magnetic properties loses its magnetism and can only become a magnet again if a magnetic field is applied. Below the curie temperature, the magnetic moments of the material are aligned in one direction but above it, they get randomly aligned which cancels out the net magnetism to zero.

Earth's magnetism is caused by activity in the core at a macro scale, it is not caused by an intrinsic property of the material in the core. The magnetic moments of the material inside the core or their alignment has nothing to do with this.


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