Why is the Earth magnetic?

[brassmonkey]

What causes the Earth to have a magnetic field? 

[CT]

It's caused by a dynamo like affect in the earth's mantle. Flowing of metallic fluids effectively creates an electric current. This gives rise to the magnetic field. Someone else will probably go into more detail about it. Can't remember all the info off the top of my head.

[brachiopod]

That's basically it, CT.

The mantle itself is only about 5-10% molten, so is not fluid enough to have sufficient flow velocity to generate an 'electro-magnetic' field.
Secondly, there is very little free iron in the mantle - most being bound up in the form of iron-rich silicates (often in association with magnesium).

The Earth's outer core, however, is mainly iron and is in a molten (fluid) state.
{Note that molten iron is not magnetic in itself - if you heat a bar magnet up to a high enough temperature, it will lose its magnetism. This is known as the Curie point, and occurs at about 500�C (off the top of my head) - whilst the temperature of the outer core is somewhere around 4000�C.}

It was Bullard and someone-whose-name-I-can't-remember that proposed the 'self-exciting dynamo' theory in the core - whereby convected liquid iron moving through a small, residual magnetic field, would produce an electric current, which would then generate its own magnetic field etc etc..

It is still poorly understood, and not entirely described mathematically to a satisfactory degree. Although the principle is feasible and it remains the most likely explanation. Polarity is not critical either, which may help account for the relatively frequent (geologically speaking) polarity reversals.

[brassmonkey]

So if it needs to feed off a 'residual magnetic field',  where does that come from?

[mibn2cweus]

Its all them compasses pointin north what does it.

[fo3nix]

CT is correct. But this is only a theory. We really don't _know_ what the cause is.