What is the cause of Earth's magnetic field?

[Rislam169]

I am a student of science.I know that the compus works for magnetic field on earth.But I don't know the actual cause of this magnetic field.Compus always set in north and south direction. Sometime I thing ,Is there any magnetic hill on north or south poll?Or this is the cause of spin of the earth.

But what is right description of this magnetic field?

 

[Drakkith]

The Earth's magnetic field is generated by a dynamo effect in its core/mantle.

From wiki: http://en.wikipedia.org/wiki/Dynamo_theory

In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales. A dynamo is thought to be the source of the Earth's magnetic field, as well as the magnetic fields of other planets.