What causes the earth’s magnetic field?
(Lansing State Journal, March 20, 1996)
Geologic samples provide a record of geomagnetic activity that spans many billions of years of earth history. Along with modern observations, some from satellites, a history of the ebb and flow of Earth’s magnetic field can be constructed.
It is stable over periods of millions of years, and its acts as a magnetic "dipole"; that is, it exhibits a north-and-south magnetic pole, much like a bar magnet. This axis of this dipole does not exactly correspond to the axis of rotation of the earth, making the geographic and magnetic poles distinct. Amazingly, every millennia or so, the earth’s magnetic field suddenly switches polarity!
Current theory suggests that the mechanism for field generation is the "geodynamo". The outer core of the earth is an electrically conduction fluid and acts as a generator (or dynamo). The outer core rotates relative to the earth’s rotation along its axis and moves across existing lines of magnetic force. The magnetic field is thus regenerated, using the fluids energy of motion.