This is a report of a computer numerical model of the electromagnetic, fluid dynamical processes of Earth’s interior that reproduced key features of the magnetic field over more than 40,000 years of simulated time in which the computer-generated field reversed itself. By itself, the reversal is strong confirmation of the dynamo theory model, and other details – magnitude and structure of the field – also agree with surface features of Earth’s field. The simulation also offers insight into the dynamics that sustain the magnetic field and generate reversals.