Gary A. Glatzmaier of the Institute of Geophysics & Planetary Physics at Los Alamos National Laboratory has done extensive work in this area. He replies:

"The Earth's magnetic field is thought to be generated by fluid motions in the liquid, outer part of the Earth's core, which is mainly composed of iron. The fluid motions are driven by buoyancy forces that develop at the base of the outer core as the Earth slowly cools and iron condenses onto the solid, inner solid core below. The rotation of the Earth causes the buoyant fluid to rise in curved trajectories, which generate new magnetic field by twisting and shearing the existing magnetic field. Over 99 percent of the Earth's magnetic energy remains confined entirely within the core. We only observe the small portion of the magnetic field that extends to the surface and beyond, where its basic structure is a dipole--that is, a simple north-south field like that of a simple bar magnet. There are also smaller, non-dipolar structures in the Earth's field; these change locally and very slightly on a century timescale.

"The dipole part of the field is usually aligned fairly closely with the Earth's rotation axis; in other words, the magnetic poles are usually fairly close to the geographic poles, which is why a compass works. Occasionally, however, the dipole part of the field reverses, causing the locations of the north and south magnetic poles to switch. This reversal process can be seen in the paleomagnetic record, locked into rocks of the ocean floor and in some lava flows. The reversal process is not literally 'periodic' as it is on the sun, whose magnetic field reverses every 11 years. The time between magnetic reversals on the Earth is sometimes as short as 10,000 years and sometimes as long as 25 million years; the time it takes to reverse is only about 5,000 years.

"The first dynamically-consistent, three-dimensional computer simulation of the geodynamo (the mechanism in the Earth's fluid outer core that generates and maintains the geomagnetic field) was accomplished and published by Paul H. Roberts of the University of California at Los Angeles and myself in 1995. We programmed supercomputers to solve the large set of nonlinear equations that describe the physics of the fluid motions and magnetic field generation in the Earth's core. The simulated geomagnetic field, which now spans the equivalent of over 300,000 years, has an intensity, a dipole-dominated structure and a westward drift at the surface that are all similar to the Earth's real field. Our model predicted that the solid inner core, being magnetically coupled to the eastward fluid flow above it, should rotate slightly faster than the surface of the Earth. This prediction was recently supported by studies of seismic waves passing through the core.

"In addition, the computer model has produced three spontaneous reversals of the geomagnetic field during the 300,000-year simulation. So now, for the first time, we have three-dimensional, time-dependent simulated information about how magnetic reversals can occur. The process is not simple, even in our computer model. Fluid motions try to reverse the field on a few thousand-year timescale, but the solid, inner core tries to prevent reversals because the field cannot change (diffuse) within the inner core nearly as quickly as in the fluid, outer core. Only on rare occasions do the thermodynamics, the fluid motions and the magnetic field all evolve in a compatible manner that allows for the original field to diffuse completely out of the inner core so the new dipole polarity can diffuse in and establish a reversed magnetic field. The stochastic (random) nature of the process probably explains why the time between reversals on the Earth varies so much."

For more detailed explanations of the geodynamo, the simulated magnetic reversals and the super-rotation of the Earth's inner core, Glatzmaier recommends the following papers:

"A Three-Dimensional Self-Consistent Computer Simulation of a Geomagnetic Field Reversal" by Gary A. Glatzmaier and Paul H. Roberts in Nature, Vol. 377, pages 203-209; 1995.

"Rotation and Magnetism of Earth's Inner Core" by Gary A. Glatzmaier and Paul H. Roberts in Science, Vol. 274, pages 1887-1891; 1996.

Edwin S. Robinson is a professor of geophysics at Virginia Polytechnic Institute & State University in Blacksburg, Virginia.

He adds some additional background information:

"The main geomagnetic field of the earth is produced by the flow of electrically charged particles in the liquid part of the earth's core. This liquid zone extends from a depth of 2,900 kilometers to a depth of 5,100 kilometers. Currents of flowing liquid are caused by the difference in temperature between the top and the base of this zone. These currents are something like the movement of water in a boiling kettle. Rotation of the earth on its axis imparts symmetry to the pattern of liquid core currents. Therefore, there is a somewhat symmetrical electrical current in the liquid core that is the result of the movement of the electrically charged particles.

"We know from the principles of physics concerning electromagnetic induction that an electrical current always has an associated magnetic field. In the earth's liquid core, a dynamo is created. Because the core current is somewhat symmetrical around the axis of rotation, the associated magnetic field is similar to that of a bar magnet. For reasons not clearly understood, the balance between the effect of the earth's rotation and the effect of temperature on the core dynamo becomes upset from time to time, causing the pattern of the core current to be disrupted. Following such a disturbance, it is theoretically possible for the dynamo to reconstitute itself with an opposite direction of current flow. The associated magnetic field, then, will have an opposite polarization.

"Because we cannot get down into the liquid core to observe what actually is happening, we have to make inferences based on measurements made on or above the earth's surface. Therefore our knowledge of the core is quite incomplete. We simply don't know enough about the core to predict when pole reversals will occur in the future or how long it takes to complete such a reversal or what upsets the delicate balance of the factors that produce the core current. But we do have convincing information obtained from magnetized mineral grains in rocks that tells us that geomagnetic polarity reversals have occurred a great many times in the history of the earth.