evidence: "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. [...] 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: [...] "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.