A change in the magnetic direction of the Earth. It was discovered that volcanic lava flows, which like kilns and fired clay record the magnetism at the time they were hot, retained measurable magnetization in a reversed direction. Over the last 4 million years, magnetic direction changed at least 10 times. It switched to normal, as we know it, about 700,000 years ago. The direction of the dipole component reverses, on an average, about every 300,000 to 1,000,000 years. This reversal is very sudden on a geologic time scale, apparently taking about 5,000 years. The time between reversals is highly variable, sometimes occurring in less than 40,000 years and at other times remaining steady for as long as 35,000,000 years. No regularities or periodicities have yet been discovered in the pattern of reversals. A long interval of one polarity may be followed by a short interval of opposite polarity. These reversals have proved an important dating aid to archaeology.