We have known since at least the 16th century that the Earth's magnetic field varies with time as well as geographic position. This secular variation (SV), which has been monitored instrumentally at various locations since that time, is fundamentally important to a complete understanding of the geomagnetic field and its origin. Unfortunately, the instrument record for SV is very short in duration and if we wish to extend this record backward through time we must use a very different recording process.Paleomagnetists have considered this problem for well over 100 years, and have discovered two processes by which the SV may be commonly recorded. The first, thermoremanent magnetization (TRM), essentially causes a rock, which has been heated above a characteristic temperature, to acquire a remanent magnetization parallel to the field present when the rock cools. In this manner, lava flows and fired pottery or bricks record an instantaneous picture of the field during their cooling process.
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