Thermoremanent magnetization

When an igneous rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see isothermal remanence). This remanence can also be very stable, lasting without significant change for millions of years. TRM is the main reason that paleomagnetists are able to deduce the direction and magnitude of the ancient Earth's field.

As early as the eleventh century, the Chinese were aware that a piece of iron could be magnetized by heating until it was red hot and then quenched in water. While quenching it was oriented in the Earth's field to get the desired polarity. In 1600, William Gilbert published De Magnete (1600), a report of a series of meticulous experiments in magnetism. In it, he described the quenching of a steel rod in the direction of the Earth's field, and he may have been aware of the Chinese work.

In the early 20th century, a few investigators found that igneous rocks had a remanence that was much more intense than remanence acquired in the Earth's field without heating; that heating rocks in the Earth's magnetic field could magnetize them in the direction of the field; and that the Earth's field had reversed its direction in the past.

It has long been known that a TRM can be removed if it is heated above the Curie temperature ${\displaystyle \scriptstyle T_{\text{C}}}$ of the minerals carrying it. A TRM can also be partially demagnetized by heating up to some lower temperature ${\displaystyle \scriptstyle T_{1}}$ and cooling back to room temperature. A common procedure in paleomagnetism is stepwise demagnetization, in which the sample is heated to a series of temperatures ${\displaystyle \scriptstyle T_{1},T_{2},\ldots }$, cooling to room temperature and measuring the remaining remanence in between each heating step. The series of remanences can be plotted in a variety of ways, depending on the application.

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