What is "thermoremanent magnetization"

n. The magnetization of an igneous rock that was acquired at the time it cooled from the molten state; it provides information on the Earth's magnetic field at that time.

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.