thermoremanent magnetization

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The magnetization that an igneous rock acquires, usually from the magnetic field in which it is located, when the temperature of the magma or lava from which it forms falls below the Curie point during the cooling and solidification process.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.