iron-titanium oxide minerals in basalt and other igneous rocks may preserve the direction of the earth's magnetic field when the rocks cool through the curie temperatures of those minerals. the curie temperature of magnetite, a spinel-group iron oxide, is about 580°c, whereas most basalt and gabbro are completely crystallized at temperatures below 900°c. hence, the mineral grains are not rotated physically to align with the earth's field, but rather they may record the orientation of that field. the record so preserved is called a thermoremanent magnetization (trm). because complex oxidation reactions may occur as igneous rocks cool after crystallization, the orientations of the earth's magnetic field are not always accurately recorded, nor is the record necessarily maintained. nonetheless, the record has been preserved well enough in basalts of the ocean crust to have been critical in the development of theories of sea floor spreading related to plate tectonics. trm can also be recorded in pottery kilns, hearths, and burned adobe buildings. the discipline based on the study of thermoremanent magnetisation in archaeological materials is called archaeomagnetic dating.