This instantaneous magnetic field expresses itself in solidified material upwelled from the mantle at the Earth's surface. Like a tape recorder, the magnetized material in the extrusive rocks serves as a record of the Earth's magnetic history. This type of magnetization is referred to as remanent magnetization. Remanent magnetization is a record of how a particular material has responded to the past magnetic fields of the Earth. The other type of magnetization solidified surface material may retain is induced magnetization.
Induced magnetization occurs when a material is exposed to a present day magnetic field.
Some materials only contain one type of magnetization, while others retain both induced and remanent magnetization. Together, both types of magnetism are used as tools in determining the past geomagnetic arrangement of the Earth. Rocks along the midoceanic ridges began being studied using paleomagnetism as a dating correlation method. If it could be shown that the solidified mantle material along the midoceanic ridges expressed some change in magnetic field direction over time, then it could be hypothesized that the midoceanic ridges were the source of new crust, and thus ocean floor spreading was occurring as well. While the first two assumptions were proven to be correct, it was discovered that the different midoceanic ridges spread at different rates.