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The term crustal magnetism refers to magnetic fields that do not stem from the core of a world (planet or moon), but are from magnetic material on or near the surface, i.e., in the planet's crust. Earth has crustal magnetism that is not merely part of the dipole magnetic field through its core. In the case of Earth, it is considered to be a remnant within rocks that include iron (and/or other metals that retain magnetism), magnetized by Earth's earlier magnetic field, and often subsequently moved and/or reoriented through Earth geological activity such as continental drift. Some worlds, such as the Moon and Mars have magnetism at the surface but no core magnetic field, and the crustal magnetism is considered a possible remnant of an earlier core magnetic field, which implies a dynamo earlier in its life.