Final answer:
Magnetism can be preserved in rocks if they contain magnetic minerals and solidify in the presence of an external magnetic field.
Step-by-step explanation:
Magnetism can be preserved in rocks if they contain magnetic minerals. When a rock forms, the magnetic field at that time can be preserved if the rock solidifies in the presence of an external magnetic field. This is because the magnetic minerals in the rock align themselves with the external magnetic field and become permanently magnetized.
For example, when magma or lava cools and solidifies to form igneous rocks, the magnetic minerals in the magma or lava align themselves with the Earth's magnetic field at that time. The minerals retain this alignment as the rock solidifies, preserving the magnetic field.
However, not all rocks contain magnetic minerals, so not all rocks preserve the magnetic field. Rocks that are composed primarily of non-magnetic minerals, such as most sedimentary rocks and metamorphic rocks, do not preserve the magnetic field.