The law which states that the variations of electric field causes magnetic field is

Explanation

Some of the important but probable causes of earth.s magnetism are listed below although exact causes are not known :
1. Ionised gas in earth.s atmosphere due to primary cosmic rays contributes a bit to earth.s magnetism. When earth rotates, electric currents are set up due to movement of charged ions.
2. It may be there due to rotation of the earth. Every substance is made of charged particles protons and electrons. Rotating charge is equivalent to circulating current.
3. It may be due to molten charged metallic fluid in the core of the earth. These charges when rotate alongwith the earth may produce magnetic field.