what is origin of the earth magnetic field

Where is the origin of the earth magnetic field? Deep down on the earth crust. We can find the core It is widely believed this core is made up of molten iron,perhaps mixed with nickel and sulphur. But its still a mystery of why the core is not the origin of the earth magnetic field. Because the earth magnetic field does not covers all over the earth surfaceNobody has ever taken the mythical journey to the centre of the Earth, but by studying the way shockwaves from earthquakes travel through the planet, physicists have been able to work out its likely structure but its still can't explain the origin of magnetic field.