The same energy source that powers plate tectonics is also responsible for for the Earth's dynamo. Energy is released when material from our planets liquid core cools and solidifies onto the solid core. This energy stirs up the motions of the liquid core, and these motions generate a magnetic field.While there is also fluid motion in the mantle, it does not produces an appreciable magnetic field : The silicon rich rocks that comprises the mantle are poor conductors of electricity, and their motions are million times slower than those in the outer core. Hope this helps!
