The earth was shaped into a sphere by the force of

Earth is spherical due to gravity, which pulls all matter toward the center of mass. During Earth's formation, the molten material was pulled equally inward in all directions by gravitational force, naturally forming a sphere (the shape with minimum surface area for a given volume. This is why all large bodies in space (planets, stars) are spherical. Magnetism attracts only certain materials, and heat would cause expansion, not spherical shaping.