How can moving liquid metal inside Earth sustain a magnetic field?

A compass at Earth's surface responds to a magnetic field generated largely thousands of kilometers below it. The source is not a giant permanent bar magnet. Earth's liquid outer core contains iron-rich material that conducts electricity, meaning electric charge can move through it. The fluid's continuing motion helps maintain a natural generator called the geodynamo.

When conducting fluid moves through an existing magnetic field, electromagnetic induction can drive electric currents within it. Those currents generate magnetic fields of their own. For suitable, sufficiently complex patterns of motion, this generated field helps sustain the field involved in induction. Motion, current and magnetism form a coupled process rather than three independent steps.

The loop still needs energy. Electrical resistance continually converts some electrical energy into heat, so a field would not be maintained indefinitely by merely drawing a circular arrow. Thermal and compositional differences create buoyancy: material with lower density than its surroundings tends to rise while other material moves to replace it. This convection supplies motion, and Earth's rotation influences how the flows are organized.

Some of the field produced in the core extends through the mantle to the surface and into surrounding space. A compass aligns with the local horizontal field, which need not point exactly toward geographic north. The field changes because the moving generator changes. Surface measurements and physical models let scientists investigate that hidden system, although details of its internal flows remain an active research subject.