Which statement describes the mechanism generating Earth's magnetic field?

• A. The movement of liquid iron in the outer core generates the field.
• B. The atmosphere's winds generate the field.
• C. The Moon's gravity generates the field.
• D. The oceans generate the field.

Answer: (A)

The main idea being tested is that Earth’s magnetic field comes from a dynamo in the planet’s deep interior. In the outer core, molten iron and nickel are a highly conducting fluid. Heat-driven convection makes this liquid move, and because the Earth is rotating, those flows organize into structured patterns under the Coriolis effect. As this electrically conducting fluid moves, electric currents are generated, and those currents produce magnetic fields. Those magnetic fields interact with the moving fluid in a self-sustaining loop, so the field is kept alive much like a dynamo in a generator. The result is a large-scale magnetic field that is roughly dipolar and extends far into space.

The other sources don’t explain the global field. Winds in the atmosphere aren’t strong enough conductors deep inside Earth to generate the sustained, planet-wide field. The Moon’s gravity doesn’t create a magnetic field. Ocean currents can induce small, local magnetic signals, but they don’t drive the planet’s overall magnetic field.