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Chapter 3: Drifting Continents and Spreading Seas
Feature Articles
The Rest of the Story: Generating Earth's Magnetic Field
by Stephen Marshak
Explantatons for how the Earth generates a magnetic field rely on the theory of electronmagnetism: an electric current generates magnetism, and the movement of an electrical conductro (such as metal wire) in a magentic field produces electricty. Because iron allow is a good electrical conductor (electrons flow through it easily), the flow of iron alloy makes the Earth's outer core into an electromagnet.
To better understand the generation of Earth's magnetic field, consider an electric power plant. In a power plant, water or wind power spins a wire coil (an electrical conductor) around an iron bar (a permanent magnet). This apparatus is a dynamo. The motion of the wire in the bar's magnetic field generates an electric current in the wire, which in turn generates more magnetism. Similarly, in the Earth, convection and the Earth's rotation cause liquid iron alloy (an electrical conductor) to flow in Earth's magnetic field (see figure). This flow generates an electric current, which in turn generates more magnetism.
The Earth differs from an electric power plant in that there is no permanent magnet in the center of the Earth; at the very high temperatures of the core, permanent magnets can't exist, because thermal energy makes atoms vibrate and tumble so fast that their tiny magnetic dipoles can't lock into alignment. Instead, the Earth is a "self-exciting dynamo," meaning that the magnetism produced by elecric currents in the outer core is the magnetism that led to the generation of an electric current in the flowing iron alloy in the first place—this system perpetuates itself.