How is It Formed?
That’s a very good question. The easy answer is that much of the magnetic field is formed as a result of electric currents generated deep beneath our feet. In reality, it’s much a much more complex set of processes that also includes a small amount of magnetism that already exists in rocks in Earth’s crust as well as magnetic fields generated from interactions with the solar wind. For this discussion, I’m going to focus on the action at the heart of our planet. This is where 97 percent of our magnetic field is generated and has been for the last 3.5 billion years.
The action that generates much of our magnetic field occurs in the liquid outer core that surrounds a solid inner core at the center of the planet. This diagram at left is a model of what geologists use to explain the process.
How does this process work?
A lot of times you’ll hear scientists refer to “Earth’s geodynamo” almost as if there’s a huge engine at the center of our planet that is generating the field. Actually, it’s a very nice analogy for what is a very complex set of actions. I’ll give you a “rough” outline here, since the actual mechanics are more complex than I want to get into here. However, if you’re interested in the complexities, visit the the United States Geological Survey Geomagnetism page, or the Comprehensive Modeling of the Magnetic Field page at Goddard Space Flight Center, or When North Goes South, a page that explains how Earth’s magnetic field changes over time.
To generate and maintain a field such as Earth’s (or any of the planets, for that matter), you need electrically conducting fluids interacting with each other and with the existing magnetic field. Inside Earth, the fluid material is molten iron (and some nickel) that conducts electricity. You also need motion to create gentle currents (sort of like currents you see in the ocean) in the molten iron. That motion is supplied by the rotation of Earth’s various layers around our planet’s spin axis. The core rotates the fastest, the outer core rotates at a slightly slower rate, and the rest of Earth’s layers and the surface rotate even more slowly. The fluid iron sloshes around, adding to the electrical generation environment.
The currents flow across the already-existing magnetic field lines and this keeps the whole magnetic field-generating mechanism going. If the flow stopped (say if Earth stopped turning or somehow the fluid outer core was somehow cooled down to the point where it was no longer as fluid), the generating “dynamo” would shut down and the magnetic field would diminish over time. There would still be some “remnant” of the field emanating from rocks, but that would be it. Luckily, our geodynamo isn’t likely to shut off anytime soon, but if you want to see a planet that doesn’t seem to have a dynamo at its core, check out Mars. it does havve remnant magnetic fields from concentrations of magnetized rocks. But, there’s no overarching huge magnetic field being generated as there is here at Earth. That, in itself, is an interesting clue to Mars’s evolutionary past. Studying it could help us understand what happens to a planet when it loses its magnetic field.