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Basics of Magnetism 2: The Geodynamo

Here’s the second in a series that explains the basic ideas in paleo-, geo-, and rock magnetism. I’m hoping to separate the real-life mysteries and wonder from the jargon that sometimes makes magnets seem like magic tricks.  Have a question about any of these posts? Or about any aspect of paleomagnetism? I’d love to hear it. Please comment!

If you’ve taken an intro-level geology class, or if you’ve read much about magnetism, you have probably heard that Earth acts like a giant magnet because of something in its core. Earth’s core is a giant lump of metal at our planet’s center. We’ve never been there and have no samples of it, even though, as the crow flies, it’s just a little further from here than Chicago. We do know three important things about the core:

  1. It is dense, probably because it’s mostly made of iron and nickel.
  2. It has a molten outer shell surrounding a solid inner nugget.
  3. It is hot.

More on all of those later. We also think that Earth’s core the giant magnet responsible for Earth’s magnetic field. But here’s the weird thing about Earth’s core. When I say that the core is a “giant magnet,” I don’t mean it in the sense of the things that stick to your fridge. Although iron-nickel alloys like the core would probably stick to your fridge if they were suitably magnetized, they would lose their magnetic stickiness at the high temperatures deep in the Earth (more on that later, too). So how could the core be at such a high temperature and still be a magnet, producing Earth’s magnetic field?

The answer has to do with those giant electromagnets you might have seen at auto wrecking yards. These have an enormous coil of wire through which runs an electrical current. The electric current produces a strong magnetic field, allowing the coil to hold up big iron things like cars.  In the Earth, though, the electric current isn’t passing through wires – it’s caused by the swirling around of molten iron in the outer core.

Earth’s core is more complicated than a coil of wire. In the wrecking yard, the coil of wire becomes a magnet when it’s hooked up to an electrical generator – forcing a current through it. In the Earth, the outer core is both generator and electromagnet [1]. Electrical generators work by moving conductive wires through magnetic fields. In Earth’s core, the flow of molten iron and nickel in the outer core moves the conductive material through Earth’s magnetic field instead. That is the same magnetic field produced by the electrical currents in the molten metal. This confusing process is an example of a feedback loop.

Physicists love feedback loops (as do other scientists and mathematicians). Physical systems with feedback behave in interesting ways. You could imagine starting the molten outer core flowing in a certain way under a very weak magnetic field, maybe from the Sun or something else. Suppose this situation is not strong enough to cause a big electric current in the core. Earth’s core, then, might not be able to sustain its electric generating activity for very long. Alternatively, you might be able to imagine a pattern in which the outer core fluid flows in a way that causes big electrical currents. such a pattern could make Earth’s magnetic field stronger as time goes on.

The flow of molten metal in Earth’s outer core is controlled by a bunch of other factors besides the magnetic field. For example, the outer core loses more heat where the mantle above it is cold [2]. The formation of the inner core, heat due to radioactive elements, and the rotation of the Earth, all make the behavior of the outer core very difficult to predict. The unpredictable behavior of the core can make Earth’s magnetic field strengthen, decay, wander, and even reverse itself.  Nonetheless, over the past ten or so years, observations of Earth’s magnetic field through geological time have become numerous enough [3], and models of core behavior [4] have become precise enough, that we can draw some conclusions about some features of our planet’s core, which will be a topic for later. [5]

[1Dynamo is another term for electrical generator. Earth’s outer core is sometimes referred to as the geodynamo. For more information on this topic, see Glatzmaier, G.A., and Olson, P., 2005, Probing the Geodynamo: Scientific American, v. 292, no. 4, p. 50–57, doi: 10.1038/scientificamerican0405-50.
[2] For example, we think that the lowermost mantle is cold where old slabs of subducted lithosphere have piled up. We can actually image this through a technique called seismic tomography (a topic for another day).
[3] The state of the art in crunching together high-resolution records of past magnetic fields is described in Korte, M., Constable, C., Donadini, F., and Holme, R., 2011, Reconstructing the Holocene geomagnetic field: Earth and Planetary Science Letters, v. 312, no. 3-4, p. 497–505, doi: 10.1016/j.epsl.2011.10.031. Definitely not beginner material.
[4] For information about a geodynamo simulation that includes reversals, see Gary Glatzmaier’s website.
[5] Want additional information? See David Stern’s The Great Magnet, The Earth.


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