Why are earth's poles flipping out?

Magnetic earth

A while ago I joked with a friend about some mishaps in my life, flippantly quipping that they must be due to solar flares. “Or earth’s demagnetizing poles!” He joked. “Yeah, where does all that iron go?” I shot back. Then I wondered, No really, what happens?

I knew that the earth’s magnetic poles flipped periodically, and that these events are recorded in volcanic rocks and “magnetic striping” in the seafloor, but I didn’t know why the poles flipped. Or what happened to animals that rely on magnetic sensing during these events. So I pulled my old geology textbook off my bookshelf (no, I’m not telling you what year I took GEO-2010!), and did a little digging on the internet to fill in the holes. I learned enough for at least two posts — one on the earth processes (today), and a second on the magnetic sensing in animals (later). Here goes:

Earth core

Most geologists accept a hypothesis that the earth’s magnetic fields are generated by electrical currents in the earth’s molten outer core, which are in turn caused by convection that churns at an accelerated rate compared to convection in the mantle (we’re talking about several orders of magnitude difference in the rates here, like a million times faster).  But until the mid-1990s, apparently no one had a plausible model to describe why the earth’s magnetic field was self-sustaining (the temps at the core apparently prevent any material from being permanently magnetized), why the magnetic poles are roughly aligned with the rotational axis, and why the poles flipped periodically.

Then in 1995 a pair of researchers published the first major study to explain these mechanisms {1}.  You can read the detailed explanation at GeoDynamo. A crude two-line summary might go like this: as the earth rotates, parts of the conductive,  liquid outer core solidifies on to the inner core, in a process where the crystallization of iron gives off latent heat (despite the high temps in the core, the pressure is so great that the material “freezes” onto the

Helical fluid swath within outer core. Credit: G. A. Glatzmaier

inner core). This heat instigates thermal buoyancy in the liquid outer core, causing fluid swaths to rise and be less viscous than other parts. Compositional buoyancy is also created by the lighter materials left in the outer core as the heavier iron particles accrete on to the inner core.  At the same time, because the earth rotates, the Coriolis effect causes a shearing effect on the liquid, resulting in twisted, helical convection currents within the outer core. This combination of convection and twisting results in an eastward flow closer to the inner core boundary and a westward flow closer to the outer core-mantle boundary. This convective movement within the conductive core kick-starts an electric current, and the current itself generates a magnetic field too, which explains the self-sustaining generation of both the electric current and the magnetic field lines. Apparently, the Glatzmaier- Roberts model was the first to give a plausible mechanism for how the earth’s magnetic field continually renews itself . Okay, that is more than a two-line summary but I am not known for my brevity. (Whew, I hope I got that all right… if I mangled something, drop me a line.)

Even better, when allowed to play out over time, the Glatzmaier- Roberts model mimicked a magnetic polar flip. From what I understand, they didn’t structure their model to try to mimic this, but because they factored in the earth’s rotation, the polar flipping happened as a result of this parameter. Apparently the magnetic lines emanating from the twisted convecting outer core can become tangled, leading to periodic “islands” of flipped polarity at the earth’s surface that tend  to forecast a reversal event. These “islands” and the tangled magnetic lines lead to an overall weakening of the magnetic field (forces cancel each other out). The fluctuations grow stronger, causing declining magnetism, and then the chaotic system flips and stabilizes again.

When a reversal occurs, instead of the magnetic lines departing from the south pole and diving back down into the north pole, as we are used to observing, they suddenly shoot out of the north pole and ride back to the core via the south pole. For a really neat visual of how this works, visit NOVA’s Magnetic Storm pages, where they have a QuickTime video of the model {2}. We know these flips have occurred in the past because they are recorded in volcanic rocks and the seafloor in what scientists call the paleomagnetic record. What’s even neater is that Mars used to have a magnetic field too, but it de-activated about 4 billion years ago.

The paleomagnetic record shows that the earth’s poles have flipped roughly – very roughly – every 250 thousand years or so. It’s definitely not a highly regularized phenomenon, and yet it occurs with some regularity the farther out you dial the geological time scale. The record also shows that it’s been about 720 thousand years since the last flip-flop, and that today’s magnetic field has been weakening for about 300 years. The entire flipping process can take several hundred to several thousand years.

Solar wind and magnetosphere

Because the earth’s magnetic field protects the surface from solar radiation and solar wind {3}, I’m curious to learn what happens to ecosystems, and animals that rely on magnetic sensing, when the magnetic field weakens and flips. Stay tuned, and I’ll let you know what I find out…

NOTES:
{1} G.A. Glatzmaier and P.H. Roberts, “A three-dimensional self-consistent computer simulation of a geomagnetic field reversal,” Nature, 377, 203-209 (1995).
{2} NOVA science programming, Magnetic Storms, original air date November 18, 2003, retrieved online August 2, 2009.
{3} Physical Geology Revealed, 2nd edition. David McGeary and Charles Plummer. pg 38-39.

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