Compass Needles to Flip, Models Show

Nov. 15, 2006 — Compasses could start pointing south soon — as in mere thousands of years — according to new simulations of Earth's internal magnetic field.

Mathematical models indicate that the more tidily the undulations in Earth's magnetic field align with the equator, the more prone the field is to reversing its polarity.

Such a reversal would involve a fading out in the magnetic field and then a restarting, causing compass needles to swing south.

The theory, which seems to be backed up by remnants of actual magnetic fields frozen in ancient volcanic rocks, also suggests that the opposite is true: When the hum of Earth's magnetic field isn’t in sync with the equator — which is just a product of the spin axis of the Earth — the field is a lot less likely to flip.

Right now Earth has a very tidy field, which is weakening, so the chances are high that a reversal is in the offing — although it could take millennia to complete. Over the last 10 to 20 million years the magnetic field has flip-flopped at a rate of about five times per million years.

On the other hand, there was a weird period of stability which lasted from 84 million to 125 million years ago — what’s known as the Cretaceous Normal Synchron (CNS). During those 41 million years the field flipped only once or twice.

"The average reversal rate has varied enormously over the long term," said Robert Coe of the University of California at Santa Cruz. He and colleague Gary Glatzmaier report on the matter in the latest issue Geophysical Review Letters. "There’s a lot of interest focused on the Cretaceous Normal Synchron."

For decades researchers have been trying to figure out what sorts of changes could be going on inside Earth’s core to account for the different reversal rates. One possibility is that the solid inner core of the Earth — which is surrounded by a circulating, convecting outer core that generates the magnetic field — is getting bigger.

Theoretically, that growth should be steady over the eons as Earth’s interior cools and more of that liquid outer core solidifies and fuses on the inner core.

Glatzmaier modeled this, along with different rates that heat might flow out from the outer core to the Earth’s mantle to see which would produce more stable magnetic fields. That’s where they found the correlation between the in-tune magnetic fields and more reversals, and the out-of-tune fields and fewer reversals.

As for how that matches up with the geological record of magnetic fields locked in the magnetic minerals in rocks, it’s looking pretty good, says Coe.

"We have data from (ancient) lava flows that indeed show the anti-symmetric (out-of-tune fields) were reversing slowly or not at all," said Coe. "So it fits very well."

But it’s just a correlation, he points out. It doesn’t explain why or predict when the magnetic field will flip.

"It’s not a simple thing to figure out why the field goes through reversals," said geologist William Sager of Texas A&M University.

Except for larger scale trends like that of the CNS, the specific, finer-scale switching process appears to be chaotic.

"People have thought for a very long time that there’s something big going on in the outer core," Sager said.

Geoscientists do, however, have a pretty good idea how a reversal proceeds. According to high-resolution magnetic records found in marine sediments, it looks like magnetic reversals begin with a weakening of the field over 10 to 20 millennia, Sager explained.

When the actual flip happens, it’s still not overnight. It can take a couple thousand years to complete.

During that final switch period Earth would have no strong, north-south, bar-magnet-type magnetic field, which ought to make it more vulnerable to dangerous charged particles from space.

Despite the predicted increased risk, Sager said there's nothing to suggest we're in big trouble, should Earth's magnetic field flip.

"There isn’t any real solid evidence that (these are) catastrophic events," he said.