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This is Scientific American 60 Second Science.

I'm Annie Sneed.

Earth's magnetic field, which creates our planet's north and south pole, is far from fixed.

In fact, the field is quite active, sometimes it weakens and even reverses, causing Earth's polarity to switch.

Reversals don't happen very often, though, only about every 100,000 to million years.

That's part of why this phenomenon has largely remained a mystery for scientists.

However, a recent study may help researchers better understand how long and how complicated

Earth's magnetic field reversals really are.

The last polarity reversal took place some 770,000 years ago.

In a new study, researchers use lava flow records, along with sedimentary and Antarctic

Ice Corps data, to examine that event.

They found that the reversal took about as long as many scientists previously believed it did, just a few thousand years. But the researchers also

examined the period prior to that final reversal process. And they discovered that a lot was

happening with Earth's magnetic field thousands of years beforehand.

There's clear evidence from the volcanic rocks of a major excursion happening at about 795,000 years ago.

Brad Singer, a geoscientist at the University of Wisconsin-Madison, who led the study.

And that was followed by another excursion, which is the unexpected finding of this study

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