科学美国人60秒:地球磁极倒转历时数千年

This is Scientific American's 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 used lava flow records, along with sedimentary and Antarctic ice core 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, at about 784,000... The two excursions that we've discovered in the lava record, and are seen in some of the sedimentary records, are a sign that the dynamo is beginning to undergo the reversal process 20,000 years earlier than the final reversal took place. So our arguments would be that the reversal process is complicated and long-lived, and it gets underway, it gets initiated well before the final reversal takes place."
All this activity prior to the final reversal is vital for our grasp of the process.
"The ultimate goal here is, we want to understand what drives reversals—what really happens in the dynamo? And if you just start and look at this short period right around the reversal, you're missing all this unusual behavior that happens in thousands of years prior to that."
We need to know if Singer's findings hold true for magnetic field reversals in general. Alterations in the field will mess with critical human systems, such as the GPS satellites that help us navigate. Fortunately, whenever the next reversal happens, it looks like we'll have plenty of time to prepare.
Thanks for listening for Scientific American — 60-Second Science. I'm Annie Sneed.