Archaeologists discover 1,000-year-old weak spot in Earth's magnetic field

Patches of ground in southern Africa where huts were once burned down reveal surprising clues about the history – and future – of our planet's geomagnetic shield.

|
John Tarduno/University of Rochester
Grain bins much like these modern ones, photographed in southern Africa, were ritually burned down during Africa's Iron Age. The scorched ground beneath them conserved rare clues about the Earth's magnetic field.
|
M. A. Shea, Geophysics Directorate, Philips Laboratory
This chart maps the location of memory failures, in yellow, for an old satellite, UoSAT-2. They happened much more frequently as it passed through the South Atlantic Anomaly. During solar storms, objects passing through the anomaly are much more strongly affected by damaging cosmic rays.
|
Michael Osadciw/University of Rochester
This depicts the lower strength of the Earth's protective magnetic field over the Southern Atlantic Anomaly, overlaid with the hot, dense mantle rock of an underlying feature deep below. John Tarduno and other researchers think the irregularity below may cause the weakened field above.

The fiery demise of ancient huts in southern Africa 1,000 years ago left clues to understanding a bizarre weak spot in the Earth's magnetic field — and the role it plays in the magnetic poles' periodic reversals.

Patches of ground where huts were burned down in southern Africa contain a key mineral that recorded the magnetic field at the time of each ritual burning. Those mineral records teach researchers more about a weird, weak patch of Earth's magnetic field called the South Atlantic Anomaly and point the way toward a possible mechanism for sudden reversals of the field.

"It has long been thought reversals start at random locations, but our study suggests this may not be the case," John Tarduno, a geophysicist from the University of Rochester in New York and lead author of the paper, said in a statement. [How Earth's Magnetic Field Shielded Us from 2014 Solar Storm]

Tarduno told Space.com in an interview that data from the huts suggest that the strange weak patch "forms, and it decays away, and it forms, and it decays away; eventually, one might form and get really large, and then we might actually have a geomagnetic reversal."

Something strange in the South Atlantic

The South Atlantic Anomaly is a dent in Earth's shield against cosmic radiation, 124 miles above the ground (200 kilometers). It may be the most dangerous place in the Earth's sphere for satellites and spacecraft to traverse, because anything electronic traveling through it is vulnerable to strong radiation from space and tends to malfunction.

Even the Hubble Space Telescope takes no measurements when passing over  the anomaly. It's an area where, instead of pointing outward, part of the Earth's magnetic field actually ushers energetic particles down instead of repelling them, weakening the overall field in the area. And it has been growing.

"Some have postulated that the Earth's magnetic field is leaking out the wrong way at that particular spot," Rory Cottrell, a geologist also at the University of Rochester and co-author of the new paper, told Space.com. "One theory is that changes in the South Atlantic Anomaly could be responsible for the decrease in the overall magnetic field that we're seeing, because these patches are growing or changing over time."

Many researchers have speculated that this kind of anomaly is temporary, caused by changes of flow within the Earth's outer, iron core, which generates the planet's magnetic field. Such anomalies, in weakening the magnetic field, may bring the Earth closer to a magnetic reversal — when the magnetic north and south poles on Earth switch places, rearranging the magnetic field over the course of 1,000 to 10,000 years (although it can happen faster). The process generally happens every 200,000 to 300,000 years, after the magnetic field weakens enough, but the last magnetic-field reversal occurred 780,000 years ago.

The new data from the African burnings suggests that the South Atlantic Anomaly was up to its same field-weakening tricks over 1,000 years ago; if it's caused by something permanent near the Earth's core, it might play an important role in the Earth's magnetic-pole reversals.

Burn it all down

Modern magnetic records only stretch back for the past 150 years or so, and within that time frame, researchers have seen the Earth's magnetic field rapidly decrease in intensity. But the researchers used the Iron Age remnants of African villages to extend their view even further back, from A.D. 1,000 to A.D. 1,850 — and the record reveals that the South Atlantic Anomaly was going strong at that time, too. [Earth Quiz: Do You Really Know Your Planet?]

Throughout that time, the inhabitants of ancient African villages would burn down the huts and grain bins in their villages on a regular basis, giving scientists key, consistent data throughout that time period.

"They had this ritualistic burning of villages," Tarduno told Space.com. "Particularly in times of drought, the conclusion would be that there might have been some offence in the village, so the solution was to have a burning down of the village." The process was intended to cleanse the village, their collaborator archaeologist Thomas Huffman, from Witwatersrand University in South Africa, said in the statement.

At the very least, it cleansed the ground: The burning villages would reach temperatures of over 1,800 degrees Fahrenheit (1,000 degrees Celsius), which would melt the magnetic compounds like magnetite in the clay floors. The magnetite would become remagnetized by the Earth's magnetic field at the precise instant it cooled, ready to be analyzed centuries later.

"The hut floors are actually very good magnetic recorders," Tarduno said. "Sort of like minimagnetic observatories back in time."

Researchers had obtained very little historical data in the southern hemisphere, and none in southern Africa before these findings. The new baked-clay records revealed an eerily familiar picture of the Earth's magnetic field: Just like today, the Earth's magnetic field at the time was steadily weakening, with a focus on that same South Atlantic Anomaly. The effect did not appear to be continuous, but rather seemed to be a recurring event in that part of the globe, whose weakening power comes and goes over time.

Digging deeper

To Tarduno's group, that consistently recurring spot of weakening suggests that a permanent feature deep below the Earth's surface may be generating the South Atlantic Anomaly and might therefore play a role in the reversal of the Earth's magnetic field.

That feature is a section of particularly hot and dense mantle rock just above the Earth's outer core. The section is 1,860 miles (3,000 km) below southern Africa and the Atlantic, and it's about as wide as the distance between New York and Paris. Scientists call it the Large Low Shear Velocity Province, and Tarduno's group suspects that its sharp boundaries might disrupt the flow of iron within the Earth's core, creating a strange, field-weakening eddy that could lead to reversals time and time again.

The researchers' model is only one of many theories about magnetic pole reversal, and they're focusing on refining the mathematics and gathering more, even earlier data from southern Africa to further track the weak spot.

"No one knows what causes reversals, and there is no agreement on whether we can ever even find convincing evidence to forecast a reversal," Ron Merrill, a geophysicist from the University of Washington, who was not involved in the study, told Space.com in an email.

While the new magnetic field records in Africa are useful in their own right, he wrote, it will take much more testing and theory to make a solid connection between the feature near the Earth's core and the magnetic field's weakening and reversal (and the long-lasting nature of the South Atlantic Anomaly).

The new research can't predict the next magnetic field reversal, but finding a connection between the ancient irregularity near the Earth's core and a weakening magnetic field would be one more step toward deciphering the incredibly complex magnetic system that protects humanity from the harsh radiation of space.

This research is detailed in the July 28 edition of the journal Nature Communications.

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com

Copyright 2015 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to Archaeologists discover 1,000-year-old weak spot in Earth's magnetic field
Read this article in
https://www.csmonitor.com/Science/2015/0730/Archaeologists-discover-1-000-year-old-weak-spot-in-Earth-s-magnetic-field
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe