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The African country where compasses go haywire

What’s behind the Bangui Magnetic Anomaly? Some experts see an ancient meteor strike

Until the last year, when the Central African Republic’s civil war became a humanitarian crisis too dire to ignore, most Americans thought little about the country at all. It has a low global profile in part because it is exceedingly poor, with four out of five people living on less than $2 a day. It has some natural resources, but because it is landlocked by other troubled countries—Chad, Sudan, Congo, and Cameroon—even if a lull in the war allowed it to extract those from the ground, it would still face formidable problems in exporting them.

But for one group, the Central African Republic is anything but ignorable, and in fact is home to an enduring scientific mystery. Geophysicists who map the earth’s magnetic fields have identified a disturbance in the earth’s natural magnetic fields within the Republic. They still have few clues about what causes it, but at least some think it could be key to understanding one of the most dramatic events in the history of the planet.

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When geophysicists look at the globe, they don’t see national borders. Instead they see geological features like fault lines and tectonic plates, or, if they study geomagnetism, zones where the earth’s normal, needle-points-north magnetic field seems to go haywire. They map these anomalies by satellite and with ground surveys. When they look at the Central African Republic, something strange appears in the center of the country: a massive aberration known as the Bangui Magnetic Anomaly, named for the country’s capital. At 600 miles across, it is one of the largest such anomalies on earth.

“If you were on the ground there and you had a magnetic compass, you’d need to correct for it,” says Patrick T. Taylor, a NASA geophysicist who has studied the anomaly closely. “The compasses would go berserk.”

The earth’s magnetic field is generally consistent, which means hikers and sailors have been able to use it to avoid getting lost since the invention of the navigational compass a thousand years ago in China. Compasses point to magnetic north, a point in the Arctic near the geographic North Pole. But a handful of anomalies are large enough to alert scientists that something unusual is happening in the earth’s crust. When the anomalies can’t be explained, they serve to show just how poorly we understand certain details of earth’s underlying structure. (Arthur C. Clarke and Stanley Kubrick used this mystery to great effect in “2001: A Space Odyssey,” which featured a magnetic anomaly in a crater on the moon that turns out to have been planted by extraterrestrials as a beacon.)

Some large anomalies on earth have obvious origins: The Kursk anomaly in far western Russia is clearly the result of an iron deposit. Not so the Bangui anomaly. First identified 60 years ago by French mappers, it continues to perplex scientists. Iron is usually the explanation for a huge positive anomaly, Taylor says, and one theory explains the Bangui anomaly by proposing that the culprit is a large, dense upwelling of magnetic material, possibly iron-rich, from the earth’s mantle a few miles beneath the surface. But the Central African Republic also has a very negative gravity anomaly—things weigh less there—which suggests that the country is sitting on rock less dense than a huge upwelling of the mantle.

In 1991, Taylor, R. W. Girdler, and J. J. Frawley pointed out features of the Central African Republic that hinted at another origin, first proposed in 1976. They found that the surface of the country resembled a huge, shallow crater, with a circular rim that mapped closely onto the area with strange magnetism and gravity.

The center of the bull’s-eye, Taylor says, may be the site of the largest meteor strike still visible on the surface of the earth, from over a billion years ago. “When meteorites strike the earth, you get a crater and a rim,” he says. And at this size, this one would be “bigger than any other known impact, by an order of magnitude.” Under this theory, a huge meteor would have hit, heating up the impact site, making the iron there more magnetic, and leaving a great pock on the earth that hasn’t disappeared after a billion or more years of erosion.

“Every solid body in the solar system—every moon, every planet, even comets—are covered by impact structures, except for earth,” Taylor says. That’s because the earth has water and an active atmosphere that eventually wash away the evidence. “It’s like they’ve been erased.”

As a result, geologists have become sympathetic to the hypothesis that many geological features that are not obviously craters are, in fact, sites of ancient impacts. “We’ve been slow to recognize the number of craters on the earth,” says Raymond Jeanloz, a planetary geophysicist at the University of California at Berkeley. “We’ve probably overlooked a few dozen.” Jeanloz points out that the most famous such site, the Chicxulub site in the Yucatan peninsula, was found and explored only in the last 35 years or so. It is now thought to be the strike that killed the dinosaurs.

Jeanloz classifies the Bangui meteor hypothesis as “interesting but unproven.” Only a handful of geophysicists have examined the Bangui anomaly in any detail, and all of them seem to agree that more work would be necessary to advance the hypothesis.

One hint, proponents believe, might be found in one of the Central African Republic’s few exports. The country has a small diamond industry—its total annual product could fit in a single suitcase. But in addition to gemstones, it boasts a peculiar type of industrial diamond. Taylor and colleagues note that both the Central African Republic and Bahia, Brazil—areas that were once close to each other before plate tectonics shifted the continents into their present positions—produce “carbonados.” Most other diamonds are found in pipes of a rock called kimberlite. But carbonados seem to be scattered, and now Taylor and others believe they might be the remnants of a meteor made mostly of diamond. The carbonados could, Taylor says, be part of the “splatter” of the strike.

The simplest way to check this hypothesis would be to go rock-hunting in the Central African Republic. Taylor would like to visit the areas that look like the rim of the crater, and study rocks for signs that they were deformed in a meteor strike. “That would be the smoking gun.” But for the moment he’ll have to wait. The Central African Republic was never an easy country to do research in—it has few paved roads and no scheduled commercial domestic plane routes. Now, the war has made research almost unthinkable. “It’s very hard to get in there and do proper work,” Taylor says.

Even if Taylor makes it to the Central African Republic, he’ll be examining the coldest of geological cold cases. Over a billion years have passed since the event he thinks happened. And we simply have very little idea what a huge meteor crater looks like a billion years after the fact. “What do you look for, when you’re looking for ancient impacts?” Taylor asks. “We don’t really know.”

Related:

MIT scholar fights malaria with magnets

Alan Guth: What made the Big Bang bang

NASA spacecraft launched in 1977 enters interstellar space

Harvard-Yale team on trail of electron’s mysteries

Graeme Wood is a contributing editor at The Atlantic.
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