North by northwest: the planet's wandering magnetic poles help reveal history of Earth and humans.
In other locales, a compass needle slews either to the left or the right of true north by a certain angle, a process commonly known as declination. That's because a compass isn't attracted to the north geographic pole, the point at which Earth's rotational axis pierces the Arctic ice. Instead, the needle is attracted to the north magnetic pole, the spot where the planet's invisible magnetic field lines burst from the surface and point directly upward.
Astronomers have long known that a compass doesn't always point true north, a bearing in the night sky that lies within 0.5[degrees] of Polaris, the North Star. Their measurements of declination, along with those made by seafaring explorers, enabled 16th-century sailors to better navigate along their trade routes, especially those linking the New World to the Old. What many scientists didn't appreciate until the 1600s, after they had compiled a few decades' worth of precise measurements at astronomical observatories, was that declination varied through time. Suddenly, they realized: The magnetic pole moves!
What difference does this make in today's world, where pilots, navigators, and even backcountry campers increasingly depend on satellite-based technology such as Global Positioning System equipment to find their way? In practice, not much: Earth's magnetic poles are located in remote regions and in recent times they've moved, at most, only a few dozen kilometers a year.
However, a slowly wandering magnetic pole is a boon for archaeologists and other researchers who study the past. Often magnetic substances in rock, paint, and other materials become aligned with Earth's magnetic field under certain conditions, enabling researchers to, say, determine when a mural was painted, when a town was built, or when a fireplace was used for the last time.
WANDERING POLES Draw a line between the north and south geographic poles, and it runs smack through the center of the planet. Earth's rotation around this axis once each 24 hours produces the familiar cycle of day and night. Unlike the geographic poles, however, our planet's north and south magnetic poles aren't located directly opposite one another, says Nils Olsen, a geophysicist at University of Copenhagen.
Earth's geographic poles are fairly stable, wobbling back and forth across the landscape only a few meters every year or so (SN: 8/12/00, p. 111). The north and south magnetic poles are far more mobile, and they move independently of one another, says Olsen. Now located in the Arctic Ocean just north of Canada, the north magnetic pole is moving northwest toward Siberia by about 50 km each year. The south magnetic pole, just off the Antarctic coast south of Australia, is also--for now--heading northwest, but only at around 5 km/yr, Olsen and Mioara Mandea, a geophysicist at the National Research Center for Geosciences in Potsdam, Germany, report in the July 17 Eos.
Such wanderings stem from irregularities in the process that generates the magnetic field, says Olsen. Although Earth's inner core is solid and primarily composed of iron, its outer core is a molten mix of iron and lighter metals that is constantly on the move. The flow of that material, which carries charged particles and conducts electricity, produces the magnetic field, says Olsen. Long-lived eddies and swirling currents in the fluid, which moves at an average speed of about 20 km/yr and is no more viscous than water, make the magnetic field deep within Earth much more complex than it is at the planet's surface. "It's a highly chaotic system," says Olsen.
In particular, he notes, that turbulence can create "reversed-flux patches," regions on the surface of the outer core where magnetic field lines point opposite to those predominant at the Earth's surface. Variations in the size and strength of these patches significantly affect the location and the motion of the magnetic poles. For instance, the growth and movement of a reversed-flux patch beneath northern Canada is causing the north magnetic pole to surge toward Siberia.
At its current rate, the north magnetic pole will pass within 400 km of the north geographic pole in 2018, Olsen and Mandea report. Because of the chaotic nature of the field-generating processes in the outer core, predicting the pole's location more than a decade into the future is tricky, says Olsen. Nevertheless, the pole has been moving toward the northwest, although with varying speed, for more than a century.
In the past few decades, the strengthening of reversed-flux patches--especially ones beneath Canada and the South Atlantic Ocean--has weakened Earth's magnetic field, says Olsen. If the field's overall strength keeps dropping at today's rate, it will reach zero in a few hundred years. However, he notes, it's not clear whether recent fluctuations in field strength are routine variations or the prelude to a full-blown reversal of Earth's magnetic field--something that happens, on average, every quarter-million years or so.
Although the strength of the Earth's magnetic field is now dropping, it is 50 percent stronger than the estimated average for the past 60 million years, says Lisa Tauxe, a paleomagnetist at the Scripps Institution of Oceanography in La Jolla, Calif. At its most recent peak, about 2,000 years ago, the magnetic field was about twice as strong as it is now. "Data is spotty, but we have a crude idea of what's going on [with the magnetic field]," she notes. The data also suggest that "the field can change rapidly over a shorter time than [scientists] had thought."
AROUND THE WORLD Only in the past couple of centuries have scientists visited the Earth's magnetic poles. The first explorers to find the north magnetic pole did so at Cape Adelaide, on the west coast of Canada's Boothia Peninsula, in 1831. An expedition 73 years later discovered that the pole had moved about 50 km to the northeast. In the following century, the pole moved more than 1,300 km toward the northwest, along the same path it is taking today.
Despite this limited history of direct observations, researchers can use various clues to estimate the size, strength, and polarity of Earth's magnetic field at many times in the past. For instance, some minerals that crystallize as lava cools can record the direction of the planet's magnetic field at the time the eruption occurred, says Steven T. Johnston, a geologist at the University of Victoria in British Columbia. In many cases, such information enables scientists to establish the latitude where pieces of Earth's crust originated and thereby infer their long-term tectonic motion, he notes.
As long as magnetized minerals aren't heated above a characteristic temperature known as a Curie temperature, the alignment of the magnetic materials contained therein remains intact. If the rocks are heated beyond the Curie temperature, which typically lies between 500[degrees]C and 600[degrees]C, the stored magnetic information gets scrambled, says Cathy Batt, a paleoarchaeologist at the University of Bradford in England. Then, when the rocks cool, their magnetic materials realign themselves with the planet's magnetic field (SN: 3/13/04, p. 174). Because fires usually are hotter than a mineral's Curie temperature, magnetic materials lining a hearth record the strength and direction of magnetic field lines at the last time a fire had been lit there--a finding of great interest to an archaeologist, for example.
By combining data gathered by geologists and archaeologists, researchers have tracked the motions of the magnetic poles for the past 7,000 years or so, says Mandea. During that time, the magnetic poles have wandered through all longitudes, roughly circling the geographic poles, she notes. While the north magnetic pole has remained well within the Arctic Circle, the south magnetic pole has recently roamed farther away from the south geographic pole and is now around 64[degrees]S.
Using information collected in Britain, mostly from England and Wales, Batt and her colleagues have compiled a record of how magnetic declination has varied in that region during the past 4,000 years. To provide a more useful comparison among sites, the researchers adjusted each measurement to replicate what the magnetic field would have been like at Meriden, England, a town about 150 km northwest of London. The model should be valid for any site within 500 km of that town, which is roughly the center of the England-Wales region, Batt and her colleagues note in the Feb. 16 Physics of the Earth and Planetary Interiors.
The team's data also include information about magnetic dip, the angle between the Earth's magnetic field lines and a horizontal plane. Only a few of the 858 sets of measurements, most notably the 238 data points taken at observatories since the 1600s, include data about paleointensity, or how strong the planet's magnetic field was at the time data were gathered. The combination of two or more of these parameters enables researchers to better estimate the age of an artifact when other clues don't provide a clear answer, says Batt.
Most centuries during the past 4 millennia are represented by at least 10 data points. However, few archaeological sites have been dated to the centuries between A.D. 600 and 800, which historians often refer to as the Dark Ages. Data for the centuries before 1000 B.C. are similarly sparse.
During the past 4 millennia, magnetic declinations in Britain have varied through an angle of 70[degrees] and their magnetic dips have ranged about 25[degrees], the researchers report.
To test their model, Batt and her colleagues analyzed an archaeological site that was exposed during construction in downtown Exeter, England. The city has been continuously populated since the Roman period, so sites there often include a jumble of artifacts from different periods. One sample the team analyzed probably came from a fireplace in a home or other structure. Another sample was, most likely, just a spot of burned soil.
The combination of declination and dip found in the fireplace sample suggest the material could have been last heated during any of three intervals during the past 4 millennia, says Batt. However, because two of those intervals long predate known occupation in the area, the researchers dismiss those possibilities. Therefore, the fireplace probably last hosted a fire in the 11th century, during Europe's early-medieval period. The burnt spot of soil is a century or so older than that, the magnetic data suggest.
Using paleomagnetic data offers archaeologists "a good tool to figure out who occupied a particular area at a particular time, and what they were doing," says Batt.
PROBING THE PAST One debate among English historians regards what stimulated urban development in the fledgling nation. King Alfred, who with his brother unified the nobility in the mid-800s, commissioned earthwork defenses in many areas of southern England after Viking attacks in the 860s. One big question: Were the cities encircled by those earthworks well developed before construction of the defenses, or did those fortifications provide the protection needed for small villages to grow into thriving cities?
Information unearthed during a construction project in downtown Winchester, about 90 km southwest of London, could help settle the debate, says Ben Ford of Oxford (England) Archaeology and director of excavation at the site. During a 5-month investigation at the 2,000-square-meter construction site, he and his colleagues uncovered the remnants of many ancient structures, including some blacksmith shops. The researchers drilled samples from each of 17 hearths, estimated their ages using paleomagnetic dating techniques, and then carbon-dated organic material such as ash, burned seeds, and small sticks--presumably the remnants of the hearths' last fires--to verify the results.
Most of the ancient structures were found in an area measuring 60 m long and 12 m wide, a hint that the densely packed buildings sat along an established road, says Ford. The full range of estimated ages of the Winchester hearths, runs from the 9th century to the 14th. Preliminary results for two of the samples suggest that those structures were last used in the 840s and the 850s--decades that clearly predate the earthworks commissioned by King Alfred, Ford notes.
"These [findings] provide detail in the historical record for an area that isn't well known," says Mark Hounslow, a geographer at Lancaster (England) University, who has worked at the Winchester site.
Finding so many hearths of different ages at one site will be a boon for paleomagnetists, says Ford. Results of the team's paleomagnetic analyses can be added to comprehensive data-bases like Batt's, he notes. And, the new findings may allow scientists to fine-tune the patterns of magnetic pole movement inferred from those data.
By using paleomagnetic data, researchers no longer have to infer the ages of strata from the presence of easily dated objects such as coins or distinct forms of pottery, says Ford. "Now," he notes, "we can write history from archaeological data."
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|Date:||Dec 22, 2007|
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