Polar wandering may explain why north pole moves

Navigators have known for centuries that there is a discrepancy between geographic, or true, north and the magnetic north pole. On many navigational charts, this discrepancy, known as declination, is clearly marked in degrees.

For example, one chart shows Washington, D.C., having a magnetic declination of 6 degrees west and Sitka, Alaska, as being 30 degrees east. This means, at Washington, D.C., geographic north is 6 degrees to the west of where a compass is pointing to the magnetic north pole. For Sitka, geographic north is 30 degrees east of magnetic north. These discrepancies would be no more than a nuisance to navigation but for the fact that the magnetic pole does not stay in one place and, in fact, wanders in an almost erratic fashion at varying rates of speed.

Robert Irion reported on a startling new theory as to why this happens in the Aug. 18 issue of New Scientist. In 1600, William Gilbert, who was personal physician to Queen Elizabeth, described the Earth as having a giant bar magnet running through its center. Indeed some elementary science texts can still be found with this simplified explanation for the Earth’s magnetic poles.

The analogy cannot be a true one, of course, for two reasons. One is, as already mentioned, the magnetic poles wander, which they could not do if some gigantic permanent magnet was buried inside the planet. The other is that, in 1820, a Danish physicist named Hans Christian Oersted showed that magnet fields result from the motion of charged particles such as electrons.

This led to the current theory, writes Ladislaw Reday in the Jan. 22, 1986, issue of Technology Review, that the Earth’s magnetic field is the result of the motion of charged particles, or ions, circulating in the planet’s fluid core. The motion of these ions could generate random magnetic fields that show up as polar wandering.

Others link polar wandering to the fact that the Earth reverses the direction of its poles on a periodic basis. Cesare Emiliani, in his book “Planet Earth,” says that the Earth’s magnetic field decreases to zero over a period of about 10,000 years, reverses its direction over 2,000 years, and then returns to its peak intensity over another 10,000 years.

Little is known about the workings of this strange phenomena but Michael de Laine, writing in the February 1999, issue of Physics World, says Danish geophysicists have connected pole reversal and wandering. The magnetic north pole has moved over 600 miles towards the geographic north pole in the past century, says de Laine. He says this observation, coupled with the fact that the Earth’s magnetic field has decreased by 1 percent over the past decade, led the physicists to their conclusion.

In some instances, polar wandering is so pronounced that it is hard to attribute it to variations in the core’s magnetic field or to reversal of the poles. The Danes, from a point in Greenland, found the magnetic pole to be moving at a rate of 20 kilometers, or over 12 miles a year, says de Laine.

Even this is slow compared to a report given by Susan West in the June 9, 1979, issue of Science News. She says that magnetic studies of polar wandering in ancient rocks have shown that New England and the Canadian Maritime have virtually galloped across the planet in the past several million years. Maine, writes West, was once located about where Georgia is today some 350 million year ago. In this case, rapid polar wandering appears to be a consequence of equally rapid changes in the locations of the continents themselves which is the basis of the most recent theory discussed in Irion’s article.

In 1997, a team at Caltech led by geobiologist Joseph Kirschvink made the startling proposal that there are times the planet is so off balance that continent-sized chunks of the crust slide around on the underlying mantle. This process, which Irion calls “true polar wander,” takes millions of years but is a geological heartbeat when compared to the hundreds of millions of years associated with continental drift.

This is how Kirschvink’s hypothesis works. First the continents join together to form a giant supercontinent. That this happens has been established with the last supercontinent being Pangaea some 200 million years ago. Molten rock beneath the supercontinent heats up and flows towards the center just as a bulge of water appears in a pan of hot water before it boils. The bulge continues to grow until the planet is thrown off balance and great pieces of the supercontinent break away and float rapidly toward the equator in order to stabilize the planet’s spin. Kirschvink says that it would be possible for these pieces to move from the poles to the equator within 10 million years.

Is there any evidence this has happened? Kirschvink’s team measured the magnetic properties of four continents and found evidence the poles moved 90 degrees, corresponding to a swing from the north pole to the equator between 535 to 520 million years ago. Such a catastrophic move would have altered sea levels and made previously icebound continents into tropical paradises, says Irions. Today’s polar wanderings do not approximate the rapid shifts that would accompany Kirschvink’s theory. Yet, according to de Laine, the magnetic north pole appears to be increasing its movements by about 2 kilometers a year.

If, as the Danish scientists believe, this presages a pole reversal within the next 1,000 years, people around at the end of this millenium will have more to worry about than the Y2K computer crash. If the planet’s magnet shield is at zero, there will be nothing to shield Earth’s inhabitants from the full fury of the sun’s cosmic radiation.

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College.