Magnetic Pull Tied to Earth’s Hot, Cold Spots

Hot and cold spots in the thick layer of rock beneath the Earth’s crust may pull the planet’s magnetic field into shape and explain why compasses point north, scientists reported Wednesday.

The report in the British journal Nature is the latest in a series of discoveries by a team of Harvard University geophysicists who are making inroads into understanding the restless, constantly churning interior of the Earth.

Scientists believe the movement of molten iron in the Earth’s core acts like a generator and creates the planet’s magnetic forces.

Beneath Antarctic

Jeremy Bloxham, a Harvard researcher in geophysics, said his research indicates that most of the forces of the magnetic field exit from the inner core through two cold spots in the Earth’s rocky mantle beneath the Antarctic continent.

The forces then loop northward across the planet and re-enter the core through two more cold spots in the mantle, one underneath northern Canada and another in Siberia.

Bloxham said the cold spots in the mantle, a thick shield of rock that surrounds the molten core and extends to within 20 miles of the planet’s surface, are created when rock material from the surface sinks.

He said his discovery helps explain why the magnetic field moves toward the North Pole, forcing compass needles to point north.

Directional Changes Seen

Scientists believe the magnetic field shifts from year to year and that every million years or so the entire direction of the magnetic field reverses itself. Bloxham said the directional changes are probably dictated by hot and cold spots in the mantle.

“In a million years, a compass could point south,” he said. “It’s a very slow process.”

Bloxham traced the movements of the magnetic field by using sensory equipment that translated the magnetic pull into field lines, much the same way weather forecasters use lines to indicate cold and warm fronts on a map.

“There are field lines coming out and entering the Earth in other places, but they are concentrated at these spots,” he said. “If field lines come out of the core, they must go back in eventually.”

Hot and cold spots in the Earth’s mantle were first discovered several years ago by scientists who measured every earthquake and tremor in the world, said John Woodhouse, professor of geophysics at Harvard.

Studied Earthquake Data

“With seismic data you can infer about the internal structure of Earth and map out a mantle,” he said. “It’s like if you listen very carefully to a bell ring, you can determine its shape.”

Woodhouse said these scientists determined cool spots in the same areas where Bloxham’s magnetic field lines were concentrated.

“In the past 20 years, we have been mapping the magnetic field and mapping the internal structure of Earth and now they are coming together,” he said. “We are seeing a relationship.”