Developments in Brief : Scientists at Work on the Question: Why Does the Compass Point North?

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 say.

The report in the British journal Nature is the latest in a series of discoveries by a team of Harvard geophysicists who are gradually discovering the inner workings of the Earth’s restless, constantly churning interior.

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

Jeremy Bloxham, a Harvard geophysicist, reported last week that his research indicates that most of the forces of the magnetic field exit from the inner core through two cold spots in 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.

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

Scientists believe that 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.”

The geophysicist 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.

Hot and cold spots in the Earth’s mantle were discovered several years ago by scientists who measured earthquakes and tremors, said John Woodhouse, professor of geophysics at Harvard. “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 very 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.”