Why Does Earth's Magnetic Field Flip?

John Roach
for National Geographic News
September 27, 2004

Earth's magnetic field has flipped many times over the last billion years, according to the geologic record. But only in the past decade have scientists developed and evolved a computer model to demonstrate how these reversals occur.

"We can see reversals in the rocks, but they don't tell us how it happens," said Gary Glatzmaier, an earth scientist and magnetic field expert at the University of California, Santa Cruz.

Based on a set of physics equations that describe what scientists believe are the forces that create and maintain the magnetic field, Glatzmaier and colleague Paul Roberts at the University of California, Los Angeles, created a computer model to simulate the conditions in the Earth's interior.

The computer-generated magnetic field even reverses itself, allowing scientists to examine the process.

Computer Model

Scientists believe Earth's magnetic field is generated deep inside our planet. There, the heat of the Earth's solid inner core churns a liquid outer core composed of iron and nickel. The churning acts like convection, which generates electric currents and, as a result, a magnetic field.

This magnetic field shields most of the habited parts of our planet from charged particles that emanate from space, mainly from the sun. The field deflects the speeding particles toward Earth's Poles.

Our planet's magnetic field reverses about once every 200,000 years on average. However, the time between reversals is highly variable. The last time Earth's magnetic field flipped was 780,000 years ago, according to the geologic record of Earth's polarity.

The information is captured when molten lava erupts onto Earth's crust and hardens, much in the way that iron filings on a piece of cardboard align themselves to the field of a magnet held beneath it.

Most scientists believe our planet's magnetic field is sustained by what's known as the geodynamo. The term describes the theoretical phenomenon believed to generate and maintain Earth's magnetic field. However, there is no way to peer 4,000 miles (6,400 kilometers) into Earth's center to observe the process in action.

That inability spurred Glatzmaier and Roberts to develop their computer model in 1995. Since then, they have continued to refine and evolve the model using ever more sophisticated and faster computers.

Continued on Next Page >>


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