Earth burps and magnetic flips.

Like an unruly child, Earth's magnetic field displays some pretty erratic behavior. Throughout the planet's history, the field has flipped its orientation thousands of times, sometimes pointing toward the north and outer times aiming in the opposite direction. But 123 million years ago, the field became remarkably stable, assuming a consistent orientation for a period of 40 million years.

Two geophysicists have now developed a theory to explain this magnetic "superchron" and other, shorter periods during which the field has remained well behaved. Roger L. Larson of the University of Rhode Island in Narragansett and Peter Olson of Johns Hopkins University in Baltimore suggest that the magnetic field stabilizes when the number of thermal plumes increases inside the planet. These plumes -- the geologic equivalent of burps -- are streams of rock that rise from the deep mantle and erupt at the surface as basalt volcanoes, creating such features as the Hawaiian Islands and submerged oceanic plateaus. As evidence for their theory linking field stability with plumes, Larson and Olson report that the superchron period was a time of tremendous basaltic eruptions from rising plumes.

Geoscientists believe the magnetic field arises from convecting currents of liquid iron within Earth's outer core. Plumes are thought to originate from a boundary layer that separates the core from the overlying rocky mantle.

In Larson and Olson's model, the surge in plume development 123 million years ago thinned that boundary layer, allowing more heat to escape into the mantle and enhancing convection within the outer core. The vigorous convection stabilized the magnetic field, preventing it from reversing its orientation. Later, when the convection died down, the field began its reversals again.