Heat flow causes magnetic reversals
Earth’s magnetic field has done hundreds of somersaults over the last few billion years and if it flipped again tomorrow, compass needles would swing south instead of north. On average, these reversals happen only once every couple of hundred thousand years, but the number of switches has increased 100-fold during the last 10 million to 20 million years, compared with the time period between 125 million and 84 million years ago, leading researchers to wonder what has caused the recent surge in reversal rate.
A new study, published Nov. 8 in Geophysical Research Letters, is a step toward answering that question, as it sheds some light on what causes the geomagnetic field to flip. Robert Coe and Gary Glatzmaier, geophysicists at the University of California at Santa Cruz, report a new analysis of computer simulations of the geodynamo — the process by which Earth’s magnetic field is produced — and found that the way the core transfers its heat to the mantle influenced the stability of the geomagnetic field, and hence the number of its reversals.
Scientists think that Earth’s magnetic field is generated deep inside the planet where the outer core, a huge pool of molten iron, flows around the inner core of solid iron. Both layers spin at different speeds and the interaction between them creates the geodynamo.
Earth’s inner core has been growing larger since the planet’s birth, and the heat it releases in the process helps drive the flow of the outer core, fueling the geodynamo. But Earth doesn’t like to release its heat uniformly, Glatzmaier explains. “The core prefers to push its heat out either in polar or in equatorial regions, but not so much in mid-latitudes,” he says.
Where the heat actually manages to slip out of the core, however, is dictated by the uneven distribution of heat in Earth’s mantle, the thick layer of rock between the outer core and the crust. Heat from the core moves toward the colder regions in the mantle, but because the mantle itself is constantly moving and changing its formation, those pathways are also changing over time.
When Coe and Glatzmaier simulated different mantle formations, they saw that the geomagnetic field was very stable when cold mantle material was sitting on top of the polar and equatorial regions. However, when the mantle forced the heat out of the core in mid-latitude regions, the fluid dynamics in the core became very unstable.
“When we created conditions at the top of the core that were not as compatible with its natural fluid dynamics, we basically created storms in the outer core,” Glatzmaier says. These storms cause the flow inside the outer core to become more chaotic, causing the geomagnetic field to change constantly and increasing the chances for it to switch its polarity entirely.
“This means that the reversal rate is lower in times when the mantle picks up the heat exactly where the core wants to push it out, and higher when the mantle forces the heat out in other regions,” Glatzmaier says. The heat flow from the core into the mantle affects the geomagnetic field’s symmetry, he says, adding that the simulations show that asymmetric fields are more stable, and result in fewer reversals than symmetric fields.
“These results confirm what other scientists predicted about 15 years ago based on paleomagnetic records found in volcanic rocks,” says Ronald Merrill, a geologist at the University of Washington in Seattle. “But at that time, dynamo theory was not developed well enough to support this hypothesis from a theoretical standpoint,” he says.
The findings are a significant step forward in “understanding our geological history,” Glatzmaier says. “They help us understand how Earth got to where it is today.”