Earth's present magnetic field isn't 'same old, same old'

Turns out, Earth's ancient magnetic field was significantly different than the present day field, originating from several poles rather than the familiar two.

Scientists are able to reconstruct the planet's magnetic record through analysis of ancient rocks that still bear a signature of the magnetic polarity of the era in which they were formed. This record suggests that the field has been active and dipolar, having two poles, through much of our planet's history.

The geological record also doesn't show much evidence for major changes in the intensity of the ancient magnetic field over the past 4 billion years. A critical exception is in the Neoproterozoic Era, 0.5 to 1 billion years ago, where gaps in the intensity record and anomalous directions exist. Could this exception be explained by a major event like the solidification of the planet's inner core?

In order to address this question, Carnegie's Peter Driscoll modeled the planet's thermal history going back 4.5 billion years. His models indicate that the inner core should have begun to solidify around 650 million years ago. Using further 3-D dynamo simulations, which model the generation of magnetic field by turbulent fluid motions, Driscoll looked more carefully at the expected changes in the magnetic field over this period.

His results showed that around 1 billion years ago, Earth could have transitioned from a modern-looking field, having a "strong" magnetic field with two opposite poles in the north and south of the planet, to having a "weak" magnetic field that fluctuated wildly in terms of intensity and direction and originated from several poles. Then, shortly after the predicted timing of the core solidification event, Driscoll's dynamo simulations predict that Earth's magnetic field transitioned back to a "strong," two-pole one.

"These findings could offer an explanation for the bizarre fluctuations in magnetic field direction seen in the geologic record around 600 to 700 million years ago," Driscoll noted. "And there are widespread implications for such dramatic field changes."

It is published in Geophysical Research Letters.