New research suggests a polarity reversal of the planet takes about 22,000 years, significantly longer than former estimates.
Swirling around the solid inner core of our planet, more than 1,800 miles below the surface, hot liquid iron generates a magnetic field that stretches beyond the atmosphere.
This field provides us with everything from compass directions to protection from cosmic rays, so it’s no surprise that scientists were alarmed earlier this year when they noticed that the northern magnetic pole was rapidly drifting towards Siberia. While geophysicists scrambled to release an updated model of Earth’s magnetic field ahead of its five-year schedule, the migrating pole posed an urgent question: Is the Earth’s magnetic field preparing to flip?
The magnetic state of our world is constantly changing, with the magnetic north and south poles wandering by a few degrees every century or so. Occasionally the magnetic field experiences a complete polarity reversal, causing the magnetic north and south poles to switch places, although no one knows exactly what causes this turnabout. (In fact, the north pole of the planet is a magnetic south pole right now, but it is still referred to as “magnetic north” to correspond with our geographic measurements.)
In a study published today in Science Advances, researchers report a new estimated timeline of the last polarity reversal, named the Brunhes-Matuyama reversal, which happened around 780,000 years ago. Using a combination of lava samples, ocean sediments and ice cores, they were able to track the progression of this reversal and demonstrate that its pattern was longer and more complex than suggested by previous models. The findings could enable better understanding of how our planet’s magnetic environment evolves and hopefully guide predictions for the next major disturbance.
“[Polarity reversal] is one of the few geophysical phenomena that is truly global,” says Brad Singer, professor of geoscience at the University of Wisconsin–Madison and lead author of the study. “It’s a process that gets started in the deepest parts of the Earth, but it manifests itself in rocks across the entire surface of the planet and effects the atmosphere in pretty important ways. … If we can establish chronology for the timing of reversals, we have markers that we can use to date rocks all over the planet and know common time points around the entire Earth.”
The generation of Earth’s magnetic field starts at its very center. Heat from the solid inner core produced by radioactive decay warms the surrounding liquid iron, causing it to circulate like a pot of water on a stovetop. The fluid motion, or convection, of the iron creates an electric current, which generates a magnetic field. As the Earth spins, the magnetic field roughly aligns with the axis of rotation, creating the magnetic north and south poles.
Over the last 2.6 million years, Earth’s magnetic field flipped 10 times and nearly flipped more than 20 times during events called excursions. Some researchers believe polarity reversals are caused by a disturbance in the balance between Earth’s rotation and the temperature at the core, which alters the fluid motion of the liquid iron, but the exact process remains a mystery.