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On its face, this fact is simple: our planet's magnetic poles have traded places with some frequency over Earth's history. At points in the past, compass needles would point south instead of north. But look into the details of these transitions and things will get considerably more complicated. What exactly is it like during the times when the poles flip, for example? And what is it about the “geodynamo” of Earth's liquid iron outer core that causes this behavior?
Records of these transitions exist in several forms. Small bits of the mineral magnetite in sediment will tend to orient themselves with the Earth's magnetic field as they settle into place. Isotopes in ice cores can record changes in the magnetic field's ability to deflect away charged particles from space. And lavas—on land or the seafloor—contain magnetite crystals that are locked into place when the lava solidifies.
A new study led by the University of Wisconsin's Brad Singer uses the latest dating techniques to put together a timeline of the most-recent pole reversal (which occurred a little over 770,000 years ago) based on sequences of lava flows around the world.
History written in stone
The records come from lavas in Chile and the islands of Tahiti, Guadeloupe, La Palma, and Maui. All of them have been studied previously for tracking the history of our magnetic field, as they host multiple lava flows that each provide a snapshot around the time of the reversal. But the method used to date these rocks—based on isotopes of the element argon, which gets trapped in crystals as they solidify—has been improved enough over the last few years that the rocks were worth revisiting to get more accurate dates for each flow. The new measurements come with error bars in the neighborhood of just ±5,000 years for 780,000-year-old lavas.
The new dates help lay out an interesting timeline. Although individual records in some places have seemed to record an incredibly rapid reversal of the poles, these lavas show a complex process playing out over something like 22,000 years.
To put the whole picture together, the researchers also compiled a handful of existing magnetic records based on seafloor sediment cores and ice cores. Ice cores can only tell you how strong the magnetic field was, while sediments will record pole locations (although probably less reliably than lavas). Sediments do have the advantage of forming continuously over time, while you can only get a lava flow data point whenever a volcano feels like puking up an eruption.
Flips and flops
The researchers interpret this additional data as showing a major weakening of the magnetic field starting 795,000 years ago before the pole flipped and strengthened slightly. But around 784,000 years ago, it became unstable again—a weak field with a variable pole favoring the southern end of the planet. That phase lasted until about 773,000 years ago, when it regained strength fairly quickly and moved to the northern geographic pole for good.
The team compared this timeline to ideas about how pole flips work. A key 2012 study proposed a common, halting pattern to all pole reversals. This pattern includes a halfway flip followed by an actual full flip that reverts halfway back before stabilizing in the new orientation, all over 9,000 years or less. Rather than fitting this fairly simple pattern, the new researchers point to a 2011 model simulation they say is more similar. Although that model took 50,000 years to make the transition, it showed a matching pattern of rising and falling field strength and pole variability.
Because of that match, they argue this model simulation would “provide an excellent starting point from which to design future simulations” that dig into what would make this reversal event behave that way. Like a first-time driver learning a stick shift, it seems to take ages before the rough fits and starts finally smooth out.