The Earth’s magnetic field, which protects us from potentially dangerous solar radiation, is gradually losing its stability. No need to move underground or build space colonies just yet, though: the changes are taking place over millions of years.
You might assume that compasses will always point north, but in fact the magnetic poles have swapped places many times in the Earth’s history. Earth scientists have long suspected that these flips are becoming more frequent, and that the magnetic field was less prone to pole reversals in the distant past.
Now the most detailed analysis of the geological evidence to date suggests that the field really is slowly destabilising. Whereas in the distant past it reversed direction every 5 million years, it now does so every 200,000 years.
Earth’s magnetic field is powered by the heart of the planet. At its centre is a solid inner core surrounded by a fluid outer core, which is hotter at the bottom. Hot iron rises within the outer core, then cools and sinks. These convection currents, combined with the rotation of the Earth, are thought to generate a “geodynamo” that powers the magnetic field.
Because of changing temperatures and fluid flows, the strength of the magnetic field varies, and the positions of the north and south magnetic poles shift.
These shifts leave traces in rocks. When lava cools, metal oxide particles within the rock become frozen in the direction of the prevailing magnetic field. So scientists can work out the historic positions of the magnetic poles by examining and dating lava samples.
Researchers believe the inner core is slowly growing, as the outer core cools and solidifies. That should mean more frequent flips. Simulations by Gary Glatzmaier of the University of California, Santa Cruz and his colleagues, suggest that a bigger inner core would be more of an obstruction to currents in the outer core, making for a more unstable magnetic field.
But it is hard to verify this, because in older rocks the evidence of magnetic field direction is less well preserved. So Toni Veikkolainen of the University of Helsinki in Finland, assembled a swathe of existing data from rock samples between 500 million and 3 billion years old.