If you could travel back in time 41,000 years to the last Ice Age, your compass would point south instead of north. That’s because for a period of a few hundred years, Earth’s magnetic field was reversed.
Such reversals have happened repeatedly over the planet’s history, sometimes lasting hundreds of thousands of years. We know this from the way it affects the formation of magnetic minerals that we can now study on the Earth’s surface.
There are various takes on why magnetic-field reversals happen. One of these just became more plausible. My colleagues and I discovered that regions on top of the Earth’s core could behave like giant lava lamps, with blobs of rock periodically rising and falling deep inside our planet.
This could affect its magnetic field and cause it to flip. The way we made this discovery was by studying signals from some of the world’s most destructive earthquakes.
Nearly 2,000 miles below our feet – 270 times further down than the deepest part of the ocean – is the start of the Earth’s core, a liquid sphere of mostly molten iron and nickel. At this boundary between the core and the rocky mantle above, the temperature is almost 4,000C, similar to that on the surface of a star, with a pressure more than 1.3 million times that at the Earth’s surface.