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, the Earth’s magnetic field was reversed. Magnetic 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 alignment of magnetic minerals, that we can now study on the Earth’s surface.
Several ideas exist to explain 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.
Around 3,000km 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 4,000℃ degrees, similar to that on the surface of a star, with a pressure more than 1.3m times that at the Earth’s surface.
On the mantle side of this boundary, solid rock gradually flows over millions of years, driving the plate tectonics that cause continents to move and change shape. On the core side, fluid, magnetic iron swirls vigorously, creating and sustaining the Earth’s magnetic field that protects the planet from the radiation of space that would otherwise strip away our atmosphere.
Because it is so far underground, the main way we can study the core-mantle boundary is by looking at the seismic signals generated by earthquakes. Using information about the shape and speed of seismic waves, we can work out what the part of the planet they have travelled through to reach us is like. After a particularly large earthquake, the whole planet vibrates like a ringing bell, and measuring these oscillations in different places can tell us how the structure varies within the planet.
In this way, we know there are two large regions at the top of the core where seismic waves travel more slowly than in surrounding areas. Each region is so large that it would be 100 times taller than Mount Everest if it were on the surface of the planet. These regions, termed large-low-velocity-provinces or more often just “blobs”, have a significant impact on the dynamics of the mantle. They also influence how the core cools, which alters the flow in the outer core.