We all know that the Earth has a magnetic field – whenever you look at a compass, you can see it in action, making the needle point at magnetic north. We also know what drives the magnetic field – molten metal, mostly iron, in the planet’s outer core swirling around the solid iron inner core.

This swirling can be caused by thermal convection – you can see this in a pot of boiling water. Cool, dense material sinks, while warm, less dense matter rises. Within the Earth, the result is what is known as the geodynamo action, which creates the magnetic field.

But there’s a problem here – it is not easy to cause thermal convection of liquid metal, because heat is transported by conduction very fast in metal. Earth scientists have carefully calculated how much heat would have to be lost for thermal convection, and they realized that the core would have to have been incredibly hot in the past. Evidence for such an extremely hot core and mantle would show up in the rock record. But it doesn’t, so thermal convection alone can’t explain how the early magnetic field has been sustained in the last 3.5 billion years and possibly longer.

There must be some other source of energy helping things along.

Researchers at the Earth-Life Science Institute (ELSI) of the Tokyo Institute of Technology (Tokyo Tech) think they’ve found a solution – “compositional convection.” They believe that as the planet cooled over billions of years, its liquid core started crystallizing, and this removed ingredients from the mix. Liquid with a lower amount of ingredients is denser and thus sinks, contributing to the movement that generates the magnetic field.

To test this idea, the ELSI scientists, led by Prof. Kei Hirose, simulated the conditions at the center of the Earth in their lab. They took tiny dust-size fragments with various amounts of each element that was present in the early core and squeezed them between precision-cut diamonds. At the same time they heated the dust up to thousands of degrees with a laser beam. Then, they looked at the samples with a powerful electron microscope.

They were surprised to find that they had made “quartz” crystals – the same as the ones you can find at the surface of the Earth. They realized that this could be a new way for currents to flow – as the crystals form, the elements in them – silicon and oxygen – are removed from the mix, which alters the properties of the remaining liquid. This contributes to the movement of the liquid.

The research also sheds light on the chemical composition of the core. “The core is mostly iron and some nickel, but also contains some light alloys such as silicon, oxygen sulfur, carbon, hydrogen and other compounds,” Prof. Hirose said.

John Hernlund of ELSI, a co­author of the study, said: “We were excited because our calculations showed that crystallization of silicon dioxide crystals from the core could provide an immense new energy source for powering the Earth's magnetic field.”

This is important for understanding how the Earth formed, how it generated and maintained its early magnetic field, and the conditions when it was young. The scientists think this process could be at work in other planets as well.