The Earth’s magnetic field is generated by currents of molten iron-rich metal in the outer core swirling around the solid iron inner core, producing a dynamo effect. Most scientists believe that the swirling is the result of thermal convection driven by heat escaping from the core, at least before the birth of the inner core.

But the mechanism of the early field, when the Earth was first forming, is a bit of a mystery – in particular what powered the long-running dynamo that generated the early field. This is known as the “new core heat paradox.” Judging by the high thermal conductivity of metallic iron, the temperature of the core would have been extraordinarily high more than 3 billion years ago to sustain permanent magnetism by thermal convection and the resulting dynamo action. So there must be something else going on, some other energy source, to keep the ball rolling.

Crystal energy

Research at the Earth­Life Science Institute (ELSI) at the Tokyo Institute of Technology (Tokyo Tech), by a team led by Prof. Kei Hirose, indicates that this additional energy source could be “compositional convection.” The idea is that, under the extreme conditions of the core, crystals of silicon dioxide form and precipitate out of the liquid metal. This depletes the silicon and oxygen from the mix, which changes the buoyancy of the liquid, and this induces convection as the less buoyant liquid sinks.

The ELSI team reported in the scientific magazine Nature on their investigations into this mechanism. They examined what happens to a variety of alloys, spanning a range of compositions, under conditions that mimic those in the Earth’s core.

The scientists used precision-cut diamonds as “anvils” to squeeze tiny dust­size samples to the pressures that exist at the Earth's core, and a laser to simulate the high temperatures. They then examined the textural and chemical characteristics of the resulting samples under an electron microscope.

They were surprised to find that the small amounts of silicon and oxygen in the starting sample had combined together to form silicon dioxide crystals ­­ the same composition as mineral quartz found at the surface of the Earth.

The search of alloys began to yield interesting results when Hirose and his collaborators began mixing more than one alloy, including iron. “In the past, most research on iron alloys in the core has focused only on the iron and a single impurity,” Hirose said. “But in these experiments we decided to combine two different light elements, silicon and oxygen, which we strongly believe existed in the core.”

In addition to core cooling and a possible energy source for the geomagnetic field, the research gives some indications of the chemical composition of the core. “The core is mostly iron and some nickel, but also contains about 20% light alloys, such as silicon, oxygen, sulfur, carbon and hydrogen,” Hirose said. “We think that many alloys are simultaneously present, but we don't know the proportion of each candidate element.”

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.” He added, “This result proved important for understanding the energetics and evolution of the core.”

The team has also explored the implications of these results for the formation of the Earth. As the crystals form, the composition of the entire core changes as silicon and oxygen are gradually removed over time.

The ELSI team’s experiments go a long way toward resolving the mystery of what powered the long-running geodynamo in the early Earth. In addition, this compositional convection could be powering magnetic fields elsewhere in the Solar System.