The Earth’s magnetic field has existed for at least 3.5 billion years, according to the paleomagnetic record. But the mechanism of the early field has been a mystery, in particular the energy source that powered the long-running dynamo that produces the field. Judging by the high thermal conductivity of the Earth’s core, the temperature of the core must have been too high to sustain permanent magnetism,– unless there was some other source of energy available.

Currently, it is widely accepted that the swirling of liquid metal in the Earth’s fluid outer core, which surrounds its solid inner core, induces a convective dynamo. The solid inner core is roughly the size of the moon but is as hot as the surface of the sun, and is mostly iron. The conventional model is that movement in the fluid outer core is driven by thermal convection upon cooling at the core-mantle boundary.

However, this scenario of thermally driven core convection and resulting geodynamo as the origin of the Earth’s magnetic field has recently been challenged by (1) the upward revision of the thermal conductivity of the core, which implies an inner-core nucleation younger than 1 billion years and (2) a total long-term cooling of the core in excess of 1,000 K to drive thermal convection before 1 billion years ago, which would be required to explain the long-term palaeomagnetic record.

The Earth’s core is about 10% less dense than pure iron, which suggests that it contains light elements as well as iron. Modeling of core formation at high pressure and temperature in a deep magma ocean predicts that both silicon and oxygen are among the impurities in the liquid outer core. However, until now, only alloys of Fe-Si and Fe-O have been studied in detail at high pressures, and little is known about the compositional evolution of an Fe–Si–O alloy under core conditions.

A team at the Earth-Life Science Institute of the Tokyo Institute of Technology, led by Prof. Kei Hirose, recently reported in Nature on their investigations into this mechanism. They examined what happens to cooling liquid alloys in a range of compositions that simulate the presumed conditions in the Earth’s core. They used a diamond anvil and a laser to apply core-like pressures and temperatures to a variety of samples, then examined the textural and chemical characteristics of the resulting samples using an electron microscope.

“In the past, most research on iron alloys in the core has focused only on the iron and a single alloy,” Hirose said. "But in these experiments we decided to combine two different alloys containing silicon and oxygen, which we strongly believe exist in the core."

The researchers were excited to find that when they examined the samples in an electron microscope, the small amounts of silicon and oxygen in the starting sample had combined together to form silicon dioxide crystals (Fig. 2) – the same composition as the mineral quartz found at the surface of the Earth.

They also found that the range of temperatures and pressures at which crystals can precipitate out of liquid metal is much wider than was previously thought. If the crystallization happens at the top of the core, there would have been plenty of buoyancy to power core convection and a magnetic field, from as early as the Hadean eon, more than 4 billion years ago, when the Earth first started to form.

New constraints

The findings also provide insights into the makeup of the inner Earth in the early solar system – there are limits on silicon and oxygen concentrations in the present-day outer core. The results narrow down the possible composition of the Earth’s core. In particular, the outer core cannot include both silicon and oxygen at concentrations higher than is permitted by SiO2 saturation.

The team has also explored the implications of these results for the formation and evolution of the Earth’s core. Crystallization changes the composition of the core by removing dissolved silicon and oxygen gradually over time. Eventually the process of crystallization will stop when the core runs out of its ancient inventory of either silicon or oxygen.

Before the growth of the inner core (which provides a compositional buoyancy source to drive core convection at present), the team estimates that crystallization of SiO2 on top of the core allows for a long-term core cooling rate almost four times less than is required for unassisted thermal convection alone, and the requisite heat flow to sustain the dynamo is also much smaller. Even if one assumes a relatively high core thermal conductivity, as recent studies have proposed, a sufficiently powerful geodynamo could be maintained by core convection by crystallizing SiO2 and cooling at a relatively modest rate.

“Even if you have silicon present, you can’t make silicon dioxide crystals without also having some oxygen available,” said ELSI scientist George Helffrich, who modeled the crystallization process for the study. "But this gives us clues about the original concentration of oxygen and silicon in the core, because more than just one silicon:oxygen ratio is compatible with this model.”

The ELSI team’s experiments go a long way toward resolving the mystery of what powered the long-running geodynamo in the early Earth, as well as the current conditions in the core. In addition, the model is potentially as applicable to magnetic fields operating elsewhere in the Solar System as it is to the geodynamo operating in our own Earth.