What ancient rocks reveal about Earth's inner core

New data indicates that our planet’s innermost core is much younger than we originally thought.

The inner core, the Earth’s deepest layer, is a ball of solid iron surrounded by a liquid outer core of similar composition. Scientists have disagreed on its age, with recent models yielding estimates ranging from 0.5 to 2 billion years ago.

Now, an international team of scientists say they can narrow that range. They analyzed the magnetic signature of ancient igneous rocks and found that there was a sharp increase in the strength of the Earth's magnetic field between one billion and 1.5 billion years ago.

That jump in field strength corresponds to the first occurrence of solid iron at Earth’s center, they argue, the point in Earth history when solid iron first started to "freeze" out from what had been an all-molten core.  

"This finding could change our understanding of the Earth's interior and its history," said lead author Andy Biggin in a press release.

"The timing of the first appearance of solid iron or 'nucleation' of the inner core is highly controversial but is crucial for determining the properties and history of the Earth's interior," he explained.

The age "has strong implications for how the Earth's magnetic field – which acts as a shield against harmful radiation from the sun, as well as a useful navigational aid – is generated."

Currents in the liquid outer core create a dynamo that powers the planet's magnetic field. Many factors contribute to the shape and strength of those currents, but one is convection, the circular movement of hot material rising, giving up heat to the overlying mantle, and then falling again.

Once the inner core started to freeze, this convection received a strong boost in power, argue the researchers, because light, non-metallic elements remained molten in the outer core and were buoyant relative to the overlying mantle.

“The results suggest that the Earth's core is cooling down less quickly than previously thought, which has implications for the whole of Earth Sciences," said Dr. Biggin.

"It also suggests an average growth rate of the solid inner core of approximately 1 millimeter per year, which affects our understanding of the Earth's magnetic field."

Besides offering a more precise birthdate for Earth's inner core, the new analysis, published in the journal Nature, suggests Earth's core-fueled magnetic field will remain strong for a long time.

"The theoretical model which best fits our data indicates that the core is losing heat more slowly than at any point in the last 4.5 billion years and that this flow of energy should keep the Earth's magnetic field going for another billion years or more," Biggin said.

"This contrasts sharply with Mars," he noted," which had a strong magnetic field early in its history which then appears to have died after half a billion years."