Ancient crystals have revealed when Earth's inner core finally formed

You can thank your lucky stars that this happened 550 million years ago.
Christopher McFadden
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A 3D drawing of a cross section of the crust to the inner core of the earth.Christoph Burgstedt/iStock
  • Researchers traced back the history of Earth's inner core across hundreds of millions of years
  • They relied on crystals in rock to study the Earth's core indirectly
  • Understanding how the inner core changed can help us predict how it might change again

In Earth Sciences news, a group of researchers has been able to reconstruct the evolution of Earth's deep core over hundreds of millions of years by analyzing ancient rock crystals and the magnetism records that are stored within them.

Geologists currently know that the solid inner core of the Earth, a hot, compact glob of iron and nickel, is sandwiched between the molten outer core. The crust that we live on sits above the rocky mantle, which is the thickest of all the layers, that separates the crust from the core. The distance between the crust and core is incredibly large, with a distance of around 1,800 miles (2,900 kilometers).

According to the researchers' results, it appears that around 550 million years ago, Earth's inner core was beginning to crystallize into a sizable mass. The magnetic field, which had been depleted some 15 million years prior, was restored by this event, which also created the ideal conditions for the rise and explosion of life on Earth’s surface.

The outer core's turbulent liquid iron is what generates and maintains Earth's magnetic field, which, in turn, shields life on Earth from the worst effects of solar winds. However, the solid iron-nickel alloy at the center also has a crucial function to play as an energy source, as this recent study makes clear.

"The inner core is tremendously important," said geophysicist John Tarduno of the University of Rochester, New York. "Right before the inner core started to grow, the magnetic field was at the point of collapse, but as soon as the inner core started to grow, the field was regenerated," he added.

"This research really highlights the need to have something like a growing inner core that sustains a magnetic field over the entire lifetime – many billions of years – of a planet," Tarduno explained.

The ancient history of the planet is written in the rocks

Scientists rely on crystals in rock, in this case, feldspar crystals in anorthosite, to study the Earth's core indirectly because doing so would be nearly impossible due to the vast distances and extreme temperatures. In any case, these crystals serve as extremely precise magnetism recorders.

The researchers were able to determine the shift in magnetic strength, which represents a dramatic return of the Earth's magnetic field, by comparing rocks that were dated to about 565 million years ago to rocks that were dated to around 532 million years ago. Although the transformation took tens of millions of years, this is a short period of time from a geological perspective.

According to thermal models based on the findings, the inner core's structure changed about 450 million years ago, separating the innermost from the outermost inner core. These times also line up with changes in the mantle.

"Because we constrained the inner core's age more accurately, we could explore the fact that the present-day inner core is actually composed of two parts," says Tarduno.

"Plate tectonic movements on Earth's surface indirectly affected the inner core, and the history of these movements is imprinted deep within Earth in the inner core's structure," he added.

Understanding how the inner core changed to its current state can help us predict how it might change again in the future and provide a benchmark for comparison when researching other planets.

To see what would have occurred if the inner core hadn't expanded and given Earth's magnetic field the push it needed to become powerful enough to deflect dangerous solar radiation from the surface, we simply need to look at Mars.

Over the course of billions of years, the Martian atmosphere has been stripped away by solar winds because it lacks a worldwide magnetic field to protect it. This has also removed the water and oxygen necessary for life to properly flourish.

"Earth certainly would've lost much more water if Earth's magnetic field had not been regenerated," says Tarduno. "The planet would be much drier and very different than the planet today."

You can view the entire study in the journal Nature Communications.

Abstract:

“Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core.”

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