Ancient rocks hold clues to how Earth avoided a Mars-like fate

A representation of the Earth, at first without an inner core; second, with an inner core beginning to develop about 550 million years ago; third, with an outermost and innermost inner core, about 450 million years ago. Researchers from the University of Rochester used paleomagnetism to determine these two key dates in the history of the inner core, which they believe restored the planet’s magnetic field just before life exploded on Earth. Credit: University of Rochester Illustration/Michael Osadciw

New paleomagnetic research suggests that Earth’s solid inner core formed 550 million years ago and restored our planet’s magnetic field.

The swirling liquid iron in Earth’s outer core, located about 1,800 miles below our feet, generates our planet’s protective magnetic field, called the magnetosphere. Although this magnetic field is invisible, it is vital for life on the Earth’s surface. This is because the magnetosphere protects the planet from the solar wind, from the radiation fluxes from the sun.

However, around 565 million years ago, the strength of the magnetic field dropped to 10% of its current strength. Then, mysteriously, the magnetic field rebounded, regaining its strength just before the Cambrian explosion of multicellular life on Earth.

What made the magnetosphere bounce?

This rejuvenation happened within tens of millions of years according to new research by scientists at the University of Rochester. This is very rapid on geologic timescales and coincided with the formation of the Earth’s solid inner core, suggesting that the core is likely a direct cause.

“The inner core is extremely important,” says John Tarduno, William R. Kenan, Jr., professor of geophysics in the Department of Earth and Environmental Sciences and dean of research for the arts, sciences, and engineering in Rochester. “Just before the inner core started growing, the magnetic field was about to collapse, but as soon as the inner core started growing, the field regenerated.”

In the article, published on July 19, 2022, in the journal Nature Communications, scientists have determined several key dates in the history of the inner core, including a more accurate estimate of its age. The research provides new clues about the history and future evolution of Earth and how it became a habitable planet, as well as the evolution of other planets in the solar system.

Earth layers structure infographic

The layers and structure of the Earth.

Unlock information in ancient rocks

The Earth is made up of layers: the crust, where life exists; the mantle, the thickest layer of the Earth; the molten outer core; and the solid inner core, which is, in turn, composed of an outermost inner core and an innermost inner core.

Earth’s magnetic field is generated in its outer core. The swirling liquid iron causes electric currents in it, resulting in a phenomenon called geodynamo which produces the magnetic field.

Because of the relationship between the magnetic field and the Earth’s core, scientists have been trying for decades to determine how the magnetic field and the Earth’s core have changed throughout our planet’s history. They cannot directly measure the magnetic field due to the location and extreme temperatures of the materials in the core. Fortunately, minerals that rise to the Earth’s surface contain tiny magnetic particles that lock into the direction and strength of the magnetic field as the minerals cool and solidify from their molten state.

To further constrain the age and growth of the inner core, Tarduno and his team used a CO2 laser and the lab’s superconducting quantum interference device (SQUID) magnetometer to analyze feldspar crystals from the rock anorthosite. These crystals contain tiny magnetic needles that are “perfect magnetic recorders,” says Tarduno.

By studying the magnetism locked in ancient crystals – a field known as paleomagnetism – researchers have determined two new important dates in the history of the inner core:

  • 550 million years ago: the moment when the magnetic field began to rapidly renew itself after a near collapse 15 million years ago. The researchers attribute the rapid renewal of the magnetic field to the formation of a strong inner core which recharged the molten outer core and restored the strength of the magnetic field.
  • 450 million years ago: the time at which the structure of the growing inner core has changed, marking the boundary between the innermost and outermost inner core. These changes in the inner core coincide with changes at about the same time in the structure of the overlying mantle, due to plate tectonics on the surface.

“Because we limited the age of the inner core more precisely, we were able to explore the fact that the current inner core is actually made up of two parts,” says Tarduno. “Plate tectonic movements on the Earth’s surface have indirectly affected the inner core, and the history of these movements is imprinted deep within the Earth in the structure of the inner core.”

Avoid a Mars-like fate

A better understanding of the dynamics and growth of the inner core and magnetic field has important implications, not only for uncovering Earth’s past and predicting its future, but also for unraveling the ways in which other planets might form magnetic shields and maintain the necessary conditions for port life.

Researchers believe that[{” attribute=””>Mars, for example, once had a magnetic field, but the field dissipated. That left the planet vulnerable to solar wind and the surface oceanless. While it is unclear whether the absence of a magnetic field would have caused Earth to meet the same fate, “Earth certainly would’ve lost much more water if Earth’s magnetic field had not been regenerated,” Tarduno says. “The planet would be much drier and very different than the planet today.”

In terms of planetary evolution, then, the research emphasizes the importance of a magnetic shield and a mechanism to sustain it, he says.

“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.”

Reference: “Early Cambrian renewal of the geodynamo and the origin of inner core structure” by Tinghong Zhou, John A. Tarduno, Francis Nimmo, Rory D. Cottrell, Richard K. Bono, Mauricio Ibanez-Mejia, Wentao Huang, Matt Hamilton, Kenneth Kodama, Aleksey V. Smirnov, Ben Crummins and Frank Padgett III, 19 July 2022,

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