Ancient rocks hold clues to how Earth avoided a fate similar to Mars

Ancient rocks hold clues to how Earth avoided a fate similar to Mars

Formation of the Earth's inner core

A depiction of Earth, first without an inner core; second, with an inner core beginning to grow, around 550 million years ago; third, with an outermost and innermost inner core, around 450 million years ago. University of Rochester researchers 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 the explosion of life 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 the planet’s magnetic field.

Swirling liquid iron in Earth’s outer core, which lies about 1,800 miles below our feet, generates the planet’s protective magnetic field, called the magnetosphere. Although this magnetic field is invisible, it is vital to life on Earth’s surface. That’s because the magnetosphere shields the planet from the solar wind – streams of radiation from the sun.

But around 565 million years ago, the strength of the magnetic field dropped to 10 percent of its strength today. Then, mysteriously, the magnetic field bounced back, regaining its strength just before the Cambrian explosion of multicellular life on Earth.

What caused the magnetosphere to bounce back?

This rejuvenation occurred over the course of a few tens of millions of years, according to new research from researchers at the University of Rochester. This is very fast on geological time scales and coincided with the formation of Earth’s solid inner core, suggesting that the core is likely a direct cause.

“The inner core is enormously important,” says John Tarduno, William R. Kenan, Jr. Professor of Geophysics in the Department of Earth and Environmental Sciences and Dean of Research for Arts, Sciences and Engineering at Rochester. “Just before the inner core began to grow, the magnetic field was at the point of collapse, but as soon as the inner core began to grow, the field regenerated.”

In the paper, published on 19 July 2022, in the journal Nature communication, the researchers determined several key dates in the history of the inner core, including a more precise estimate of its age. The research provides new clues about the Earth’s history and future development and how it became a habitable planet, as well as the development of other planets in the solar system.

Earth layer structure infographic

Earth’s layers and structure.

Unlocking information in ancient rocks

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

The Earth’s magnetic field is generated in the outer core. Swirling liquid iron there causes electric currents, driving a phenomenon called the geodynamo that produces the magnetic field.

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

To better 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 bergartosite. These crystals have tiny magnetic needles in them that are “perfect magnetic recorders,” says Tarduno.

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

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

“Because we more precisely constrained the age of the inner core, we were able to explore the fact that today’s inner core actually consists of two parts,” says Tarduno. “Plate tectonic movements on the Earth’s surface 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 uncovering the ways in which other planets can form magnetic shields and maintain the necessary the conditions for harbor life.

Scientists think so[{” 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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