Earth’s solid inner core formed 550 million years ago

Ancient rocks hold clues.


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Earth’s protective magnetic field, generated by swirling liquid iron in the Earth’s outer core, is invisible but is vital for life on Earth’s surface. It protects the planet from the solar wind. However, the magnetic field’s intensity dropped to 10% of what it is now about 565 million years ago. Just before the Cambrian explosion of multicellular life on Earth, the field unexpectedly rebounded and regained its strength.

What caused the magnetic field to bounce back?

New research by scientists at the University of Rochester suggests that this rejuvenation occurred quickly by geological standards—within a few tens of millions of years—and coincided with Earth’s solid inner core formation, suggesting that the core is likely a direct cause.

John Tarduno, William R. Kenan, Jr., Professor of Geophysics in the Department of Earth and Environmental Sciences and Dean of Research for Arts, Sciences & Engineering at Rochester, said, “The inner core is tremendously important. 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.”

Scientists identified several critical dates in the inner core’s history, offering clues about Earth’s history and future evolution, how it became a habitable planet, and the evolution of other planets in the solar system.

Scientists have been attempting to ascertain how Earth’s magnetic field and core have altered over our planet’s history for decades because of the relationship between the magnetic field and the core. Due to the position and extremely high temperatures of the materials in the core, they are unable to detect the magnetic field directly. Fortunately, when minerals cool from their molten state, small magnetic particles in the minerals lock in the strength and direction of the magnetic field.

Using a CO2 laser and the lab’s superconducting quantum interference device (SQUID) magnetometer, scientists determined the age and growth of the inner core. The minute magnetic needles within these crystals are perfect magnetic recorders.

Scientists studied the magnetism in these ancient crystals to determine two new essential dates in the history of the inner core:

550 million years ago: the time at which the magnetic field began to renew rapidly after a near collapse 15 million years before that. The formation of a solid inner core, which refilled the molten outer core and reinforced the magnetic field, is what scientists believe is responsible for the magnetic field’s quick renewal.

450 million years ago: the time the growing inner core’s structure changed, marking the boundary between the innermost and outermost inner core. Due to plate tectonics on the surface, these changes in the inner core occur at the same time as changes in the mantel’s structure above it.

John A. Tarduno, William R. Kenan, Jr., Professor; Professor of Geophysics, Dean of Research, Arts, Sciences & Engineering, said, “Because we constrained the inner core’s age more accurately, we could explore the fact that the present-day inner core is composed of two parts. 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.”

Understanding the dynamics and growth of the inner core could offer essential clues to the conditions in which other planets might form magnetic shields and sustain the conditions necessary to harbor life.

Tarduno said“In planetary evolution, the research emphasizes the importance of a magnetic shield and a mechanism to sustain it.”

“This research 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.”

Journal Reference:

  1. Zhou, T., Tarduno, J.A., Nimmo, F., et al. Early Cambrian renewal of the geodynamo and the origin of the inner core structure. Nat Commun 13, 4161 (2022). DOI: 10.1038/s41467-022-31677-7


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