Strength

The magnetic record stored in rocks documents the liquid core's behavior and possibly when the inner core formed. Whether it formed half a billion or more ...

However, such experiments have led to [different results through different approaches](https://eos.org/features/earths-core-is-in-the-hot-seat), enough so that various scenarios of the age of the inner core are possible. A rock that cools quickly, on the order of hundreds or thousands of years, will record a snapshot of the magnetic field. [Courtney Sprain](https://sites.google.com/site/courtneyjsprain/), a paleomagnetist at the University of Florida who was not involved in this study. As Earth cooled, thermal convection—and the intensity of the magnetic field—should have tapered. This timeline led to the intriguing hypothesis that the inner core began to form sometime after 565 million years ago—remarkably young. “Even if there was more than 10,000 years between [multiple samples’] cooling times, we wouldn’t be able to resolve it [because] the ages would overlap,” Sprain said. A rock that takes many tens or hundreds of thousands of years to cool smooths out the magnetic field’s short-term variation. But continued cooling eventually led to the beginnings of Earth’s solid metal heart, which should have boosted the waning magnetic field as compositional convection overtook its thermal counterpart. The rocks that Zhang targeted are [unique aggregations](https://doi.org/10.1029/2021GC009909) of crystals known as anorthosite xenoliths that formed deep in Earth’s crust but were brought close to the surface with magma that fed lava eruptions into the rift. [study](https://www.pnas.org/doi/abs/10.1073/pnas.2202875119) published in the Proceedings of the National Academy of Sciences of the United States of America, paleomagnetist and University of California, Berkeley doctoral student [Yiming Zhang](https://eps.berkeley.edu/people/yiming-zhang) and his coauthors collected and studied rocks from the (failed) North American Midcontinent Rift—a region where 1.1 billion years ago there was voluminous volcanism. That transcription is possible as long as the rocks remain relatively untouched by high temperatures, fluids, or other traumas of tectonics. As they archive evidence of the geodynamo’s billions-of-years-long existence,