Earth’s inner core is young at heart

The iron in Earth’s inner core was once liquid. Over time, the core cooled and solidified, stabilizing the geodynamo to its more or less present configuration. But geologists have had a hard time determining when that solidification occurred. Estimates range from 0.5 billion years ago, when complex, multicellular organisms first appeared, to as early as 2.5 billion years ago, when Earth’s atmosphere began accumulating oxygen. Now Richard Bono of the University of Liverpool, John Tarduno of the University of Rochester, and their colleagues have developed an accurate age estimate by measuring the magnetism of ancient minerals that crystallized less than 600 million years ago. That lowest-ever derived time-averaged dipole moment suggests that the inner core cooled far later in Earth’s history than many geologists previously thought.

The mineral samples came from Canada’s Sept-Ȋles intrusive suite. Those rocks formed about 565 million years ago when magma rose through Earth’s crust and crystallized before reaching the surface. Tiny inclusions in the samples retain the magnetic signature they acquired when the minerals first cooled. Using the magnetometer on an ultrasensitive superconducting quantum interference device, the researchers measured individual crystals, such as the one shown in the photo here, and calculated the paleomagnetic dipole moment of the geodynamo.

The resulting dipole moment, 0.76(47) x 10²² A m² (left in the illustration), is the lowest ever reported and less than ⅒ of the present-day field strength (right). But it’s not quite low enough to arise solely from the external magnetic field generated by the solar wind. The result suggests that a weak geodynamo did exist, possibly because the inner core was only just beginning to solidify. Bono and Tarduno combined their results with a small database of paleomagnetic directional values from the same time period as the samples were from. The observations, which are consistent with numerical geodynamo simulations, support the young inner-core hypothesis and the idea that the paleomagnetic field may have undergone a period of rapid reversals or instability.

Pinning down when the inner core solidified may assist geologists in answering other scientific questions. An unambiguous age can help them not only better estimate the core’s cooling rate, which may affect volcanism, but also determine when plate tectonics began. And though geologists only have preliminary data, there may be a link between decreased magnetic shielding and biological evolution. (R. K. Bono et al., Nat. Geosci. 12 , 143, 2019 .)