The Earth's inner core has changed shape in a short period: what factors have influenced the planet's "heart"?

The history of Earth spans over 4.5 billion years, during which our planet has undergone an incredible number of changes, both internal and surface-level. For centuries, scientists have been trying to unravel the mysteries of Earth; however, some enigmas remain unsolved — for instance, we know little about the inner core of the Earth, as noted by IFLScience.

In recent decades, researchers have been continuously studying and modeling our planet, yet the inner core of the Earth is still shrouded in numerous mysteries. This is not surprising, considering that this area is hidden more than 5,100 kilometers beneath our feet. In comparison, the deepest hole ever drilled by humans reached a depth of only 12,263 meters.

Fortunately, we can learn more about what happens in the heart of the planet by observing seismic waves traveling through the Earth, as well as the lines of the planet's magnetic field, which are a result of conditions in the core. In a new study, scientists have found evidence that the inner core of the Earth has likely changed shape. Even more intriguing is that these changes have occurred over the last two decades — incredibly fast from a geological perspective.

As early as 2022, researchers utilized data collected during nuclear tests in the 1960s and 70s. Their findings indicated that the inner core had altered its orientation relative to the surface, rotating by a tenth of a degree at least once a year. The scientists concluded that the Earth's core appears to be unstable and fluctuates.

According to the lead author of the 2022 study, Professor of Earth Sciences John E. Vidale, he and his colleagues discovered that the inner core is not fixed but moves beneath our feet — scientists propose that the core shifts by a couple of kilometers every six years. However, researchers have been trying to understand whether the inner core moves gradually or if it is primarily stationary compared to everything else in the long term. The study's authors focused on understanding how the inner core formed and how it moves over time.

Subsequent research provided additional evidence that the core's rotation has been slowing down compared to the rest of the Earth since around 2010. In the new study, Vidale and his team concentrated on examining changes in seismic waves as the Earth's core rotated at different speeds.

The study's results confirmed that the inner core rotated faster and then slower than the rest of the planet over the past few decades. In their work, the researchers analyzed seismic waves crossing the inner core — these were recorded by the Eilson arrays (ILAR) and Yellowknife arrays (YKA) in northern North America from 121 repeating pairs of earthquakes between 1991 and 2023 in the South Sandwich Islands.

In their work, the scientists expanded the set of repeating earthquakes and compared pairs at moments when the inner core returned to the same position, revealing non-rotational changes in YKA, but not in ILAR between 2004 and 2008.

The team examined 168 repeating pairs of earthquakes before and after the core returned to its original position. Scientists were able to analyze whether the changes in seismic waves were caused by the rotation speed or other factors. According to the team, some of the changes in seismic waves were attributed to changes in the shape of the core due to the tension of the outer core and the relationship with the topography of the core-mantle boundary — at least, the team considers this the most likely explanation.

However, the team did not rule out other more "exotic" changes, such as the expulsion of molten material, and further research is necessary. The study's authors found that the change in shape exists alongside a more dominant signal of differential rotation of the entire inner core. Scientists believe they have managed to resolve a long-standing debate — both rotational and non-rotational changes are present, and the latter deserve careful examination to see what is moving in the core.