Earth’s inner core is changing shape, scientists have found.
The discovery resolves a long-simmering controversy about what’s happening at the heart of the planet — which was long thought to be solid and unyielding. But it also opens new questions about how changes in the core could affect the length of our 24-hour day, Earth’s magnetic field and more.
“After decades of research and debates, we are coming to an ever-clearer picture of the changing inner core,” says Xiaodong Song, a seismologist at Peking University in Beijing, who was not involved in the work.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Researchers made the discovery by analysing how seismic waves from earthquakes travelled from the South Sandwich Islands in the South Atlantic Ocean to seismometers in Alaska and Canada, on the other side of the planet. The waveforms, or shapes, of some of the seismic signals changed between 2004 and 2008. Those shifts occurred because the waves briefly penetrated Earth’s inner core — which was changing shape, the scientists say.
“For the first time we’re seeing that it’s deforming,” says John Vidale, a seismologist at the University of Southern California in Los Angeles. He and his colleagues report the finding today in Nature Geoscience.
A dynamic inner Earth
Earth’s inner core is surrounded by an outer core, and the interface between the two, around 5,100 kilometres below the planet’s surface, is a mysterious realm. Previous seismic studies revealed that the inner core is solid metal and rotates within a super-hot molten outer core, also mostly made of metal, including iron and nickel. Researchers have tracked how that rotation speeds up and slows down over time, and have discovered that it is spinning at a slightly different rate from the rest of Earth.
Some scientists have proposed that changes in seismic signals passing through Earth are caused not by the core’s rotational shifts, but instead by physical changes at the inner core–outer core boundary. The new work suggests that both explanations are right. Many changes in the waveforms from the South Sandwich Islands earthquakes can be attributed to core rotation, Vidale says. But others are probably caused by the inner core–outer core boundary deforming by developing bulges in some areas.
The study helps to illuminate a dynamic inner Earth. The inner core grows slowly over time, as iron from the outer core crystallizes onto it. This process drives churning in the outer core, which sustains Earth’s magnetic field. Changes in the inner core’s rotation can also affect the length of our day.
“Ideally, we’d like to tie all these things that we’re seeing together” to make deep Earth less mysterious, Vidale says.
This article is reproduced with permission and was first published on February 10, 2025.