Understanding the Earth's core better

To explore Earth’s interior, researchers rely on seismic waves. Despite numerous efforts, drilling has only ever reached a few kilometres beneath the surface. By observing how earthquake waves travel through the planet, scientists can draw conclusions about the materials and structures deep within the Earth. This approach has already uncovered many insights, such as the existence of a solid core and the extreme temperatures at great depths.

One of the most enduring mysteries, however, has remained unsolved for over 50 years. Why do seismic waves accelerate dramatically in what is known as the D” layer, located about 2,700 kilometres beneath the Earth's surface? Now, a team led by Motohiko Murakami, professor of experimental mineral physics at ETH Zurich, has found the answer – and it's surprising not only to the public but also to experts in the field. According to the news article, solid rock flows in this region – neither liquid like lava nor brittle like typical solid rock. The findings were published in the journal Communications Earth & Environment.

Murakami had already made a groundbreaking discovery back in 2004: the primary mineral of the lower mantle, known as perovskite, transforms under extreme pressure and temperature into a new form called post-perovskite. This transformation was originally believed to explain the unusual acceleration of seismic waves. However, as it turns out, that was only part of the story.

Using computer models, the research team discovered the true cause behind the speed changes in the D” layer. It depends on the orientation of post-perovskite crystals. The hardness of the material – and therefore the speed of seismic waves – varies depending on how these crystals are aligned. When all the crystals point in the same direction, seismic waves move significantly faster, exactly as observed in the D” layer at 2,700 kilometres depth.

The researchers also determined what causes this crystal alignment: solid mantle rock slowly flows horizontally along the bottom edge of the mantle. The researchers compare this motion to water beginning to boil in a pot, moving in slow circular patterns. Murakami and his team were able to demonstrate it in their experiments, providing evidence that mantle convection is indeed happening at the boundary between the Earth's core and mantle.

This confirms that solid rock can flow slowly but steadily at extreme depths. The findings fundamentally change how scientists understand the planet’s interior. According to the research team, this discovery shows that the Earth is active not just at the surface but deep within as well. With this new information, the dynamics of the Earth's interior can be understood even better in the future.