A groundbreaking study reveals that water from the Earth's surface reaches the core, changing its composition and suggesting more active interactions between the core and mantle and a more complex global water cycle. Decades ago, seismologists imaging deep parts of the Earth discovered a thin layer of water, just a few hundred kilometers thick. Until now, the origin of this thin layer, known as the "eosphere", has been a mystery.

Illustration showing silicon crystals flowing out of liquid metal in the Earth's outer core due to chemical reactions caused by water. Source: DanShim/University of the National Academy of Sciences

An international team of researchers, including Arizona State University scientists Dan Shim and Taehyun Kim, and Joseph O'Rourke of the School of Earth and Space Exploration, revealed that water from the Earth's surface can penetrate deep into the planet, changing the composition of the outermost region of the metallic liquid core, forming a unique thin layer.

Their research results were published in the journal Nature Geoscience on November 13.

Research shows that over billions of years, surface water has been transported deep into the Earth by descending or subducting tectonic plates. Upon reaching the core-mantle boundary, about 1,800 miles below the surface, this water triggers profound chemical processes that alter the structure of the core.

Schematic diagram of Earth's interior showing subducting water and rising magma columns. At the interface between the subducted water and the core, chemical exchange occurs, forming a hydrogen-rich layer in the uppermost outer core and dense silica at the bottom of the mantle. Source: Yonsei University

Chemical interactions at the core-mantle boundary

Shim and his team, together with YongJae Lee of South Korea's Yonsei University, used high-pressure experiments to demonstrate that subduction water chemically reacts with Earth's core materials. This reaction forms a hydrogen-rich, silicon-poor layer that changes the top outer core region into a film-like structure. In addition, the silica crystals produced by the reaction rise and merge into the mantle. This modified layer of liquid metal is predicted to be less dense and reduce seismic velocities, consistent with unusual features mapped by seismologists.

"For many years, it was thought that the exchange of material between the core and the mantle was small. However, our recent high-pressure experiments revealed a different story. We found that when water reaches the core-mantle boundary, it reacts with silicon in the core to form silica." This discovery, along with our previous observation of water reacting with carbon in molten iron to form diamonds at extreme pressure, suggests that the interaction between the core and mantle is more active, indicating a large amount of material exchange.

The discovery advances our understanding of Earth's internal processes and shows that the global water cycle is more extensive than previously appreciated. The altered core "film" has profound consequences for the geochemical cycles linking surface water circulation to the deep metallic core.